On Silica Activity and Serpentinization

Serpentinites have the lowest silica activity of common crustalrocks. At the serpentinization front, where olivine, serpentine,and brucite are present, silica activities (relative to quartz)are of the order of 10^–2·5 to 10^–5, dependingon the temperature. Here we argue that this low silica activityis the critical property that produces the unusual geochemicalenvironments characteristic of serpentinization. The formationof magnetite is driven by the extraction of silica from theFe₃Si₂O₅(OH)₄ component of serpentine, producing extremely reducingconditions as evinced by the rare iron alloys that partiallyserpentinized peridotites contain. The incongruent dissolutionof diopside to form Ca²⁺, serpentine, and silica becomes increasinglyfavored at lower T, producing the alkalic fluids characteristicof serpentinites. The interaction of these fluids with adjacentrocks produces rodingites, and we argue that desilication isalso part of the rodingite-forming process. The low silica activityalso explains the occurrence of low-silica minerals such ashydrogrossular, andradite, jadeite, diaspore, and corundum inserpentinites or rocks adjacent to serpentinites. The tendencyfor silica activity to decrease with decreasing temperaturemeans that the presence of certain minerals in serpentinitescan be used as indicators of the temperature of serpentinization.These include, with decreasing temperature, diopside, andraditeand diaspore. Because the assemblage serpentine + brucite marksthe lowest silica activity reached in most serpentinites, thepresence and distribution of brucite, which commonly is a crypticphase in serpentinites, is critical to interpreting the processesthat lead to the hydration of any given serpentinite. KEY WORDS: serpentinization; serpentinites; silica activity; oxygen fugacity; rodingites; magnetization of serpentinites     

Geochemical models of metasomatism in ultramafic systems: serpentinization, rodingitization, and sea floor carbonate chimney precipitation

In a series of water-rock reaction simulations, we assess the processes of serpentinization of harzburgite and related calcium metasomatism resulting in rodingite-type alteration, and seafloor carbonate chimney precipitation. At temperatures from 25 to 300°C (P = 10 to 100 bar), using either fresh water or seawater, serpentinization simulations produce an assemblage commonly observed in natural systems, dominated by serpentine, magnetite, and brucite. The reacted waters in the simulations show similar trends in composition with decreasing water-rock ratios, becoming hyper-alkaline and strongly reducing, with increased dissolved calcium. At 25°C and w/r less than ∼32, conditions are sufficiently reducing to yield H₂ gas, nickel-iron alloy and native copper. Hyperalkalinity results from OH⁻ production by olivine and pyroxene dissolution in the absence of counterbalancing OH⁻ consumption by alteration mineral precipitation except at very high pH; at moderate pH there are no stable calcium minerals and only a small amount of chlorite forms, limited by aluminum, thus allowing Mg²⁺ and Ca²⁺ to accumulate in the aqueous phase in exchange for H⁺. The reducing conditions result from oxidation of ferrous iron in olivine and pyroxene to ferric iron in magnetite. Trace metals are computed to be nearly insoluble below 300°C, except for mercury, for which high pH stabilizes aqueous and gaseous Hg°. In serpentinization by seawater at 300°C, Ag, Au, Pd, and Pt may approach ore-forming concentrations in sulfide complexes. Simulated mixing of the fluid derived from serpentinization with cold seawater produces a mineral assemblage dominated by calcite, similar to recently discovered submarine, ultramafic rock-hosted, carbonate mineral deposits precipitating at hydrothermal vents. Simulated reaction of gabbroic or basaltic rocks with the hyperalkaline calcium- and aluminum-rich fluid produced during serpentinization at 300°C yields rodingite-type mineral assemblages, including grossular, clinozoisite, vesuvianite, prehnite, chlorite, and diopside.     

The production of iron oxide during peridotite serpentinization: Influence of pyroxene

Serpentinization produces molecular hydrogen(H2)that can support communities of microorganisms in hydrothermal fields;H2 results from the oxidation of ferrous iron in olivine and pyroxene into ferric iron,and consequently iron oxide(magnetite or hematite)forms.However,the mechanisms that control H2 and iron oxide formation are poorly constrained.In this study,we performed serpentinization experiments at 311℃ and 3.0 kbar on olivine(with 5% pyroxene),orthopyroxene,and peridotite.The results show that serpentine and iron oxide formed when olivine and orthopyroxene individually reacted with a saline starting solution.Olivine-derived serpentine had a significantly lower FeO content(6.57±1.30 wt.%)than primary olivine(9.86 wt.%),whereas orthopyroxene-derived serpentine had a comparable FeO content(6.26±0.58 wt.%)to that of primary orthopyroxene(6.24 wt.%).In experiments on peridotite,olivine was replaced by serpentine and iron oxide.However,pyroxene transformed solely to serpentine.After 20 days,olivine-derived serpentine had a FeO content of 8.18±1.56 wt.%,which was significantly higher than that of serpentine produced in olivine-only experiments.By contrast,serpentine after orthopyroxene had a slightly higher FeO content(6.53±1.01 wt.%)than primary orthopyroxene.Clinopyroxene-derived serpentine contained a significantly higher FeO content than its parent mineral.After 120 days,the FeO content of olivine-derived serpentine decreased significantly(5.71±0.35 wt.%),whereas the FeO content of orthopyroxene-derived serpentine increased(6.85±0.63 wt.%)over the same period.This suggests that iron oxide preferentially formed after olivine serpentinization.Pyroxene in peridotite gained some Fe from olivine during the serpentinization process,which may have led to a decrease in iron oxide production.The correlation between FeO content and SiO_2 or AI_2 O_3 content in olivine-and orthopyroxene-derived serpentine indicates that aluminum and silica greatly control the production of iron oxide.Based on our results and data from natural serpentinites reported by other workers,we propose that aluminum may be more influential at the early stages of peridotite serpentinization when the production of iron oxide is very low,whereas silica may have a greater control on iron oxide production during the late stages instead.     

The stoichiometric effects of ferric iron substitutions in serpentine from microprobe data

ABSTRACT

Given that secondary magnetite is common in serpentinites, it is clear that serpentinites are oxidized rocks. Questions remain, however, concerning the distribution of ferric iron among magnetite and serpentine minerals and the role of ferric iron-rich serpentine in the formation of secondary magnetite. Direct determination of ferric iron in serpentine is not possible using an electron microprobe. We show, however, that the stoichiometic effects of ferric iron substitutions are detectable, although not quantifiable, by microprobe. First, we demonstrate that for studies that provide both microprobe analyses of major elements of serpentine and Mössbauer analysis of ferric iron, substitution effects are obvious. Next, it is equally clear that the early veins forming at the onset of olivine hydration (type 1 veins) show no indication of the presence of ferric serpentine, although a small amount of ferric ‘brucite’ may occur. Finally, we show that secondary (type 2) veins, which form as the system becomes open to fluids in equilibrium with plagioclase or pyroxene, contain, in addition to significant alumina, stoichiometric indications of ferric iron substitution. The serpentine in these veins is magnesian, usually with Mg#s around 96–98. Thus, even if a significant proportion of this iron is ferric, it comprises only a small fraction of the total ferric iron budget of the rock. Given that reduced iron is known to be abundant in early-formed brucite and early-formed serpentine and given that brucite, in particular, is absent from evolved serpentine veins, we propose that most magnetite in serpentinites forms as a tertiary product via oxidation of brucite.     

A Chemical Study of Serpentinization--Burro Mountain, California

Serpentinized dunites and harzburgites from the Burro Mountainperidotite show no change in the ratio of iron and magnesiato silica when compared with the same ratio for the unserpentinizedequivalents. The mineral assemblage resulting from serpentinizationconsists of lizardite-chrysotile, brucite, and magnetite andis determined by the original bulk composition of the peridotite.The chemical and mineralogical data indicate that serpentinizationproceeded under isochemical conditions except for the introductionof water into the peridotite. Expansion accompanies serpentinizationbecause the serpentine products occupy a greater volume thanthe peridotite protolith. Tectonic emplacement of the BurroMountain peridotite was facilitated by serpentinization andthe attendant expansion.     

Serpentinization: a review

The common alteration assemblage produced by serpentinization of ultramafic rocks is: lizardite, chrysotile, magnetite±brucite±antigorite. Lizardite-chrysotile serpentinites are more common than antigorite; the presence of antigorite indicates that the serpentinite has undergone prograde metamorphism or that the periootite was serpentinized in a higher P,T regime than lizardite and chrysotile. The iron subsitution into serpentine minerals and brucite is a function of temperature at low f_O2, with increased temperature enhancing magnetite formation. The presence of awaruite and native Fe are strong evidence for a locally very reducing environment. Isotopic studies have shown a wide variety of origins for the fluids involved in serpentinization. The increased boron content of serpentinized rocks when compared to boron contents of the parent ultramafic body indicates a possible sea water origin for the fluids. Serpentinization takes place under both constant volume and constant chemical composition conditions. The factors in evaluating the importance of the two processes for an individual serpentinite are: (1) determination of the mineral assemblage and its paragenesis, (2) the structural and tectonic relationship of the ultramafic body to its country rock, (3) fluid access to the rock in duration and amount, and (4) timing of serpentinization - before, during or after emplacement into the crust.     

Impact-induced phyllosilicate formation from olivine and water

Shock-recovery experiments on mixtures of olivine and water with gas (air) were performed in a previous study to demonstrate water-mineral interactions during impact events (Furukawa et al., 2007). The products of these former experiments were investigated in the present study using transmission electron microscopy, scanning electron microscopy, and X-ray powder diffraction with the aim of finding evidence of aqueous alteration. Serpentine formed on the surface of shocked olivine with well-developed mosaicism. The yield of serpentine depended on the water/olivine ratio in the starting material, indicating progressive serpentinization under water-rich conditions. Comminution and mosaicism were developed in shocked olivine grains. The temperature and pressure changes of the samples during the experiments were estimated by constructing Hugoniots for mixtures of olivine and water, combined with the results of an additional fracturing experiment on a shocked container. Pressures and temperatures reached 4.6-7.2 GPa and at least 230-390 °C, respectively, for 0.7 μs during in-shock compression. Post-shock temperatures reached a maximum of ∼1300 °C, when the shock wave reached the gas in the sample cavity. The serpentine formed after the post-shock temperature maximum, most likely when temperatures dropped to between 200 and 400 °C. This is the first experiment to demonstrate the formation of phyllosilicates using heat supplied by an impact. The present results and estimations suggest that phyllosilicates could form as a result of impacts into oceans as well as by impacts on terrestrial and Martian crustal rocks, and on some asteroidal surfaces, where liquid or solid H₂O is available. A significant amount of phyllosilicates would have formed during the late heavy bombardment of meteorites on the Hadean Earth, and such phyllosilicates might have affected the prebiotic carbon cycle.     

Present day serpentinization in New Caledonia,Oman and Yugoslavia

Geochemical evidence for modern low-temperature serpentinization has been found in three new localities. Apparently the low-temperature reactions are a common mode of formation of the lizardite-chrysotile and brucite assemblage. Possibly the ¹⁸O content of serpentine formed at low temperatures is in part inherited from the pyroxene and olivine.     

青海茫崖石棉矿区变质超基性岩的蚀变演化序列：岩石化学及矿物学证据

青海茫崖石棉矿区超基性岩体是由原岩以纯橄岩、辉橄岩和橄辉岩为主体组成的富镁质超基性岩体,经历自变质和后期多期热液的叠加变质蚀变作用,经蛇纹石化后形成蚀变完全的蛇纹岩岩体,其中部分蛇纹岩又进一步发生滑石化及碳酸盐化蚀变为滑石菱镁片岩、菱镁滑石片岩、滑石片岩和菱镁岩等。本文在野外地质调查基础上,在室内通过镜下岩矿综合鉴定、全岩化学成分分析以及电子探针成分分析等手段进行了岩石化学特征、矿物学特征及其蚀变演化过程研究。结果表明,该变质超基性岩体蛇纹岩主要特征组合矿物为蛇纹石(利蛇纹石、叶蛇纹石、纤蛇纹石)、磁铁矿、菱镁矿、滑石、水镁石、铬铁矿,变余矿物有斜方辉石、单斜辉石和铬铁矿,滑石菱镁片岩类主要组成矿物为菱镁矿、滑石、蛇纹石及磁铁矿,局部可见石英脉。该地区变质超基性岩体较完整地记录了橄榄岩水化、滑石化及碳酸盐化作用过程的各个阶段,超基性岩蚀变演化过程主要有两个作用阶段:(Ⅰ)橄榄石、辉石类矿物的蛇纹石化作用及蛇纹石绿泥石化作用;(Ⅱ)富Ca、CO2流体交代蛇纹石、滑石及水镁石的碳酸盐化作用。蛇纹石化等变质蚀变作用促进了Si、Mg及Fe元素化学活动性,使元素发生富集与迁移,对于次生矿物的形成与演化起到了一定的催化作用。多期不同组成流体热液的交代作用过程,清晰地展示了利蛇纹石、纤蛇纹石和叶蛇纹石的演化序列,以及滑石、水镁石、铬铁矿和磁铁矿的形成过程及标形特征。     

Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks

In recent years, serpentinized ultramafic rocks have received considerable attention as a source of H₂ for hydrogen-based microbial communities and as a potential environment for the abiotic synthesis of methane and other hydrocarbons within the Earth’s crust. Both of these processes rely on the development of strongly reducing conditions and the generation of H₂ during serpentinization, which principally results from reaction of water with ferrous iron-rich minerals contained in ultramafic rocks. In this report, numerical models are used to investigate the potential influence of chemical thermodynamics on H₂ production during serpentinization. The results suggest that thermodynamic constraints on mineral stability and on the distribution of Fe among mineral alteration products as a function of temperature are likely to be major factors controlling the extent of H₂ production. At high temperatures (>∼315 °C), rates of serpentinization reactions are fast, but H₂ concentrations may be limited by the attainment of stable thermodynamic equilibrium between olivine and the aqueous fluid. Conversely, at temperatures below ∼150 °C, H₂ generation is severely limited both by slow reaction kinetics and partitioning of Fe(II) into brucite. At 35 MPa, peak temperatures for H₂ production occur at 200-315 °C, indicating that the most strongly reducing conditions will be attained during alteration within this temperature range. Fluids interacting with peridotite in this temperature range are likely to be the most productive sources of H₂ for biology, and should also produce the most favorable environments for abiotic organic synthesis. The results also suggest that thermodynamic constraints on Fe distribution among mineral alteration products have significant implications for the timing of magnetization of the ocean crust, and for the occurrence of native metal alloys and other trace minerals during serpentinization.     

Redox evolution and mass transfer during serpentinization: An experimental and theoretical study at 200 °C, 500 bar with implications for ultramafic-hosted hydrothermal systems at Mid-Ocean Ridges

Highly reducing and high-pH vent fluids characterize moderately low temperature ultramafic-hosted hydrothermal systems, such as the recently discovered Lost City hydrothermal field at 30°N Mid-Atlantic Ridge Ridge (MAR). To better understand the role of mineral reaction rates on changes in fluid chemistry and mineralization processes in these and similar systems, we conducted an experimental study involving seawater and peridotite at 200 °C, 500 bar. Time series changes in fluid chemistry were monitored and compared with analogous data predicted using experimental and theoretical data for mineral dissolution rates. Although there was qualitative agreement between predicted and measured changes in the chemical evolution of the fluid for some species, the rate and magnitude of increase in pH, dissolved chloride and H₂ did not agree well with predictions based on theoretical modeling results. Experimental data indicate that dissolved H₂ abruptly and intermittently increased, reaching a value only approximately 20% of that predicted assuming magnetite as the primary Fe-bearing alteration phase. The distribution and valence of Fe in primary and secondary minerals reveal that the most abundant secondary mineral, serpentine, contained significant amounts of both ferric and ferrous Fe, with the less abundant brucite, also being Fe-rich (X_Fe = 0.3). Surprisingly, magnetite was present in only trace amounts, indicating that H₂ generation was largely accommodated by the formation of Fe-chrysotile. Accordingly, the diversity of Fe-bearing secondary minerals together with rates of serpentinization less than theoretically predicted, account best for the relatively low dissolved H₂ concentrations produced. Thus, the experimental data can be used to obtain provisional estimates of thermodynamic data for Fe-bearing minerals, enhancing the application of reaction path models depicting mass transfer processes during serpentinization at mid-ocean ridges. Similarly, the observed differences between theoretically predicted and experimentally measured pH values result from constraints imposed by complex patterns of mass transfer inherent to the experimental system. In particular, the experimental observation of a late stage increase in Na/Cl ratio likely results from the dissolution of a Na₂O component of clinopyroxene, which causes pH to increase sufficiently to induce precipitation of a Ca-bearing phase, perhaps portlandite. As with the redox variability observed during the experiment, this event could not be predicted, underscoring the need to use caution when modeling alteration processes in the chemically complex ultramafic-hosted hydrothermal systems at elevated temperatures and pressures.     

Oxygen isotopic variability associated with multiple stages of serpentinization, Duke Island Complex, southeastern Alaska

Ultramafic rocks of the Duke Island Complex in southeastern Alaska crystallized in a supra-subduction zone setting, but the serpentinization of olivine-bearing rocks involved the incursion of late-stage meteoric waters. Three textural types of serpentine (primarily lizardite) have been identified which in part reflect progress in reactions during multiple stages of fluid infiltration. The overall mesh texture of serpentine has been subdivided into a massive-type, found in dunites and wehrlites, and a dendritic-type found in wehrlites and olivine clinopyroxenites. Serpentine veins represent a late-stage in the hydrothermal alteration process. Both FeO contents and δ¹⁸O values of the three textural types of serpentine are variable at the centimeter scale. Magnetite abundance in association with serpentine is also variable with up to 5 vol% of magnetite found in samples with dendritic serpentine. Continued reaction of FeO-bearing serpentine with fluid appears to control the formation of most magnetite. Oxygen isotope ratios of the three textural types of serpentine are distinct, with the massive variety characterized by δ¹⁸O values between −3‰ and 3‰, the dendritic variety showing values between 2‰ and 6‰ and the veins having the highest values between 4‰ and 10‰. Although the δ¹⁸O values may vary by as much as 5‰ on the centimeter scale, δD values tend to show relatively less variation with over 90% of the measured values between −100‰ and −120‰. The O and H isotopic values are consistent with the involvement of meteoric water that had undergone variable degrees of isotopic exchange with country rocks prior to reacting with olivine in the Duke Island Complex. Small-scale variability in both serpentine FeO content and δ¹⁸O values suggests that chemical and isotopic equilibria may have not been attained at larger than centimeter scales. Oxygen isotopic variability in serpentine produced during relatively low-temperature hydrothermal alteration is in large part a function of exchange mediated via fluid flow through microfractures.     

Partage du nickel entre serpentine et brucite au cours de la serpentinisation du péridot

When nickeliferous peridot is submitted to a serpentinisation reaction with formation of brucite, the peridotic nickel is partitioned between the two minerals (serpentine and brucite) with a distribution coefficient K=[(Ni/Mg) brucite]/[(Ni/Mg)] serpentine=1,63 (at 350°C and 1 kbar), in favour of brucite.     

High temperature alteration of Abyssal ultramafics from the Islas Orcadas Fracture Zone,South Atlantic

Ultramafic rocks dredged from the Islas Orcadas Fracture Zone, along the SW Indian Ocean Ridge (6° E and 54° S), show evidence of progressive hydration beginning at temperatures greater than 600° C (and perhaps as high as 900° C) and continuing to less than 50° C. There are two principal types of alteration present in the ultramafic rocks, both of which are the result of hydration reactions. The first type of alteration involves hydration of original clinopyroxene, orthopyroxene and olivine to amphibole, talc, secondary olivine, and serpentine. The second is a vein type of alteration and results in the formation of veins of amphibole, chlorite, talc and serpentine. — The alteration appears to be episodic. The sequence of events suggested by the petrography is: 1) clinopyroxene altering to amphibole; 2) orthopyroxene altering to talc, or talc + olivine; 3) supersolvus hornblende veining; 4) coexisting actinolite + hornblende veining; 5) chlorite, chlorite + actinolite, or chlorite + secondary clinopyroxene veining; 6) talc veining; 7) serpentine veining; and 8) pervasive serpentinization. — The alteration fluid is most likely seawater. It is suggested that the high temperature alterations may reflect seawater circulation into the upper mantle.     

大洋橄榄岩的蛇纹岩石化研究进展评述

橄榄岩的蛇纹石化是大洋中不可忽略的重要地质过程,近年来引起广泛关注。大洋橄榄岩的蛇纹石化主要发生在洋中脊和汇聚板块边缘等环境中,大洋蛇纹岩典型的矿物组合包括:蛇纹石±磁铁矿±滑石±水镁石±角闪石。其中蛇纹石根据其矿物的晶体结构特征可分为利蛇纹石、纤蛇纹石和叶蛇纹石3种类型;偏光显微镜下可将蛇纹石结构划分为3类:假晶结构、非假晶结构和过渡结构。橄榄岩的蛇纹石化不仅会改变岩石的物理性质,如导致岩石密度的减小和地震波速的降低、影响橄榄岩的磁性等,而且也会对橄榄岩的流变性产生重要影响。大洋超基性岩系热液系统的发现,进一步激发了研究者们对大洋橄榄岩蛇纹石化研究的兴趣。与橄榄岩蛇纹石化相关的喷口流体含有较高的H2和CH4含量,此外,蛇纹石化是一个放热反应,可以驱动热液循环,导致Lost City等中低温型热液系统的出现。     

Vent fluid chemistry of the Rainbow hydrothermal system (36°N, MAR): Phase equilibria and in situ pH controls on subseafloor alteration processes

The Rainbow hydrothermal field is located at 36°13.8′N-33°54.15′W at 2300 m depth on the western flank of a non-volcanic ridge between the South AMAR and AMAR segments of the Mid-Atlantic Ridge. The hydrothermal field consists of 10-15 active chimneys that emit high-temperature (∼365 °C) fluid. In July 2008, vent fluids were sampled during cruise KNOX18RR, providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at Rainbow. Data suggest that the Mg concentration of the hydrothermal end-member is not zero, but rather 1.5-2 mmol/kg. This surprising result may be caused by a combination of factors including moderately low dissolved silica, low pH, and elevated chloride of the hydrothermal fluid. Combining end-member Mg data with analogous data for dissolved Fe, Si, Al, Ca, and H₂, permits calculation of mineral saturation states for minerals thought appropriate for ultramafic-hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations. These data indicate that chlorite solid solution, talc, and magnetite achieve saturation in Rainbow vent fluid at a similar pH_(T,P) (400 °C, 500 bar) of approximately 4.95, while higher pH values are indicated for serpentine, suggesting that serpentine may not coexist with the former assemblage at depth at Rainbow. The high Fe/Mg ratio of the Rainbow vent fluid notwithstanding, the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0.78 and 0.22, respectively. In situ pH measurements made at Rainbow vents are in good agreement with pH_(T,P) values estimated from mineral solubility calculations, when the in situ pH data are adjusted for temperature and pressure. Calculations further indicate that pH_(T,P) and dissolved H₂ are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution-fluid equilibria. Indeed, the predicted correlation between dissolved silica and H₂ defines a trend that is in good agreement with vent fluid data from Rainbow and other high-temperature ultramafic-hosted hydrothermal systems. We speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions, which, in turn are variably replaced by chlorite + magnetite + talc ± tremolite, with important implications for pH lowering, dissolved sulfide concentrations, and metal mobility.     

CO2 sequestration and extreme Mg depletion in serpentinized peridotite clasts from the Devonian Solund basin, SW-Norway

The conglomerates of the Solund Devonian basin of SW-Norway contain numerous (locally up to 20 vol.%) peridotitic clasts with concentric mm- to 10-cm thick zones of varying red to black color. The peridotite clasts show a clear, alteration-related textural evolution. The least-altered rocks are partly serpentinized peridotites, showing a typical mesh texture with veins of serpentine, magnesite and Ni-rich magnetite surrounding olivine (Fo₉₁) relicts and its Mg-depleted, clay-like alteration product (deweylite assemblage). In the more advanced ophicarbonate stage, the mesh cells contain calcite, silica and are surrounded by talc. In the final stage, quartz, calcite, and hematite dominate the mineralogy and occur together with minor amounts of chromite, talc, Cr-chlorite, and Cr-hydroandradite. In tandem with this textural evolution is a decrease in MgO from 40 to 2.5 wt% and a CaO increase from 1 to 35 wt%. All peridotite clasts are characterized by high Cr and Ni concentrations. The chemistry and the textural evolution show that the clasts formed by an extreme Mg-mobilization from the peridotite, with development of secondary porosity and subsequent precipitation of calcite. MgO removed from the clasts after burial is in part consumed by replacement reactions in the sediment matrix around the clasts where Mg-free minerals (e.g., almandine) are replaced by Mg-bearing minerals (e.g., talc). Calculated apparent ⁸⁷Sr/⁸⁶Sr ratios of the clasts at 385 Ma (0.7124-0.7139), corresponding to the inferred age of sediment deposition and incipient clast alteration, indicate interaction with diagenetic basinal fluids. We explain the reaction history as a three stage process involving (a) partial serpentinization of olivine in an oceanic environment (b) breakdown of olivine relicts to the deweylite assemblage resulting in mobilization of MgO under (near-) surface conditions in a tropical Devonian climate and (c) further Mg-mobilization and replacement of the deweylite assemblage by calcite and quartz after diagenesis. Sedimentary basins with abundant weathered peridotite represent potential sites for a permanent CO₂ storage by formation of calcite in a low-temperature environment.     

Genetic implications of Zn‐ and Mn‐rich Cr‐spinels in serpentinites of the Tidding Suture Zone,eastern Himalaya,NE India

Metamorphosed serpentinites of the Tidding Suture Zone (TSZ), eastern Himalaya, contain variably altered Cr‐spinels that are concentrically zoned from high‐Cr, low‐Fe³⁺ spinel at the core to Cr‐magnetite at the rim. Two types of Cr‐spinel have been recognized, based on back‐scattered electron imaging in conjunction with microprobe analytical profiles. Cr‐spinel type‐I is present in the least metamorphosed serpentinite (Cr# = 0.78–0.85, Mg# = 0.38–0.45) and Cr‐spinel type‐II is present in the most highly metamorphosed serpentinite (Cr# = 0.86–0.94, Mg# = 0.10–0.34). Primary igneous compositions are preserved in the type‐I chromites whereas these compositions have been partly or completely obscured by metamorphism and alteration in type‐II grains. The enrichment of Mn and Zn increases from the type‐I (MnO = 1.86–2.42 wt.%, ZnO = 0.77–1.67 wt.%) to type‐II (MnO = 2.72–4.04 wt.%, ZnO = 1.33–3.22 wt.%) and the strong similarity in their distribution patterns implies that these elements were introduced during low‐grade metamorphism and serpentinization. The abundance of Mg‐rich chlorite and serpentine minerals suggest that olivine was the predominant silicate phase before serpentinization. Zn and Mn enrichment in the core zone of the Cr‐spinel is due to the substitution of Mg²⁺ and in part of Fe²⁺, by Zn and Mn. These elements were probably supplied from olivine upon serpentinization during and after obduction of the ophiolitic mélange along the Tidding Suture Zone in the eastern Himalaya, NE India. Copyright © 2012 John Wiley & Sons, Ltd.     

Hydrothermal altered serpentinized zone and a study of Ni-magnesioferrite–magnetite–awaruite occurrences in Wadi Hibi,Northern Oman Mountain: Discrimination through ASTER mapping

Economic important minerals and ore deposits are common in hydrothermal altered serpentinized zone. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite sensor is capable of discrimination of such hydrothermal mineralized zone and detection of hydrothermal altered minerals. In the present study, the hydrothermal altered serpentinized harzburgites of Wadi Hibi area of Northern Oman Mountains have been discriminated by using ASTER VNIR–SWIR spectral bands by image processing methods and the occurrences of Ni-magnesioferrite–magnetite–awaruite in the rocks are studied. The color composite RGB image developed using ASTER spectral bands 8, 4 and 1, mapped well the occurrence of weathered peridotites by pale green to dark blue in colors and discriminated the hydrothermally altered serpentinized rocks by pale brown to dark blue colors due to the strong absorption of OH and Mg–OH molecules that occurred in the serpentine minerals of the rocks in the study area. The ASTER band ratios 4/7, 4/1, and 2/3 × 4/3 RGB images studied are capable of discrimination of hydrothermal mineralized areas more clear by pale blue to purple colors due to the strong absorption of such hydroxyl bearing serpentine minerals. The studied image processing methods are evaluated by applying to the region of Wadi Sarami situated in the Semail ophiolite (Oman). In addition to that, the occurrence of serpentine minerals namely, lizardite and antigorite in the hydrothermally altered serpentinized region are detected qualitatively and quantitatively using Spectral Angle Mapper (SAM) supervised classification image processing method and studied.The interpreted images are verified in the field and checked for the occurrences of minerals including Ni-magnesioferrite, magnetite, pentlandite and awaruite and are confirmed through laboratory studies. Petrographic study of serpentinized harzburgites shows that the rocks consist predominantly of antigorite and lizardite serpentines, olivine and have the opaque minerals assemblage of Ni-magnesioferrite + magnetite + awaruite + pentlandite developed during serpentinization of the rock. The occurrences of such minerals are confirmed by XRD, electron microprobe analyses and spectral measurements in the laboratory.ASTER sensor proved its capability in discriminating the hydrothermal altered serpentinized zone and detecting the mineral occurrences and thus the study recommends the technique to the exploration geologists, scientists and mining geologists for mapping of such rocks and minerals in the similar arid region.     

Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30°N): Insights from B and Sr isotope data

The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30°N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, O, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150-250 °C and at high integrated W/R ratios that led to a marked boron enrichment (34-91 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in δ¹¹B (11-16‰) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature-pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270-350 °C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater-gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of δ¹¹B values (9-10‰), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings.