The Origin of Highly Silicic Glass in Mantle Xenoliths from the Canary Islands

Transcription

1 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 PAGES The Origin of Highly Silicic Glass in Mantle Xenoliths from the Canary Islands E.-R. NEUMANN AND E. WULFF-PEDERSEN MINERALOGISK-GEOLOGISK MUSEUM, UNIVERSITY OF OSLO, SARSGT. 1, N-0562 OSLO, NORWAY RECEIVED APRIL 5, 1997; REVISED TYPESCRIPT ACCEPTED JULY 1, 1997 Spinel harzburgite, lherzolite, dunite and wehrlite mantle xenoliths KEY WORDS: Canary Islands; silicic glass inclusions; mantle xenoliths; from the Canary Islands (La Palma, Hierro, Tenerife and Lanzarote) melt wall-rock reactions contain a spectrum of silicate glasses as inclusions in minerals, along grain boundaries, and in interstitial glass pockets. These glasses show a range in composition from basaltic (~44 wt % SiO 2 ), to highly silicic, TiO 2 FeO MgO CaO P 2 O 5 -poor INTRODUCTION types (up to 71 wt % SiO 2 ). Glasses in spinel harzburgites and Highly silicic glass (up to 72 wt % SiO 2 ) is commonly lherzolites are generally silica oversaturated, whereas those in spinel found as inclusions and as interstitial glass pockets in dunites and wehrlites have somewhat lower SiO 2 contents and are upper-mantle peridotites (e.g. Frey & Green, 1974; Frangenerally silica undersaturated. Glasses in xenoliths from La Palma cis, 1976, 1987; Jones et al., 1983; Siena et al., 1991; and Tenerife are rich in K 2 O compared with those from Hierro and Schiano et al., 1992, 1994, 1995; Ionov et al., 1994; Lanzarote. Daughter minerals coexisting with highly silicic glass Schiano & Clocchiatti, 1994; Neumann et al., 1995; in polyphase inclusions are similar in composition to the main Zinngrebe & Foley, 1995; Wulff-Pedersen et al., 1996a). phases in the host xenoliths ( Fo>90, Cr-diopside, chromite), Reported host xenoliths include both anhydrous and whereas those in less silicic glasses are richer in Al 2 O 3,TiO 2 and hydrous spinel-bearing harzburgites, lherzolites and du- FeO, and poorer in MgO. The systematic relations found to exist nites from continental, oceanic and island-arc tectonic between glass composition, mineralogy of the host xenolith and settings (e.g. Schiano et al., 1992, 1994, 1995; Schiano locality (island) cannot reflect random variations in the geochemistry & Clocchiatti, 1994; Ionov et al., 1994; Neumann & of exotic melts infiltrating the mantle lithosphere, but instead Wulff-Pedersen, 1995). suggest a cogenetic relationship between the melts and their mantle The origin of silicic glass in mantle xenoliths, and its host xenoliths. The silicic glasses are interpreted as the products of role in mantle processes, has been the subject of vigorous reactions at 8 12 kbar between infiltrating alkali basaltic magmas debate. A number of workers (e.g. Frey & Green, 1974; and peridotitic wall-rocks which, in orthopyroxene-bearing rock- Francis, 1976) have attributed the formation of silicic types, involves formation of silicic melt+olivine at the expense of melts to the breakdown of amphibole in response to orthopyroxene. In xenoliths from La Palma and Tenerife, where decompression during transport of the xenoliths to the interstitial phlogopite is commonly present, phlogopite has been surface, and heating by the host lava. It has also been partly or totally consumed by the reactions between relatively mafic proposed that silicic melts may form by partial melting melts and peridotite, giving rise to silicic glasses with high K 2 O of mantle xenoliths during short residence times (up to contents and K 2 O/Na 2 O ratios. The low K 2 O concentrations and a few years) in crustal magma chambers during ascent K 2 O/Na 2 O ratios in glasses in anhydrous xenoliths suites from to the surface in the host magma (Klügel et al., 1996). Hierro and Lanzarote are believed to result from reactions between Such melts would have no bearing on mantle processes. infiltrating melts and anhydrous and/or amphibole-bearing mantle Other models imply formation of silicic melts at mantle wall-rocks. The silicic melts appear to have been mobile over depths. These include immiscible separation of a single distances exceeding the diameter of a xenolith, that is, at least melt into coexisting silicic and carbonate melts (Schiano cm. et al., 1994); small degrees of partial melting of subducted Corresponding author. Extended Data Set can be found at Oxford University Press 1997

2 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 crust followed by percolation of the melts into the over- and dunites; lherzolites and wehrlites are relatively rare, lying depleted mantle wedge in volcanic arcs (Schiano et but have been included where available. This study has al., 1995); infiltration by migrating metasomatic melt revealed systematic relationships between glass composition phases genetically unrelated to the mantle rock in which and type of host xenolith, mode of occurrence they are found (Edgar et al., 1989; Schiano et al., 1992, of the glass, and locality (island), which strongly support 1994, 1995; Schiano & Clocchiatti, 1994); reactions between the infiltrating melt wall-rock reaction model and in- infiltrating basaltic melts and peridotite wall-rock dicate that the compositions of highly silicic melts to a (Zinngrebe & Foley, 1995; Wulff-Pedersen et al., 1996a); large extent are controlled by the modal and chemical in situ melting involving breakdown of composition of the mantle wall-rock. We found no re- amphibole±phlogopite (Amundsen, 1987); in situ melting lationship between the compositions of the glasses and of clinopyroxene + spinel±amphibole (Francis, 1987; the host lavas. Chazot et al., 1996); disequilibrium in situ melting involving largely clinopyroxene and spinel owing to reaction with migrating fluids (Ionov et al., 1994); and partial melting of peridotite which has previously been metasomatized THE HOST XENOLITHS by carbonatitic melts (Hauri et al., 1993). The silicic glasses discussed in this paper occur in ultra- Recently, experimental investigations have indicated mafic xenoliths from the Canary Islands belonging to that highly silicic melts may form by small degrees of in Group I of Frey & Prinz (1978). Group II xenoliths situ partial melting in the upper mantle. Baker et al. (1995) (wehrlites, clinopyroxenites, dunites with Fo<87 in olivine showed that at 10 kbar, near-solidus melts (melt fraction and relatively Ti Fe-rich clinopyroxene and spinel) from F= ) are enriched in SiO 2 (57 wt % SiO 2 at the same localities contain basaltic glasses (<50 wt % SiO 2 ; F=0 02), Al 2 O 3 and Na 2 O, and depleted in FeO total, Neumann & Wulff-Pedersen, 1995), but these glasses will MgO and CaO relative to melts formed by higher degrees not be discussed here. The Group I xenoliths comprise of melting, and exhibit a strong increase in SiO 2 and refractory spinel harzburgites, rare spinel lherzolites, alkalis and decrease in FeO, MgO and CaO with de- spinel dunites, and rare spinel wehrlites. All these rockcreasing melt fraction at near-solidus conditions. Similar types contain Mg-rich olivine ( Fo>89), Cr-diopside and results were obtained by Drury & FitzGerald (1996) chromite; orthopyroxene is a major phase in harzburgites with a melt fraction of Draper & Green (1997) and lherzolites, but is generally absent in dunites and observed that under upper-mantle pressures and tem- wehrlites. Phlogopite is a common accessory phase in all peratures, silicic (56 62 wt % SiO 2 ), aluminous, alkaline types of xenoliths from La Palma and Tenerife, but has melts, typical of silicic glasses found in mantle xenoliths, not been observed in xenoliths from Lanzarote, and only have near-liquidus mineral assemblages and mineral compositions in one Group I xenolith from Hierro. The Group I which indicate equilibrium with a harzburgite xenoliths are interpreted as fragments of the oceanic residue, both in the presence of a CO 2 H 2 O fluid and lithospheric mantle that have been subjected to al- under anhydrous conditions. Draper & Green (1997) ternating episodes of partial melting and metasomatic proposed that silicic, aluminous, alkaline melts may form enrichment in highly incompatible trace elements (Neu- by low-degree partial melting of peridotite enriched in mann et al., 1995; Wulff-Pedersen et al., 1996a). Detailed alkalis, volatiles, and other low-melting-temperature components. discussion of the petrography and mineral chemistry, chemical composition and origin of the xenoliths may The aim of this study is to establish the origin of silicic be found in Johnsen (1990), Neumann (1991), Hansteen glasses in unveined upper-mantle xenoliths from the et al. (1991), Neumann et al. (1995) and Wulff-Pedersen Canary Islands. Wulff-Pedersen et al. (1996a) suggested et al. (1996a). A summary of these data is given below. that highly silicic glass in veined xenoliths from La Palma P T estimates, based on conventional mineral geothermometry have formed as the result of reactions between infiltrating and densities of CO 2 inclusions, give minihave basaltic melts and peridotite wall-rock. An important mum temperatures of about 900 C, and minimum pressures question is whether this model has general application of origin of 12 kbar for Hierro (Hansteen et al., to silicic melts in peridotites in the Canary Islands (and 1991; Neumann, 1991), and 6 8 kbar for Lanzarote other localities). Our approach in the present study is to ( Johnsen, 1990; Neumann et al., 1995). test if any relationship exists between glass composition The discussion is based on 674 analyses of glasses in and type of host xenolith, mode of occurrence (or relative 50 mantle xenoliths (29 spinel harzburgites, 5 spinel age) of the glass, and/or locality. As a basis for this study lherzolites, 14 spinel dunites and 2 spinel wehrlites). The we have chosen ultramafic xenolith suites from four xenoliths were collected in one locality in each of the islands: essentially anhydrous xenoliths from Hierro and islands La Palma, Hierro and Tenerife, and four localities Lanzarote, and hydrous xenoliths from La Palma and in Lanzarote (Fig. 1); each locality contains different types Tenerife. The main rock types are spinel harzburgites of xenoliths. All the xenoliths were collected in cinder 1514

3 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Fig. 1. Map of the Canary Islands showing xenolith localities as black crosses. cones. In Hierro and Tenerife the xenoliths appear to clinopyroxene + spinel±olivine. Equidimensional neobe concentrated in a single layer. Sample identifications blasts with interlocking grain boundaries are often found are of the type XXii-jj, where the letters XX indicate along the rims of phenocrysts with very irregular grain island (PAT, La Palma; H, Hierro; TF, Tenerife; LA, boundaries, and in narrow zones crosscutting por- Lanzarote), the number ii gives locality in that island, phyroclasts, and appear to grow at the expense of these. and the number jj is sample number at that locality Interstitial grains of clinopyroxene frequently enclose (sample TF14-52 thus means sample 52 from locality 14 corroded grains of olivine and orthoin Tenerife). pyroxene, and/or vermicular spinel. Clinopyroxene neoblasts often exhibit spinel exsolution lamellae, and orthopyroxene neoblasts occasionally have exsolution lamellae of clinopyroxene. The neoblast generation has Spinel harzburgite and lherzolite xenoliths been related to in situ heating and metasomatism during Harzburgite and lherzolite xenoliths are protogranular or the Canary Islands magmatism (Neumann, 1991; Neuporphyroclastic [following the nomenclature of Mercier & mann et al., 1995; Wulff-Pedersen et al., 1996a). In ad- Nicolas (1975)]; three generations of crystal growth may dition to these textures, many spinel harzburgites from be distinguished. The oldest generation consists of highly Tenerife exhibit large, irregular, poikilitic, mildly strained strained porphyroclasts of olivine (up to about 25 mm or unstrained orthopyroxene (up to about 6 mm long) and long) and orthopyroxene (containing exsolution lamellae clinopyroxene grains (up to about 4 mm long), enclosing of spinel±clinopyroxene) with highly irregular grain smaller grains of rounded to corroded olivine, pyroxene boundaries. In La Palma xenoliths large rounded spinel and spinel; fluid inclusions are rare. Some samples contain grains are also interpreted as porphyroclasts. Porfree exsolved orthopyroxene porphyroclasts with exsolution- phyroclasts are commonly very rich in glass and fluid domains surrounding olivine or clinopyroxene grains inclusions (see below). The second generation of (up to 1 mm in diameter). Where several such domains crystal growth is represented by mildly strained to unpyroxenes. occur close to one another, they resemble the poikilitic strained neoblasts of olivine+cr-diopside+chromite Poikilitic orthopyroxene and Cr-diopside thus phlogopite±orthopyroxene. Neoblasts occur as granular, appear to have formed as the result of extensive re- equidimensional grains with interlocking grain boundinteresting crystallization during the neoblast generation. Another aries (<1 0 mm in diameter), as irregular, interstitial feature is closely spaced, parallel rows of grains, or as symplectitic intergrowths of minute, platy spinel inclusions that may sometimes be 1515

4 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 followed continuously from orthopyroxene por- daughter minerals (clinopyroxene, spinel, phlogopite; Figs phyroclasts into olivine neoblasts (Neumann, 1991). Some 2d, 4a and b). With the exception of some dunites from samples show similar parallel rows of spinel inclusions Lanzarote that contain CO 2 + N 2 (Andersen et al., 1995), cutting straight through clusters of olivine neoblasts of fluid-bearing inclusions in xenoliths from the Canary different crystallographic orientations (Fig. 2). Interstitial Islands have been found to consist of pure CO 2 (Hansteen silicic glass is occasionally present in such areas. Olivine et al., 1991; Frezzotti et al., 1994; Neumann et al., 1995; neoblasts with parallel rows of minute spinel inclusions Wulff-Pedersen et al., 1996a). In some samples glass are believed to have formed by incongruent melting of inclusions or glass-bearing inclusions also contain sulph- orthopyroxene, and to have inherited the spinel inclusions ide globules (e.g. H1-4). from the pre-existing orthopyroxene. Phlogopite (in xen- Olivine and clinopyroxene neoblasts frequently exhibit oliths from La Palma and Tenerife) occurs as interstitial a central domain rich in small inclusions of colourless grains, and in polyphase inclusions (glass + to pale brown glass±fluid±spinel±phlogopite; these phlogopite ± clinopyroxene ± spinel). Interstitial phlo- inclusions have negative crystal shape, or they are gopite often encloses spinel or small neoblasts of olivine, rounded to vermicular (Fig. 3a). Their mode of ocand is frequently associated with interstitial glass. In currence suggests that they are primary, and represent Hierro phlogopite has only been found in a single poly- melt trapped during growth of the neoblasts. Small, phase inclusion in olivine (glass + phlogopite + spinel) scattered, vermicular glass±fluid inclusions also occur in a spinel harzburgite xenolith. Sulphide globules occur in the rims of olivine porphyroclasts. as scattered monomineralic inclusions in minerals, as Orthopyroxene porphyroclasts commonly contain parts of polyphase inclusions, and as inclusions in inter- numerous, scattered inclusions of colourless glass. Small stitial glass. The third, and youngest, generation of crystal glass inclusions (a few micrometres in diameter) frequently growth consists of microlites of spinel, olivine, and clino- exhibit negative crystal shape, whereas larger ones are pyroxene in interstitial glass pockets. irregular and are often found along the rims of euhedral to subhedral olivine neoblasts located inside porphyroclasts (Fig. 4a). These inclusions may contain fluid bubbles and/ Spinel dunite and wehrlite xenoliths or daughter minerals of chromite and/or Cr-diopside, Spinel dunites and wehrlites are porphyroclastic to equiand in some cases sulphide globules. In xenoliths from granular. Olivine porphyroclasts are strongly deformed La Palma and Tenerife the phase assemblage in glass- whereas neoblasts and equigranular rocks are moderately bearing inclusions in orthopyroxene porphyroclasts may deformed. Orthopyroxene is rarely present, and the include phlogopite. The observed transition from exxenoliths do not contain glass olivine aggregates which solved orthopyroxene with silicic glass + olivine in- might be interpreted as the result from incongruent clusions, to large clear poikilitic orthopyroxene in melting of pre-existing orthopyroxene. Phlogopite is cominclusions cutting several olivine neoblasts, indicates that Tenerife xenoliths, and parallel rows of platy spinel mon in dunites from La Palma and Tenerife. Amphibole (pargasite) has only been observed in Group I dunites the neoblast generation involved formation of silicic and wehrlites from La Palma where it is a rare accessory glass + olivine±clinopyroxene at the expense of ortho- phase in dunites, but present in considerable amounts in pyroxene. wehrlites. Sulphide globules are more common in dunites Spinel porphyroclasts often contain numerous rounded and wehrlites than in harzburgites and lherzolites. to vermicular inclusions and tunnels consisting of glass± clinopyroxene ± orthopyroxene ± olivine ± fluid. These inclusions are commonly concentrated along the rims, but may be found throughout large grains. Locally MICROSTRUCTURES OF GLASS glass-filled tunnels continue into interstitial glass. The Glass shows different modes of occurrence and relative host spinels have embayed, highly irregular outlines and age. Glass inclusions in olivine porphyroclasts generally are zoned with higher Cr 2 O 3, FeO total and TiO 2 in inform trails, indicating a secondary origin. The inclusion clusion-rich than in inclusion-free domains (Wulff- trails sometimes stop at the boundaries of neoblasts, Pedersen et al., 1996a). indicating an age younger than the porphyroclasts but In addition, many xenoliths contain brownish or col- older than the neoblasts. The inclusions in these trails ourless interstitial glass. Contacts between olivine grains range from negative crystal shape (Fig. 3a) to elongate often display a thin coating of very small, glass droplets. irregular shapes (Fig. 3b and c), indicating different stages Locally these layers expand into continuous films of glass of healing. The glass is colourless or brownish, and both along grain boundaries, and larger domains or glass types may occur in the same thin-section (e.g. spinel pockets, and may continue into narrow glass + fluid harzburgites H1-4 and H1-7). Glass inclusions generally veinlets that cut porphyroclasts. These veinlets are clearly contain fluid bubbles (Fig. 3b) and in some cases also younger than the inclusion trails. However, we wish to 1516

5 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Fig. 2. (a) Closely spaced parallel rows of myriads of minute, platy spinel inclusions (black) cutting several olivine neoblasts of different crystallographic orientation in spinel dunite PAT2-113 from La Palma (crossed polars). (b) Same in ordinary light. The spinel lamellae show a sharp termination inside a larger olivine grain. This is believed to mark the boundary of the pre-existing orthopyroxene grain. (c) Clear glass (gl) (medium grey), skeletal lazurite (laz) (bright), and euhedral clinopyroxene (cpx) in interstitial glass pocket in spinel dunite PAT emphasize that the interstitial glass and associated veinlets highly Na 2 O-rich, silica-undersaturated, colourless glass are local phenomena inside the xenoliths; they are not (SiO wt %) present in inclusion trails in olivine connected with the enclosing basaltic magma (host and in interstitial glass pockets (Fig. 2c). In sample PATmagma), or veinlets of host magma penetrating into the 116 the silicic glass carries sulphide globules in addition xenoliths. Large pockets of interstitial glass are most to lazurite. common around the rims of orthopyroxene and spinel Contacts between colourless, silicic glass inclusions and porphyroclasts. In xenoliths from La Palma and Tenerife, host minerals are sharp, showing no evidence of reaction. glass is also commonly found along the rims of interstitial This is in direct contrast to the contacts between xenolith phlogopite, which shows corroded contacts against this minerals and the enclosing alkali basalt and basaltic glass. Interstitial glass commonly contains large, rounded, veinlets, which are frequently marked by a reaction empty vesicles, sometimes microlites of zone. Reaction zones may consist of (a) a vermicular olivine±clinopyroxene±spinel, and occasionally sulphide intergrowth of clinopyroxene and spinel, both of which globules. Sulphide globules are more common in are considerably more Ti Al-rich than the clinopyroxene glass in spinel lherzolites, dunites and wehrlites than in and spinel inside the xenoliths; (b) an intergrowth of harzburgites, and more common in hydrous than in Ti Al-rich clinopyroxene and amphibole, (c) formation anhydrous xenoliths, and are believed to be related to of clinopyroxene±olivine±spinel±glass at the expense metasomatism. In two spinel dunites from La Palma of orthopyroxene, or (d) a combination of (a) (c). Similar (PAT2-27 and PAT2-116) lazurite {Na 5 7 K 0 2 Ca 2 0 reaction zones are found against basaltic glass in veined [Al 5 9 Si 6 1 O 24 ](SO 4,S) 1 6 ; E. Wulff-Pedersen & E.-R. spinel harzburgite and dunite from La Palma (Wulff- Neumann, unpublished data, 1996} has been found in Pedersen et al., 1996a). 1517

6 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Fig. 3. (a) Secondary trail of glass (g) + fluid (F) inclusions with negative crystal shape in olivine porphyroclast (o) cutting fluid inclusion trails with very small inclusions (spinel dunite TF14-4). The glass is in the early stages of devitrification. (b) Secondary trail of irregular glass (g) + fluid (F) inclusions cutting fluid inclusion trails in olivine porphyroclast (o) (spinel harzburgite PAT2-59). (c) Glass (g) in poorly healed fracture through olivine porphyroclast (o) (spinel dunite PAT2-116). (d) Polyphase inclusion consisting of phlogopite (ph), glass (g) and fluid (F) in spinel dunite PAT2-49. ANALYTICAL PROCEDURE Major element analyses of glasses and minerals were obtained using an automatic wavelength-dispersive CA- MECA Camebax Microbeam electron microprobe fitted with a LINK energy dispersive system at the Min- eralogisk-geologisk museum in Oslo. Glass was analysed by scanning an area of 5 μm 5 μm to20μm 20 μm, counting light elements first, and minerals were analysed by point analyses, using an acceleration voltage of 15 kev, sample currents of 10 na for glass, 20 na for minerals, and counting times of s per element. The composition of groundmass in host lavas was estimated as the average of scanning analyses (20 μm 20 μm) of adjacent areas, avoiding phenocrysts and xenocrysts. Oxides and natural and synthetic minerals were used as standards. Matrix corrections were performed by the PAP procedure in the CAMECA software. Analytical precision (2σ) evaluated by repeat analyses of individual grains is <1% for oxides in concentrations of [20 wt %, <2% for oxides in the range wt %, <5% for oxides in the range 2 10 wt %, and <10% for oxides in the range wt %. Representative analyses of glass in different types of mantle xenolith are listed in Table 1, together with (CIPW) normative quartz, nepheline and leucite. An Fe 3+ /Fe total ratio of 0 5 was arbitrarily chosen for the norm calculations. As the glasses contain very little iron, the estimated amounts of quartz and feldspathoids in the norms are only marginally influenced by the choice of Fe 3+ /Fe total ratio. A reduction in this ratio from 0 5 to 0 0 causes an increase in normative quartz (or reduction in nepheline + leucite) of <1%. The compositional ranges of glass inclusions in different types of host xenolith from each of the islands included in this study are given in Table 2. CHEMICAL COMPOSITIONS OF THE GLASSES Glasses in mantle xenoliths from the Canary Islands exhibit a considerable range in composition (e.g wt % SiO 2, wt % TiO 2, <1 8 wt % MgO, 1518

7 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Fig. 4. (a) Inclusion consisting of clear glass (g) and fluid (F) (empty) and euhedral olivine daughter mineral or neoblast (o) inside orthopyroxene porphyroclast (p) (spinel harzburgite PAT2-61). The presence of small, scattered, primary glass (g) and spinel (s) inclusions in the olivine grain should be noted. (b) Polyphase inclusion consisting of glass (g) + cpx (c) + fluid (F) (empty) in olivine porphyroclast (o) (spinel harzburgite PAT2-59). The compositions of different phases are given in the tables to the right of each figure; analysis locations are indicated in the figures by numbers corresponding to the column numbers. 1519

8 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Table 1: Representative major element analyses and normative quartz (Qz), nepheline+leucite (Ne) contents of glass inclusions and interstitial glass in mantle xenoliths from the Canary Islands, listed by island and type of host xenolith Island: La Palma Rock type: Sp harz Sp lherz Sp dunite Sp dunite Sp dunite Sp wehr anhydr anhydr anhydr amph phl/amph amph/phl Sample: PAT2-41 PAT2-59 PAT2-25 PAT2-42 PAT2-116 PAT2-74 Incl in: opx por sp por opx por ol por ol por ol por ol por ol por Other phases: sp+f lz+f F F Interstitial: pocket pocket pocket SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O n.d Cl Sum CIPW norm Qz Ne <1 15 wt % CaO, <1 12 wt % K 2 O, 1 2 wt % P 2 O 5 most glasses in harzburgites and lherzolites are moderately and wt % Cl), and show general trends of to strongly silica oversaturated, whereas those in decreasing concentrations and ranges of TiO 2, FeO total, dunites and wehrlites tend towards silica undersaturation MgO, CaO and P 2 O 5 with increasing SiO 2 (Tables 1 (Fig. 7). We have found no systematic compositional and 2; Figs 5 and 6). Na 2 O and K 2 O exhibit considerable differences between glasses in anhydrous and hydrous scatter (K 2 O/Na 2 O ) with the highest concentrations xenoliths within the same island, nor between harz- in the most silicic glasses, whereas the Al 2 O 3 burgites and lherzolites. However, Amundsen (1987) re- contents appear to reach a maximum at wt % ported higher K 2 O contents and K 2 O/Na 2 O ratios in SiO 2. The compositional ranges of glass inclusions in glasses in phlogopite-bearing than in phlogopite-free spinel harzburgites and lherzolites from Hierro and Lanzarote xenoliths from Gran Canaria (Canary Islands). Available obtained by us overlap those published by Schiano data (Fig. 7) indicate that silicic glass in spinel harzburgite et al. (1994), whereas Siena et al. (1991) reported a slightly and lherzolite xenoliths from other locations and tectonic higher SiO 2 range for glasses in spinel harzburgites from settings also tends towards silica oversaturation. Unfortunately, Lanzarote ( Table 2). very few data on glass in spinel dunites and In each island the most SiO 2 -rich, and TiO 2 wehrlites are as yet available. The glasses also show FeO MgO CaO P 2 O 5 poor glasses are found in spinel inter-island differences with respect to Na 2 O CaO K 2 O harzburgites and lherzolites ( Table 2; Figs 5 and 6), that relations. Most of the glasses in the anhydrous xenolith is, in orthopyroxene-bearing rock-types. In La Palma all suites from Hierro and Lanzarote are poorer in K 2 O glasses in harzburgites and lherzolites fall within the and richer in CaO and show lower K 2 O/Na 2 O ratios range wt % SiO 2 (Table 3; Fig. 5). Furthermore, than glasses with similar SiO 2 contents in the xenolith 1520

9 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Table 1: continued Island: Hierro Rock type: Sp harz Sp harz Sp harz Sp lherz Sp dunite anhydr anhydr anhydr anhydr anhydr Sample: H1-4 H1-19 H1-58 H1-7 H1-13 Incl in: opx por ol por ol por sp neo ol por cpx neo oxp por ol por ol por ol por ol por Other phases: opx+f ol+sulph F ol+f cpx+f F F F Interstitial: grbd/opx pocket/cpxveinlet/ol grbd/ol SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Cl Sum CIPW norm Qz Ne

10 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Table 1: continued Island: Tenerife Rock type: Sp harz Sp harz Sp harz Sp dunite Sp dunite Sp wehrlite anhydr phlog phlog phlog phlog phlog Sample: TF14-40 TF14-42 TF14-41 TF14-11 TF14-50 TF14-46 Incl in: opx neo ol neo ol neo sp neo phlog ol por ol por ol ol ol por ol por sp neo Other phases: F F F sp+f F F cpx cpx Interstitial: pocket pocket pocket pocket SiO TiO Al 2 O FeO MnO n.d n.d n.d MgO CaO Na 2 O K 2 O P 2 O n.d Cl n.a. n.a Sum CIPW norm Qz Ne Lc

11 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Table 1: continued Island: Lanzarote Rock type: Sp harz Sp dunite anhydr anhydr Sample: LA6-35 LA6-25 Incl in: ol por opx por opx por ol ol ol Other phases: ol+f F F F Interstitial: grbd/ol grbd/sp SiO TiO Al 2 O FeO MnO n.d. n.d. n.d MgO CaO Na 2 O K 2 O P 2 O Cl Sum CIPW norm Qz Ne Additional analyses of glass and coexisting minerals in polyphase inclusions are given in Table 3 and Fig. 3. Anhydr, anhydrous; amph, amphibole; phlog, phlogopite; ol, olivine; opx, orthopyroxene; cpx, clinopyroxene; sp, spinel; sulph, sulphide globule; lz, lazurite; por, porphyroclast; neo, neoblast; grbd, grain boundary; grbd/ol, grain boundary along olivine; pocket, glass pocket; F, fluid. n.d., not determined; n.a., not analysed. suites from La Palma and Tenerife, where phlogopite is a common accessory phase (Figs 5 and 6). In an Na 2 O CaO K 2 O diagram, glasses in harzburgite and lherzolite xenoliths from La Palma and Tenerife define trends towards the central part of the Na 2 O K 2 O sideline, whereas those from Hierro and Lanzarote fall close to the Na 2 O CaO tie-line (Fig. 8). Glasses in dunites show less clear differences in Na 2 O CaO K 2 O relations. We have also tested compositional differences against mode of occurrence: (a) glass in secondary inclusion trails in olivine porphyroclasts (assumed to be the oldest generation of glasses), (b) irregular glass-bearing inclusions in orthopyroxene porphyroclasts, and (c) glass in interstitial glass pockets. Clusters of (primary) inclusions in olivine and clinopyroxene neoblasts (intermediate in age between type a and type c) are generally too small to allow analysis; we have therefore not been able to test those as a separate group. Glasses in spinel harzburgites and lherzolites from La Palma show no compositional relation to mode of occurrence. In the other islands there is a weak tendency for glass found in secondary inclusion trails in olivine and as inclusions in orthopyroxene to show a more restricted range in high-sio 2 compositions and higher average SiO 2 than interstitial glasses (Fig. 5). Dunites show no chemical differences between glass inclusions in olivine and interstitial glass (Fig. 6). MINERALS COEXISTING WITH GLASS IN INCLUSIONS AND GLASS POCKETS The compositions of daughter minerals and microlites are clearly correlated with the coexisting glass. Euhedral to subhedral daughter minerals in polyphase inclusions associated with highly silicic glass have compositions typical of the main phases in the host xenoliths, that 1523

12 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Table 2: Compositional ranges in glass inclusions and interstitial glasses from different types of mantle xenoliths from the Canary Islands Island: La Palma Hierro Rock type: harz/lherz harz/lherz dun dun wehr harz/lherz dun harz/lherz (1) anhydr hydr anhydr hydr hydr anhydr anhydr n (rocks): incl incl incl incl incl incl incl incl SiO ±0 35 TiO ±0 02 Al 2 O ±0 13 FeO ±0 11 MgO ±0 18 CaO ±0 19 Na 2 O ±0 02 K 2 O ±0 05 P 2 O ±0 02 Host (Fo) Island: Tenerife Lanzarote Rock type: harz/lherz harz/lherz dun dun wehr harz/lherz dun harz (2) dun (2) harz/lherz (1) anhydr hydr anhydr hydr hydr anhydr anhydr anhydr anhydr anhydr n (rocks): incl incl incl incl interst incl incl interst interst incl SiO TiO Al 2 O FeO MgO CaO Na 2 O K 2 O P 2 O Host (Fo) Data published by (1) Schiano et al. (1995) and (2) Siena et al. (1991) are included for comparison. n, number of rock samples; incl, inclusions in orthopyroxene and olivine; interst, interstitial glass. is, olivine with Fo>90, and Mg Cr-rich, Ti Al-poor pyroxenes (Table 3, Fig. 4). The same is true for microlites in glass pockets with highly silicic glass. Less silicic glass, in contrast, contains minerals which are markedly richer in TiO 2,Al 2 O 3 and FeO, and poorer in MgO than those in the mantle wall-rock. Minerals of similar compositions are typically found as phenocrysts in alkali basalts, and in Group II wehrlites and clinopyroxenites. The latter are interpreted as mantle cumulates formed from alkali basaltic Canarian magmas (Hansteen et al., 1991; Neumann, 1991). Similarly, veined xenoliths from La Palma show a continuous shift in mineral compositions from relatively Fe-rich olivine and Ti Al Fe-rich clinopyroxene, amphibole and phlogopite in the least silicic glass, to Mg-rich olivine and Ti Al-poor, Mg Cr-rich clinopyroxene and phlogopite coexisting with the most silicic glass (Wulff-Pedersen et al., 1996a). Corresponding relations between the compositions of glass and daughter minerals microlites were observed by Zinngrebe & Foley (1995) in mantle xenoliths from Gees, Germany. 1524

13 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Fig. 5. CHEMICAL COMPOSITIONS OF THE glass) for a number of samples. Representative analyses are presented in Table 4 and plotted in Figs 5 7. Like HOST LAVAS Canary Islands basalts in general, the host lavas of the As the xenoliths were collected in cinder cones, their xenoliths are TiO 2 rich, and silica saturated to underhost lavas were often difficult to analyse. However, we saturated. K 2 O/Na 2 O values range from 0 31 to obtained data on host lava (bulk rock, groundmass, or Within each locality we found only minor compositional 1525

14 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Fig. 5. Compositional variations vs SiO 2 content in glasses in anhydrous (left column) and hydrous spinel harzburgite and lherzolite xenoliths (right column) from the Canary Islands. Groundmass and glass in host lavas of the xenoliths are indicated by letters: P, La Palma; H, Hierro; T, Tenerife; L, Lanzarote. For comparison are also shown trends defined by glasses in the vein system of veined spinel harzburgite from La Palma, PAT2-4 (grey field; Wulff-Pedersen et al., 1996a), by aphyric lavas in Tenerife (field outlined by dotted line; E.-R. Neumann & E. Wulff- Pedersen, unpublished data, 1996), aphyric lavas in Hierro (field outlined by continuous line; data from Pellicier, 1977, 1979), and mafic MORB (star). Incl, inclusions in orthopyroxene and olivine. The horizontal dashed line in the K 2 O SiO 2 figure shows the lower limit of the range defined by glasses in hydrous xenoliths. (See text for discussion.) 1526

15 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Fig

16 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Fig. 6. Compositional variations vs SiO 2 content in glass inclusions in olivine and interstitial glass in anhydrous (left column) and hydrous (right column) Group I spinel dunite xenoliths in the Canary Islands. Symbols as in Fig. 5. (See text for discussion.) differences between host lava attached to different xeno- highly silicic melts are in, or close to, equilibrium with liths. This means that different types of xenoliths are the spinel peridotites in which they are found, whereas hosted by lava of the same composition. Furthermore, the basaltic melts are not. This is indicated by the sharp host lavas from different localitites and islands fall on, or contacts between silicic glass and peridotite minerals, in close to, the trends defined by aphyric lavas in Tenerife contrast to the reaction-type contacts between xenoliths (E.-R. Neumann & E. Wulff-Pedersen, unpublished data, and basaltic glass seen in both unveined (this study) 1996) and Hierro (data from Pellicier, 1977, 1979; and veined xenoliths. Furthermore, the compositions of Figs 5 7). daughter minerals and microlites in highly silicic glass are similar to those of the main phases in the host xenolith ( Mg-rich olivine Cr-diopside, chromite; Fig. 4, Table 3), whereas those in basaltic glass are not (e.g. Ti Fe-rich DISCUSSION augite and titanomagnetite; Figs 5 and 6). Another im- The origin of the silicic glasses portant result of this study is that it reveals systematic We noted above that silicate glasses in peridotite xenoliths relations between glass composition and type of host from the Canary Islands cover a wide range in comand xenolith, mode of occurrence (or relative age) of the glass, positions from basaltic, to SiO 2 Al 2 O 3 Na 2 O K 2 O-rich, locality (island). The mineral melt relations, and TiO 2 FeO MgO CaO P 2 O 5 -poor types (Tables 1 and contact relations between glass and peridotite minerals 2; Figs 5 and 6). There is considerable evidence that the found for unveined Canary Islands xenoliths closely 1528

17 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Table 3: Representative compositions of euhedral to subhedral crystals coexisting with highly silicic glass in polyphase inclusions (incl) and interstitial glass (interstitial), compared with wall-rock phases (host rock) Rock type: Sp harzburgite Sample: H1-4 H1-8 H1-12 TF14-14 TF14-53 anhydr anhydr anhydr phlog phlog interstitial incl in sp interstitial host rock incl in opx incl in ol Phase ass: gl+ol gl+ol gl+ol+cpx+sp+f gl+ol+f gl+sp+cpx+f Phase: glass ol glass ol glass ol cpx sp cpx por sp glass ol glass cpx sp SiO TiO Al 2 O Cr 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Cl S 0 05 Sum mg-no Fo (host)

18 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Table 3: continued Rock type: Sp Sp dunite Sp lherzolite wehrlite Sample: TF14-5 PAT2-34 TF14-4 LA6-12 PAT2-74 anhydr anhydr phlog anhydr amph/ phlog incl in ol host rock incl in ol host rock incl in ol host rock intersititial incl in ol host rock Phase ass: gl+cpx+f gl+ sp+f sp por gl+cpx+f gl+ol+cpx+f gl+cpx+f Phase: glass cpx cpx glass sp sp(core) sp(rim) glass cpx cpx glass ol glass cpx cpx SiO TiO Al 2 O Cr 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Cl S Sum mg-no Fo (host) Phase ass., phase assemblage. Other abbreviations as in Table 1. Additional data are given in Fig

19 NEUMANN AND WULFF-PEDERSEN MANTLE XENOLITHS FROM CANARY ISLANDS Fig. 7. Normative quartz and nepheline + leucite (CIPW) plotted against SiO 2 for glass inclusions in olivine and orthopyroxene and interstitial glass in different types of host xenoliths in the Canary Islands (a d, f ). The figure shows that glasses in spinel harzburgites and lherzolites are generally silica oversaturated, whereas those in spinel dunites and wehrlites are silica undersaturated. The open-star symbol represents average N-MORB. Groundmass and glass in host lavas of the xenoliths are indicated by letters: P, La Palma; H, Hierro; T, Tenerife; L, Lanzarote. Trends defined by glasses in the vein system of veined spinel harzburgite PAT2-4 and veined spinel dunite PAT2-62, are shown as grey fields in (a) and (b), and in (c) and (d), respectively (grey fields; Wulff-Pedersen et al., 1996a). Trends are defined by aphyric lavas from Tenerife (field outlined by dotted line; E.-R. Neumann & E. Wulff-Pedersen, unpublished data, 1996), and Hierro (field outlined by continuous line; data from Pellicier, 1977, 1979). (f ) Published data on glass in spinel harzburgite and lherzolite xenoliths from other locations: Yemen interstitial glass, glass in glass pockets and veinlets in spinel lherzolites (Chazot et al., 1996); W Eifel interstitial glass in spinel harzburgites (Zinngrebe & Foley, 1995); Mongolia glass in glass pockets in spinel lherzolites from Mongolia (Ionov et al., 1994); Oceanic and Continental glass inclusions in spinel harzburgites and lherzolites from various oceanic and continental localities, respectively (Schiano et al., 1992, 1994; Schiano & Clocchiatti, 1994); Philippines glass inclusions in spinel harzburgites from Philippine arc lavas (Schiano et al., 1995). Incl, inclusions in orthopyroxene and olivine. resemble those found in the vein systems of veined xenoliths from La Palma (Wulff-Pedersen et al., 1996a). The small compositional differences found to exist among the host lavas cannot account for the chemical contrasts exhibited by glasses in different types of peridotites and different localitites (islands). Most striking is the fact that the host lavas fall on a common K 2 O SiO 2 trend, whereas the silicic glasses in hydrous xenoliths are markedly richer in K 2 O than glasses with similar SiO 2 contents in anhydrous xenoliths (Figs 5 and 6). Furthermore, whereas all the host lavas fall close to the TiO 2 SiO 2 trend defined by aphyric lavas in Tenerife and Hierro (Figs 5 and 6), glasses in Hierro xenoliths appear to define both a high- TiO 2 and a low-tio 2 trend. The above relations have important implications for our understanding of the origin of silicic glasses in mantle xenoliths. They indicate that the compositional diversity found to exist among glasses in Canary Islands xenoliths does not reflect random chemical variations among infiltrating melts migrating undisturbed through a column 1531

20 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 11 NOVEMBER 1997 Fig. 8. Na 2 O CaO K 2 O relations among glasses in (a) spinel dunite and (b) harzburgite and lherzolite xenoliths from the Canary Islands. Glass in veined spinel harzburgite (PAT2-4) and dunite (PAT2-62; Wulff-Pedersen et al., 1996a) are shown as grey fields marked vein glass. Mafic basaltic lavas (MgO>7 wt %) from the islands of La Palma, Hierro, Tenerife and Lanzarote (data from Fuster et al., 1968a; Pellicier, 1977, 1979; Staudigel, 1981; Hernandez-Pacheco & Valls, 1982; Staudigel et al., 1986; E.-R. Neumann & E. Wulff-Pedersen, unpublished data, 1996) are shown as grey fields marked basalts. (c) Published data on series of glass inclusions and glass pockets in spinel harzburgites and lherzolites from other localities: GC hydr and GC anhydr glasses in phlogopite-bearing and phlogopite-free spinel harzburgite xenoliths from Gran Canaria (Amundsen, 1987); Mongolia (Ionov et al., 1994); Gees, West Eifel (Zinngrebe & Foley, 1995), Mt Lessini, Southern Alps, and Cape Verde (Siena & Coltorti, 1993). (See text for discussion.) of different mantle rock types ( Model 1: Edgar et al., melt and spinel peridotite in equilibrium should have 1989; Schiano et al., 1992, 1994, 1995; Schiano & Clocchiatti, similar TiO 2 /Al 2 O 3 ratios. Spinel harzburgites and lherdirect 1994). Instead, the observed relations imply a zolites from La Palma, Hierro and Lanzarote are char- association between melts and host xenolith which acterized by TiO 2 /Al 2 O 3 <0 07. This is within the range may arise through reactions between infiltrating basaltic of normal oceanic peridotites and primitive tholeiitic melts and peridotite wall-rock ( Model 2: Zinngrebe & basalts (highest MgO/FeO ratios) collected along nor- Foley, 1995; Wulff-Pedersen et al., 1996a), or through in mal segments of the Mid-Atlantic Ridge (Bryan et al., situ partial melting ( Model 3: Amundsen, 1987; Francis, 1981; Sigurdsson, 1981; Schilling et al., 1983; Weaver et 1987; Hauri et al., 1993; Ionov et al., 1994; Baker et al., al., 1985; Menzies, 1991). Mafic Canarian lavas (MgO 1995; Chazot et al., 1996; Draper & Green, 1997). >7 wt %), in contrast, have TiO 2 /Al 2 O 3 >0 15. Glass in Model 2 (reactions between infiltrating melts and veined spinel peridotite xenoliths from La Palma shows mantle wall-rocks) is also supported by a strong decrease a gradual decrease in TiO 2 /Al 2 O 3 from about 0 2 in in TiO 2 /Al 2 O 3 ratios from the least silicic to the most alkali basaltic glass in broad veins, to TiO 2 /Al 2 O 3 <0 08 silicic glasses (exemplified by data on Hierro in Fig. 9). in the most SiO 2 -rich glass in very narrow veinlets pen- Experiments at pressures of kbar (e.g. Mysen & etrating peridotite fragments (Wulff-Pedersen et al., 1996a; Kushiro, 1977; Jaques & Green, 1980; Falloon & Green, Fig. 9). The variation in glass chemistry in the veined 1987) indicate only moderate fractionation of Ti relative xenoliths was interpreted by Wulff-Pedersen et al. (1996a) to Al over a wide range of partial melting of spinel as the consequence of melt wall-rock reactions. These peridotite. Similar results were obtained in near-solidus reactions start with infiltration by alkali basaltic melts melting experiments (2 5% melting of spinel lherzolite) (with high TiO 2 /Al 2 O 3 ratios) out of equilibrium with at 10 kbar (Baker et al., 1995). This implies that a silicate the peridotite wall-rock, and their end-products are silicic 1532