O-isotope Equilibrium Research Articles

The Garies wollastonite deposit, Namaqualand, South Africa: High-Temperature metamorphism of a low-δ18O skarn?

ABSTRACT The Garies wollastonite deposit is located in the Bushmanland terrane of the Namaqualand Metamorphic Province and is part of a discontinuous calc-silicate unit bounded by granulite facies gneiss that experienced peak metamorphic temperatures above 800 °C. In bulk, the deposit is dominated by wollastonite, but varied proportions of garnet, diopside, quartz, calcite, and vesuvianite are also present. Mineral chemistry variations across the deposit are minor, and the absence of inclusions indicates textural and chemical equilibrium. The wollastonite-bearing rocks have unusually low mineral δ18O values: –0.6 to +2.2‰ for garnet, –0.2 to +2. 6‰ for clinopyroxene, and –0.2 to +0.4‰ for wollastonite. Calcite δ18O values range from 6.8 to 11. 8‰ and δ13C values from –6.4 to –3.2‰. Calcite δ18O values are unusually low for calc-silicate rocks, but Δcalcite-garnet values from 3 to 12‰ indicate O-isotope disequilibrium between calcite and the silicate minerals. Garnet-biotite metapelitic and diopside gneisses have unexpectedly low δ18O values (<7‰). The approach to O-isotope equilibrium displayed by coexisting silicate minerals, and low mineral δ18O values in calc-silicate and metapelite and metapsammite gneisses, is consistent with low δ18O values being acquired before peak metamorphism. Low δ18O values in the minerals of the calc-silicate rocks require interaction with external fluid at high water/rock ratio. We suggest that the deposit represents a metamorphosed skarn that developed at the contact between the original carbonate rocks and intruding felsic magmas.

Petrogenesis of peralkaline granite dykes of the Straumsvola complex, western Dronning Maud Land, Antarctica

Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI = molecular Al/[Na + K]) of the dykes is 1.20 (n = 29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na–Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb–Sr isochron age of 171 ± 4.4 Ma, with variable initial 87Sr/86Sr ratio (0.7075 ± 0.0032), and have an average e Nd of − 12.0. Quartz phenocrysts have δ18O values of 8.4–9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δ18O values of quartz and aegirine (average Δquartz−aegirine = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative e Nd (< − 10) and magma δ18O values averaging 8.6‰ (assuming Δquartz−magma = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δ18O values and much less negative e Nd (average − 3.1, n = 3). The country rock has similar δ18O values to the granite dykes (average 8.0‰, n = 108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δ18O contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization.

Oxygen isotope constraints on the alteration temperatures of CM chondrites

We report a systematic oxygen isotopic survey of Ca-carbonates in nine different CM chondrites characterized by different degrees of alteration, from the least altered known to date (Paris, 2.7–2.8) to the most altered (ALH 88045, CM1). Our data define a continuous trend that crosses the Terrestrial Fractionation Line (TFL), with a general relationship that is indistinguishable within errors from the trend defined by both matrix phyllosilicates and bulk O-isotopic compositions of CM chondrites. This bulk-matrix-carbonate (BMC) trend does not correspond to a mass-dependent fractionation (i.e., slope 0.52) as it would be expected during fluid circulation along a temperature gradient. It is instead a direct proxy of the degree of O-isotopic equilibration between 17,18O-rich fluids and 16O-rich anhydrous minerals. Our O-isotopic survey revealed that, for a given CM, no carbonate is in O-isotopic equilibrium with its respective surrounding matrix. This precludes direct calculation of the temperature of carbonate precipitation. However, the O-isotopic compositions of alteration water in different CMs (inferred from isotopic mass-balance calculation and direct measurements) define another trend (CMW for CM Water), parallel to BMC but with a different intercept. The distance between the BMC and CMW trends is directly related to the temperature of CM alteration and corresponds to average carbonates and serpentine formation temperatures of 110 °C and 75 °C, respectively. However, carbonate O-isotopic variations around the BMC trend indicate that they formed at various temperatures ranging between 50 and 300 °C, with 50% of the carbonates studied here showing precipitation temperature higher than 100 °C. The average Δ17O and the average carbonate precipitation temperature per chondrite are correlated, revealing that all CMs underwent similar maximum temperature peaks, but that altered CMs experienced protracted carbonate precipitation event(s) at lower temperatures than the least altered CMs. Our data suggest that the Δ17O value of Ca-carbonates could be a reliable proxy of the degree of alteration experienced by CM chondrites.

Constraints on paleofluid sources using the clumped-isotope thermometry of carbonate veins from the SAFOD (San Andreas Fault Observatory at Depth) borehole

The San Andreas Fault Observatory at Depth (SAFOD), near Parkfield, California, is a borehole drilled through two active deforming zones of the San Andreas fault, the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ). These zones accommodate displacement by seismic slip and aseismic creep. Elevated fluid pressures and fluid–rock interactions have been proposed to explain the low apparent strength and aseismic creep observed, but the origin of the fluids and existence of high fluid pressures remains uncertain. We use clumped-isotope thermometry and δ18O–δ13C compositions of calcite in veins to constrain the origin of paleofluids and compare these results to the isotopic composition of modern-day pore fluids from the SAFOD borehole and nearby areas. We observe that: (1) calcite vein temperatures vary from 81 to 134°C, which overlaps the current ambient borehole temperatures of 110–115 °C at sampled depths; (2) vein calcite is not in carbon isotope equilibrium with modern-day pore fluids; (3) the δ18O values of paleofluids close to the SDZ and CDZ, calculated from vein δ18O and temperature data, are not in equilibrium with local modern-day pore waters but approach equilibrium with modern pore waters far from these zones; and (4) syntectonic vein calcite is only in C- and O-isotopic equilibrium with their host rocks within the SDZ and CDZ. Spatial patterns of δ18O and δ13C show little evidence for across-fault fluid-flow. Clumped isotope temperatures are consistent with locally-derived fluid sources, but not with continuous or episodic replenishment of fluids from shallow sedimentary brines or deep fluid sources. Our findings are compatible with flow of meteoric fluids from the southwestern damage zone into the SDZ and CDZ, which would have favored the formation of weak phyllosilicates and contributed to the present day weakness of the two actively deforming zones.

ORIGIN OF TANZANITE AND ASSOCIATED GEMSTONE MINERALIZATION AT MERELANI, TANZANIA

Tanzanite is the blue vanadium-bearing variety of zoisite. The only known source of gem-quality tanzanite, to date, is the Merelani area of northeastern Tanzania, a structurally complex region consisting of granulite facies metacarbonate and metasilicate rocks overprinted by amphibolite to greenschist facies retrogression. Gem-quality green grossular garnets, known as tsavorite, are also found in the area, as well as a variety of other rare minerals and gemstones (such as axinite, dravite, fluorapatite, prehnite and tremolite). The δ18O values of tanzanite, tremolite, tsavorite and kyanite from the Merelani area are all high (average of 19.1, 19.5, 18.5 and 18.7‰, respectively) and are consistent with high-T O-isotope equilibrium, and the minimal involvement of externally derived fluids. The whole-rock δ18O values of graphite-bearing gneisses that host the tanzanite range from 16.8 to 21.6‰, whereas the marbles have somewhat higher values (24.0 and 25.0‰). Graphite-free garnet-bearing gneisses from the Telele Hills, ~33 km south of the mining area, have much lower δ18O values (~7 to 10‰). The high loss on ignition (LOI) values, ~8 weight % and ~10 weight %, of the graphite-bearing gneisses from the mining area do not correlate with CaO or MgO content, which is consistent with the presence of up to 10% graphite with minimal carbonate being present. The average δ13C value of the graphite from the garnet-kyanite gneiss in the mining area is −23.8‰, whereas most of the associated carbonate rocks have δ13C values close to zero. This lack of C-isotope equilibration between the adjacent carbonate and silicate rocks, together with the lack of change in bulk-rock δ18O values at the contact between carbonate and silicate units suggests that the high δ18O value of the graphite-bearing gneiss reflects that of its protolith. The average δD value of the tanzanite and tremolite is −77 and −27‰, respectively, which is consistent with their formation at in equilibrium with fluids having δD values of −36 and −11‰, respectively, assuming a temperature of formation of ~630°C. High δ18O values calculated for the fluids in equilibrium with tanzanite are consistent with a strongly rock-buffered system and hence a ‘metamorphic origin’. Calculated fluid δD values are close to zero, and suggest that the fluid is formation water, ultimately of marine or meteoric origin. The negative δ13C of the graphite and the high δ18O values of the rocks and minerals are consistent with previous suggestions that the protolith of the graphite-bearing gneisses was black shale. The major and trace element compositions of the rocks and minerals are consistent with this hypothesis

Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part II: Thermal peak conditions and cooling rates inferred from oxygen-isotope fractionations

Anhydrous mineral assemblages in felsic and mafic rocks from the Saxon Granulite Massif, part of the central European Variscan Belt, preserve O-isotope equilibrium fractionations recording temperatures of 1000–1020 °C. Although ultrahigh-temperature (UHT) metamorphism has been reported from Precambrian shields and Phanerozoic fold belts, this is the first time that an independent confirmation of UHT conditions based on both O-isotopic and chemical equilibria in the very same set of samples is presented. The evidence comes from feldspathic prismatine granulite and garnet-two-pyroxene granulite, where decompression-related minerals and the whole rocks are in isotopic equilibrium. The rocks thus behaved as closed systems with respect to oxygen at least during early stages of exhumation. Based on isotopic fractionations between the whole rock and minerals that cease to exchange oxygen at high temperatures, the closed-system behaviour enabled us to estimate near-peak temperatures, even if the predominant minerals were reset during cooling. This has been demonstrated in several samples mainly for whole-rock/garnet fractionations, which indicate temperatures similar to or slightly higher than the multiphase equilibria. Average cooling rates calculated from O-isotopic disequilibrium assemblages using the program COOL (Jenkin et al. , 1991) are in the range of 50–80 °C/Ma and correspond to an average exhumation rate of 10 mm/a. The preservation of the UHT isotopic and chemical equilibria is due partly to rapid cooling, partly to anhydrous conditions during peak metamorphism and early cooling. Partial chemical and isotopic homogenisation of peak-metamorphic mineral compositions under anhydrous conditions suggests that the chemical and isotopic exchange reactions were essentially controlled by volume diffusion.

Stable isotope study of the Archaean rocks of the Vredefort impact structure, central Kaapvaal Craton, South Africa

The Vredefort dome in the Kaapvaal Craton was formed as a result of the impact of a large meteorite at 2.02 Ga. The central core of Archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the Witwatersrand Supergroup, exposing a substantial depth section of the Archaean crust. Orthogneisses of the core show little variation in whole-rock δ 18O value, with the majority being between 8 and 10‰, with a mean of 9.2‰ (n = 35). Quartz and feldspar have per mil differences that are consistent with O-isotope equilibrium at high temperatures, suggesting minimal interaction with fluids during subsequent cooling. These data refute previous suggestions that the Outer Granite Gneiss (OGG) and Inlandsee Leucogranofels (ILG) of the core represent middle and lower crust, respectively. Granulite-facies greenstone remnants from the ILG have δ 18O values that are on average 1.5‰ higher than the ILG host rocks and are unlikely, therefore, to represent the residuum from the partial melting event that formed the host rock. Witwatersrand Supergroup sedimentary rocks of the collar, which were metamorphosed at greenschist-to amphibolite-facies conditions, generally have lower δ 18O values than the core rocks with a mean value for metapelites of 7.7‰ (n = 45). Overall, through an ∼20 km thick section of crust, there is a general increase in whole-rock δ 18O value with increasing depth. This is the reverse of what is normal in the crust, largely because the collar rocks have δ 18O values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide. The collar rocks have δD values ranging from −35 to −115‰ (average −62‰, n = 29), which are consistent with interaction with water of meteoric origin, having a δD of about −25 to −45‰. We suggest that fluid movement through the collar rocks was enhanced by impact-induced secondary permeability in the dome structure.

Effect of polymorphic transition on oxygen isotope fractionation between aragonite, calcite, and water: A low-temperature experimental study

The polymorphic transition from aragonite to calcite is a common phenomenon in nature and laboratory experiment, but its effect on O-isotope fractionation between secondary calcite and water is not known. Low-temperature synthesis experiments were carried out in this study to decipher the behavior of O-isotope fractionation between CaCO 3 and H 2 O during this process. The initial aragonite was synthesized by one- and two-step overgrowth techniques at 50 and 70 °C, respectively, with a difference in the duration of O-isotope exchange between dissolved carbonate and water prior to initial aragonite precipitation. Calcite was then synthesized by in situ polymorphic transition from the precursor aragonite in aqueous solution at the same temperatures. SEM observations show that the phase transition from aragonite to calcite proceeded in the mechanism of dissolution-reprecipitation. Although different approaches were employed to synthesize the starting aragonites, O-isotope analyses of product calcite and coexisting water unanimously yield consistent calcite-water fractionations relative to temperature. This finding demonstrates that calcite derived from polymorphic transition of aragonite can readily achieve O-isotope equilibrium with water regardless of its precursor composition. In particular, not only is the calcite-water fractionation relationship obtained in this study at 50 to 70 °C in agreement with that extrapolated from the literature data by slow precipitation experiments at 0 to 40 °C, but also the experimental calcite-water fractionations available at 25 to 70 °C are in close agreement with theoretically calculated fractionations. These results indicate that O-isotope equilibrium between calcite and water may prevail in calcite formed by the mechanism of dissolution-reprecipitation in aqueous solution despite different reaction paths.

Microscale oxygen isotopic exchange and magnetite formation in the Ningqiang anomalous carbonaceous chondrite

We report in situ measurements of O-isotopic compositions of magnetite, olivine and pyroxene in chondrules of the Ningqiang anomalous carbonaceous chondrite. The petrographic setting of Ningqiang magnetite is similar to those in oxidized-CV chondrites such as Allende, where magnetite is found together with Ni-rich metal and sulfide in opaque assemblages in chondrules. Both magnetite and silicate oxygen data fall close to the carbonaceous-chondrite-anhydrous-mineral line with relatively large ranges in δ 18O in magnetite (−4.9 to +4.2‰) and in silicates (−15.2 to −4.5‰). Magnetite and silicates are not in O-isotopic equilibrium: the weighted average Δ 17O (=δ 17O − 0.52 × δ 18O) values of magnetite are 1.7 to 3.6‰ higher than those of the silicates in the same chondrules. The petrological characteristics and O-isotopic disequilibrium between magnetite and silicates suggest the formation of Ningqiang magnetite by the oxidation of preexisting metal grains by an aqueous fluid during parent body alteration. The weighted average Δ 17O of −3.3 ± 0.3‰ is the lowest magnetite value measured in unequilibrated chondrites and there is a positive correlation between Δ 17O values of magnetite and silicates in each chondrule. These observations indicate that, during aqueous alteration in the Ningqiang parent asteroid, the water/rock ratio was relatively low and O-isotopic exchange between the fluid and chondrule silicates occurred on the scale of individual chondrules.

Oxygen isotope composition of the Denchai sapphire, Thailand: a clue to its enigmatic origin

Alluvial gem sapphires from the Denchai area, northern Thailand, have O-isotope compositions ranging from +4.7‰ to +6.1‰ (15 samples), with one outlier, +8.4‰. It is suggested that these sapphires are not in O-isotope equilibrium with and must be xenocrysts from the nearby alkali basalts, which contain olivine phenocrysts with δ 18O=+4.9‰ to +5.1‰ and are the presumed sources of these gem sapphires. Homogeneous δ 18O values within individual sapphire crystal indicate limited interactions with the carrier magmas during transport to the surface. The O-isotope compositions of these sapphires suggest that crustal rocks might have been involved in the genesis of those sapphires with δ 18O values higher than +5.5‰. A hybrid origin model for the Denchai sapphires is, therefore, postulated, which involves different degrees of interactions between a fractionated partial melt from (metasomatized) mantle and a lower/mid-crust Al-rich rock.

Oxygen isotope variations in primitive tholeiites of Iceland: evidence from a SIMS study of glass inclusions, olivine phenocrysts and pillow rim glasses

Oxygen-isotope compositions are reported for glass inclusions, their host olivines (Fo 87–90) and tholeiitic pillow rim glasses from the neovolcanic rift zone of Iceland. The inclusion compositions range from enriched ([La/Sm] n=1.4–4.8) to very depleted ([La/Sm] n=0.08–0.35). The whole rocks and matrix glasses display little variation ([La/Sm] n=0.41–0.51). Glass inclusions vary somewhat stronger in 18O/ 16O ratios (δ 18O=4.0–6.2±0.6‰) than the matrix glasses (δ 18O=4.6–5.6±0.6‰). The range of olivine phenocryst composition is even wider (δ 18O=2.4–6.9±0.5‰), and in some cases the variations within individual crystals reach 1.0–2.5‰. Most glass inclusions are in O-isotopic equilibrium with the surrounding host olivine but not with the central parts of the host crystals; similarly, most olivines are out of equilibrium with their host matrix glasses. This implies that the glass inclusions and olivine phenocrysts can preserve O-isotopic compositions inherited from contrasting parental mantle-derived magmas, which might have mixed in a common volcanic system. The isotopic heterogeneity of individual olivine crystals allows one to estimate their residence times in the magma reservoir using the rate of 18O diffusion in olivine. The time required to achieve isotopic homogeneity over 90–520 μm ranges from 25 to 1030 yr. The δ 18O values found in depleted glass inclusions and their host olivines are unlikely to have resulted from magma contamination by upper crustal rocks. We suggest two possibilities, implying either an O-isotope heterogeneity of the Icelandic mantle or a mixing of two end-members: a refractory component with ‘normal-mantle’ or somewhat elevated δ 18O values, high Ca/Na and strong light rare earth element depletion mixed with a component having lower δ 18O and Ca/Na but also depleted in incompatible elements. Based on δ 18O–La/Sm and δ 18O–δ 11B relationships, the variations of δ 18O between 4.6 and 6.2‰ in enriched and depleted glass inclusions are interpreted to reflect those of primary magmas and, consequently, of their source region.

Magma ascent and contamination beneath one intraplate volcano: evidence from S and O isotopes in glass inclusions and their host clinopyroxenes from Miocene basaltic hyaloclastites southwest of Gran Canaria (Canary Islands)

We report ion microprobe analyses of S and O isotopes in glass inclusions and their host clinopyroxenes (Cpx) from Miocene basaltic hyaloclastites drilled during Ocean Drilling Program (ODP) Leg 157. This is the first study where high-precision in situ δ34S and δ18O measurements were applied in order to constrain the isotopic variability of intraplate magmas and to have insights into establishment of isotopic equilibrium between the melt and crystallizing phenocrysts. Glass inclusions range from tholeiitic to alkali basaltic composition and contain varying to very high proportions of sulfur dissolved as sulfate (S6+/Stotal = 0.40 to 0.87) at S contents of 800 to 2100 ppm. The δ34S values varying from -1.0 to +8.5 ± 1.5‰, exceed significantly the range reported for fresh submarine midocean ridge basalt (MORB) and oceanic island basalt (OIB) glasses, and positively correlate with Cl concentrations and S6+/Stotal ratios. The δ18O values in glass inclusions and their host Cpx crystals analyzed within 50 μm distance away from inclusions are also strongly variable (inclusions: +5.0 to +8.2 ± 0.7‰; Cpx: +5.7 to +8 ± 0.7‰) and are in agreement with O-isotope equilibrium between Cpx and basaltic melt. A broad δ18O range (+5.6 to +7.3‰) decoupled from that of the melt inclusions was also found in the Cpx crystals measured >500 μm away from inclusions.The variations of δ34S and δ18O in glass inclusions are interpreted as resulting from a combination of degassing and assimilation processes. Hydrothermally altered basaltic crust (δ34S = +4.0 to +9.6‰, δ18O = +9.0‰) mixed with oceanic sediments (δ18O ≈ 25‰) and containing up to 5 wt.% of seawater sulfate (δ34S = +21‰, δ18O = +9.5‰) represents the most probable contaminating end-member. The oxygen isotope heterogeneity found in most Cpx crystals and their glass inclusions implies that magma contamination and inclusion-trapping occurred during a few years only, i.e., too short for O isotopic homogeneity to be reestablished in the entire crystals.

Fluid flow in metacarbonates associated with emplacement of the Bushveld Complex, South Africa

The 2.06Ga Bushveld Complex (BC) intrudes the Transvaal Supergroup. Calcsilicate xenoliths within the mafic phase of the BC (Rustenburg Layered Suite, or RLS) preserve xenoliths with high-temperature (∼1200°C), anhydrous mineralogies that were later metasomatised at 2059±1.1Ma (U–Pb titanite age) by hydrous, retrograde (∼600–700°C) fluids. These fluids were in approximate O-isotopic equilibrium with host mafic rocks. In the aureole, marbles show significant (∼5–10‰) but variable lowering of O-isotope values with increasing metamorphic grade. Mineral assemblages were typically buffered to higher XCO2 with increasing grade, except at the highest grades, where skarns and low-XCO2 assemblages occur locally. These data are generally consistent with variable flow of fluid derived from devolatilisation of interlayered metapelites up-temperature towards the intrusion. Limited, low-temperature channelling of low-18O fluids that had equilibrated with the RLS occurred along fault zones within the aureole.

Formation conditions of vesicle/fissure–filling smectites in Penghu basalts: a stable-isotope assessment

AbstractStable isotope and chemical compositions of vesicle- and fissure-filling smectites (i.e. montmorillonite and saponite) in Penghu subaerial basalts were analysed to establish their possible formation conditions. The results show that montmorillonites have higher Fe contents (as FeOtotal), as well as higher O-isotope and H-isotope compositions than saponites. These differences may largely reflect dissimilar formation conditions and/or fluid compositions. It is suggested that saponites probably precipitated from fluids dominated by meteoric groundwaters at 40–50°C, whereas montmorillonites were probably formed at 20–40°C from mixtures of groundwaters and seawater. Considering all the possible uncertainties, the above estimated temperature conditions are suggested as reasonable lower limits.It is established that the associated calcite is not in O-isotope equilibrium with the montmorillonite. The calcite might have been formed in environments with ambient fluid dominated by meteoric groundwaters at temperatures lower than those for montmorillonite.

Low [formula omitted], ultrahigh- P garnet-bearing mafic and ultramafic rocks from Dabie Shan, China

The Maowu mafic–ultramafic complex from the Dabie ultrahigh-pressure metamorphic belt of China is enclosed in coesite-bearing gneiss. The complex mainly consists of layered garnet ortho- and clinopyroxenites in addition to minor harzburgite, eclogite and rare omphacitite. The rocks contain pyrope-rich garnet, enstatite, diopside and forsterite and minor clinohumite, magnesite, chromite, rutile, monazite and apatite with rods of monazite. Thermobarometry and coesite inclusions in omphacite constrain peak metamorphism at about 700±50°C and 40–60 kbar. The δ 18 O (SMOW) values of garnet, olivine, orthopyroxene, clinopyroxene and omphacite from seven samples were determined by in situ analysis with an Ultraviolet Laser Microprobe. Except for omphacite, the δ 18 O data define a narrow range between +2.9 and +4.4 per mil; omphacite has the lowest δ 18 O values (+1.5±0.3). Furthermore, fractionation of 18 O and 16 O between coexisting mineral pairs (e.g., ΔGrt-Cpx, ΔOl-Cpx, and ΔGrt-Opx) is less than ±0.5 indicating high-temperature O-isotope equilibrium. High REE contents and extreme LREE-enrichment [(La/Yb) N up to 17 400] for whole rock, as well as low δ 18 O data of minerals suggest the ultramafic–mafic rocks were metasomatized by a LREE-enriched fluid and reacted with meteoric water at a shallow crustal level prior to ultrahigh- P metamorphism at upper mantle depths.

Stable isotope and modeling studies of fluid-rock interaction associated with the Snake Range and Mormon Peak detachment faults, Nevada

Stable isotope and fluid-inclusion data were obtained from rocks from traverses within and above the Snake Range and Mormon Peak detachments in Nevada in order to evaluate fluid sources and the nature of fluid flow associated with detachment faults during faulting, and to determine whether the initial depth of the detachment fault influenced the nature of syntectonic fluid flow. Oxygen and hydrogen isotope data indicate the detachment faults were infiltrated by meteoric water over a range of structural levels; however, only the upper-plate rocks and brittlely deformed portions of the faults exhibit significant O-isotopic shift. Although all traverses included limestones, δ 18 O of detachment fault breccia, veins, and upper-plate rocks differed significantly depending on the specific limestone involved. In one traverse in the Snake Range, where thin-bedded limestone of the Cambrian Lincoln Peak Formation was sampled, the δ 18 O of detachment fault breccia, veins, and stylolitically deformed upper-plate limestone near the detachment fault is typically 15‰±2‰ standard mean ocean water (unexchanged limestone has δ 18 O of ≈20‰), and the matrix was in O-isotopic equilibrium with vein fluid. Elsewhere in the Snake Range, where the detachment fault lay in massive medium-grained Cambrian and Ordovician limestones, δ 18 O values of detachment fault breccia and veins was much lower, typically 2‰±3‰, whereas δ 18 O of limestone matrix was between 16‰ and 20‰, far out of O-isotopic equilibrium with vein fluid. The sampled portion of the Mormon Peak detachment lay in medium-bedded Cambrian and late Paleozoic limestone: early veins have high δ 18 O values of 23‰–28‰ (unexchanged limestone has δ 18 O of ≈28‰), whereas later veins and detachment fault breccia have δ 18 O between 6.6‰ and 17.9‰. Thus, in the Snake Range, fluids were either in isotopic equilibrium with wall rock throughout the sampled fault history, implying intergranular flow, or were far out of equilibrium with it, implying channeling via a fracture network. The fluids in the Mormon Peak detachment were initially in isotopic equilibrium with wall rock, becoming increasingly 18 O depleted and out of O-isotopic equilibrium with wall rock with time. The difference in isotopic exchange history in the detachment faults and related rocks is evidently not a function of initial structural depth, but of permeability and its distribution between matrix and fractures. Combined thermal, fluid flow, and oxygen isotope exchange modeling demonstrate that the observed isotopic composition of rocks in and associated with the detachment fault could have been produced either by influx of meteoric water from topographically high areas downdip of the sampled area during detachment faulting, or by convection in the upper plate induced by elevated geothermal gradient and deformation-enhanced permeability. Locally derived meteoric fluids also infiltrated the detachment fault system in places, but are not required in order to account for low δ 18 O detachment breccias and veins. The model computations indicate that cumulative syntectonic fluid flux along the detachment faults was between 1700 and 11 000 kg/cm 2 , depending on location. Time-averaged fault permeabilities are estimated by modeling to have been between 2 and 20 mD. Thus, the model results not only verify fluid source, but also provide insight into fluid flow mechanism and important hydrogeologic properties of the detachment faults during deformation.