Serpentinization, element transfer, and the progressive development of zoning in veins: evidence from a partially serpentinized harzburgite

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1 Contrib Mineral Petrol (206) 7:5 DOI 07/s ORIGINAL PAPER Serpentinization, element transfer, and the progressive development of zoning in veins: evidence from a partially serpentinized harzburgite Esther M. Schwarzenbach,2 Mark J. Caddick James S. Beard 3 Robert J. Bodnar Received: 5 July 205 / Accepted: 25 November 205 Springer-Verlag Berlin Heidelberg 205 Abstract Serpentinization is an important geochemical process that affects the chemistry and petrophysical properties of the oceanic lithosphere and supports life through abiogenic formation of hydrogen. Here, we document through detailed mineralogical evidence and equilibrium thermodynamic models the importance of water (H 2 O) and silica (SiO 2 ) activities on mineral assemblages produced during progressive serpentinization of a harzburgite. We describe a harzburgite from the Santa Elena Ophiolite in Costa Rica that is ~30 % serpentinized. Serpentine + brucite ± magnetite veins occur in, Al-rich serpentine + talc veins occur in orthopyroxene, and Al-rich serpentine ± talc ± brucite veins occur at the boundary of orthopyroxene and. Bulk vein chemistry and element distribution maps demonstrate distinct chemical zonations within veins and chemical gradients between orthopyroxene- and -dominated areas. Specifically, the sample records () varying brucite composition depending on whether or not it is associated with magnetite, (2) formation of magnetite from Fe-rich brucite (±Fe-rich serpentine) Communicated by Othmar Müntener. Electronic supplementary material The online version of this article (doi:07/s ) contains supplementary material, which is available to authorized users. * Esther M. Schwarzenbach esther.schwarzenbach@fu berlin.de 2 3 Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 2406, USA Institut für Geologische Wissenschaften, Freie Universität Berlin, 2249 Berlin, Germany Virginia Museum of Natural History, Martinsville 242, VA, USA during hydration, where magnetite coexists with brucite Mg#96 and serpentine Mg#99, (3) chemical gradients in Si, Al, Cr, and Ca within and between orthopyroxeneand -hosted veins, and 4) local (different) equilibrium assemblages within different zones of veins. The studied sample preserves rarely observed textures documenting continuous replacement of, rather than individual vein generations and overprinting that is typically observed in more intensely serpentinized peridotites. Furthermore, the presence of a discrete sequence of vein textures and mineralogy allows direct comparison between mineral textures and equilibrium thermodynamic models and permits new insights into mineral reactions during serpentinization. Keywords Costa Rica Olivine hydration Peridotite Serpentinization Perple_X Introduction Serpentinization is a common alteration process that occurs when water interacts with the primary minerals and pyroxene in ultramafic rocks to form a rock dominated by serpentine (Mg 3 Si 2 O 5 (OH) 4 ). Serpentinites preserve some of the most reducing geological environments found on Earth as oxidation of ferrous iron in results in hydrogen formation (Frost 985; Moody 976; Neal and Stanger 983) and some of the lowest aqueous silica activities found in crustal rocks (e.g., Allen and Seyfried 2003; Frost 985; Frost and Beard 2007; Palandri and Reed 2004; Peretti et al. 992). The process of serpentinization has been studied for decades (e.g., Barnes and O Neil 978; Macdonald and Fyfe 985; Mével 2003; Moody 976; Wenner and Taylor 97; Wicks and Whittaker 977) and has gained widespread interest because serpentinization

2 5 Page 2 of 22 of ultramafic rocks impacts many large-scale tectonic processes and biogeochemical cycles. Specifically, the transformation of and pyroxene to primarily serpentine and magnetite affects the rheology (Escartin et al. 997, 200), petrophysical, and seismic properties (Dyment et al. 997; Miller and Christensen 997) of the oceanic lithosphere, while the chemical exchange between seawater and rock influences the chemical budget of the oceans (Früh- Green et al. 2004; Snow and Dick 995) and the transport of elements and water into the mantle and, ultimately, to arc magmas (Alt et al. 203; Hacker 2008; Hattori and Guillot 2007; Scambelluri et al. 995, 2004; Ulmer and Trommsdorff 995). Additionally, the microbial communities that are found in association with rocks undergoing serpentinization have potentially important implications for the origin of life on Earth and other planets (Kelley et al. 2005; Martin et al. 2008; Martin and Russell 2007; McCollom 999; Russell et al. 200). Serpentine is the dominant mineral in serpentinites; however, brucite, talc, and magnetite are important products of serpentinization reactions and provide evidence for the temperature of water rock interaction, water rock ratios, and the chemical composition of the interacting fluid (e.g., Bach et al. 2004; Frost and Beard 2007; Klein et al. 204). Additionally, the formation of magnetite during serpentinization is important for the production of hydrogen (e.g., Klein et al. 2009; McCollom and Bach 2009; Neal and Stanger 983) and the interpretation of magnetization signatures in the oceanic lithosphere (Klein et al. 204; Oufi et al. 2002). In the process of understanding serpentinization reactions, and specifically the mechanisms of magnetite formation, various authors have suggested that serpentinization is a two-stage process and that magnetite formation increases with degree of serpentinization, replacing Fe-rich brucite and/or serpentine (Bach et al. 2006; Oufi et al. 2002; Toft et al. 990). Numerous studies (e.g., Beard et al. 2009; Frost et al. 203; Katayama et al. 200; Miyoshi et al. 204) reported and identified several generations of vein formation associated with serpentinization and suggested that early veins form in a rock-dominated system, while later, magnetite-bearing veins form in a fluid-dominated, open system. In most of these studies, an increase in silica activities, for example derived from the decomposition of pyroxene, is interpreted to be a key factor for brucite to break down to form magnetite (e.g., Bach et al. 2006; Beard et al. 2009), while a more recent experimental study showed that magnetite is absent when silica activity is high (Ogasawara et al. 203). In contrast to these studies, Evans (2008) suggests that magnetite is formed by the simple reaction of + water = serpentine + magnetite (+iron alloys) and that high Mg# in serpentine is the result of an Fe Mg exchange potential between or orthopyroxene and serpentine. Contrib Mineral Petrol (206) 7:5 Unraveling the detailed mineralogical processes and identifying the factors that control the progress of serpentinization, specifically the formation of magnetite as a function of the degree of serpentinization, can significantly advance our ability to interpret seismic and magnetic signals from midocean ridge to subduction zone settings and provide new insight into the extent of hydrogen production during serpentinization. Here, we present a study of a harzburgite sample from an ophiolite sequence that has been affected by low degrees of serpentinization, preserving detailed structures of vein formation during replacement of and orthopyroxene. Serpentinized peridotites generally preserve several generations of vein formation and reactivation of older veins, obscuring the primary vein textures. The sample studied here, however, is insufficiently serpentinized to have experienced such overprinting, and records direct mineralogical evidence of the temporal evolution of the reactions that control serpentinization and magnetite formation. Using petrography, Raman spectroscopy and mineral chemical data from numerous - and orthopyroxene-hosted veins combined with simple thermodynamic models, we document the influence of water and silica activities on the mineralogical and chemical distributions within veins, tracking the progression of serpentine, brucite, talc, and magnetite formation upon and orthopyroxene hydration. Analytical methods Petrography Mineral chemistry was determined by electron microprobe (EMP) analysis using a Cameca SX-50 electron microprobe at the Department of Geosciences at Virginia Tech. Accelerating potential was 5 kv, and a 20 na beam current and -μm spot size were used. Calibration was accomplished using natural and synthetic mineral standards. Element distribution maps were produced using the EDS (energy-dispersive spectrometers) and were run for 2 8 h at either 40 or 00 na depending on run time. Analytical reproducibility (σ) for the major elements is < % and for the minor elements generally <4 %. Scanning electron microscope (SEM) images were carried out on a ZEISS SUPRATM 40 VP Ultra SEM at the Institute of Geological Sciences at the Freie Universität Berlin. Raman spectroscopy The mineralogy of serpentine and the possible presence of talc and brucite in veins were determined by Raman spectroscopy using a JY Horiba LabRam HR800 with 600 grooves/mm gratings. The slit width was set at 50 µm and the confocal aperture at 400 µm. Excitation was provided by a nm

3 Contrib Mineral Petrol (206) 7:5 Page 3 of 22 5 (green) Ar + laser, with an output of 50 mw at the source and <0 mw at the sample. The detector was an electronically cooled open electrode CCD. The laser spot size (analytical area) was on the order of μm using a 00 objective. Geological background Santa Elena peninsula Nicaragua Costa Rica Caribbean Sea In this study, we examined a partly serpentinized harzburgite sample from the Santa Elena Ophiolite in Costa Rica. The ophiolite is located on the northwest coast of Costa Rica and comprises a 40-km-long and 6-km-wide complex including mafic and ultramafic lithologies that were emplaced during the Upper Cretaceous (Fig. ; Baumgartner and Denyer 2006; Denyer and Gazel 2009; Gazel et al. 2006). The Santa Elena Ophiolite comprises variably serpentinized peridotites, dunites, locally layered gabbros, and two generations of mafic dikes that intrude the peridotites: one generation preserving chilled margins and another without chilled margins, suggesting that the latter was emplaced into a hot mantle host preceding serpentinization (Gazel et al. 2006). The mafic sequences have been dated at ~0 25 Ma with geochemical evidence for melt formation associated with an ultra-slow to slowspreading oceanic ridge and variable subduction input from a presumed underlying subduction zone (Baumgartner and Denyer 2006; Hauff et al. 2000; Madrigal et al. 205). Additionally, some of the mafic lithologies contain a secondary mineral assemblage that has been ascribed to ocean floor metasomatism (albite + epidote + actinolite + chlorite) and local rodingitization (the presence of tremolite, diopside, hydrogrossular, zoisite) (Gazel et al. 2006), while the sulfide mineral and metal assemblages in the peridotites indicate highly reducing conditions and low (<) water rock ratios during serpentinization (Schwarzenbach et al. 204). The tectonic evolution of this ophiolite sequence is controversial and subject to current research (Madrigal et al. 205; Schwarzenbach and Gazel 203; Schwarzenbach et al. 204). Overall, there is increasing evidence that serpentinization of the peridotite occurred when sections of the upper mantle were exposed to seawater along an ultra-slow to slow-spreading ridge, but that fluid influx was limited (Madrigal et al. 205; Schwarzenbach et al. 204). Precise temperatures of water rock interaction are currently unknown, but a previous study of this ophiolite suggests serpentinization temperatures <250 C based on mineralogical observations (Schwarzenbach et al. 204). Sample description The peridotites of the Santa Elena Ophiolite are lherzolites, cpx-rich harzburgites, and dunites and have a degree of (a) Pacific Ocean Pacific Ocean Panama km Santa Elena Nappe Santa Rosa Accretionary Complex Pillow and massive Dike swarm basalts Dolerite dikes Faults Santa Elena thrust Fig. a Location of the Santa Elena peninsula in Costa Rica and b geological map of the Santa Elena Ophiolite (rectangle in a) with the star indicating the sampling location of the studied harzburgite. After Gazel et al. (2006) serpentinization ranging between 30 and 00 %. Numerous samples were examined during this and a previous study (Schwarzenbach et al. 204), with samples that are only partially serpentinized showing consistent textures (at the microscope scale) in the orthopyroxene- and -hosted veins. We focus here on the least serpentinized sample studied, which also preserves the best-developed textures. The studied sample is ~30 % serpentinized, with replacement occurring along veins in orthopyroxene and, with a mesh texture developed around grains (Fig. 2a). Replacement of is significantly more advanced than replacement of orthopyroxene, while rare clinopyroxene has experienced little replacement presumably reflecting both thermodynamic stability and differential kinetics of the replacement reactions at this temperature (e.g., Bach et al., 2006). Olivine has a uniform Mg# (=Mg/(Mg + Fe) 00) of 90 with NiO contents of wt% and traces of MnO (<0.24 wt%), TiO 2 (<0.2 wt%), and Cr 2 O 3 (<0.24 wt%) (Table ).

4 5 Page 4 of 22 Contrib Mineral Petrol (206) 7:5 Fig. 2 Thin section images (in transmitted light) of the studied harzburgite. a Overview of partly replaced orthopyroxene and, where replacement of by serpentinite is more advanced than replacement of orthopyroxene. Rectangle A indicates the area shown in b and in Fig. 9, rectangle B indicates the area shown in c and in Fig. 0. White dashed lines outline areas dominated by hybrid serpentine veins, i.e., areas where is replaced by serpentine in the vicinity of an orthopyroxene contact. b Vein formation during hydration of. Small veins (<0 μm) are optically homogeneous and dominated by serpentine (subsequently termed stage veins). Larger veins preserve a variably brucite-rich center reflected by brownish color (stage 2 and stage 3 veins). Veins >50 μm can contain opaque magnetite in the vein center (stage 4 veins). c Orthopyroxene and adjacent areas with greenish serpentine veins that cut oblique to the cleavage of the primary orthopyroxene Orthopyroxene has a Mg# of 90 and contains clinopyroxene exsolution lamellae that are resistant to serpentinization. Orthopyroxene typically contains wt% Al 2 O 3, <.87 wt% CaO, and <0.83 wt% Cr 2 O 3 (Table ). Clinopyroxene has a diopsidic composition with wt% Al 2 O 3, wt% Cr 2 O 3, wt% TiO 2, wt% Na 2 O, and traces of MnO (<0.6 wt%). Spinel has a Cr# (=(Cr/Cr + Al) 00) of 9.3 and Mg# of Several different types of serpentine ± brucite ± magnetite ± talc veins can be observed in the studied sample, with the most common veins hosted in or orthopyroxene, while rare clinopyroxene contains few veins. Veins replacing are dominated by serpentine, but a brownish color in some veins may suggest the presence of variable amounts of brucite, which was confirmed by Raman spectroscopy, as pure serpentine is typically colorless. Small veins (<0 20 μm) are optically homogeneous (i.e., lack zonation). Veins >20 μm typically contain a brown center that is variably defined (i.e., a distinct line vs. gradational) and locally contain magnetite in veins exceeding ~50 μm in width (Fig. 2b). These magnetite-bearing veins are characterized by a complex mineralogical zonation (Fig. 2b). The mineral zoning is consistent with chemical variations determined by EMP analyses and chemical mapping, and is a major focus of this study. Veins replacing orthopyroxene are typically <80 μm wide, are dominated by greenish serpentine, and are most commonly cut oblique to the cleavage of the orthopyroxene (Fig. 2a, c). These veins are typically relatively homogeneous and only rarely show internal vein structures. Where orthopyroxene abuts, the green serpentine veins in the orthopyroxene extend into the (e.g., Fig. 2c). These veins are later referred to as hybrid serpentinites. The opaque phases in the sample are mostly pentlandite, with traces of pyrrhotite, awaruite, native copper, and sugakiite. The typically observed intergrowth of pentlandite + magnetite + awaruite is attributed to desulfurization of primary pentlandite due to the highly reducing conditions during

5 Contrib Mineral Petrol (206) 7:5 Page 5 of 22 5 Table Averages and ranges of electron microprobe analyses of, orthopyroxene, - and orthopyroxenehosted veins, and hybrid serpentinite (in wt%) SiO 2 TiO 2 Al 2 O 3 Cr 2 O 3 MgO CaO MnO FeO Cl Total Average SD Average opx SD Mgt-free -hosted veins (n = 603) Average Min 7.2 <l.o.d. <l.o.d. <l.o.d <l.o.d. <l.o.d..46 <l.o.d Max Mgt-bearing -hosted veins (n = 295) Average Min 0.4 <l.o.d. <l.o.d. <l.o.d <l.o.d. <l.o.d. <l.o.d. <l.o.d Max All -hosted veins (n = 898) Average Min 0.4 <l.o.d. <l.o.d. <l.o.d <l.o.d. <l.o.d. <l.o.d. <l.o.d Max Opx-hosted veins (n = 39) Average Min <l.o.d..82 <l.o.d Max Hybrid serpentinites (n = 445) Average Min 5.74 <l.o.d. <l.o.d. <l.o.d <l.o.d. <l.o.d <l.o.d Max Detection limits Averages are reported for analyses above detection limits serpentinization (Schwarzenbach et al. 204). Otherwise, magnetite is rare; it only occurs in the centers of hosted veins that are >50 μm (Fig. 2b; see detailed discussion below) and is absent from orthopyroxene-hosted veins. Carbonate veins that are common in variably serpentinized peridotites were not observed in this sample. Bulk vein chemistry Overall chemical variability EMP analyses were performed on numerous veins to reveal the chemical and mineralogical variability of different vein generations and veins hosted by either or orthopyroxene (Table ; supplementary Table S). Although EMP analyses of very fine-grained serpentinites (or other fine-grained materials) typically fail to resolve individual mineral phases (which form < μm intergrowths), large numbers of spot analyses taken together can help define the overall chemical variability of serpentinites based on knowledge of the phases that could be mixed in each analysis. EMP data indicate that there are three groups of serpentinite in the Santa Elena sample (Table ): one occurring as veins within protolith, one as veins in protolith orthopyroxene and a hybrid serpentinite formed when is replaced by serpentinite near an orthopyroxene grain boundary. The use of the term serpentinite is deliberate and refers to a polymineralic assemblage commonly dominated by, but not necessarily consisting exclusively of, serpentine. Note that the variations described here can, in most areas, be correlated with distinct patterns of margin-parallel zoning in individual veins. The immediate description, however, is restricted to overall host-dependent vein chemistry and the implications for chemical fluxes. Serpentinite derived from The serpentinites hosted by are best described as mixtures of serpentine and brucite, with or without magnetite. The average composition of the veins is approximately that expected from simple hydration of the host. For the majority of the magnetite-free veins, the most silicic compositions in the data set approximate stoichiometric serpentine, but the data set taken as a whole represents

6 5 Page 6 of 22 Contrib Mineral Petrol (206) 7:5 (a) Mg Cations/7 ox Mg# Mg Cations Brucite intercepts Mg Cations = Mg#78 Mg# = Mg#72 r 2 = 0.63 Serpentine intercepts Mg cations = Mg# 95 Mg# = Mg#94 serpentine r 2 = Si + Al cations/7 ox Mgt-bearing veins Mgt-free veins Mg Cations/7 ox. (c) magnetite trend mgt-free veins Si + Al cations/7 ox. (d) Fe Cations/7 ox. 6 magnetite trend 4 2 r 2 = Si + Al cations/7 ox. Fig. 3 Bulk EMP analyses of serpentinite veins in (due to submicron intergrowth structures, each analysis samples a mixture of mineral phases). a Variation of Mg with Si + Al in magnetite-free serpentinite veins. Brucite at Si = 0, serpentine at Si = 2. Serpentine Mg# (Mg/(Mg + Fe)) and Mg cations based on intercept value of 2.8 out of 3 Mg cations in the serpentine formula (intercept at 6.53 out of 7 on y-axis shown), for brucite based on 5.49 out of 7 Mg cations in the brucite formula. Al concentration is negligible and included only for purposes of later comparison with high-al serpentinites in orthopyroxene-hosted veins. b Mg# variation with Mg content. In magnetite-free veins, Mg content increases as Mg# decreases. In magnetite-bearing veins, two trends are observed: coupled Mg# and Mg content increase/decrease (attributable to a magnetite component) and a trend of near constant Mg# with increasing Mg. This trend and that of the magnetite-free veins are most easily explained by a brucite component: Mg-rich brucite in the magnetite-bearing veins and more Fe-rich brucite in the magnetite-free veins. c Mg# variations in magnetite-bearing veins. Low-Mg# analyses include a magnetite component. High-Mg# analyses are largely magnesian (Mg# 90+) brucite. Note the divergence at high Mg# between magnetite-bearing and magnetite-free veins in. The field of magnetite-free vein compositions and the regression line are from a. d Fe variation in hosted veins with the regression line through magnetite-free veins. Note the very Fe-poor, Si-poor compositions in magnetite-bearing veins, which reflect the presence of Mg-rich brucite coexisting with magnetite in the vein centers variable serpentine brucite proportions (Fig. 3a). Regression analysis of the whole data set for magnetite-free veins suggests that the serpentine end member is more magnesian (Mg# = 94 95) than the host (Fo 90), while the brucite end member is less magnesian (Mg# = 72 78), which is evident in the regression lines shown in Fig. 3a. These results are consistent with previous studies of partially serpentinized rocks (e.g., Beard et al. 2009; Frost et al. 203). Also note the anti-correlation between Mg cations and Mg# in these veins (Fig. 3b), a result consistent with the presence of a Mg-rich, Si-poor phase, with brucite being the obvious candidate. The less abundant magnetite-bearing veins differ from other veins in that some spot analyses are very Fe rich (i.e., magnetite rich; see trends in Fig. 3c, d). In the magnetitebearing veins, serpentine-rich (i.e., silica-rich) analyses are similar to those from magnetite-free veins (Fig. 3d). More noteworthy is the presence of Mg-rich brucite (Mg# = 96) in the magnetite-bearing veins (e.g., Fig. 3c). The divergence of brucite compositions in the two vein types is apparent from the deviation of low-si, low-fe, high-mg analyses in the magnetite-bearing veins from the regression lines calculated for the magnetite-free veins (Fig. 3d). In this sample, the most magnesian brucite is restricted to the centers of magnetite-bearing veins. Serpentinite derived from orthopyroxene Serpentinites derived from orthopyroxene are chemically distinct from those derived from. First, they are more silicic, with the only overlap in composition with -derived serpentinites in the vicinity of stoichiometric serpentine (Fig. 4a). The average (and typical) orthopyroxene-derived serpentinite can be described as a mixture of serpentine and talc (Fig. 4a). Second, on average, orthopyroxene-derived serpentines (average Mg# = 87.8 ±.8)

7 Contrib Mineral Petrol (206) 7:5 Fig. 4 Major cations in magnetite-free -hosted veins contrasted with veins in orthopyroxene. a Veins in orthopyroxene are more silicic than veins in, with the only overlap in the vicinity of stoichiometric serpentine, and strongly depleted in silica relative to the protolith orthopyroxene. b Although there is substantial overlap, veins in orthopyroxene are on average less magnesian than those in and slightly less magnesian than the protolith orthopyroxene. c, d Hybrid serpentinite veins have a bulk chemistry intermediate between serpentinite veins in and orthopyroxene. Note that, for the most part, the hybrid veins form by replacement of at and near the orthopyroxene grain boundaries (a) Mg cations/7 ox. (c) Mg cations/7 ox Si+Al cations/7 ox veins in Opx veins in Ol average vein compositions average protolith average Ol average vein serpentinites hybrid serpentinites Ol vein field serpentine average Opx talc Opx vein field Si+Al cations/7 ox. (d) average Ol Ol vein field Page 7 of 22 5 Mg(OH) 2 cronstedtite serpentine average Opx talc Si+Al cations/7 ox. Opx vein field Si+Al cations/7 ox. Fig. 5 Concentrations of minor cations in a and b orthopyroxene- and -hosted and c, d hybrid serpentinite veins. Al and Cr are elevated in host orthopyroxene and in veins in orthopyroxene. Hybrid serpentinites have intermediate compositions (a) Al cations/7 ox veins in Opx veins in Ol averages Cr cations/7 ox. 3 2 (c) Si cations/7 ox. (d) Si cations/7 ox. 3 Al cations/7 ox. 0.4 hybrid serp Opx Field Ol Field Si cations/7 ox. Cr cations/7 ox. 2 Ol Field Opx Field Si cations/7 ox. have lower Mg# than -derived serpentinites (average Mg# = 90.8 ± 6.6), despite the orthopyroxene-derived serpentinites having a wider range in Mg# and, unlike the -derived material, are slightly less magnesian than their host (Fig. 4b). Third, minor and trace elements that are enriched in orthopyroxene, specifically Cr, Al, Ca, and Ti, are enriched in orthopyroxene-derived serpentinite as well (Fig. 5a, b; Table S), a characteristic that is typically used to identify serpentine after pyroxene (Dungan 979). Note that the average composition of serpentinite derived from orthopyroxene is substantially less silicic than would be expected from simple hydration of orthopyroxene (Fig. 4a, b), suggesting significant open-system behavior. Hybrid serpentinites Hybrid serpentinites are formed by hydration of in the vicinity of an orthopyroxene contact, consist

8 5 Page 8 of 22 of variably Al-enriched serpentine ± brucite ± talc, and always lack magnetite. As the name implies, they have chemical characteristics suggesting input from both orthopyroxene and during their formation. Both major (Fig. 4c, d) and minor (Fig. 5c, d) element compositions of hybrid serpentinites straddle the boundaries between the and orthopyroxene fields (Table ). One interpretation is that individual analyses are recording protolith composition, i.e., the composition of the mineral that is replaced. However, this is clearly not the case since the mineral being replaced in all areas of hybrid serpentinites is. The hybrid serpentinites are characteristically enriched in Cl (up to.9 ± 2 wt%), more so than any analysis from - or orthopyroxene-hosted veins (Tables, S). Structural and mineralogical variations within veins Olivine hosted veins Petrographic observations show that -hosted veins preserve various types of mineral and chemical zoning (Fig. 2b). Using a combination of petrography, EMP traverses through individual veins, and element distribution maps, we identified four stages of vein formation (i.e., hydration) representing increasing reaction progress. These four stages were identified based on their distinct mineralogical, textural, and chemical characteristics, but they probably only represent snapshots at different stages of vein evolution. In other words, the initial fractures in the minerals developed at different times and then evolved from stage toward stage 4. Fractures that developed later may only have evolved to stage or 2 before the rock was exhumed to the surface, whereas fractures that developed earlier may have evolved through all four stages. Additionally, the different vein stages rare crosscutting relationships suggest that all veins evolved at a similar time. Given this, and the absence of data suggesting differently, we assume that evolution of stages 4 occurred at approximately constant temperature. Locally, magnetite-bearing veins do crosscut older stages 3 veins, supporting our assertion that the magnetite-bearing veins represent the most evolved vein stage. Because we associate increasing alteration of with an increase in the vein width, we identify the thinnest (<0 20 μm), homogeneous, and unzoned veins (Fig. 2b) as representing the earliest stage of replacement. EMP traverses across these stage veins show that they consist of serpentine brucite mixtures. The serpentine brucite mixture has an average Mg# of ~92 and an Si/ (Mg + Fe) of 3 (Fig. 6a), but is too fine grained Contrib Mineral Petrol (206) 7:5 to permit separate analysis of each mineral. Increasing width of -hosted veins is typically associated with the development of a brownish vein center (Fig. 2b). EMP analyses reveal that this central zone has a slightly higher brucite content (Mg# = 9 92; Si/(Mg + Fe) = ) than the main part of the vein (zone 2: Mg# ~9; Si/ (Mg + Fe) = 0 3; Fig. 6b). We identify these veins as stage 2 veins. Petrographically, the next stage of vein formation is very similar to stage 2 and cannot be distinguished using microscopy alone. We identify stage 3 veins based on their more complex chemical zonation (Fig. 7a) and as a transitional stage between stages 2 and 4. Stage 3 veins comprise four zones between the boundary of the grain and the vein center: At the contact with, a thin serpentine-rich zone (zone 2*, Fig. 6c) can be identified locally, which is also seen in Fig. 7a (as Srp*). The main zone between and the vein center (zone 2; Fig. 6c) consists of a serpentine brucite mixture (Mg# = 90 92, Si/ (Mg + Fe) = 2 6) that is chemically similar to zone 2 in stage and stage 2 veins. A thin serpentine-rich zone generally forms between zone 2 and the center of the vein (Mg# = 93 94; Si/(Mg + Fe) = ; Figs. 6c, 7a, Srp ). The center of the vein is enriched in Fe (Mg# 85; Si/(Mg + Fe) 0.25) and is brucite enriched (as magnetite is not identified by microscopy) compared to the center in stage 2 veins. This suggests the local presence of Fe-rich brucite, which is also confirmed by SEM analyses (Fig. 8 and supplementary material S.2). We interpret the magnetite-bearing veins as representing the latest (stage 4) and the most evolved veins for the following reasons: () the chemical zoning of stage 4 veins has significant similarities (except for the presence of magnetite) to the zoning of stage 3 veins (Fig. 6c, d), suggesting that stage 4 veins evolved from stage 3 veins, and (2) stage 4 veins locally crosscut stages 3 veins. A relatively complex zoning in magnetite-bearing veins is apparent from microscopic observations (Fig. 2b) but is pronounced in EMP traverses across individual veins (Fig. 7b), and in element distribution maps of Si, Mg, and Fe (Fig. 9). A serpentine-rich zone (Mg# = 9 93; Si/ (Mg + Fe) = ) forms at the immediate interface with (Fig. 6d, zone 2*, and bright pink regions in Fig. 9e). The main part of the vein consists of a serpentine brucite mixture (zone 2 in Fig. 6d; Fig. 7b) that ranges from very fine-grained to coarser intergrowths. These coarse-grained mixtures are visible in SEM images (Fig. 8d) and characterized by highly variable Mg# and Si/ (Mg + Fe) (Mg# = 85 9; Si/(Mg + Fe) = 0.4 7) as shown by the zig-zag pattern in the compositional profile shown in Fig. 7b. Toward the center of the vein, three distinct, thin zones can be distinguished: a brucite-rich zone (Mg# = 85 90; Si/(Mg + Fe) = ) that

9 Contrib Mineral Petrol (206) 7:5 Page 9 of 22 5 (a) 2 Stage Ol Srp +Brc Ol Stage Ol Srp +Brc brc dom. Srp +Brc Ol (c) 2* * Stage Ol srp dom. Srp +Brc srp dom. brc dom. srp dom. Srp +Brc srp dom. Ol (d) 2* * Stage Ol srp dom. Srp +Brc brc dom. srp mgt brc srp brc dom. Srp +Brc srp dom. Ol Fig. 6 Schematic description of the stages of vein formation within showing the different zones determined by EDS mapping and average ranges of Mg/(Mg + Fe) (=Mg#/00) and Si/(Mg + Fe) for each zone from profile analyses of ~80 different veins. In all developmental stages, zone contains with Mg/(Mg + Fe) = 0.9 and Si/(Mg + Fe) =. The dashed gray line represents serpentine (Si/ (Mg + Fe) = 0.67). a Stage vein representing initial formation of serpentine + brucite in. b Stage 2 vein contains a more brucite-dominated (brc dom.) vein center. c Stage 3 vein has slightly serpentine-dominated (srp dom.) zone next to and a distinct brucite-dominated vein center with lower Mg# than stage 2 vein centers. d Stage 4 veins with magnetite and almost pure brucite in the vein center (zones 5 + 6), next to a zone of almost pure serpentine that has Mg# of up to 99 (zone 4) and a slightly brucite-dominated zone (zone 3) next to it. See text for detailed discussion may be absent in some areas (Fig. 9c), followed by almost pure serpentine with very high Mg# (Mg# = 96 99; Si/ (Mg + Fe) = ) and magnetite coexisting with almost pure brucite that has Mg# up to 96 (Fig. 7b). These coexisting phases are also distinguishable by SEM (Fig. 8d). In summary, the development of veins during alteration is characterized by an increase in vein width accompanied by an increase in the complexity of the vein zoning. The most convincing evidence that stage 4 veins most likely evolve from stage to stage 2 to stage 3 veins is that the main zone (zone 2 in Fig. 6a d) generally shows relatively constant chemical composition (i.e., Mg# and Si/ (Mg + Fe)), while the vein center experiences chemical and mineralogical changes as the vein interacts with fluid and evolves chemically and mineralogically. Evidence for increasing fluid flow in the center of the vein is discussed in detail below.

10 5 Page 0 of 22 (a) (c) Srp Srp Srp* Ol Brc Srp Srp* Srp 0.8 Srp* Ol Srp + Brc Srp + Brc Mgt 0. Brc pyroxene Opx serpentine vein Distance (µm) Fig. 7 EMP analyses of three vein traverses (point analyses with constant analysis spacing of 2 μm) showing variations in Mg/ (Mg + Fe) (black solid line) and Si/(Mg + Fe) (black dashed line). a Olivine-hosted stage 3 vein preserving slightly higher Si/ (Mg + Fe) ratios directly next to a brucite-dominated vein center (Srp ). b Olivine-hosted and magnetite-bearing stage 4 vein showing distinct serpentine, brucite, and magnetite zoning. c Orthopyroxene-hosted serpentine vein with Si/(Mg + Fe) of suggesting fine-grained intergrowth of serpentine and traces of talc. Gray lines = Si/(Mg + Fe) of the standard minerals: orthopyroxene Si/(Mg + Fe) = ; Si/(Mg + Fe) = ; serpentine Si/ (Mg + Fe) = 0.67; and brucite and magnetite Si/(Mg + Fe) = 0 (not shown in the figure) Orthopyroxene hosted veins In the studied sample, orthopyroxene is less serpentinized than, with serpentine veins generally cutting across the cleavage of the protolith orthopyroxene. Unlike in, it is not possible to clearly distinguish different vein types. Contrib Mineral Petrol (206) 7:5 Most veins in orthopyroxene have an Si/(Mg + Fe) of (Fig. 7c) and have Mg# slightly lower (86 89) than the host orthopyroxene (Mg# = 90). An Si/ (Mg + Fe) of >0.67 suggests the presence of traces of talc, as confirmed by Raman spectroscopy, most likely as a fine-grained intergrowth that cannot be resolved by EMP. A few veins have Si/(Mg + Fe) = 0.67 indicative of pure serpentine, while some veins contain thin, unusually Alrich zones, where Al 2 O 3 contents can reach wt%. These Al enrichments are either at the edge or in the center of the serpentine vein. Where clinopyroxene exsolutions impinge on the veins, it is apparent that these exsolutions are less altered than the orthopyroxene host, consistent with the relative stability of clinopyroxene in this sample. Distribution of hybrid serpentinites Hybrid serpentinites show two modes of occurrence, both related to contacts between and orthopyroxene: () In areas where substantial is preserved near that contact, the hybrid serpentinites occur as flames or wedge-like structures replacing. These structures occur at the point where serpentinite veins in the orthopyroxene impinge on the orthopyroxene grain boundary (Figs. 2c, 0). (2) To the left of and above the large orthopyroxene grain in Fig. 2a, is largely replaced by regions of extensive meshes of hybrid serpentinite and any relationship between specific veins in orthopyroxene and the areas of hybridization is obscured. In neither case is there any regular compositional variability in the hybrid serpentinite areas. Instead, wispy or irregular areas of element variability are especially evident for Si and Al (Fig. 0c, e). Although irregular, these variations are significant as shown by EMP analyses (Figs. 4, 5). Also note in Fig. 0f the overall high concentrations of Cl in the hybrid serpentinite, which was also confirmed by EMP point analyses. Temporal relationships Element distribution maps collected at numerous locations along the contact of and orthopyroxene suggest that the hybrid serpentinites are a relatively early-formed feature, specifically reflecting breakdown of orthopyroxene. The hybrid serpentinites are sometimes cut by -hosted serpentinite veins, which typically have high Mg, low Al, significantly lower Fe, and low Cl contents (Fig. 0). Furthermore, there is evidence that a later stage fluid interacted with orthopyroxene to produce serpentinite veins that are variably enriched in Fe and Al. These veins are best distinguished from earlier serpentine veins associated with orthopyroxene due to their distinct chemical composition and because they crosscut the hybrid serpentinites

11 Contrib Mineral Petrol (206) 7:5 Page of 22 5 (a) (d) brucite (c) Fe-rich brucite 00 µm 5 µm (c) brucite+ serpentine (d) magnetite brucite+ serpentine Fe-rich brucite brucite+ serpentine brucite 5 µm 0 µm Fig. 8 a SEM image of the area shown in Fig. 2b, where is replaced by serpentine + brucite ± magnetite veins, with b, c, and d representing enlarged areas. b Variably Fe-rich brucite in the center of a magnetite-bearing vein. c Local Fe-rich brucite in the center of a stage 3 vein. d Brucite (Mg#96) next to magnetite in the center of a stage 4 vein. The coarse-grained brucite serpentine mixture of the stage 4 veins is also recognizable (e.g., Fig. 0e). Overall, the element distribution maps suggest effectively concurrent alteration of and orthopyroxene, possibly with a later stage fluid that mostly affected orthopyroxene. Raman spectroscopy Each of the vein types was analyzed using Raman spectroscopy to confirm the mineralogy and to identify the dominant serpentine phases (Fig. ; supplementary material S.3). We recognize that relative peak intensities can be affected by the orientation of the mineral relative to the polarization of the incident laser. Because the mineral phases are fine grained in most areas and many grains are included in the analytical volume, we can assume a random orientation of grains and use the variation in relative peak intensities as a first approximation of the relative abundances of different phases in different areas of the veins. Thus, by comparing the variation in relative peak intensities of, for example, one of the dominant peaks for lizardite versus one of the dominant peaks for brucite, we can approximate the relative abundances (in a qualitative sense) of these two phases in two different areas that were analyzed. We emphasize, however, that the primary reason for Raman spectroscopy here is for phase identification. Olivine hosted veins Raman spectroscopy confirms that lizardite is a dominant mineral phase within -hosted veins. Typical Raman bands of lizardite occur at 29 3, cm (assigned to vibrations of the O H O groups), cm (vibrational modes of the SiO 4 tetrahedra) and cm (symmetric Si O Si stretching vibrations), with the OH-stretching bands at and 3705 cm (Groppo et al. 2006; Kloprogge et al. 999; Rinaudo and Gastaldi 2003). The lizardite bands are most intense (relative) in the early stage veins and in the serpentine brucite mixtures in stage 2 and stage 3 veins. Based on peak intensities, lizardite is a dominant phase along the vein contact with, within the serpentine brucite mixtures, and in the serpentine Mg#99 zone adjacent to the vein center. In many veins, distinct bands at 35 and 62 cm indicate the presence of some chrysotile intergrown with lizardite (Kloprogge et al. 999; Rinaudo and Gastaldi 2003). These

12 5 Page 2 of 22 Contrib Mineral Petrol (206) 7:5 BSE (a) Brc (Mg#96) Srp () Mgt Mg Mgt Brc (Mg#96) 00µm (c) Srp (Mg#99) Fe-Brc Brc Brc () Si (d) Fe-Brc Srp () Brc Mgt Fe-Brc Fe Mgt Fe-Brc (e) Olivine Srp-dominated ± brucite (srp()) Stage 4 vein Serpentine + brucite Brucite-dominated (brc ()) Serpentine (Mg# up to 99) Brucite (Mg# up to 96) Magnetite (Mgt) Stage 3 vein Serpentine ± brucite Fe-brucite-dominated Fig. 9 Element distribution maps of -hosted veins: a BSE image, b element distribution map of Mg, c Si, and d Fe. e Schematic description and enlargement of the area shown in a (white rectangle) showing a magnetite-bearing vein. The mineralogical and chemical variations in Si, Mg, and Fe are indicated by different colors (explained on the right of panel e), mostly representing different mixtures of serpentine, brucite, and magnetite. The pale blue arrows in e indicate fluid flow as discussed in the text bands are most intense in serpentine Mg#99 in the center of magnetite-bearing veins. Raman analyses in the center of the stage 4 veins show distinct bands at 279 and 444 cm and pronounced OH-stretching bands at 3645 and 3650 cm (Lutz et al. 994), confirming coexistence of almost pure Mg brucite (Mg#96) and magnetite (Fig. a, b). Brucite was also confirmed within most of the serpentine brucite mixtures and in the brucite-rich cores of stage 2 and stage 3 veins. These serpentine brucite mixtures usually show bands at 279, 444, and 3645 cm, but generally lack the stretching band at 3650 cm that was observed in most analyses of pure brucite in magnetite-bearing veins. 3

13 Page 3 of 22 Contrib Mineral Petrol (206) 7:5 (a) 5 opx hybrid serp wedge 00µm BSE Mg (d) (c) brucite present brucite absent Si Fe (e) (f) Al Cl Fig. 0 Element distribution maps of an orthopyroxene grain boundary, with an initially orthopyroxene-hosted serpentine vein impinging into an -dominated area. a BSE image, b element distribution map of Mg, c Si, d Fe, e Al, f Cl. The low resolutions of the Fe and Cl maps reflect the low concentrations. The element distribution maps agree with the bulk vein chemistry, showing, e.g., that orthopyroxene-hosted veins are depleted in Si, but slightly enriched in Mg compared to the host orthopyroxene. Note, Al enrichments and high Cl contents of hybrid serpentinite along the orthopyroxene grain boundary, and the relation between -hosted veins and hybrid serpentinite in the Mg map. Light colors = high concentrations; dark colors = low concentrations Orthopyroxene hosted veins cm. In contrast to -hosted serpentine veins, analyses consistently lack the OH-stretching band at ~3705 cm. Additional bands occur at , , and cm. A band at cm can be ascribed to an antisymmetric stretching mode of the Si O groups in chrysotile (Kloprogge et al. 999; Rinaudo and Raman spectra of orthopyroxene-hosted serpentine veins are similar to those from veins hosted by indicating a dominance of lizardite. Typical bands occur at 3, 23, , and cm with an OH-stretching band at 3

14 5 Page 4 of 22 (a) 50,000 Al bearing serpentine Contrib Mineral Petrol (206) 7:5 Intensity (counts) 40,000 30,000 20,000 0, ,000 5,000 4,000 3,000 2, Serpentine with high Al contents observed in orthopyroxene-hosted veins and hybrid serpentinites is identified by Raman spectroscopy because bands typically at 23 and 388 cm are shifted to and cm, respectively, which was also observed by Groppo et al. (2006). Additionally, the chrysotile bands that occur at 350 cm, 620 cm, around 00 cm and the OHstretching band at 3706 cm, are almost entirely lacking in these Al-rich serpentine phases, suggesting that chrysotile is relatively rare in these areas. Intensity (counts) (c) Intensity (counts) 30,000 20,000 0, ,000 0,000 8,000 6,000 4,000 2,000 6,000 5,000 4,000 3,000 3,500 3,000 2,500 2,000, , ,000,500 2,000 2,500 3,000 3,500 4,000 Raman Shift (cm-) Gastaldi 2003). Hence, traces of chrysotile are likely intergrown with lizardite. The presence of talc is suggested by a distinct band at 96 cm in several veins, which does not appear in the Raman spectrum of lizardite, chrysotile, or brucite. Raman spectra of talc typically show peaks at 362 and 675 cm, with the OH-stretching band at 3675 cm. These peaks are indicated by broad bands at 360, 688, and 368 cm, respectively, but are largely obscured by overlap with the lizardite peaks (Fig. 0c) serpentine bands brucite bands talc bands Fig. Raman spectra of a brucite in the center of a magnetite-bearing, -hosted vein, b a variably brucite-dominated zone in the center of a magnetite-bearing vein, showing brucite and lizardite, and c an orthopyroxene-hosted serpentine vein indicating the presence of serpentine and talc Discussion Various studies have recently shown that hydration of typically results in serpentine brucite veins and that magnetite forms as a secondary product of hydration (Bach et al. 2006; Beard et al. 2009; Frost et al. 203; Katayama et al. 200). Importantly, most previous studies that look at several generations of vein formation describe textures that suggest reopening of older veins. In contrast, the textures of the harzburgite studied here imply continuous mineral evolution during vein formation, suggesting that individual veins were continuously open to fluid flow during this process, but that different veins were generated at different times. Hence, the results obtained here are consistent with previous studies, but due to the unusual preservation of mineralogical and chemical textures, we can give new insights into the processes during water rock interaction associated with the beginning of serpentinization. In particular: () The sample studied here records chemical gradients between orthopyroxene and implying transfer of elements such as Si, Al, Cr, and Ca. (2) The chemistry of brucite in the studied sample varies depending on whether it occurs with magnetite (brucite Mg#96) or without magnetite (brucite variably Fe rich). These variations are linked to the mineral and chemical zoning within -hosted veins that imply magnetite formation from Fe-rich brucite. (3) The mineral zoning in -hosted veins records chemical gradients within individual veins, most likely due to focused fluid input into the vein center. These specific observations are discussed in detail below and are compared to thermodynamic models, also discussing the implications of progressive hydration and evolving fluid chemistry. Implications for chemical fluxes The geochemistry of the Santa Elena serpentinites and the element distribution maps demonstrate that serpentinization is an open-system process on the scale of mineral grains

15 Contrib Mineral Petrol (206) 7:5 (< mm), allowing chemical transfer between orthopyroxene-hosted and -hosted veins. The most obviously mobile element is Si, with the bulk vein chemistry indicating a net transfer of Si from orthopyroxene into the hybrid serpentinites that locally replace (Fig. 4a). Si is transported up to ~200 μm from the orthopyroxene grain boundary into -hosted veins (Fig. 0c), promoting in the hybrid serpentinite replacement of brucite by serpentine according to reaction (): 3Mg(OH) 2 + 2SiO 2 (aq) = Mg 3 Si 2 O 5 (OH) 4 + H 2 O As evinced by the element distribution maps (Fig. 0), the migration of Si is tracked by other elements such as Al, Ca, Cr, and Ti (Table ; Fig. 5c,d) that were initially concentrated in the protolith orthopyroxene and are transferred over approximately the same length scale. Taken together, this suggests a one-way element transport from the orthopyroxene toward the during serpentinization to form hybrid serpentinites. This apparent one-way element transport suggests that gradients in silica activity between and orthopyroxene are the driving force for silica mobility, as has been suggested previously (e.g., Beard et al. 2009; Beard and Hopkinson 2000; Frost and Beard 2007; Frost et al. 2008), and reflects the instability of the hydration assemblage (serpentine brucite) at the elevated silica activity imposed by the orthopyroxene hydration assemblages. However, it is evident that apart from Si, elements such as Al, Cr, Ca, and Ti develop similar chemical potential gradients between orthopyroxene and. Serpentinization of distal from the orthopyroxene grain boundary, on the other hand, appears to be approximately isochemical (Fig. 4a), with the obvious exception of water. Without exception, veins in distal to the orthopyroxene contain brucite, as demanded by stoichiometry and thermodynamic constraints. The local brucite in limit is shown in Fig. 0c, marking the boundary where availability of SiO 2 (aq) is insufficient to allow complete conversion of brucite to serpentine. Progressive hydration and formation of magnetite Progressive hydration of has recently been studied with major focus on the formation of hydrogen and its correlation with magnetite formation and the partitioning of Fe into brucite and serpentine (Andreani et al. 203; Klein et al. 2009, 203, 204; McCollom and Bach 2009). However, the sequence of reactions during hydration is still a matter of debate (e.g., Bach et al. 2006; Evans 2008; Frost and Beard 2007) as the hydration reactions and the initial textures are rarely well preserved. In the following, we use simple thermodynamic models to track the role of water and silica activities during initial hydration of the studied sample. () Page 5 of 22 5 The low degree of serpentinization of the harzburgite sample studied here preserves the relatively early stages of serpentinization (serpentinization degree 30 %) and records a critical stage in the initiation of magnetite formation (Bach et al. 2006). Accordingly, the low abundance of magnetite in this sample agrees with studies that compare the magnetic susceptibility and rock density as a proxy for degree of serpentinization, showing that increasing degree of serpentinization and magnetite formation are initially decoupled (Bach et al. 2006; Beard et al. 2009; Oufi et al. 2002; Toft et al. 990). This has previously been taken to imply that formation of magnetite during serpentinization is a two-step process, with initial formation of Fe-rich brucite, which is subsequently replaced by magnetite upon reaction with a silica-rich fluid. The sequence of vein formation within described here clearly confirms this, but also emphasizes that vein evolution was continuous during interaction with a fluid, notably in the absence of an external silica source with preservation of local equilibria in the vein. In addition, the crosscutting nature of multiple veins at different stages of evolution (Fig. 2) implies that all stages also evolved at approximately the same temperature. The initial reaction of with water is preserved locally in the mineralogy of stage to 3 veins, showing that serpentinization initiates by the formation of serpentine and brucite that is variably Fe rich, without forming magnetite. Mass balance calculations suggest that in these veins the Mg# of brucite varies between 65 and 9. Raman analyses of stage and brucite-cored stage 2 and 3 veins confirm the presence of both serpentine (lizardite ± chrysotile) and brucite. As neither Raman nor EMP analyses are able to resolve single grains, we suggest that serpentine and brucite are intergrown at the sub-micrometer scale. Mg# and Si/(Mg + Fe) in stage veins are higher in the serpentine brucite mixture than in the initial, indicating that either some Fe is lost to the fluid during early reaction or that the composition of along the reaction surface is changed. No clear change in composition is preserved within remnant grains, which is also consistent with Evans (200) for serpentinization temperatures below 300 C. Brucite is dominant in the center of stage 2 veins (Fig. 2b), but locally becomes less abundant toward the vein rim, particularly in more evolved veins (Fig. 6c, d). Assuming that the stages of vein formation presented above represent gradual reaction progress during replacement of, we infer that an increase in water rock ratio accompanies vein formation, with fluid flow in the vein center maintaining a high local water activity that diminishes toward the vein margin. This is confirmed by model calculations: Modeling of simple dissolution of pure forsterite into seawater shows that dissolution of Mg-bearing species substantially lowers ah 2 O at low fluid rock ratios

16 5 Page 6 of 22 Contrib Mineral Petrol (206) 7:5 (a) phase abundance (volume %) (c) log activity of Si-bearing species Fluid abundance SiO2(aq) H3SiO4 - H2O activity Brucite abundance Chrysotile abundance Reaction progress upon addition of forsterite to seawater -0 NaH3SiO4 - H2SiO Reaction progress upon addition of forsterite to seawater H2O activity Mg-bearing solution species (mol/l) (d) log ah2o (aq) 0. e-3 e-4 e-5 e-6 e-7 e-8 e Mg ++ MgOH + MgCl + MgHCO3 + MgCO3 Mg4(OH4) ++++ Brucite Forsterite Chrysotile MgCl + Mg4(OH4) ++++ MgCO Reaction progress upon addition of forsterite to seawater log asio2 (aq) Fig. 2 Water interaction, calculated with Geochemists Workbench at 200 C. x-axes in panels a c represent progressive dissolution of 520 g of forsterite into 00 g of seawater with an initial composition of 9 mg/l Cl,.26 mg/l Mg 2+, mg/l aqueous SiO 2, 0 mg/l Na +, 400 mg/l Ca 2+, 40 mg/l HCO 3 and ph 8.0. Reaction progress = refers to the point at which fluid saturation is lost: a precipitated phases and the activity of H 2 O, b abundance of dissolved Mg-bearing species, c activity of SiO 2 upon progressive reaction, and d forsterite and hydrated products as a function of H 2 O and SiO 2 activities, consistent with previous observations that brucite is thermodynamically more stable than serpentine only at high H 2 O activities and low SiO 2 activities (e.g., Frost and Beard 2007) (Fig. 2a, b), but that log asio 2 is buffered to ~ 5.8 while forsterite remains in thermodynamic equilibrium (Fig. 2c). The serpentine-dominated outer margin of stage 3 and stage 4 veins (Fi.g 6c, d) thus implies protracted evolution distal to the H 2 O source (i.e., and fluid are physically separated by the early-formed serpentine brucite rind ), consistent with -buffered equilibration at log asio 2 above ~ 6.2 and log ah 2 O below ~ 0.8 (Fig. 2d). This indicates that within a single vein, the progression of the serpentinization reaction involves the migration of H 2 O along a gradient from high ah 2 O in the vein center toward low ah 2 O at the surface. Further fluid infiltration toward the contact with is most likely facilitated by grain boundaries of the finely intergrown serpentine brucite mixture (Fig. 9e), which serve as suitable fluid pathways that eventually allow hydration of at the vein wall. This occurs concurrently with reactions in the vein center that alter initial reaction products, eventually resulting in the formation of magnetite from brucite, as preserved in the transition from stage 3 to stage 4 veins and expressed by the following simple reactions (Eqs. 2 and 3; with Mg# derived from EMP analyses and the thermodynamic calculations discussed below): Ol(Fo90) + H 2 O = Mg-rich Srp(Mg# 93 95) + Fe-rich Brc(Mg# 82 84) (2)

17 Contrib Mineral Petrol (206) 7:5 Page 7 of 22 5 Mg-rich Srp(Mg# 93 95) + Fe-rich Brc(Mg# 82 84) + H 2 O Fe 2+ (in silicate) = H 2 + Fe 3+ (in magnetite) was permitted (available = Mgt + Mg-rich Srp(Mg# 99) + Mg-rich Brc(Mg#96) + H 2 (3) in supplementary material S4). Possible incorporation of Fe 3+ into serpentine has been suggested by several studies This sequence of reactions also agrees with previous studies of natural serpentinites (Beard et al. 2009; Frost et al. 203; Klein et al. 2009; Miyoshi et al. 204) and experimental studies (Lafay et al. 202), implying that initial alteration of produces serpentine with Mg# of that coexists with less magnesian brucite (with reported Mg# as low as 65). However, temperature also likely affects the Mg#, particularly if Fe 3+ is incorporated into serpentine (Klein et al. 204). Previous studies have attributed subsequent magnetite formation to either increasing or decreasing silica activity (destabilizing Ferich brucite or Fe-rich serpentine, respectively) (Andreani et al. 203; Bach et al. 2006; Beard et al. 2009; Frost et al. 203; Frost and Beard 2007; Miyoshi et al. 204). We show below that, although changing Si activity may accompany magnetite formation, it is not a requirement, in accordance with magnetite formation in -dominated (orthopyroxene absent) domains. (Klein et al. 2009, 203; McCollom and Bach 2009), but suitable thermodynamic data and mixing models are as yet incomplete so we do not focus on this here (see discussion below). Minor components such as Al, Cr, Ti, Mn, and Cl were also excluded. x = 0, y = in Fig. 3 thus represents the composition Mg.8 Fe 0.2 SiO 4 (, or.8 mol MgO mol FeO + mol SiO 2 ). The x-axis represents addition of water to this, reflecting a composition of 2 mol H 2 O + mol (Mg.8 Fe 0.2 SiO 4 ) at x =, y =. The y-axis represents changing Si/(Mg + Fe) ratio, from less silica rich than below y = to orthopyroxene (Mg.8 Fe 0.2 Si 2 O 6 ) at y =. We show the resultant mineral stability fields as a function of molar ratios of water and silica to rather than water and silica activities because this allows us to predict the thermodynamic equilibrium mineral assemblages as a function of the actual local water abundance. Figure 3 was calculated at 200 C and 300 bar following a previous study that suggests serpentinization conditions Controls on the mineral assemblages in hosted veins: further implications from thermodynamic modeling of <250 C for this sample (Schwarzenbach et al. 204). We note that moderate changes in temperature and pressure would not notably affect the mineral stability fields within the boundaries where serpentinization is most likely to occur. We first calculated equilibrium stability fields for each mineral assemblage by free energy minimization with Perple_X (Connolly 2005). From this, we calculated the Mg# of brucite and serpentine in the equilibrium assemblage and the volume proportions of, brucite, and serpentine in their respective stability fields (normalized to exclude the proportion of coexisting fluid). The use of similar thermodynamic models to study metasomatic alteration of ultramafic rocks has recently been evaluated in detail by Evans et al. (203) who suggested that this type of thermodynamic modeling is an ideal technique to simulate equilibrium thermodynamics on small length scales such as veins, where local equilibrium can be assumed. As described above, there is strong evidence that the different stages of alteration preserved in the Santa Elena peridotite generally represent a single phase of protracted vein evolution, with the reactions being controlled by fluid flow into the vein center. In addition, bulk vein chemistry distal from orthopyroxene grains suggests that hydration occurred in the absence of an external silica source (Fig. 4a, b). A better understanding of the role of water and silica abundance during serpentinization can be gained with further simplified thermodynamic calculations, assuming that a lack of overprinting in the studied sample preserves early-formed equilibrium assemblages. We simulate here the equilibrium assemblages formed by simple interaction of Mg Fe silicates with water, in a system in which oxidation of the Fe component is prohibited (Fig. 3, where ratios are expressed in molar terms). Fluid here is modeled as pure H 2 O, ignoring for simplicity all possible dissolved ions (such as the Mg-bearing species shown in Fig. 2b). Results accordingly indicate the final equilibrium state for a given composition, without necessarily revealing pathways by which this equilibrium is reached. For additional simplicity, and in the absence of unambiguous analysis of the extent of Fe 3+ incorporation in serpentine, all Fe is fixed as FeO. This prohibits magnetite and H 2 formation, which is beyond the scope of this contribution, though a subsidiary set of calculations were undertaken in which the reaction H 2 O + The role of Fe 3+ serpentine Ferric iron in serpentine has been a focus of several recent studies, which presented reaction path modeling (Klein et al. 2009, 203; McCollom and Bach 2009) or petrographic analyses of variably serpentinized peridotites (Andreani et al. 203; Evans 2008), all concluding that incorporation of ferric iron can affect the amount of H 2 produced during serpentinization. Although we agree that Fe 3+ incorporation into serpentine could control H 2 formation, we do not include Fe 3+ serpentine because its inclusion introduces several significant issues: () The most appropriate end member for Fe 3+ -serpentine is unclear, with

18 5 Page 8 of 22 Contrib Mineral Petrol (206) 7:5 Fig. 3 a Phase diagram for the system MgO, FeO, SiO 2, and H 2 O calculated at 300 bar and 200 C as a function of the H 2 O/ ratio [addition of H 2 O toward the right] versus Si/(Mg + Fe) ratio. Calculated volume proportion of the solid phases of b, c brucite, d serpentine, e calculated Mg# of brucite, and f Mg# of serpentine. Black dashed lines in b f are the stability fields shown in a. ol, opx orthopyroxene, tlc talc, brc brucite, srp serpentine, per periclase, mgt magnetite X(SiO 2 ): ratio X(SiO 2 ): ratio X(SiO 2 ): ratio 0.75 (a) ol + opx + tlc ol + srp + tlc serp + tlc + H 2 O serpentine 2b serp + brc + H 2 O ol + srp + brc 2a 3 2b serpentine serpentine 2b a b 2b 2b serpentine 2a ol + brc + per (c) (e) a brucite fraction brucite Mg# (d) (f) serpentine fraction fraction a a serpentine Mg# X(H 2 O): H 2 O/(Mg.8 Fe 0.2 SiO 4 ) ratio X(H 2 O): H 2 O/(Mg.8 Fe 0.2 SiO 4 ) ratio Fe 2 3+ Mg 2 SiO 5 (OH) 4 (Evans et al. 203), Fe 2 3+ Si 2 O 5 (OH) 4 (Klein et al. 2009), or Fe 2 2+ Fe 3+ (Si,Fe 3+ )O 5 (OH) 4 (cronstedtite) being candidates. Testing shows that the choice of these end members strongly affects the thermodynamic calculations, as, for example, the replacement of Fe 3+ for Si can prevent formation of brucite, due to the lower Si contents in cronstedtite compared to Fe-free serpentine. However, especially during initial serpentinization, brucite is clearly present, as shown here. We thus stress that the choice of ferric end member is crucially important, and an inappropriate choice can substantially modify phase equilibria. (2) Thermodynamic data for all Fe 3+ serpentine end members are limited, and simple calculation of thermodynamic data, as suggested previously, as well as constructing suitable mixing models may not be sufficiently accurate to model the typically very low abundance of Fe 3+ serpentine formed during initial stages of serpentinization (Evans 2008; Klein et al. 2009). (3) At temperatures 300 C, Fe 3+ /ΣFe ratios in serpentine decrease with decreasing water rock ratios (Evans 2008; Klein et al. 2009;