A Reappraisal of Redox Melting in the Earth s Mantle as a Function of Tectonic Setting and Time

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1 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 PAGES1363^ doi: /petrology/egq061 A Reappraisal of Redox Melting in the Earth s Mantle as a Function of Tectonic Setting and Time STEPHEN F. FOLEY* GEOCYCLES RESEARCH CENTRE AND INSTITUTE OF GEOSCIENCES, UNIVERSITY OF MAINZ, BECHERWEG 21, MAINZ, GERMANY RECEIVED FEBRUARY 2, 2010; ACCEPTED SEPTEMBER 14, 2010 ADVANCE ACCESS PUBLICATION NOVEMBER 3, 2010 Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH 4 -andh 2 -bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks.this oxidation mechanism causes an increase in the activity of H 2 O by the reaction of CH 4 with O 2, and the increased ah 2 O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-date the first mechanism in the same geographical area, explaining the sequence of igneous rock types from lamproites to ultramafic lamprophyres that occurs during the development of rifts through cratons. The CRM mechanism relies on the oxidation of solid carbon as graphite or diamond that has accumulated in the lithosphere over time. The solidus temperature for rocks with both CO 2 and H 2 O is lower than in conditions with H 2 O alone; it does not occur at depths less than 65 km, but has recently been confirmed experimentally to depths of at least 200 km. Melts produced by HRM are not SiO 2 -undersaturated, even at depths of 200 km, and may often resemble lamproites or SiO 2 -rich picrites, whereas melts produced by CRM are always SiO 2 -undersaturated and range from carbonatitic to ultramafic lamprophyric or melilititic with increasing degree of melting. The operation of redox melting may be more common than has been recognized because the oxidation state of the upper mantle is not uniform as a function of depth, geodynamic setting or geological time. The general decrease of oxygen fugacity (fo 2 )ofc.0 7 log units per 1GPa pressure increase dictates that rapidly subducted oceanic lithosphere will be considerably more oxidized than ambient mantle peridotite at depths of 200^300 km. Hydrothermal alteration (serpentinization), addition of continental or carbonate sediments, and dehydration reactions during subduction all contribute to the heterogeneity of oxidation states in the subducted slab, which may vary over 6 log units; this raises the potential for redox reactions on local and regional scales. The oceanic lithosphere has a lower average fo 2 than either continental or cratonic mantle lithosphere at a given depth, so that the HRM mechanism dominates in recycled blocks and at the base of the continental lithosphere. The higher thermal gradients dictate that HRM is more common in the modern Earth beneath ocean islands and in upwelling mantle currents than in subduction zones. The oxidation state of the mantle is often described as having been constant since 3 5 Ga, but this overlooks the bias towards continental samples. Redox melting of oxidized recycled blocks (at approximately the fayalite^magnetite^quartz buffer) in the mantle was not important in the Hadean and Archaean, as it had to await the gradual oxidation of the mantle and the establishment of the subduction process, as well as the stabilization of the continents. The lack of CRM explains the lack of carbonatites before 2 7 Ga. However, the lower fo 2 of the Archaean asthenosphere and higher volatile contents caused more prevalent HRM in the Hadean and Archaean mantle. Degassing is controlled by solubility of volatile species in melts, which are H 2 O-rich but C-poor in reducing conditions. Silicate melts under reduced conditions contain much less carbon but more nitrogen than melts in the modern mantle, arguing for a nitrogen-rich, CO 2 -poor early atmosphere. KEY WORDS: redox melting; craton; recycled crust; Archean; mantle degassing; fo 2 *Corresponding author. Telephone: þ Fax: þ foley@uni-mainz.de ß The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@ oup.com

2 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 INTRODUCTION The oxidation state of the Earth s mantle has remained controversial for decades. Both the chemistry of volcanic rocks and their constituent minerals and minerals in mantle peridotites have been used as oxygen sensors; however, the information they provide often does not match up. Whereas volcanic rocks are usually found to lie in the oxygen fugacity (fo 2 ) range of the nickel^nickel oxide (NNO) or fayalite^magnetite^quartz (FMQ) oxygen buffers (Carmichael & Nicholls, 1967; Haggerty & Tompkins, 1983; Foley, 1985; Carmichael, 1991; Ballhaus, 1993; Canil, 1997; Be zos & Humler, 2005; Lee et al., 2005; Mallmann & O Neill, 2009), the mantle peridotite samples, whether from xenoliths in volcanic rocks or from alpine peridotite bodies, show a greater range at lower oxygen fugacities between the FMQ and iron^wu«stite (IW) buffers (Fig. 1; Eggler, 1983; O Neill & Wall, 1987; Wood & Virgo, 1989; Wood, 1991; Woodland & Koch, 2003; Woodland et al., 2006; Frost & McCammon, 2008). An increasing body of evidence indicates that the deeper mantle is even more reduced and may be metal-saturated (Ballhaus & Frost, 1994; Rohrbach et al., 2007; Frost & McCammon, 2008). Figure 1 indicates the position of the oxygen buffers referred to above and shows the range of fo 2 typical for most volcanic and mantle rocks. The wide range of oxygen fugacities exhibited by mantle peridotite samples indicates that, provided that oxidation states vary over small distances, there is a large potential for redox-controlled reactions that will include melting reactions. This would be in keeping with modern conceptions of the mantle as containing a complex intermingling of pyroxenites, eclogites and other mineral assemblages with peridotite (Alle' gre & Turcotte, 1986; Foley et al., 2001; Jacob, 2004; Sobolev et al., 2007), thus resembling a migmatite more than the uniform peridotite as in older notions. The melting points of peridotite and other ultramafic rocks likely to be present in the mantle have been shown by experimental studies to be extremely susceptible to the presence of even small amounts of volatile components such as H 2 O, CO 2 and CH 4 (Green, 1973; Eggler, 1976; Wyllie, 1978; Taylor & Green, 1988; Dasgupta & Hirschmann, 2006; Foley et al., 2009). The term redox melting was coined by Taylor (1985) for melting caused by the increase in water activity as a result of the oxidation of reduced H 2 O þch 4 fluids, accompanied by the precipitation of solid carbon. It was, however, anticipated by Wyllie (1980), who wrote if reduced oxygen fugacity raises the solidus for the system peridotite^c^h^o... partial melting would then only occur where temperature were raised or oxygen fugacity was increased. Here, the concept of redox melting is extended to include a second mechanism of melting in more oxidized conditions in which a further drop in the melting point is achieved by the introduction of CO 2 into the system by the oxidation of carbon to carbonate. Recent experimental studies at 4^6 GPa (Foley et al., 2009) confirmed the findings of Wallace & Green (1988) at lower pressures (53 1GPa) that the melting temperature of peridotite in the presence of both H 2 O and CO 2 is lower than with H 2 OorCO 2 alone. The change in melting temperature under oxidizing conditions is not as large as that under reducing conditions, but the melting points are still lower, making the appearance of the effects of this second mechanism in the rock record more likely, particularly in cratonic areas. The melting curves of mantle peridotite at various oxidation states were surprisingly poorly constrained until recently, and there is still considerable room for improvement under reduced conditions. Furthermore, the involvement of ultramafic assemblages other than peridotite is now thought to be important during melting of the mantle (Foley, 1992a, 1992b; Hirschmann & Stolper, 1996; Pertermann & Hirschmann, 2003; Sobolev et al., 2005, 2007); however, the effect of oxygen fugacity on the melting of these components is yet to be studied. In the context of this study, I argue that melting of the mantle as a result of redox reactions is more important than has been realized, and in some geodynamic situations, such as cratonic mantle lithosphere and the deep recycling of lithospheric blocks, it may be more important than temperature changes. The rejuvenation of cratonic blocks by episodic infiltration of small-degree partial melt is likely to result from the first, reduced, redox melting mechanism, here termed hydrous redox melting (HRM), and later by the second, more oxidized, mechanism (carbonate redox melting; CRM). The consecutive action of both redox melting mechanisms is a logical consequence of the erosion of cratonic lithosphere mantle (Foley, 2008). The HRM mechanism may have been more important in the first half of Earth history before oxidation of the uppermost parts of the mantle and the surface. The operation of redox melting during the deep recycling of lithospheric mantle blocks and in melting processes in the early Earth and in the evolution of the Earth s mantle is discussed below. THE OXIDATION STATE OF THE MANTLE LITHOSPHERE AND ASTHENOSPHERE It is now recognized that the oxygen fugacity in the Earth s upper mantle varies by several orders of magnitude both vertically and laterally (Ballhaus & Frost, 1994; Woodland et al., 2006; Foley, 2008; Frost & McCammon, 2008; Mackwell, 2008). However, available data on the upper mantle oxidation state are often generalized to give an average value that lies around FMQ 1 tofmq(fig.1). Average values tend to be emphasized [e.g. FMQ 0 41 for mid-ocean ridge basalt (MORB) glasses, Be zos & 1364

3 FOLEY REDOX MELTING IN MANTLE Fig. 1. A comparison of commonly used oxygen buffers plotted in terms of oxygen fugacity against temperature (a) and pressure (b). OIE, olivine^iron^enstatite corrected for Mg-number ¼ 90 indicates the approximate fo 2 of metal saturation in mantle peridotite, although a Ni-rich metal appears at around IW (O Neill & Wall, 1987). EMOG ¼ enstatite-magnesite-olivine-graphite. CW is not a buffer, but indicates the position of the maximum H 2 O content in COH fluids. The plot against pressure (b) is less commonly used, but shows the divergence of FMQ from NNO and the higher relative fo 2 of CW at high pressures, which promotes the importance of the reduced realm in Fig. 9. Oxygen fugacities throughout this study are given relative to FMQ. Volcanic rocks range mostly around and slightly above FMQ^NNO (grey box; Carmichael, 1991) whereas xenolith samples of mantle rocks range from slightly above FMQ down to IW (very few samples in the light grey part of box). These ranges are not temperature-specific, but are intended to show the generally higher oxidation state of volcanic rocks relative to their mantle sources. 1365

4 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 Humler, 2005; FMQ þ 0 24 for continental spinel lherzolites, Bryndzia & Wood, 1990], whereas exceptions, deviations and regional variations that are potentially important for the operation of redox melting are not. Therefore, the information on the redox state of the uppermost mantle is summarized here with a view to assessing the heterogeneities resulting from juxtaposition of blocks because of geodynamic movements, or from possible changes in the oxidation state through the evolution of the Earth. Information on the oxidation state of the upper mantle can potentially be gleaned from several sources, including the Fe 3þ /Fe 2þ ratios and redox-sensitive trace element ratios of volcanic rocks, oxybarometry on mantle-derived minerals and xenoliths, on diamonds and mineral inclusions within them, and from evidence for the presence of minerals such as carbonates whose stability is restricted to particular fo 2 conditions. The most studied, and consequently best understood, region of the mantle in the context of oxygen fugacity is the continental lithosphere, based on extensive calculations with oxybarometers that depend on the exchange of iron as Fe 3þ and Fe 2þ between component minerals of peridotites (Mattioli & Wood, 1986; O Neill & Wall, 1987; Wood & Virgo, 1989; Ballhaus et al., 1991). There are several calibrations, each with an uncertainty of around 0 5 log units fo 2, so that even a mantle with uniform fo 2 would show a scatter of calculated fo 2 values. However, I show here a compilation of the available data as a series of histograms (Figs 2^4), which shows that the variations are much larger than the uncertainties, such that blocks with contrasting fo 2 are likely to become juxtaposed in the mantle. A complicating factor is that the fo 2 of peridotites should decrease by c. 0 7 FMQ units per 1GPa increase in pressure (Ballhaus & Frost, 1994). For this reason, the information contained in the histograms is summarized in Fig. 5, which shows a series of parallel lines representing the average fo 2 for peridotites in various tectonic settings. The orthogonal distances between the parallel lines represent real, pressureindependent differences in the fo 2 of peridotites in these tectonic settings. Garnet peridotites sampled by kimberlites in cratonic regions have consistently been shown to have lower fo 2 and to show a greater range of fo 2 between FMQ and FMQ 5, with most values falling between FMQ 2 and FMQ 4 (Frost & McCammon, 2008). These are shown here to have an average of FMQ 2 83 (Fig. 2, top left panel), although there appear to be craton-specific regional differences as shown particularly by the lower average fo 2 of FMQ 4 05 for the Baltic craton (Fig. 3, left panels; Woodland & Peltonen, 1999). The large range for cratonic peridotites is shown in Fig. 5 (after Frost & McCammon, 2008) together with the slope expected from the pressure effect (Ballhaus & Frost, 1994; thick blue line). This allows a pressure correction for comparison with peridotites from other tectonic settings. The effect of pressure is illustrated by the difference between non-cratonic lithosphere and abyssal peridotites, which show a similar range in the histograms (Fig. 2), but are 41 log unit fo 2 apart in Fig. 5. This is because of the lower equilibration pressure of the abyssal spinel lherzolites (assumed average of 1GPa or 33 km depth, corresponding to the approximate depth of separation of melts from their mantle source) in comparison with the deeper-derived continental xenoliths, many of which contain garnet. For the oceanic mantle, much less information is available, and the age information is much more restricted because of the ephemeral nature of oceanic crust and lithosphere (the ocean crust is restricted to the last 54% of Earth history). An indirect method of estimation of the oxidation state at mid-ocean ridges is the Fe 3þ /Fe 2þ ratio in mid-ocean ridge basalts. The first compilation by Christie et al. (1986) found fo 2 values of FMQ 1 35 for Atlantic MORB glass rinds and FMQ 1 5 for Pacific MORB glass rinds ( MORB-1 in Fig. 5), whereas the more slowly crystallized basalt cores of pillows showed the more oxidized values of FMQ þ 0 5 for the Atlantic and FMQ þ 0 3 for the Pacific. This information was emphasized in a previous contribution on redox melting (Foley, 1988), not only for the more reduced state of oceanic relative to continental basalts, but also as evidence for late-stage oxidation of the melts between rapid quenching of the glassy rims and slower cooling of the basalt pillow cores. The first line of evidence has been called into question by a more recent compilation of glass analyses: Be zos & Humler (2005) found a more oxidized average value of FMQ 0 41 ( MORB-2 in Fig. 5), and this has been used as confirmation of the homogeneity of mantle redox states (Frost & McCammon, 2008). However, the discrepancy of 41 log unit fo 2 in pressure-corrected values between abyssal peridotites (Bryndzia et al., 1989; Bryndzia & Wood, 1990) and continental peridotites remains (Fig. 5), and a more reduced state of the oceanic mantle is also favoured by the expectation that melts will oxidize during emplacement (Ballhaus, 1993), as was also indicated by the difference in Fe 3þ /Fe 2þ ratios between pillow centres and their rapidly chilled rims (Christie et al., 1986). Assuming the same pressure dependence of 0 7 FMQ units per GPa pressure (Ballhaus & Frost, 1994), the average value for abyssal peridotites of FMQ 1 05 can be extrapolated to higher pressures to form an array for the convecting sub-oceanic asthenosphere (dashed line in Fig. 5) that is about 0 7 log units below that of the cratonic lithosphere and 41 log unit below the non-cratonic continental lithosphere. This difference means that the average fo 2 of abyssal peridotites at 35^45 km beneath mid-ocean ridges 1366

5 FOLEY REDOX MELTING IN MANTLE Fig. 2. Variation of oxidation states in mantle rocks as a function of tectonic setting on a global scale, expressed as the deviation (FMQ) from the FMQ buffer (see Fig. 1 for position relative to other buffers). The average for the oceanic mantle is lower than for the non-cratonic continental lithosphere, which itself shows considerable variation. Higher values are typical of supra-subduction zone and ocean island settings. The limited data for rift-influenced continental mantle include modifed Archean mantle, and may therefore manifest a considerable oxidation effect relative to the generally uniformly lower oxidation state of cratonic lithosphere. Data sources: cratonic mantleçluth et al. (1990), Woodland & Peltonen (1999), McCammon et al. (2001), Woodland & Koch (2003), McCammon & Kopylova (2004), Creighton et al. (2009, 2010), Lazarov et al. (2009); oceanic and abyssal peridotitesçbryndzia & Wood (1990), Nasir (1996), Canil et al. (2006); continental lithosphere (non-cratonic)ç Wood & Virgo (1989), Luth et al. (1990), Ionov & Wood (1992), Qi et al. (1995), Luhr & Aranda-Gomez (1997), Nasir et al. (2006, 2010); supra-subduction zone xenolithsçwood & Virgo (1989), Canil et al. (1990), Brandon & Draper (1996), Parkinson & Arculus (1999), McInnes et al. (2001), Parkinson et al.(2003), Bryant et al.(2007),wanget al. (2008, 2009); oceanic within-plate xenolithsçballhaus (1993); rift-influenced continental mantleçrudnick et al. (1994), Foley et al. (2006); orogenic massifsçwoodland et al. (1992,1996, 2006), Song et al. (2009). corresponds to about 85^95 km in the continental lithosphere. Further variation is visible when one considers rift-related xenoliths and those from supra-subduction zone environments. The rift-influenced peridotites are from cratonic (Tanzania) or close to cratonic (eastern Antarctica) regions, and show higher pressure-corrected fo 2 than the rest of the continental mantle. This is barely visible from the histogram (Fig. 2), but variations within the sample set from both areas show that the lowest fo 2 values are from samples least affected by overprinting (Rudnick et al., 1994; Foley et al., 2006). This is interpreted to indicate the oxidizing effect of melt infiltration in the plume-related uplift of the Tanzanian craton and beneath the developing Lambert^Amery rift in Antarctica (Foley et al., 2006). 1367

6 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 Fig. 3. Variation in the oxidation state of cratonic lithosphere of Archaean age (left panels) and in supra-subduction zone peridotites from continental arc (upper right) and island arc (lower right) settings. Data sources as in Fig. 2. Whereas the variation in cratonic mantle lithosphere may be largely owing to the effect of pressure (see also Fig. 5), the arc peridotites show a larger range in oxidation states at a more restricted range of pressures. It has frequently been shown that later metasomatism of peridotites caused by the infiltration of melts leads to an increase in the fo 2 of the peridotites (McGuire et al., 1991; Frost & McCammon, 2008; Creighton et al., 2009), so that the position of the continuous blue line for cratonic peridotites in Fig. 5 would be lower by c. 1 log unit if it is to represent the original pre-metasomatic redox state. Information from orogenic peridotite massifs (average FMQ 0 21; Fig. 2) documents this oxidizing effect of melt infiltration (Woodland et al., 1996). Histograms for orogenic peridotites with textural and chemical evidence for melt infiltration are compared with those for probably unaffected (or less affected) peridotites in Fig. 4. The average for the metasomatically overprinted peridotites is FMQ þ 0 38, 0 86 log units higher than for the unaffected peridotites (FMQ 0 48). It is increasingly recognized from textural relationships and chemical zonation that clinopyroxenes and garnets in continental peridotites are often introduced later (Glaser et al., 1999; Simon et al., 2007; Rehfeldt et al., 2008). This is important for the reconstruction of mantle conditions during Archaean times; as melt infiltration tends to correlate with increasing oxidation state, the implication is that the cratonic mantle originally had lower fo 2 than it now displays. The fo 2 of peridotites from the mantle wedge above subducting slabs is often summarized as being more oxidized than in stable continental areas (McInnes et al., 2001; Canil, 2002; Parkinson et al., 2003; Malaspina et al., 2009). The average of available data is FMQ þ 0 51 (Fig. 2, top right panel), including a substantial proportion of Fig. 4. Oxidation state of peridotites from orogenic massifs showing the oxidizing effect of metasomatic melt infiltration averaging c.0 86 log units fo 2. Data sources as in Fig. 2. Melt infiltration by similar processes is thought to be widespread in the mantle lithosphere, particularly the lower continental lithosphere. rocks with fo 2 4FMQ þ1. There is also a distinct difference between peridotite xenoliths from island arcs (average FMQ þ1 3) and those from continental arcs (average FMQ 0 16; Fig. 3, right panels). The pressure-corrected average (Fig. 5) is 1 6 log units above the cratonic mantle, and 2 3 log units above the asthenosphere value. These more oxidized values are usually attributed to the infiltration of melts or water-rich fluids associated with the subducting slab. However, evidence for the sense of redox 1368

7 FOLEY REDOX MELTING IN MANTLE Fig. 5. A summary of oxygen fugacity data from mantle samples as a function of their pressure of origin (modified after Frost & McCammon, 2008). The continuous blue line has a slope of 0 7 FMQ units per GPa (Ballhaus & Frost, 1994) indicating the pressure effect on fo 2 that explains most of the cratonic array with a scatter of 1 log units. The dashed line is drawn parallel, but originates from the average value for abyssal peridotites (yellow square at FMQ 1 05; see Fig. 2), showing that the asthenospheric mantle should be more reduced than even the cratonic peridotites in their present state. MORB-1 is the average fo 2 of MORB after Christie et al. (1986), and MORB-2 the corrected value after Be zos & Humler (2005). The more oxidized position than average abyssal peridotites reinforces the conclusion of Christie et al. (1986) that MORB melts oxidize on their way to the surface. Further lines for the non-cratonic continental lithosphere (green), rift-influenced continental lithosphere (red) and supra-subduction zone peridotites (orange) are plotted parallel, but through the average values shown in the histograms of Fig. 2 plotted at the depths indicated by thermobarometry. The grey symbols show the positions at 1GPa, a reasonable depth for extensive melting beneath mid-ocean ridges, to facilitate direct comparison of the relative oxidation state in different tectonic settings. The array of lines indicates a variation of at least 2^3 log units fo 2 at any depth. change during subduction processes is complex and partly contradictory; there are examples of subduction metasomatism with fo 2 between 1 and 5 log units below FMQ (Song et al., 2009; Wang et al., 2009), and metal-bearing peridotites are known from the sub-kamchatka mantle (Ishimaru et al., 2009). A more recent development in the estimation of redox states in the mantle source regions of magmas uses the V/Sc ratio (Li & Lee, 2004; Lee et al., 2005). This is based on the similar incompatibility of the two elements during melting, so that their fractionation at various degrees of melting should be minimal, coupled with the fact that vanadium exists in several oxidation states, whereas Sc occurs only in the 3þ state in geological conditions. There are now enough results to compare melts from different geodynamic environments, but these are not consistent with those from oxygen barometry: current estimates see no consistent difference between MORB, ocean island basalts and island arc basalts (Lee et al., 2005; Mallmann & O Neill, 2009) despite the differences delineated above from oxygen barometry of peridotites. The V/Sc ratio of basalts has also been used to suggest that the oxidation state of basalt sources has remained essentially constant through geological time, with a V/Sc average for modern MORB of 6 74 compared with an average for Archaean basalts of 6 34, indicating a change of less than 0 3 FMQ units since 3 5 Ga (Li & Lee, 2004). A similar result was obtained from Cr abundances in volcanic rocks by Delano (2001). However, the oxybarometry results for cratonic peridotites indicate a great range of redox states, with some appreciably more reduced than the extrapolation of abyssal peridotite values (Fig. 5; Ballhaus, 1993; Woodland & Peltonen, 1999; Kadik, 2003), leading to an apparent contradiction. Rare occurrences of 1369

8 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 coexisting cohenite (Fe 3 C) and Fe-metal in cratonic diamonds (Jacob et al., 2004) would appear to correspond to the lowest possible fo 2 conditions for diamond formation (Wood, 1993). A possible solution to the paradox is that the data on Archaean basalts considered by Delano (2001), Li & Lee (2004), and Berry et al. (2008) are restricted to continental settings, which show only the more oxidized part of the upper mantle range on the modern Earth. The same is true for late Archaean komatiites, which are also no more reduced than modern continental basalts (Canil, 1997; Berry et al., 2008). The best estimates for the composition of Archaean oceanic crust are provided by cratonic eclogite xenoliths in kimberlites; a compilation of the limited available V/Sc data gives an average of 7 38, encompassing eclogites from West and South Africa, Siberia and Canada (Jacob & Foley, 1999; Barth et al., 2001; Jacob et al., 2005; Smart et al., 2009), with regional averages ranging from 4 7 to Care must be taken here to eliminate fractionated rock compositions, which tend to have higher V/Sc, because many eclogites are thought to represent former gabbros and not volcanic rocks (Jacob, 2004). Li & Lee (2004) introduced a selection window of 8^12% MgO to restrict their attention to unfractionated volcanic rocks. If we restrict our attention to the very few data for eclogites inferred from their compositions and high d 18 O values to be picritic volcanic rocks and not gabbros (Jacob & Foley, 1999), then the V/Sc range of 4 9^5 9 may indicate slightly more reducing conditions for oceanic crust production at 2 6 Ga than the current global MORB average (6 74; Li & Lee, 2004). The exact difference in terms of fo 2 depends on the degree of melting (Li & Lee, 2004), but is between 0 5 and 0 7 log units, bearing in mind the picritic composition of the volcanic late Archaean eclogites (Jacob & Foley, 1999). If the oxidation of the mantle was gradual and continuous, then the early Archaean mantle was probably more than one log unit more reduced than in modern MORB magma sources. This contrasts with the conclusions from Cr and V/Sc in continental Archaean volcanic rocks, and may indicate that redox contrasts between tectonic settings in the Archaean were at least as large as those seen in Figs 2^5 for the modern Earth. The continental signal from the Archaean is not typical, it is just preferentially preserved. The picture emerging from these various lines of evidence provides abundant potential for the geodynamic juxtaposition of blocks with contrasting redox state, both in terms of average values and the variation of redox states in all geodynamic environments (Figs 2^5). The pressure effect dictates that more reduced conditions will prevail in the lower cratonic lithosphere and also in the asthenospheric mantle in close proximity to it. In these regions the HRM mechanism is likely to be widespread, whereas the CRM mechanism will be commoner closer to the surface, or where more oxidized blocks are found at great depth. Another implication of the pressure effect is that the hypothesized initiation of melting beneath mid-ocean ridges at c. 300 km in the presence of CO 2 (Dasgupta et al., 2007) must be considered very unlikely because the redox state will generally be far too low to allow the stability of CO 2. Instead, methane and/or diamond would dominate the carbon species and the mantle may even be metal-saturated at these depths (Ballhaus & Frost, 1994; Rohrbach et al., 2007; Frost & McCammon, 2008). The most important conclusions for the following discussion are that the boundary between the lithosphere and convecting asthenosphere is likely to correspond to a redox front, and that the range of redox states at any given depth is 42 log units even without the presence of recycled blocks, leaving ample potential for redox melting reactions. REDOX MELTING BY HYDRATION (HRM): OXIDATION OF REDUCED FLUID COMPONENTS The redox melting mechanism originally proposed by Taylor (1985) is referred to here as redox melting by hydration (HRM) and operates in relatively reduced conditions slightly above the IW buffer at depths corresponding to the lower cratonic lithosphere (Fig. 1). Given the evidence summarized above for the oxidation state of mantle rocks, it can be expected to apply to melt production in deeper levels of the mantle (150^250 km), except directly below mid-ocean ridges where it may operate at shallower levels. Melting is a indirect product of the oxidation of methane to form water and solid carbon: CH 4 þo 2 ¼ 2H 2 O þ C: The increase in ah 2 O results in depression of the melting temperature by hundreds of degrees in the presence of water (Green, 1973). The effect is shown in Fig. 6a on a plot of the C/(C þ H 2 ) ratio against fo 2. All mixtures of C þ H þ O fluid components falling within the grey shaded region are carbon-saturated, so that solid carbon in the form of either graphite or diamond coexists with a fluid composition lying on the continuous curved line (Frost, 1979). The C, H and O components of a fluid under reducing conditions around the IW buffer consist principally of CH 4,H 2 O and H 2, and will release water whilst precipitating carbon on encountering more oxidized rocks. The increase in water activity may result in melting if the solidus is depressed sufficiently to meet the ambient geotherm. Whether melting occurs will depend on the concentration of volatile components and thus the amount of water released by the redox mechanism and the variation in geothermal gradient with geodynamic setting. The rapid change in the H 2 O/CH 4 ratio of fluids in the fo

9 FOLEY REDOX MELTING IN MANTLE Fig. 6. The mechanism of redox melting by hydration (HRM). (a) Carbon saturation curve for COH fluids at realistic mantle melting conditions of 3 GPa and 1300 K (10278C). Compositions in the pale grey area consist of solid carbon plus a fluid with a composition on the curved line (carbon saturation curve). Fluids at intermediate fo 2, about halfway between IW and FMQ, consist of 496% H 2 O at this pressure and temperature, effectively separating a region of reduced CH 4 þ H 2 O fluids from oxidized CO 2 þ H 2 O fluids.when reduced fluids come into contact with oxidized material they move vertically in the diagram until carbon saturation is reached, and then increase in H 2 O content, which can lead directly to melting by the depression of melting point by increased water activity (see Fig. 8). The CH 4 /H 2 O ratio is strongly dependent on fo 2 at about IW þ1, as is the CO 2 /H 2 O ratio at FMQ 0 5. (b) Species abundance as a function of fo 2 between FMQ and IW ^ 1, showing the rapid transition from CO 2 -toh 2 O-rich fluids at FMQ 0 5, and from H 2 O- to CH 4 -rich fluids at IW þ1 (FMQ 3toFMQ 4). The line water-maximum denotes the fo 2 at which H 2 O content of the fluid is highest. region 0 5^1 5 log units above IW (at the temperature^ pressure conditions of Fig. 6) restricts the operation of HRM to this narrow range of fo 2 : this is clearer in Fig. 6b, in which the molar amounts of fluid species are shown plotted against fo 2 for the same pressure^temperature conditions as in Fig. 6a. The carbon saturation curve indicates that fluid compositions pass through a maximum H 2 O content (496% in Fig. 6a) between 1 5 and 2 0 log units above IW, and then experience a rapid increase in CO 2 /H 2 O over the next 1 log unit, reaching extremely CO 2 -rich and H 2 O-poor compositions whilst still below FMQ. This will be important for the second, CRM, mechanism at a later stage. The possibility of melting as a result of HRM depends on the geothermal gradient and on the magnitude of the increase in ah 2 O. The effect of increasing ah 2 O on the solidus of peridotite is estimated in Fig. 7 from the limited experimental data on the reduced solidus by Taylor & Green (1988). This diagram only approximates conditions in the Earth s mantle because it does not account for dissolved silicates in the fluid, which will increase in importance towards higher pressures, possibly resulting in termination of the solidus at the second critical end point (Kessel et al., 2005). Above this pressure, supercritical liquids that compositionally mimic melts will occur at temperatures below the extrapolation of the solidus (white dotted line; 1371

10 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 Fig. 7. Dependence of the melting curve (solidus) of peridotite on water activity (after Taylor & Green, 1988). The intermediate curves for ah 2 O ¼ 0 35, 0 70 and 0 85 are most relevant for redox melting (HRM), whereby the increasing solubility of silicate material in fluids towards the higher pressures shown here decreases the ah 2 O in fluids. The higher temperatures relative to water-saturated conditions dictate that HRM is a realistic mechanism for the continental lithosphere as indicated by the SE Australia geotherm, and may also occur in the lower reaches of cratons where the deposition of diamond will result. The white circle indicates the approximate position of the second critical end-point in peridotitic systems: the solidus does not exist at higher pressures than this (dotted line), but is replaced by a continuum between fluid- and liquid-like behaviour termed supercritical liquid to emphasize its liquid-like element partitioning behaviour (Kessel et al., 2005). Kessel et al., 2005). The position of the second critical end point, shown in Fig. 7 as a white circle, is very poorly known, with experimental estimations varying from 3 8 to 10 GPa (Stalder et al., 2001; Mibe et al., 2007). The position shown at a depth of c. 220 km is probably a minimum for peridotite, as this is only slightly above the pressure determined for eclogite (Kessel et al., 2005), which has a much lower MgO concentration. Because of its high content of MgO and low contents of SiO 2, Na 2 O and Al 2 O 3, the correct position of the second critical end point for natural mantle peridotite may be deeper than the lower end of Fig. 7. As the second critical end point is approached, the amount of silicate material dissolved in the fluid increases greatly and this will result in a drop in ah 2 O, leading to an increase in the solidus temperature. A third uncertainty concerns the solidus of peridotite in H 2 O-rich conditions. Grove et al. (2006) determined the solidus temperature to be as low as 8508C at pressures between 2 5 and 3 GPa with 14 5wt % H 2 O, which is 150^ 2008C lower than that shown in Fig. 7. However, Green et al. (2010) interpreted the glasses in Grove et al. s experiments as being quenched out of a fluid phase: their results indicate a solidus temperature of 13508C at 6 GPa, which is close to the ah 2 O ¼ 0 85 line in Fig. 7, possibly because of high silicate solute concentrations. Despite these uncertainties, Fig. 7 illustrates the effect of increasing ah 2 O in decreasing the melting temperature, and thus the principle of the HRM mechanism. Referring to the example of Fig. 6, the water activity at fo 2 ¼ IW before the HRM mechanism begins is likely to be well below 0 35 as a result of the abundance of CH 4 at this fo 2. The solidus for these redox conditions (ah 2 O ¼ 0 35) is far above the geotherms for continental areas (Fig. 7). Following oxidation of methane accompanied by the deposition of carbon, the ah 2 O will have increased to around the 0 85 line, and the peridotite solidus will be very close to the geothermal gradient. It should be noted here that CH 4 þ H 2 O fluids are strongly non-ideal (Taylor, 1985; Matveev et al., 1997), so that a small molar CH 4 content results in a higher ach 4, and correspondingly lower ah 2 O, thus accentuating the role of changing CH 4 /H 2 O in the fluid. At higher pressures than those 1372

11 FOLEY REDOX MELTING IN MANTLE higher pressures where fluids with 499% H 2 O are stable over several log units fo 2 (Woermann & Rosenhauer, 1985). The HRM mechanism remains constrained to a small fo 2 range at the lower fo 2 end of this water maximum, so that the fo 2 range applicable to HRM moves towards lower fo 2 at higher pressures as shown in Fig. 9. This diagram divides oxygen fugacity^pressure space into reduced and oxidized realms (Foley, 1994), separated by the lightly coloured central region in which all fluids are extremely water-rich, so that no important change in fluid composition can occur to cause melting in this region. In terms of the potential for redox melting, this area of intermediate fo 2 is relatively inert or barren. The conditions of HRM are indicated by the diagonally lined region as being limited to a very restricted range of fo 2 (Fig. 9). The rapid change from H 2 O-rich to CO 2 -rich fluids in less than one log unit fo 2 above the water maximum also constrains melting to be restricted to a very small fo 2 range. Thus, redox melting is active only in the diagonally lined regions in Fig. 9 despite the existence of a wide range of oxidation states in peridotite samples (Arculus, 1985; Frost & McCammon, 2008). Fig. 8. Pressure effect on the width of the water maximum. Curved lines show the carbon saturation curves (as in Fig. 6) for 3, 5, 7 and 10 GPa. Fluids below the curve are CH 4 þ H 2 O mixtures, whereas those above the curve are CO 2 þ H 2 O mixtures. These regions are separated by a point of extreme H 2 O content in the fluid (circles). Squares indicate the positions of intersection with the IW buffer. The pale shaded area emphasizes the existence of fluids with 490% H 2 O over 6 log units fo 2 at depths of 300 km, in contrast to the darker shaded area for 3 GPa, which spreads over only 2 5^3 log units. This pressure effect leads to the polarization of redox melting conditions shown by the width of the pale shaded area in Fig. 9. corresponding to Fig. 6, the water maximum widens and the influence of CH 4 is suppressed still further (H 2 O499%; Fig. 8). Here, the relevant peridotite solidus will be essentially indistinguishable from the watersaturated solidus in Fig. 7 and melting will occur where the solidus cuts the geothermal gradient, which should apply in parts of the lower cratonic lithosphere. The water maximum is less marked at lower pressures, but HRM will nevertheless apply away from cratons because the form of the reduced solidus (Taylor & Green, 1988) is conducive to cutting the higher geothermal gradients at depths of 60^100 km. The exact form of the carbon saturation surface and its position relative to fo 2 buffers varies with pressure and temperature, whereby particularly the pressure effect is important for mantle melting by HRM. Figure 8 shows that the water maximum is more marked and wider at REDOX MELTING BY CARBONATION (CRM): OXIDATION TO CARBONATE- STABLE CONDITIONS A second mechanism by which melting of peridotite can occur as a result of a change in fo 2 without transport of heat has become apparent from recent experiments on the melting of peridotite in the presence of both H 2 O and CO 2 (Foley et al., 2009). A further increase in fo 2 from conditions corresponding to the water maximum will lead to a rapid increase in the ratio of CO 2 to H 2 O in the fluid as long as the fluid remains saturated in carbon (Fig. 6). The oxidation may occur by infiltration of fluids or melts along cracks in the lithosphere or along grain boundaries in the convecting mantle, or by simple juxtaposition of blocks with contrasting oxygen fugacities by geodynamic movements. If little or no carbon is present in the rock or in the infiltrating fluid, then the carbon will soon be exhausted and the system cannot proceed to CO 2 -rich conditions. However, saturation in carbon will be ensured wherever solid carbon is abundant in the rock in the form of either graphite or diamond, so that the previous operation of HRM in the same rock would provide the ideal pretreatment of the mantle for this second mechanism. The abundance of diamonds in the lower cratonic lithosphere, documented by peridotite and eclogite xenoliths as well as mineral inclusions in diamonds (Boyd & Gurney, 1986; Stachel & Harris, 2008) thus means that the lower cratonic lithosphere may be especially susceptible to CRM. 1373

12 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 Fig. 9. Regions of operation of the two redox melting mechanisms in pressure^fo 2 space. The dark shaded areas indicate reduced, CH 4 - bearing and oxidized CO 2 -bearing conditions (as in Fig. 6). In the palest shaded area, H 2 O accounts for 490% of the C^O^H components in the fluid, and this widens towards higher pressures, thus effectively polarizing the melting areas (diagonal line shading), so that the fo 2 values of HRM and CRM diverge towards higher pressures. Melting conditions are limited to small areas on this graph, but the mantle can be quickly oxidized across the H 2 O-rich area. The lines for asthenosphere, cratonic mantle, continental lithosphere and supra-subduction zone mantle correspond to the average oxygen fugacity lines in Fig. 5. These indicate that CRM is unlikely at lower lithosphere levels unless an unusual oxidation event triggered by the oxidation of solid carbon is realized. CRM may be important in subduction zones. In contrast, much of the mantle at depths of 100^300 km will be close to the fo 2 of operation of the HRM mechanism, which may thus be a prevalent cause of incipient melting. The rapid production of CO 2 dictates that the melting curve now corresponds to peridotite melting in the presence of CO 2 and H 2 O. Experiments on peridotite with small amounts (0 4^2 wt %) of both H 2 O and CO 2 by Wallace & Green (1988) indicated that the melting point lies at 960^9808C between 65 and 100 km and that a field for carbonatitic melt lies directly above the solidus. This corrected earlier experiments (see Olafsson & Eggler, 1983, and references therein) in which the solidus temperature may have been overestimated as a result of alkali^carbonate melt pockets being dissolved during preparation of polished mounts. However, only recently have experiments determining the position of the solidus at higher pressures (43^10 GPa) become available. First results came from melting of peridotite in the presence of CO 2 only (i.e. without H 2 O), which indicate the presence of carbonate-rich melt at temperatures close to the water-bearing solidus (Canil & Scarfe, 1990; Dasgupta & Hirschmann, 2006; Brey et al., 2008). Experiments at 4^6 GPa with similar small amounts of both H 2 O and CO 2 to those of Wallace & Green (1988) have recently confirmed that the solidus is lower for the mixed volatile phase with respect to either CO 2 or H 2 O alone (Fig. 10; Foley et al., 2009). These experiments are from a K 2 O-enriched peridotite composition, but the position of the solidus with similar amounts of H 2 O and CO 2 is confirmed in unenriched Hawaiian and 1374

13 FOLEY REDOX MELTING IN MANTLE depending on pressure; Figs 8 and 9), the oxidation of methane causes an increase in ah 2 O, which in turn depresses the melting point of the rocks. This is the HRM mechanism. In more oxidized conditions (FMQ 1 5 to FMQ 0 5 depending on pressure), redox melting is caused by the oxidation of solid carbon to carbonate. A very minor amount of water is needed to minimize the solidus temperature, but this CRM is essentially a dry mechanism steered principally by carbonation; it does not produce H 2 O but depends on reduction of ah 2 O by the resulting carbonate. The fo 2 of operation of the two redox melting mechanisms diverges to greater depths (Fig. 9). Fig. 10. Comparison of melting curves for reduced (CH 4 þ H 2 O; after Taylor & Green, 1988; Green & Falloon, 1998) and oxidized (CO 2 þ H 2 O; Foley et al., 2009) conditions. The reduced solidus follows H 2 O-undersaturated, CO 2 -free melting to 3 GPa and then a low-ah 2 O solidus at higher pressures. During hydrous redox melting (HRM), the solidus moves to lower temperatures owing to the increase in ah 2 O as indicated by the arrows. Melts are not SiO 2 -undersaturated and resemble olivine lamproites. The oxidized solidus for H 2 O þ CO 2 -bearing conditions is that determined at 4^6 GPa by Foley et al. (2009), and extrapolated as a dashed line to higher pressures parallel to the solidus for CO 2 alone (Dasgupta & Hirschmann, 2006). Melts just above the solidus are carbonate-rich; the transition to SiO 2 -undersaturated silicate melts (25^40 wt % SiO 2 ) needs experimental clarification at pressures 46GPa. However, these solidi may cease to exist if the second critical end-point is reached (white-filled circles show possible approximate positions). This possibility is indicated by the pale grey areas in which supercritical liquids will resemble melts more than fluids in terms of their element compositions (Kessel et al., 2005). (See text for further discussion.) UML ¼ultramafic lamprophyre. MORB-pyrolite (S. F. Foley et al., unpublished data). The same uncertainties in the position of the second critical end point apply for Fig. 10 as for Fig. 7. This melting curve means that melting is likely to occur in the lower reaches of cratonic mantle lithosphere as long as the appropriate mixture of volatiles is available. Thus, both HRM and CRM mechanisms have the potential to cause melting in the lower cratonic lithosphere and in areas of the mantle in other geodynamic settings at depths of 120^300 km (Figs 7, 8 and 10). A comparison of the P^T positions of cratonic and asthenospheric geotherms with the volatile-free peridotite solidus indicates that melting at this depth is otherwise unlikely (McKenzie & Bickle, 1988; Wyllie, 1988). In summary, redox melting at two distinct and well-defined fo 2 conditions in the mantle is caused by different petrological mechanisms. In reduced conditions (IW þ 0 5 to IWþ1 5 or FMQ 3 5 to FMQ 4 5, MELT COMPOSITIONS PRODUCED BY THE HRM AND CRM MECHANISMS The question of whether redox melting as opposed to decompression melting or melting as a result of an increase in temperature is most important in the natural conditions corresponding to diverse geodynamic situations will eventually be answered by comparing the melt compositions found in high-pressure experiments with those of natural volcanic rocks or interstitial phases in mantle samples. Redox melting processes may often be concentrated in areas enriched in ultramafic rock types other than peridotite. However, experimental determinations of melt compositions in mantle assemblages such as garnet pyroxenite (Irving, 1974; Adam et al., 1992; Pertermann & Hirschmann, 2003) or those containing abundant hydrous minerals (Lloyd et al., 1985; Thibault et al., 1992; Foley et al., 1999) are much rarer and less comprehensive than studies of peridotite melting, and the effects of redox state on melt composition are still only poorly known. Here, melt compositions expected for CRM and HRM are compared with those likely to be produced by other mechanisms under various conditions to form the basis for an assessment of the relative importance of redox melting that can be improved by future experimental investigations. If the convecting mantle contains only trace amounts of H 2 O and CO 2, as is generally thought (Wyllie, 1980; Dasgupta et al., 2007), then the solidus of peridotite will generally lie considerably above the geothermal gradient expected for the mantle defined by a conductive lid overlying an adiabatic gradient within the convecting mantle (Wyllie, 1988). The position of this geotherm depends largely on the temperature in deeper parts of the mantle, and thus on the convection patterns, but in none of the cases illustrated in Fig. 7 does the geothermal gradient cross the volatile-free solidus. This means that the mantle is mostly in the solid state; it is only in exceptional circumstances that it melts. 1375

14 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 The most geodynamically reasonable scenarios for the partial melting of volatile-free peridotite in the upper mantle are by decompression melting of upwelling mantle beneath mid-ocean ridges at depths of 20^50 km and by melting in the laterally spreading heads of mantle plumes beneath either oceanic or continental lithosphere (Wyllie, 1988; McKenzie & Bickle, 1988). This conclusion is in agreement with geophysical observations of the upper mantle such as seismic-wave velocity (White et al., 1992) and electrical conductivity profiles (Shankland & Waff, 1977), which discount widespread extensive melting except in these regions of the uppermost 120 km of the mantle. Melting in plume heads beneath oceanic lithosphere will be concentrated at depths of 70^100 km, depending on the distance from the nearest mid-ocean spreading centre and thus on the thickness the conductive lid of lithosphere has attained, whereas the source regions of continental flood basalts are likely to be deeper (Neal et al., 1997). Extensive experimental data on the melt compositions generated by melting of dry peridotite show that there is a general increase in MgO and alkali content, and a decrease in silica activity with increase in pressure. In addition, at a given pressure, an increase in the temperature, and thus the degree of melting, leads to a decrease in alkali content and an increase in MgO (Green, 1970; Jaques & Green, 1980; Takahashi, 1986; Falloon & Green, 1987, 1988; Kinzler & Grove, 1992; Herzberg & O Hara, 1998). These trends would result in the production of tholeiitic melts at depths of 15^40 km beneath mid-ocean ridges (Falloon et al., 1988; Fujii, 1989), whereas plume-head melts at higher pressures would be olivine basalt to picrite. Low-degree melting at 460 km depth may produce alkaline, silica-undersaturated compositions that explain the alkaline magma series observed in ocean islands (Jaques & Green, 1980). Integrated studies of experimental petrology and mineral chemistry (Green et al., 2001) indicate that mantle temperatures and potential temperatures are probably higher than commonly modelled (e.g. McKenzie & Bickle, 1988), so that picritic melts may be commoner beneath both mid-ocean ridges and ocean islands than is often thought. In Archaean times, higher mantle temperatures may have led to the production of komatiitic melts (18^30% MgO; Arndt, 1977; Nisbet et al., 1993), although it is debatable if these melts were restricted to plumes (Abbott et al., 1994; Be dard, 2006), or were typical of oceanic crust in the Archaean (Arndt, 1983; Nisbet &Fowler,1983). Experimental studies of melts relevant to hydrous redox melting are extremely rare: Taylor & Green (1988) melted pyrolite in the presence of mixed H 2 O þ CH 4 fluids and delineated the melting curve between 1 5 and 3 5 GPa (corresponding to 50^120 km depth), but did not report analyses of melt compositions. Indeed, there are no published melt compositions from peridotite with this volatile mixture available to date, so that melt compositions can only be estimated from indirect information. The geodynamically reasonable scenarios for HRM are (1) close to the base of continental, particularly cratonic, lithosphere owing to the proximity of the solidus to the ambient geotherm, and (2) interaction of reduced mantle with more oxidized, recycled blocks within the asthenosphere (Green et al., 1987). In the second of these, a higher geothermal gradient applies, and melting is achieved by suppression of the solidus curve owing to the increase in water activity by the HRM mechanism (Fig. 7). Melt compositions can be estimated from the solubilities of available volatile species and their effects on the structure of silicate melts taken from studies of melt compositions that do not correspond directly to melts of garnet or spinel peridotite. C^O^H fluid compositions at upper mantle pressures and fo 2 conditions corresponding to the HRM mechanism are dominated by CH 4 and H 2 O (Fig. 6). Water is known to be very soluble at high pressures and to depolymerize the aluminosilicate network of silicate melts by breaking bridging oxygen bonds, causing a reduction in the average size of aluminosilicate species (Burnham, 1979; Stolper, 1982; Mysen et al., 1982). The petrological corollary of this effect is the expansion of the stability fields of minerals with higher ratios of network-modifying cations to network-forming cations, meaning that olivine is stabilized relative to pyroxenes during melting (Kushiro, 1975; Gupta & Green, 1988). Partial melts of peridotite with H 2 O alone may be alkaline (nephelinite or basanite; Green, 1973; Millhollen et al., 1974) because of the presence of low-degree melts over a considerable temperature range before major melting occurs (Green & Falloon, 1998). The few studies of the solubility mechanisms of methane in silicate melts indicate that only 0 2 wt % carbon dissolves in reduced form (Taylor & Green, 1987) and this may increase to a maximum of 0 5% by dissolution of CH 4 groups in less polymerized melts such as nephelinites with NBO/Tof 0 9 (Mysen, 1987; Mysen et al., 2009). This major difference in solubilities of H 2 O and CH 4 means that the position of the solidus and the melt compositions are determined principally by H 2 O, with CH 4 having little more than a dilution effect on the water activity (Taylor & Green,1988; Fig.7). Melt compositions produced by HRM will, therefore, be similar to melts of peridotite with small amounts of H 2 O; these may include alkaline compositions, but the lack of carbonate ions in the melt will prevent a strong degree of undersaturation in silica. A similar effect has been shown in lamproitic systems with mixed H 2 O and CH 4 volatiles present, for which no silica-undersaturated melts are produced at all at depths up to at least 200 km (Foley, 1993). Melt compositions produced at higher oxygen fugacities by the CRM mechanism will differ greatly from those 1376

15 FOLEY REDOX MELTING IN MANTLE produced by HRM. This is due to the presence of abundant CO 2 and the contrasting effect it has on the structure of silicate melts, which is to cause polymerization of the aluminosilicate network by forming complexes with network-modifying cations (Mysen et al., 1982). This expands the stability field of pyroxenes and garnet relative to olivine in the melting peridotite, meaning that the contribution of olivine components to the melt is greater so that melts have lower SiO 2 contents than their dry or hydrous counterparts at the same pressure. The solubility of carbon in the form of carbonate at pressures 420 kbar is high, of the order of 20 wt % compared with 50 5wt % at 51kbar (Brey, 1976; Wyllie & Huang, 1976), so that its effects on melt compositions are strong. Experiments on peridotite with CO 2 alone at upper mantle pressures have shown initial melt fractions to be carbonatitic with less than 10 wt % SiO 2 (Sweeney, 1994; Dasgupta et al., 2007; Brey et al., 2008). At higher melt fractions, the SiO 2 contents of melts increase, but the resulting carbonate-bearing silicate melts are still notably SiO 2 -poor compared with anhydrous melts, corresponding to melilitites or ultramafic lamprophyres. Current results differ as to whether the transition from carbonatitic to silicate melts is abrupt (Moore & Wood, 1998; Dasgupta et al., 2007) or continuous (Brey et al., 2008; Foley et al., 2009; Litasov & Ohtani, 2009a). Experiments with both H 2 O and CO 2 are most relevant to the CRM mechanism, as these correspond to the stable volatile mixture (Fig. 6) and together suppress the melting point more than CO 2 alone (Fig. 10). First results by Wallace & Green (1988) at 3 GPa emphasized the high Na 2 O contents of initial carbonatitic melts, whereas later experiments have shown that K 2 O can also be enriched in carbonatitic melts (Thibault et al., 1992; Sweeney, 1994; Foley et al., 2009; Ghosh et al., 2009). A potentially important effect of increasing pressure is that initial melts may become less carbonatitic towards 50^60 kbar (Foley et al., 2009). The difference in melt types produced by the two redox melting mechanisms in comparison with melts of dry peridotite is best illustrated by considering the melt compositions expected at similar pressure conditions to those commonly resulting from decompression melting of dry peridotite. For example, beneath ocean islands, melting will be concentrated beneath the oceanic lithosphere at depths of 70^100 km, where initial melts will be slightly nepheline-normative and higher degree of melts will be picritic (Jaques & Green, 1980). Melting in reduced, H 2 O þ CH 4 -bearing conditions at similar depths will occur at lower temperatures owing to the influence of H 2 O, and melt compositions will be mildly alkaline. Here, the influence of water on depressing the melting temperature is more important than its depolymerizing effect on the melt structure. This is because the drop in solidus results in a wide temperature interval of low-degree melting referred to by Green (1990) as the incipient melting regime. Within this temperature interval, melts remain alkaline, and would become SiO 2 -richer than dry melts only at higher temperatures within the major melting regime. At the same depths, melting by CRM will occur at even lower temperatures than HRM, and melts will be markedly lower in SiO 2, resembling melilitites or much lower SiO 2 melts resembling ultramafic lamprophyres that are probably not seen at the surface in an unreacted state. DISCUSSION AND APPLICATIONS Partial melting in the upper mantle as a result of one of the redox melting mechanisms can occur wherever redox state varies greatly over relatively small distances, and may operate in a greater variety of conditions and geodynamic settings than the original definition of redox melting intended (Taylor, 1985; Taylor & Green, 1987). This was restricted to HRM at low oxygen fugacities close to the IW buffer (Figs 3 and 5), whereas the CRM mechanism may operate in different geodynamic situations, and partly in the same settings at a later stage of development (Foley, 2008). Figures 2^5 demonstrate that variation of fo 2 in mantle rocks is characteristic of all tectonic settings, and that the redox contrast of blocks derived from differing tectonic settings (e.g. oxidized subducted blocks in a reduced deep mantle environment) will in many cases be strong. Here, three situations are considered in which redox melting may be or have been most important through the evolution of the Earth; (1) the rejuvenation of cratonic lithosphere by thinning and erosion as a precursor to continental rifting; (2) the interaction of recycled lithospheric blocks from subduction or delamination processes with ambient mantle at deeper levels; (3) redox melting in reduced conditions in the upper mantle of the Hadean to Archaean Earth. This is not an exhaustive list of possibilities, as the juxtaposition of rock types with contrasting lithologies and oxidation states may be common in the convecting upper mantle (Alle' gre & Turcotte, 1986; Foley et al., 2001; Sobolev et al.,2007). Rejuvenation of cratons and rifts through cratons Cratons are typified by the long-term stability of the crust and lithospheric mantle beneath it; however, evidence is mounting that there may be more magmatic activity at the base of cratons than has generally been assumed. This evidence comes from the proven removal of the lithosphere beneath the North China craton (Xu, 2001; Gao et al., 2004), from young melt infiltration events in peridotite xenoliths (Konzett et al., 2000; Simon et al., 2007; Rehfeldt et al., 2008), geochemical investigations of inclusions in diamonds (Richardson et al., 1993; Shimizu & Sobolev, 1377

16 JOURNAL OF PETROLOGY VOLUME 52 NUMBERS 7 & 8 JULY & AUGUST 2011 Fig. 11. Redox melting during the rejuvenation and breakup of cratons may be due to operation of both HRM and CRM mechanisms consecutively. During the initial stages of breakup of the North Atlantic craton to produce the Labrador Sea, melting in reducing conditions produced lamproites at 1400^1200 Ma. Following the erosion of the base of the craton root, impingement of the more oxidized upwelling asthenosphere caused redox melting owing to depression of the solidus from reduced conditions (CH 4 þ H 2 O; dashed line, right panel) to oxidized conditions (CO 2 þ H 2 O; continuous line, right panel). Incipient melting following the initial oxidation event first causes veining of the overlying mantle wedge that is still reduced, and with further development of the rift base, the solidus depression moves as a wave upwards and re-melts the recently enriched wedge. The resulting melts derived by CRM are ultramafic lamprophyres, emplaced mostly during two episodes at around 610 and 55 Ma. Further development of the rift resulted in melililtic to nephelinitic magmatism during the Mesozoic, derived from depths of 100^120 km, with nearby reactivation of ultramafic lamprophyre melting as a result of the steeply sloping sides of the Archaean cratonic lithosphere, which still allows CRM at high pressures (left panel). G = graphite, D ¼ diamond. Diagrams combined and modified after Tappe et al. (2006, 2007). 1995; Jacob et al., 2000), and from rift magmatism. The most extreme form of reactivation of cratonic lithosphere is manifested in successful rifts through cratons that proceed to the production of oceanic lithosphere, as in the Labrador Sea rift between Canada and Greenland. In other areas, similar processes can be seen in peridotite xenoliths and volcanic rocks of unsuccessful and current rifts at the margins of cratonic blocks, as in Antarctica (Foley et al., 2006) and Congo^Tanzania (Link et al., 2010). The Labrador Sea area provides evidence for the series of igneous rocks produced during the development of cratonic rifts and has been described in detail by Tappe et al. (2006, 2007, 2008). It results from several episodes prior to and including the rifting event that eventually produced new oceanic crust: (1) a 1400^1200 Ma event produced lamproitic magmas from great depths; (2) a 610^570 Ma event produced mostly carbonate-rich ultramafic lamprophyres; (3) a Mesozoic event produced nephelinitic and similar rocks (Tappe et al., 2007, 2008). Lamproites originate at the base of the lithosphere at craton margins and their chemistry can be explained only by melting in reduced conditions, probably triggered by HRM (Foley, 1989a, 1989b). The lack of CO 2 or carbonate under these conditions results in melts that are not silicaundersaturated, despite being silica-poor, and thus lamproitic rather than belonging to the melilitite^ carbonatite series (Foley, 1993). Lamproitic melts produced by HRM may be common at the base of the lithosphere and may act as regular agents of enrichment in potassium and other incompatible elements; however, they are unlikely to reach the surface and form igneous rocks in most cases. This explains the tendency of lamproites to occur around the margins of cratons and not in their centres (Janse & Sheahan, 1995). The second event characterized by ultramafic lamprophyres can be ascribed to the effects of the CRM mechanism (Fig. 11; Tappe et al., 2006). Thinning of the lower cratonic lithosphere reactivates and oxidizes diamond that was deposited during earlier, largely local, HRM events that produced the lamproitic melts noted above by the release of H 2 O from the oxidation of CH 4. The carbon from the methane was left in the residue as diamond formed, particularly in the time period 1 4^1 2 Ga. In the later episode around 600 Ma, this diamond was oxidized by the juxtaposition of upwelling asthenospheric mantle and the thinning continental lithosphere; the resulting oxidized carbon caused a drop in the solidus temperature (Fig. 10). This CRM mechanism is common in continental rifts around the world, resulting in the frequent association of continental rifting with carbonate-rich melts. 1378