Serpentinization Processes Research Articles

Ridge segmentation and the magnetic structure of the Southwest Indian Ridge (at 50°30′E, 55°30′E and 66°20′E): Implications for magmatic processes at ultraslow‐spreading centers

The aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow‐spreading Southwest Indian Ridge. Contrary to faster spreading ridges, magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities. There is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities. We suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase, result in thinner extrusive lavas and thus control the along‐axis magnetization variations by thinning the magnetic source layer. A unique segment centered at 50°28′E shows a marked low magnetization anomaly at its center similarly to the segments of the slow‐spreading Mid‐Atlantic Ridge. We suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated. A higher rate of melt production to the west of Gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low‐magnetization lavas. To the east, magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas. Finally, we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley. Off‐axis, in thin crust areas, upper mantle rocks may become progressively more altered, as distance from the axis increases. The strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities. By contrast, in thicker crust areas, the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting, as on the Mid‐Atlantic Ridge, in slightly more positive magnetization values along the traces of axial discontinuities, regardless of polarity.

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 H 2 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 Mg 2+ and Ca 2+ 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.

U–Pb dating of serpentinization: hydrothermal zircon from a metasomatic rodingite shell (Sudetic ophiolite, SW Poland)

Abundant zircon grains were found within a metasomatic chlorite–corrensite (trioctahedral regularly interstratified chlorite/smectite) shell that surrounds rodingite from the Jordanów–Gogołów (JG) serpentinite massif (Sudetic ophiolite, Bohemian Massif). The zircon grains bear two types of primary fluid inclusions containing either liquid CO 2 or aqueous solutions. The inclusions were used to assign the P– T conditions of the zircon formation (270–300 °C, ca. 1 kbar), using the crossed-isochore method. Abraded single zircon grains give a U–Pb age of 400+4/−3 Ma, which is interpreted as the age of zircon crystallization. This zircon formed during rodingitization, i.e., a metasomatic process directly connected with the serpentinization and thus provides the opportunity of direct dating of the early serpentinization process. The zircon age from the Jordanów–Gogołów massif is consistent with the crystallization age of metagabbro protoliths from the Sudetic ophiolite as determined by Oliver et al. [J. Geol. Soc. (Lond.) 150 (1993) 355]. A Ce anomaly, combined with the mineralogical record of greenschist facies metamorphism of the mafic rocks from Sudetic ophiolite, suggests that the Sm–Nd whole-rock isotopic data of Pin et al. [Lithos 21 (1988) 195] (353±21 Ma) possibly characterize the age of later metamorphic event.

Serpentinization of the forearc mantle

A wide range of geophysical and geological data indicate that extensive serpentinization in the forearc mantle is both expected and observed. Large volumes of aqueous fluids must be released upwards by dehydration reactions in subducting oceanic crust and sediments. Subduction of oceanic lithosphere cools the overlying forearc such that low temperature hydrous serpentine minerals are stable in the forearc mantle. Over several tens of millions of years estimated fluid fluxes from the subducting plate are sufficient to serpentinize the entire forearc mantle wedge. However, fluid infiltration is probably fracture controlled such that mantle serpentinization is heterogeneous. Geological evidence for hydration of the forearc mantle includes serpentine mud volcanoes in the Mariana forearc and serpentinites present in exposed paleo-forearcs. The serpentinization process dramatically reduces the seismic velocity and density of the mantle while increasing Poisson’s ratio. Serpentinization may generate seismic reflectivity, an increase in magnetization, an increase in electrical conductivity, and a reduction in mechanical strength. Geophysical evidence for serpentinized forearc mantle has been reported for a number of subduction zones including Alaska, Aleutians, central Andes, Cascadia, Izu-Bonin–Mariana, and central Japan. Serpentinization may explain why the forearc mantle is commonly aseismic and in cool subduction zones may control the downdip limit of great subduction thrust earthquakes. Flow in the mantle wedge, induced by the subducting plate, may be modified by the low density, weak serpentinized forearc mantle. Large volumes of H 2O may be released from serpentinized forearc mantle by heating during ridge subduction or continent collision.

D, O and C isotopes in podiform chromitites as fluid tracers for hydrothermal alteration processes of the Mayarí — Baracoa Ophiolitic Belt, eastern Cuba

The Mayarí — Baracoa Ophiolitic Belt (MBOB, eastern Cuba) is composed of two large, chromite-rich massifs: Mayarí —Cristaland Moa-Baracoa. The chromitites and hosting dunites were firstly affected by a regional serpentinization event, a subsequent episode of hydrothennal alteration (chloritization mainly) and, finally, these already altered bodies were crosscut by thin calcite-dominated veins. Analysed serpentines from serpentinized chromitites and dunites present very similar isotopic compositions ( δ 18O=+4.7 to +6.3 and δD= −67 to −60 , suggesting that the serpentinization process took place at moderate temperatures, in an oceanic environment. Serpentine formation by interaction with ocean water is also supported by the isotopic composition of chlorite and calcite. These results suggest that the serpentinization, chloritization and fracture filling processes of the Mayarí — Baracoa Ophiolite Belt took place in a subocean floor scenario and, thus, that the Mayarí — Baracoa serpentines represent a good example of serpentine formed during interaction with seawater. The oceanic origin of the serpentines from serpentinized chromitites and dunites from the MBOB indicate that the serpentinization of the mantle sequence occurred pre-thrusting (pre-emplacement in age).

Non-transform offsets along the Mid-Atlantic Ridge south of the Azores (38°N–34°N): ultramafic exposures and hosting of hydrothermal vents

Ten contiguous non-transform offsets (NTOs) along the Mid-Atlantic Ridge (MAR) south of the Azores (between 38°N and 35°40′N) have been studied in detail using swath bathymetric, acoustic backscatter and deep-tow high-resolution sidescan sonar (TOBI) data. In contrast with discontinuities studied elsewhere at slow-spreading ridges, these left-lateral NTOs are consistently broader and larger, with complex structural fabrics accommodating the offset. They are characterized by a range of elevated and faulted massifs detached from their segment flanks, with an irregular acoustic backscatter pattern. Some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks, and sometimes associated with high-temperature hydrothermal venting. Water column surveys adjacent to these massifs show high CH 4 and low TDM (total dissolvable manganese) concentrations, possibly resulting from the process of serpentinization of ultramafic rocks. The correlation between the shallow dome-like shaped massifs and the high concentrations of CH 4 (associated with low levels of Mn) is of particular interest to predict the outcrop of ultramafic rocks within the NTOs where no geological data are available. The exposure of the ultramafic massifs within the NTOs is favored by low magmatic supply and low-angle detachment faulting occurring at segment ends. The pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high-temperature vent sites.

Chlorine in the Elba, Monti Livornesi and Murlo serpentines evidence for sea-water interaction

The nature of the water source for serpentinization has been previously addressed mostly using oxygen isotopes, that document interaction with sea-water and point out that lizardite and chrysotile originated within the oceanic domain. Mesh textures and bastites characterize the Elba, Monti Livornesi and Murlo (Central Italy) retrograde serpentinites. These rocks, formed by hydration of harzburgitic peridotites, were sampled and analyzed to find further evidence for the origin of serpentinization fluids. Based on chemical and mineralogical compositions, we conclude that the serpentinites from Elba, Monti Livornesi and Murlo contain important amounts of chlorine. In particular, bulk analyses indicate amounts ranging from 182 to 950 ppm; contents as high as 0.6 wt.% have been observed within the serpentine pseudomorphs. Chlorine is not present as a specific phase, even in nanometer-size domains; instead, chlorine isomorphically replaces hydroxyl groups. Even if widespread, chlorine is not completely homogeneously distributed, but increases from mesh rims to mesh cores, bastites and chrysotile veins. The heterogeneous chlorine distribution matches the previously reported two-stage serpentinization process, based upon thermal fracturing of the peridotite first (formation of the mesh rim) and massive water penetration into the weakened peridotite (formation of the mesh core). Geochemical balance of the chlorine content and the textural chlorine distribution are in agreement with sea-water origin of chlorine during serpentinization.

The Serpentinization of Peridotite from Cedar Valley, Jamaica

Previously undescribed, microscopic textures in partially serpentinized peridotite shed light on the process of serpentinization. Fracturing and fragmentation of the peridotite protolith produced a modest increase in volume and opened channels to reactive aqueous fluids. Prior to, or in conjunction with, the first appearance of serpentine, micrometer-scale grains of magnetite crystallized on the surfaces of fractures. This decoration of fracture surfaces faithfully preserved the original, angular outlines of fragments of forsterite and pyroxene through all stages of serpentinization. Simple geometric considerations indicate that the volume of a given fragment did not change during replacement. Partially serpentinized forsterite shows a penetrative, micrometer-scale, sieve or sponge texture. Fifty percent or more of the volume of optically continuous forsterite can consist of micrometer-scale perforations, filled with serpentine. This sponge texture has not been described elsewhere. The delicate connections between different parts of the same forsterite grain further support strict constant-volume replacement of the fragments. Serpentinization stopped before completion (mid-reaction) when the otherwise open, magnetite-decorated channels themselves filled with serpentine, effectively sealing the rock from further access by aqueous fluids.

Blueschist blocks at Mochimaru in the Tari‐Misaka ultramafic complex: Their petrologic characteristics and significance

Blueschist tectonic blocks occur in serpentinites at Mochimaru, Hiroshima Prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with or without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high P/T metamorphism around the boundary between the blueschist and pumpellyite–actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole rock compositions. The P–T conditions are estimated to be within 200–350°C and 5–7 kbar. These are higher P/T conditions than those of the regional metamorphism of Southwest Japan. The difference in the P–T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic rocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramafic rocks. The reducing effect of serpentinization is common throughout the high P/T metamorphic terranes of Southwest Japan.

Abiogenic methane in deep‐seated mid‐ocean ridge environments: Insights from stable isotope analyses

In this paper we examine geochemical processes that control volatile chemistry at depth in mid‐ocean ridge environments by focusing on CO2‐CH4‐H2O‐H2 fluids entrapped in plutonic rocks from the Southwest Indian Ridge (SWIR), Ocean Drilling Program Hole 735B. Compositional and isotopic analyses of CO2‐CH4‐H2O and CH4‐H2O‐H2 fluids show that methane production involved two phases of magma‐hydrothermal activity, which spanned supersolidus to greenschist facies metamorphic conditions. The first phase of methane generation is characterized by fluid inclusions that contain up to 30–50 mol % CO2 and 43 mol % CH4. Isotopic analyses of CO2, CH4, and H2O released at >900°C yields δ13C(CO2) values of −24‰ to −2‰, δ13C(CH4) values of −30‰ to −19‰, δD(CH4) values of −244‰ to −128‰, and average δD(H2O) values of −43±6‰. Phase equilibria and isotopic data strongly indicate that the CO2‐CH4‐H2O fluids reflect Rayleigh distillation of evolved magmatic CO2, subsequent closed‐system respeciation, and attendant graphite precipitation at temperatures of ∼500–800°C, and at fO2 from −3 log units below, to close to quartz‐fayalite‐magnetite oxygen fugacity (QFM) conditions. The second phase of CH4 production involves CH4‐H2O±H2±C‐fluids that contain >40 mol % CH4. Phase equilibria indicate that the CH4‐H2O fluids were trapped under equilibrium conditions at 400°C, very near to QFM conditions. Our study suggests that in the absence of CO2 as a stable fluid component, extensive distillation fractionation or alteration processes are required to form this later generation of methane. The mean δ13C values of methane extracted at 500°C from the gabbros (−25±4.4‰) are remarkably similar to the range of light carbon observed in studies of mantle rocks. We conclude that the presence of reduced carbon species in oceanic gabbros and mantle peridotites is a potential source of carbon in hydrothermal fluids and that serpentinization processes play a key role in the production of methane at greenschist facies conditions. Although total methane concentrations are low (0.3–0.6 mmol/kg) in the SWIR samples, on a global scale, plutonic layer 3 comprises ∼60% of the oceanic crust and thus represents a potentially immense reservoir (∼1019 gCH4) for abiogenic methane in mid‐ocean ridge hydrothermal systems. Production of methane and hydrocarbon species should be a common process in mid‐ocean ridge systems where high‐temperature fluids interact with mafic mineral phases. This is particularly significant because carbon‐bearing fluids may provide sustenance to subsurface‐and vent‐associated microbial communities and therefore represent an important link between deep‐seated hydrothermal systems and more shallow crustal environments.

PERIDOTITES, PYROXENITES AND GABBROS ASSOCIATION WITHIN HIGH-GRADE CRYSTALLINE BASEMENT ROCKS FROM THE CALABRIAN THYRRENIAN COASTAL CHAIN, CALABRIAN ARC, SOUTHERN ITALY

Small peridotite and gabbro bodies outcrop within medium- high grade, Grt- and Sil-bearing granulitic and migmatitic rocks of the Calabrian Thyrrenian Coastal Chain, northern sector of the Calabrian-Peloritan Arc, southern Italy. These bodies generally have sharp contacts with the country rocks. The peridotites, which sometimes include folded, transposed and boudinated pyroxenite veins and/or gabbroic dykes, are heavily serpentinized, although small, less serpentinized portions are locally preserved. These fresher portions show porphyroclastic microstructure, with olivine (Fo 90-93), orthopyroxene (En 89-91; Al2O3 = 1.1 - 2.9 wt%), brown spinel (Cr/(Cr+Al) 0.07-0.75), very rare clinopyroxene (Ca/(Ca+Mg) = 0.51-0.52; Al2O3 = 2.36-3.22 wt%), amphibole (Mg-hornblende - pargasite), chlorite, antigorite, lizardite 1T and magnetite. The pyroxenite veins are represented by abundant orthopyroxenites, Amph-pyroxenites and scarce Spl-websterites. The orthopyroxenites have porphyroclastic to granoblastic polygonal microstructures, with porphyroclasts of orthopyroxene (En 88-89; Al2O3 = 3.06-3.92 wt%), olivine (Fo 90) and hercynitic spinel set up in a well-recrystallized, polygonal matrix of orthopyroxene (En 88; Al2O3 = 3.5 wt%), olivine (Fo 90), amphibole (Mg - hornblende), serpentine, chlorite and diaspore. The Amph-pyroxenites show porphyroclastic texture with large porphyroclasts of amphibole (Mg-hornblende) and clinopyroxene (Ca/(Ca+Mg) = 0.49-0.51; Al2O3 = 1.9- 2.1 wt%) set up in a well-recrystallized matrix of amphibole (pargasite to Mg-hornblende), clinopyroxene (Ca/(Ca+Mg) 0.58; Al2O3 = 1.94-2.1 wt%), orthopyroxene (En 86-87; Al2O3 = 2.75-3.05 wt%), hercynitic spinel (Cr/(Cr+Al) = 0.09-0.13) and magnetite. The Spl-websterites have a porphyroclastic microstructure, with clinopyroxene (Ca/(Ca+Mg) 0.48; Al2O3 = 0.6 wt%), orthopyroxene (En 89; Al2O3 = 0.6 wt%) and brown spinel (Cr/(Cr+Al) 0.54). The serpentinized peridotites have MgO/FeOt values ranging from 5.1 to 7.6, which coupled with their Ni-Co-Sc patterns suggest a residual character. The REE patterns, in spite of some zig-zag behaviour of L - and MREE, possibly due to the serpentinization process, are also indicative of a depleted character (CeN/YbN = 0.03-0.65). The metagabbros show a tholeiitic fractionation trend. Incompatible elements spider diagrams normalized against MORB show more or less flat patterns around 1, except for a conspicuous negative Th anomaly. The REE normalized patterns show either a positive slope from La to Eu followed by a more or less flat trend at about 10 x Ch, or almost flat trend at around 10 x Ch. Some samples show slight, positive Eu anomaly. The LaN/YbN ratios range from 0.07 to 2.39. Their geochemical signature would be consistent with an underplating magmatism. The REE abundances of the pyroxenites, as well as their Ni-Co-Sc patterns, suggest a derivation from reacting melts and/or fluids. Temperatures of equilibration of about 800°C and 750°C have been calculated for the peridotites - pyroxenites and metagabbros, respectively. These data fit quite well the inferred physical conditions (750-800°C and about 0.9 - 1.1 GPa) for the metamorphic climax of the granulitic country rocks (Piluso et al., 1998). Preliminary barometric data are also consistent with these estimates. From the above considerations and in the light of the evolution model proposed for the northern sector of the Calabrian Arc by Piluso and Morten (1999), we suggest that: i) astenophere upwelling, responsible for the thermal anomaly, melting, and granulitic metamorphism in the crustal rocks, produced lithosphere thinning (after Variscan orogenic collapse) and underplating gabbroic magmatism in the northern sector of the Calabrian Arc; ii) rising melts/fluids formed pyroxenitic and gabbroic dykes in the underlying mantle peridotites, which were coupled with the crustal rocks before or during the migmatitic-granulitic event dated at 295 Ma (Shenk, 1989); iii) the peridotitespyroxenites- gabbros and the crustal rocks subsequently shared in a common exhumation history through HT and LT detachment shear zones (Piluso and Morten, 1999), under high amphibolite, amphibolite and greenschists facies conditions.

Guatemala jadeitites and albitites were formed by deuterium-rich serpentinizing fluids deep within a subduction zone

Jadeitites and albitites from the Motagua Valley, Guatemala, are high-pressure–low-temperature metasomatic rocks that occur as tectonic inclusions in serpentinite-matrix melange. Metasomatism was driven by a fluid with a δ18OH2O value of 6‰, and a δDH2O value that is high in comparison with metamorphic fluids at other high-pressure–low-temperature localities of similar grade. We infer that the fluid was originally seawater that was entrained during subduction either as mineral-bound H2O or as free pore waters. The fluid drove serpentinization reactions in ultramafic rocks, possibly leading to deuterium enrichment of H2O, prior to forming the jadeitites and albitites at a depth of 29 ± 11 km. There are isotopic and fluid-inclusion similarities to rodingites, which are Ca-rich metasomatites found at other serpentinite localities. Our results suggest that the serpentinization process, whether it occurs within subduction zones or on the flanks of oceanic spreading ridges, may produce residual fluids that are H2O rich, have 1–8 wt% equivalent NaCl, and have high, perhaps seawater-like, δD values.

A INTERAÇÃO DE CROMO-ESPINÉLIO COM FASES SILICÁTICAS NOS PROCESSOS METAMÓRFICOS

MgAl 2 O 4 . is one of the most significative components in chrome spinel from ultramafic rocks. It reacts with silicate phases during metamorphic reequilibration depending on PT conditions and mineral phases present. The presence of Cr in some silicate phases (serpentine, chlorite, amphibole) suggests that it may be extrated from chrome spinel as well. In serpentinites, the Mg-Al component is incorporated into the silicate phases, leaving behind the chrome-iron component, which may exist as an only phase, or as two associated phases, depending on the Cr/Fe 3+ ratio and temperature. In the chlorite stability field, the spinel may exhibit compositions varying between chromite and magnetite members, with low Al contents. Under these conditions, Cr and Al contents and Al/(Al+Cr) increase in spinel with increasing metamorphic grade. With chlorite breakdown MgAl spinel is formed regardless of the previously existing spinel phases. The Mg-Al spinel thus formed may constitute individualized grains, or else grow over the pre-existing spinel grains. The occurence of chlorite + spinel associations is due to the multivariant character that this reaction acquires in the presence of other cations, such as Cr, Fe 2+ , Fe 3+ . In the Mg-Al spinel stability field associations with one or more spinels, with broad compositional variations, may occur. Low-P peridotites contain plagioclase and develop aluminous spinel through the olivine + plagioclase reaction, leading to the orthopyroxene + pargasitic amphibole + aluminous spinel association, often with coronitic textures. The formation of ferrichromite is intimately associated with pervasive and straithforward serpentinization processes, without previous reequilibration of the peridotites under progressively lower conditions.

Intense CH 4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20′N fracture zone and the Mid-Atlantic Ridge

As part of the FARA French-US Program designed to study the Mid- Atlantic Ridge (MAR) between 15°N and the Azores, twenty-three dives with the submersible Nautile were conducted during the French-US Faranaut 15N cruise on the eastern and western parts of the 15°20′N Fracture Zone/Ridge axis intersection. South of the eastern ridge-transform fault intersection, nine Nautile dives were made within the rift valley and along the western rift valley wall. CH 4 concentrations in the bottom waters reach 53.2 nmol/kg along faulted zones on top and on the east flank of the ultramafic inner corner high (15°05′N, 44°59′W) where serpentinized rocks outcrop. No 3He anomaly is associated with methane, ruling out any primary mantle component. Fourteen dives were also made in the rift valley to the north, close to the western intersection of the 15°20′N Transform. High CH 4 anomalies (up to 22 nmol/kg) are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop. Seven vertical hydrocasts carried out in the axial valley (4500 m deep) show an intense CH 4 anomaly, with a maximum (35.8 nmol/kg) at 3200 m depth. This CH 4 concentration is among the highest found along the Mid-Atlantic Ridge (Charlou and Donval, 1993; Charlou et al., 1997). CH 4 concentrations of 9.9–14.9 nmol/kg are also present on the western wall along the 3200 m isobath. The high CH 4 concentrations correspond to only weak 3He anomalies. This CH 4-rich plume is also associated with active fault zones that expose peridotite rocks. CH 4 output from ultramafic outcrops on the western and eastern intersections of the 15°20′N Fracture Zone with the MAR is believed to reflect ongoing serpentinization. These results associated with many other CH 4 anomalies measured in the water column above ultramafic outcrops found between 12°N and the Azores most likely reflect serpentinization processes extending along the whole slow spreading Mid-Atlantic ridge.

Sulfur in serpentinized oceanic peridotites: Serpentinization processes and microbial sulfate reduction

The mineralogy, contents, and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes. Serpentinization of <1 Ma peridotites at Hess Deep occurred at high temperatures (200°–400°C) and low water/rock ratios. Oxidation of ferrous iron to magnetite maintained low ƒO2 and produced a reduced, low‐sulfur assemblage including NiFe alloy. Small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled, producing low‐δ34S sulfide (1.5‰ to −23.7‰). In contrast, serpentinization of Iberian Margin peridotites occurred at low temperatures(∼20°–200°C) and high water/rock ratios. Complete serpentinization and consumption of ferrous iron allowed evolution to higher ƒO2. Microbial reduction of seawater sulfate resulted in addition of low‐δ34S sulfide (∼15 to ∼43‰) and formation of higher‐sulfur assemblages that include valleriite and pyrite. The high SO4/total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high δ34S of total sulfur (mean ∼8‰). In contrast, Iberian Margin serpentinites gained large amounts of 34S‐poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in δ34S of total sulfur (mean ≈ −5‰). Thus serpentinization is a net sink for seawater sulfur, but the amount fixed and its isotopic composition vary significantly. Serpentinization may result in uptake of 0.4–14 × 1012 g S yr−1 from the oceans, comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output.

Determination of the lower edges of magnetized bodies by using geothermal data

SUMMARY The determination of the lower edges of magnetized bodies in the Earth's crust is a complex geophysical problem, although these values can be estimated by using geothermal data. An analysis of the temperature regime and location of the lower edges of magnetized bodies has been carried out for the geosynclinal region of the southern Caucasus and the area joining the ancient platform with the Arabian Shield in Israel. Geothermal calculations for Israel have been performed for three models of the thermal regime for the Earth's crust and upper mantle. The process of ultrabasic rock serpentinization is accompanied by the transformation of iron suboxide to iron oxide. Both these processes run under identical thermodynamic conditions within an average temperature interval of 200-400 C. The Curie surface controls the position of lower edges only in fault zones where oxidation conditions hold up to great depths.

Reduction of CO2 during serpentinization of olivine at 300 °C and 500 bar

CO 2 reduction processes occurring during experimental serpentinization of olivine at 300 °C and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas. Data reveal that conversion of Fe(II) in olivine to Fe(III) in magnetite during serpentinization leads to production of H 2 and conversion of dissolved CO 2 to reduced-C species including methane, ethane, propane, and an amorphous carbonaceous phase. Hydrocarbon gases generated in the process fit a Schulz-Flory distribution consistent with catalysis by mineral reactants or products. Magnetite is inferred to be the catalyst for methanization during serpentinization, because it has been previously shown to accelerate Fischer-Tropsch synthesis of methane in industrial applications involving mixtures of H 2 and CO 2 . The carbonaceous phase was predominantly aliphatic, but had a significant aromatic component. Although this phase should ultimately be converted to hydrocarbon gases and graphite, if full thermodynamic equilibrium were established, its formation in these experiments indicates that the pathway for reduction of CO 2 during serpentinization processes is complex and involves a series of metastable intermediates.

Estimating and simulating the degree of serpentinization of peridotites using hyperspectral remotely sensed imagery

The mineral products resulting from the process of serpentinization, by which primary magnesium silicate minerals in peridotites are replaced by hydrous serpentine-group minerals, are of economic importance since Alpine-type peridotites are the host rocks for virtually all large asbestos deposits, which may be attributed mainly to the serpentine-group mineral chrysotile. Conventional field mapping of the distribution of highly serpentinized areas is time consuming and requires detailed sampling and laboratory analysis. In 0.4- to 2.5-μm reflectance spectra of serpentinized peridotites, serpentinization is responsible for a decrease in contrast of olivine-pyroxene iron absorption features and an appearance and increase in OH− absorption features near 1.4 μm and 2.3 μm associated with serpentine minerals. The degree of serpentinization is positively correlated with the depth of the 1.4-μm and 2.3-μm absorption features for samples containing more than 55 weight percent serpentine minerals. Small amounts of magnetite in a sample obscure the spectral contrast and decrease the overall brightness of weakly serpentinized samples. A methodology is used for mapping serpentine minerals in ultrabasic rocks from imaging spectrometer data, which includes (1) vegetation masking, (2) calculating the absorption band depth of the 2.3-μm absorption feature in unmasked pixels, (3) translating this value into percent serpentine minerals using an empirical linear model, and (4) estimating the degree of serpentinization at the remaining locations using conditional simulation techniques or ordinary block kriging. From the results of this study, it can be concluded that mapping the degree of serpentinization from high-spectral resolution imagery is possible within marginal statistical fluctuations. Conditional simulation reproduces the spatial and statistical variability of the data set; however, it sacrifices the local accuracy. Direct estimation using ordinary kriging provides a better local estimate but does not honor the statistics and spatial dispersion of the original data.

Fast and slow spreading ridges: Structure and hydrothermal activity, ultramafic topographic highs, and CH4 output

Different parts of the world ridge system have quite different morphologies, which reflect different constructional processes. It appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust. This hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes. The TAG (26°N) and Snake Pit (23°N) hydrothermal sites on the Mid‐Atlantic Ridge (MAR) display Mn/CH4 ratios in overlying seawater similar to those of nonsedimented East Pacific Rise sites, i.e., about 0.2 mol/L. In contrast, large methane anomalies with very low Mn/CH4 ratios of 0.005 mol/L are associated with ultramafic topographic highs near 15°N, close to the axis and on ultramafic walls of the rift valley. The association of ultramafic bodies and CH4 anomalies in seawater indicates active serpentinization processes. CH4 is produced during serpentinization according to the Fisher Tropsch reaction (Charlou et al., 1991). Many ultramafic rock assemblages have been sampled on the MAR away from fracture zones, in particular by the Deep Sea Drilling Project. Changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges.

Accessory chromium spinels: Their coexistence and alteration in serpentinites

Accessory chromium spinel occurring in serpentinites is usually altered. The extent of this alteration and its relationship to alteration processes such as serpentinization and metamorphism is not defined, complicating the use of altered spinel as a petrogenetic indicator. Here, the relationship between different alteration processes and spinel alteration is evaluated in terms of accessory chromium spinel occurring in serpentinites of progressive metamorphism in the eastern Central Alps. Two coexisting Cr-spinels were identified, i.e., an altered, Fe-rich Cr-spinel (I) with Cr > Al, and an Fe-poor, Cr-spinel (II) with Al > Cr, which exsolved a Cr-Fe 3+ phase and which resisted alteration. Alteration of Cr-spinel (I) is evident from decreasing Al, Mg, and Cr values, with a trend leading to ferrite-chromite (trend a). An enrichment in Cr (trend b) related to CO 2 in the serpentinizing fluid, indicated by the presence of carbonate, talc or tremolite, is superimposed to (a). Alteration is a decomposition of the spinel structure caused by the dissolution of Mg, Al, and Cr during serpentinization. Dissolution of Cr is favored in a reduced environment whereas reprecipitation occurs under locally oxidizing conditions, indicated by the presence of CO 2. SiO 2 detected in spinel is a relative measure of the degree of alteration. Metamorphism up to amphibolite facies recrystallized and homogenized preexisting spinel phases to various degrees. Whether or not and to what extent a spinel alters depends on its Fe content. Fe 2+, stabilized in the octahedral site (high CFSE), is responsible for an inverse and less stable spinel structure. Fe-rich spinels (Cr-spinel I) are, therefore, readily subjected to alteration, whereas Fe-poor spinels (Cr-spinel II) are resistant to alteration during the serpentinization processes.