Seafloor Alteration Research Articles

Chlorine in the Earth’s Mantle as an Indicator of the Global Recycling of Oceanic Crust

Abstract —Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth’s history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.

Petrological Implications of Seafloor Hydrothermal Alteration of Subducted Mid-Ocean Ridge Basalt

Abstract Determining the mineralogical changes occurring in subducted oceanic crust is key to understanding short- and long-term geochemical cycles. Although numerous studies have explored the mineral assemblages that form in mid-ocean ridge basalt (MORB) at different depths below the Earth’s surface, it is widely recognized that seafloor hydrothermal alteration of the uppermost portion of the oceanic crust can change its composition between a ridge and a trench prior to subduction. In this study, we use petrological modelling to explore the effects of different types of pre-subduction hydrothermal alteration on the phase changes that occur during seafloor alteration of MORB-like compositions during subduction along an average Phanerozoic geotherm. We consider a representative composition of altered oceanic crust, as well as extreme end-member scenarios (pervasive spilitization, chloritization, and epidotization). Our models show that epidotization and chloritization of MORB strongly affects phase equilibria at different depths, whereas spilitization and an average style of alteration produce relatively fewer changes on the mineral assemblage to those expected in a pristine MORB. Devolatilization of MORB during subduction occurs mostly in the forearc region, although the type and extent of alteration strongly control the depth and magnitude of fluid released. Altered compositions carry significantly more H2O to sub- and postarc depths than unaltered compositions; the H2O carrying capacity of unaltered and altered compositions is further enhanced during subduction along colder geotherms. Extremely localized areas affected by epidotization can transport up to 22 times more H2O than unaltered MORB and up to two times more than average altered oceanic crust compositions to depths beyond the arc. Regardless of the extent and style of alteration, the stability of hydrous phases, such as epidote and phengite (important trace element carriers), is expanded to greater pressure and temperature conditions. Thus, hydrothermal alteration of the subducted oceanic slab-top represents a viable, and probably common, mechanism that enhances geochemical recycling between the Earth’s hydrosphere and shallow interior.

Large iron isotope variation in the eastern Pacific mantle as a consequence of ancient low-degree melt metasomatism

Studies of mid-ocean ridge basalts (MORB) have revealed a heterogeneous asthenospheric mantle in chemical elements and radiogenic isotopes. Here we report that MORB mantle is also heterogeneous in Fe isotopes through studying the glass samples from seamounts flanking the northern East Pacific Rise between 5° and 15°N. These samples show large Fe isotope variation with δ56Fe values (+0.03‰ to +0.36‰) exceeding the known range of MORB (+0.05‰ to +0.17‰). Such highly varied δ56Fe values cannot be well explained by seafloor alteration, fractional crystallization or partial melting processes, but instead require a source mantle significantly heterogeneous in Fe isotope compositions. Importantly, the δ56Fe values of these basalts correlate significantly with major and trace elements and Sr-Nd-Pb-Hf radiogenic isotopes, reflecting melting-induced mixing of a two-component mantle with the enriched component having heavy Fe isotope compositions dispersed as physically distinct domains in the depleted mantle matrix. The major and trace element characteristics of the enriched mantle component, as inferred from these basalts, are consistent with a low-degree melting origin. Such low-degree melts with heavy Fe isotope compositions most likely formed at sites such as the lithosphere-asthenosphere boundary beneath ocean basins, which can metasomatize the overlying oceanic lithosphere by crystallizing dikes/veins of garnet pyroxenite lithologies. Recycling of these dikes/veins with isotopically heavier Fe can readily contribute to the Fe isotope heterogeneity in the MORB mantle. However, the extremely high primitive δ56Fe values of the two alkali basalts (up to 0.34‰) require an enriched source component with unusually high δ56Fe values. We suggest that partial melts from the recycled dikes/veins of garnet pyroxenite lithologies can react with the ambient peridotitic mantle and generate a secondary garnet pyroxenite with heavier Fe isotope compositions than, but similar radiogenic isotope compositions as its precursor. Melting-induced mixing between these garnet pyroxenites (recycled and newly formed) and depleted mantle matrix can readily explain the compositional variations in elements, radiogenic isotopes and Fe isotopes observed in these seamount lavas. These new data and correlated variations offer a new dimension for understanding the origin of mantle chemical and isotopic heterogeneity in the context of chemical differentiation of the Earth.

Physical volcanology and petrogenesis of the Archean Quebra Osso komatiite flow field, Rio das Velhas greenstone belt, Quadrilátero Ferrífero (Brazil)

The Quebra Osso komatiite flow field is part of the 2.9–2.7 Ga Rio das Velhas greenstone belt, in the Quadrilátero Ferrífero province, located in the Southern São Francisco craton (Brazil). It is mostly composed of metamorphosed komatiites with a minor association of pyroclastic and metasedimentary rocks. Through a detailed and systematic study, the komatiite flows were subdivided into coherent facies (massive, layered, and pillowed facies) and autoclastic facies (autobreccias and hyaloclastites), formed by effusive flows; and minor pyroclastic facies (tuffs and lapilli-tuffs) that are related to explosive flows. The igneous textures are still preserved, although their primary minerals (olivine and pyroxene) were completely pseudomorphosed by serpentine and/or chlorite. The major proportion of the komatiites of Quebra Osso Group is massive and cumulate-textured, which suggests channelized flow systems. The layered flows are subordinate and encompass an upper zone of random spinifex that grades to a lower cumulate zone. They are restricted to the marginal zones and interpreted as breakouts of lava from the main pathway. The occurrence of pillow lavas, hyaloclastites, and chemical sediments suggests that lavas erupted in a submarine environment. The abundance of autobreccias and local hybrid rocks point out to turbulent percolation of komatiites and/or multiple influxes of melts that caused the fragmentation of the solidified lava and magma mixing/mingling. The studied komatiites were variably affected by hydration due to seafloor alteration, regional metamorphism, and hydrothermal alteration. The mineral assemblage is secondary and comprises serpentine (mostly antigorite), chlorite, tremolite, talc, carbonate, and opaque minerals as accessory phases. The chemical composition of chromites suggests metamorphic overprint up to lower amphibolite facies conditions. Some komatiites are enriched in LREE (La, Sm, Nd), Rb, Ba, Th, Zr, Ti, and Y, and exhibit low [Nb/Th]MN, suggesting contamination by continental crustal. The bulk-rock and the mineral chemistry of amphibole, serpentine/chlorite, and chromite show enrichment of Al and decrease in Si contents in the spinifex-textured komatiites, as a result of crustal material assimilation and/or hybridization processes. The partial melting model for the HREE composition refers to a 50% batch melting of a garnet-free source with a composition similar to the pyrolytic mantle. The formation of Quebra Osso komatiite lavas is associated with the ascension of a mantle plume that was subsequently erupted onto the Archean sialic crust of the Santa Bárbara Complex and was later reworked by subsequent metamorphic and deformational events.

Halogen and trace element analysis of carbonate-veins and Fe-oxyhydroxide by LA-ICPMS: Implications for seafloor alteration, Atlantis Bank, SW Indian Ridge

Halogens are critical elements for transporting metals in hydrothermal solution and tracing the sources of fluids and volatiles in subduction zones. This study tested the suitability of LA-ICPMS for simultaneous measurement of Cl, Br and I with selected trace elements in altered gabbros recovered from the Atlantis Bank core complex close to the SW Indian Ridge. The aim was to better understand the causes of I-enrichment in low temperature carbonate and Fe-oxyhydroxide alteration, which is intensely developed in core recovered from between ~100 and ~600 mbsf down Hole U1473A drilled during Expedition 360 of the International Ocean Discovery Program (IODP).The analysis of reference materials including scapolites, Durango apatite, USGS and NIST reference glasses and volcanic glasses demonstrate Cl, Br and I can be measured in most minerals, but accuracy is limited by matrix dependent laser backgrounds (false signals of Cl, Br and I generated during sample ablation). Silicate reference glasses define reproducible ‘laser backgrounds’ of ~320 μg/g Cl and ~1.7 μg/g Br that point to limits of quantification of ~200 μg/g Cl and ~1 μg/g Br in silicate glasses after correction for laser backgrounds. In contrast, laser backgrounds are much lower and possibly negligible for carbonate, suggesting limits of quantification as low as ~50 μg/g Cl and 0.1 μg/g for Br, dependent on instrument sensitivity. Laser backgrounds for I varied between minerals and analytical sessions, there was no laser background for I in apatite and under favourable conditions silicate glasses have laser backgrounds of <1 μg/g. In addition to the laser backgrounds, we report a correction for interference of doubly charged rare earth elements on Br measurement, which significantly improves data quality for some samples.The Atlantis Bank alteration minerals investigated include calcite veins and Fe-oxyhydroxide, which is intermixed with, and cut by veinlets of, calcite and smectite. The Fe-oxyhydroxide contains 100 s–1000s μg/g Cl, 10s μg/g Br and 5–30 μg/g I and it is strongly enriched in other trace elements including B and As. Most calcite veins contain <100–200 μg/g Cl, <0.4–0.9 μg/g Br and 5–26 μg/g I. Among the halogens, I is uniquely correlated with divalent cations in calcite including Mg and Sr, consistent with its incorporation into the carbonate lattice as iodate. In most cases smectite contains less I than coexisting calcite or Fe-oxyhydroxide, but three smectite grains in one sample are exceptionally enriched with 150–240 μg/g I. The in situ analyses support the attribution of exceptional iodine enrichment of Atlantis Bank lithologies to preferential adsorption of iodate over halides by Fe-oxyhydroxides and subsequent remobilisation with preferential fixing of iodate in carbonate-dominated alteration. The data demonstrate the utility of LA-ICPMS for in situ halogen studies. Iodine in carbonate veins can provide information about fluid oxidation state and fluid-rock reaction history. However, further work is required to test if Cl and Br in carbonate can provide information about fluid sources.

Serpentinization at the Rainbow and Saldanha sites, Mid-Atlantic Ridge: Mineralogical, geochemical, and isotopic features

Abstract The Rainbow hydrothermal field (36°14′N) and the Saldanha seamount (36°34′N), in the Mid-Atlantic Ridge (MAR), are tectonic exposures of serpentinized upper mantle peridotites, both associated with significant hydrothermal activity. On the basis of detailed mineralogical and geochemical characterization of serpentinites from both sites, several serpentinization-related issues are discussed in the present work. As expected in oceanic environments, most of the sampled rocks are lizardite-chrysotile serpentinites exhibiting a variety of pseudomorphic through non-pseudomorphic textures, such textural evolution probably being related to changing water/rock ratios during this retrograde process. Oxygen isotope temperatures indicate that the serpentinization took place at 300–200 °C; on the other hand, isotopic data suggest that replacement of early pseudomorphic lizardite by lizardite ± chrysotile non-pseudomorphic textures requires that temperatures and/or water/rock ratios are high enough to promote the necessary dissolution–recrystallization processes. Mass-balance calculations for olivine-serpentine and orthopyroxene-serpentine pairs provided a basis for establishing serpentinization reactions likely to have produced the present rocks. Moreover, these calculations also showed that, notwithstanding some noticeable loss of MgO from olivine and of SiO2 from orthopyroxene, serpentinization of both minerals implies volume increases on the order of 26–27%, therefore potentially promoting the overall expansion of the rock. The geochemical and isotopic features of the studied rocks indicate that unmodified seawater was responsible for the serpentinization of the MAR peridotites. However, the mineralogy and REE patterns of some of these serpentinites indicate occasional subsequent interaction of the serpentinized rocks with seawater at much lower temperatures (seafloor alteration, characterized by carbonate deposition and negative Ce anomalies), or with high-temperature ore-forming hydrothermal fluids (ore-forming alteration, characterized by sulfide precipitation and steep positive Eu anomalies).

Sulfur isotope compositions of pyrite from high-pressure metamorphic rocks and related veins (SW Tianshan, China): Implications for the sulfur cycle in subduction zones

A detailed petrographic, microstructural and geochemical investigation of pyrites from four eclogites / blueschists and their veins in the Tianshan (NW China) have revealed the occurrence of four types of pyrites: δ34S values of the pyrites range from −24.7 to +13.2‰, whereby three populations can be distinguished that have distinct δ34S ranges corresponding to different paragenetic stages. Py1 pyrite grains contain micro- to nanoscale inclusions of pyrrhotite, chalcopyrite, bornite, sphalerite, galena, arsenopyrite, cobalt-pentlandite, enargite, titanite, biotite, barite, and have δ34S values of −3.8 to +2.7‰ (average δ34S = −0.6 ± 0.9‰), which is inherited from a magmatic and hydrothermal fluid input during alteration of the oceanic crust. Py2 pyrite occurs as inclusions within the cores of garnet where it coexists with fine-grained inclusions of rutile, glaucophane and rarely with omphacite. Thus, these pyrite inclusions likely formed during prograde evolution. Py2 is too small to be analyzed by SIMS and thus no isotopic compositions could be determined. The Py3 pyrite grains contain eclogite-facies mineral inclusions, such as omphacite, glaucophane, epidote, rutile and lawsonite, and are inferred to have formed during the late prograde evolution close to or at peak eclogite-facies conditions. Py3 pyrite display positive δ34S values ranging from +3.6 to +13.2‰ (average δ34S = +9.7 ± 2.8‰), suggesting the source of sulfur was either anhydrite incorporated during seafloor alteration or 34S-enriched sulfide from lower sections of the oceanic lithosphere. Py4 pyrite occurs as irregular patches, isolated euhedral to subhedral grains or as rims around Py1 or Py3 pyrites, associated with magnetite and ilmenite. Py4 grains have δ34S values of −24.7 to −3.9‰ (average δ34S = −5.0 ± 2.1‰), with the most likely source of sulfur being 32S-enriched pelagic sediments that in the Tianshan are interlayered with the metavolcanic lithologies and underwent amphibolite facies metamorphism during exhumation. This study provides insight into the sulfur cycle within subduction zones. We document that during subduction and exhumation fluids with distinct sulfur isotopic compositions driven by dehydration reactions in the downgoing slab cause metasomatism in the surrounding lithologies. In particular, we suggest that at least locally the plate interface is characterized by sediment-derived fluids that are characterized by 32S-enriched sulfur and comparatively higher oxygen fugacities.

Evolution of serpentinite from seafloor hydration to subduction zone metamorphism: Petrology and geochemistry of serpentinite from the ultrahigh pressure North Qaidam orogen in northern Tibet

Serpentinite from the North Qaidam ultrahigh pressure metamorphic belt in northern Tibet is studied to provide insight into petro-geochemical evolution of serpentinite from seafloor hydration to subduction zone metamorphism. The North Qaidam serpentinite can be divided into undeformed lizardite serpentinite that was not severely overprinted during subduction and deformed antigorite serpentinite that was well recrystallized during subduction. Petrological and geochemical analyses demonstrate that the serpentinite was originally melt-percolated refractory abyssal harzburgite. Considering the local geodynamic setting, it is inferred that the serpentinite probably originated from the oceanic lithosphere that subducted before continental subduction. Relatively uniform low δ18O (4.0‰–4.5‰) of the antigorite serpentinite indicates high temperature hydrothermal alteration of protolith harzburgite by seawater. In contrast, much lower δ18O (0.6‰–2.7‰) of the lizardite serpentinite is ascribed to exchange with meteoric water at the Earth’s surface. Relict serpentinization textures are well preserved in the lizardite serpentinite, with reactions of olivine to lizardite and magnetite, pyroxenes to talc, tremolite, and lizardite, and spinel to chromite and chlorite. The replacement of lizardite by antigorite during subduction mostly initiates along grain boundaries and interconnecting veinlets, implying fluid-assisted transformation of lizardite into antigorite. Partial decomposition of antigorite produces magnesian secondary olivine in the antigorite serpentinite, while direct breakdown of metastable lizardite generates ferroan secondary olivine in the lizardite serpentinite. The serpentinite is enriched in fluid-mobile elements, with U primarily accumulated during seafloor alteration and alkalis notably introduced by sedimentary fluids at bending faults or in accretionary wedge. Infiltration by fluids equilibrated with sediments is also supported by highly elevated 87Sr/86Sr of the serpentinite. Moreover, 87Sr/86Sr of the antigorite serpentinite (0.710649–0.713996) is higher than that of the lizardite serpentinite (0.707184–0.708502), which implies more intense interaction of sedimentary fluids with the former than the latter. However, the lizardite serpentinite contains more alkalis and less U than the antigorite serpentinite, which indicates that large proportions of alkalis were lost during partial dehydration of serpentinite, while U was not released.

Evolution of the Ca isotopic composition of the mantle

Calcium stable isotopes are mainly fractionated during low-temperature geochemical processes and have, therefore, the potential to trace weathering and incorporation of recycled carbonated material in the mantle. However, behavior of Ca isotopes during mantle processes over Earth's history is not well understood. In this study, we present Ca isotopic compositions of 27 spinifex-textured and olivine cumulate komatiites ranging in age from 2.41 to 3.55 Ga. The Ca isotopic composition of all spinifex textured komatiites analyzed is similar, with an average δ44/40Ca = 0.92 ± 0.16‰ (n = 7; 2SD) (permil deviation of the 44Ca/40Ca ratio from SRM915a). This value is identical, within error, to the previous estimates of the composition of the mantle. In contrast, olivine cumulates from the Weltevreden Formation in the Barberton Greenstone Belt are isotopically heavier compared to the rest of the komatiites studied. While igneous processes and sea-floor alteration unlikely account for this Ca isotopic signature, crystallization of an early crust from a global magma ocean is a possibility, in agreement with previously obtained O, Os, Nd, Hf isotopic data and trace elements systematics.

Cu and Zn isotope fractionation during oceanic alteration: Implications for Oceanic Cu and Zn cycles

The behaviours of Cu, Zn and their isotopes during seafloor fluid-peridotite interaction and its influence on oceanic Cu-Zn cycles were investigated by analyzing a series of unaltered to strongly altered oceanic peridotites. Most of the altered peridotites have δ 66 Zn values indistinguishable from those of the unaltered ones (0.19 ± 0.05‰; 2SD), suggesting that seafloor alteration generally induces a small Zn isotope fractionation. A few altered samples that seem to have gained Zn have elevated δ 66 Zn, probably due to adsorption by ubiquitous secondary phases (e.g. goethite). Almost all altered peridotites show Cu loss (up to 90%) and have Cu isotope ratios that are shifted toward heavy values relative to the unaltered ones. Given that Cu is organically-complexed in fluids over a wide range of temperature and such fluids have strong preferences for the heavy isotopes, alteration and weathering alone are unlikely to account for the substantial loss of Cu coupled with elevated δ 65 Cu. An alternative mechanism is that initial Cu in peridotites was partly lost by alteration and weathering, and then Cu was added via adsorption by secondary mineral phases preferring the heavy Cu isotopes. This is confirmed by leaching experiments that were designed to extract the adsorbed Cu fractions in secondary phases. The results show that the leachates and the residua have, respectively, higher and lower δ 65 Cu values in comparison with the bulk peridotites, demonstrating that adsorption is an important mechanism for generating heavy copper isotope enrichments in the altered peridotites. In any case, mass balance considerations indicate that oceanic alteration must have resulted in a net input of isotopically light Cu from oceanic peridotites into the ocean. Recent estimates found that the Cu isotopic composition of the modern ocean is imbalanced, because all characterized sinks are isotopically light relative to characterized input fluxes, which requires an additional, isotopically light input. Our results indicate that seafloor alteration of abyssal peridotites is an isotopically light input (δ 65 Cu = −0.4‰ to −0.1‰) to the modern ocean, up to ∼1‰ lighter than the riverine input (∼0.7‰). First-order flux estimates suggest that this process exerts an important control on the marine budget of Cu. Most of Zn released during seafloor alteration has δ 66 Zn similar to that of mantle peridotites and this input is thus slightly lighter than the riverine input (∼0.33‰). Current estimates of modern oceanic Zn isotope cycles require consideration of this isotopically light source.

Lithium isotope systematics of the Sumdo Eclogite, Tibet: Tracing fluid/rock interaction of subducted low-T altered oceanic crust

Abstract The Sumdo eclogite, located in the central Lhasa Terrane, Tibet, has received much attention as it has important implications for the tectonic evolution of the Tibet orogenic belt. Aiming to constrain the Lithium (Li) isotope characteristics of rocks in subduction zones, we investigated whole-rock major and trace elements, Li isotopes as well as oxygen (O) and radiogenic (Sr, Nd, and Pb) isotopes on 20 Sumdo eclogite samples. The investigated samples display REE patterns similar to N-MORB, with relatively homogeneous bulk eNd(t=260Ma) values ranging from +4.0 to +7.8. These data suggest that protoliths of the Sumdo eclogite were N-MORB-type oceanic basalts. Yet, initial 87Sr/86Sr and δ18O are heterogeneous and range from 0.703703 to 0.706645 and from +4.9 to +8.9‰, respectively. A positive correlation between Sr and oxygen isotopic compositions suggests that the MORB protoliths had experienced low temperature seafloor alteration prior to subduction. The Li contents and δ7Li values of Sumdo eclogite vary from 3.64 to 29.7 µg/g and −1.1 to +4.0‰, respectively, significantly lower than those of low T altered MORBs (AOC) that usually have high Li contents (up to 33 µg/g) and heavy Li isotopic compositions (δ7Li up to +14‰). Modelling results suggests limited Li isotopic fractionation during dehydration. After eliminating the effect of diffusion, we suggest that a rehydration stage during exhumation may be responsible for the Li isotope systematics of the Sumdo eclogite. The fluids (

Sulphur and carbon cycling in the subduction zone m\xe9lange

Subduction zones impose an important control on the geochemical cycling between the surficial and internal reservoirs of the Earth. Sulphur and carbon are transferred into Earth’s mantle by subduction of pelagic sediments and altered oceanic lithosphere. Release of oxidizing sulphate- and carbonate-bearing fluids modifies the redox state of the mantle and the chemical budget of subduction zones. Yet, the mechanisms of sulphur and carbon cycling within subduction zones are still unclear, in part because data are typically derived from arc volcanoes where fluid compositions are modified during transport through the mantle wedge. We determined the bulk rock elemental, and sulphur and carbon isotope compositions of exhumed ultramafic and metabasic rocks from Syros, Greece. Comparison of isotopic data with major and trace element compositions indicates seawater alteration and chemical exchange with sediment-derived fluids within the subduction zone channel. We show that small bodies of detached slab material are subject to metasomatic processes during exhumation, in contrast to large sequences of obducted ophiolitic sections that retain their seafloor alteration signatures. In particular, fluids circulating along the plate interface can cause sulphur mobilization during several stages of exhumation within high-pressure rocks. This takes place more pervasively in serpentinites compared to mafic rocks.

In-situ Sr-Pb isotope geochemistry of lawsonite: A new method to investigate slab-fluids

Lawsonite is a hydrous Ca-Al silicate mineral which is broadly stable under modern style subduction conditions. In order to understand the elemental fractionation and the sources of subducted materials, lawsonite crystals in lawsonite-eclogite facies basaltic and sedimentary rocks from South Motagua Mélange of Guatemala were investigated using LA-ICPMS and LA-MC-ICPMS. Mass balance calculation based on in-situ trace element and modal compositions in an eclogite with a low-variance mineral assemblage confirmed that lawsonite hosts most of the light rare earth element (LREE), Sr, Pb, Th and U in the bulk. In-situ Sr-Pb isotope analyses of eclogite-facies lawsonite crystals revealed isotopic variations reflecting their protoliths. Isotopic zoning was also detected in some crystals. Lawsonite crystals in a phengite-rich eclogite have relatively low 87Sr/86Sr = 0.70335–0.70355 with variations in 207Pb/206Pb = 0.8401–0.8512 and 208Pb/206Pb = 2.0757–2.0911. Lawsonite crystals from another eclogite are remarkably zoned. The cores have 87Sr/86Sr = 0.70558–0.70601 and the rims have relatively elevated 87Sr/86Sr = 0.70636–0.70662, exhibiting MORB-like 207Pb/206Pb = 0.843–0.844 at the cores and more enriched 207Pb/206Pb = 0.839–0.841 at the rims. The radiogenic Sr isotope composition would have been derived from seafloor alteration before subduction. In fact, lawsonite crystals in a metachert have higher 87Sr/86Sr = 0.70697–0.70757 with 208Pb/206Pb = ∼2.07. However, the isotopically zoned metabasaltic lawsonite crystals with MORB-like core compositions suggest that metasomatism by an external fluid from sediment protolith occurred during overgrowth of their rims. Our study indicates that lawsonite crystals record both the isotopic compositions of the protoliths and metasomatic fluids from different protoliths. The in-situ Sr-Pb isotope analysis has a potential to reveal records of the complicated metamorphic processes.

Triple oxygen isotope mass balance for the Earth's oceans with application to Archean cherts

The oxygen isotope composition of the Earth's oceans is buffered by high- and low-T exchange with the lithosphere. We present a triple oxygen isotope mass balance model for the Earth's oceans. The model is based on triple oxygen isotope measurements of rocks from various reservoirs including high- and low-T alteration products. The modern ocean water composition can be well-matched if the ratio between continental weathering and high-T seafloor alteration is ~25% higher than previously assumed. The mass balance suggests that putative Precambrian low-δ18O ocean water would fall on a trend with slope λ = 0.51 passing through “modern” ice-free-world seawater. Exemplified application to a published Phanerozoic and Archean chert data suggest precipitation in cool oceans with modern-like δ18O followed by diagenetic alteration with involvement of meteoric water.

Cretaceous seawater and hydrothermal fluid compositions recorded in abiogenic carbonates from the Troodos Ophiolite, Cyprus

We evaluate the potential of ophiolites as archives of paleoseawater and hydrothermal fluid compositions by analysing the chemical and isotopic composition of abiogenic carbonates, precipitated from fluids within the oceanic crust of the 91 Ma Troodos Ophiolite, Cyprus. Calculated variations in fluid Mg/Ca, Sr/Ca and 87Sr/86Sr with temperature within the upper sections of the ophiolite are similar to those from drilled oceanic crust, and yield literature values for late Cretaceous seawater Mg/Ca, Sr/Ca and 87Sr/86Sr. This indicates that carbonates from ophiolites could be used to estimate the composition of ancient seawater at times before the age of the oldest preserved in-situ oceanic crust. Whereas most carbonates recovered from in-situ oceanic crust were precipitated at temperatures <60 °C, abiogenic carbonates from the Troodos Ophiolite formed over a temperature range of 7 °C to 218 °C. These provide unique insights into the chemical and mineralogical processes that transform seawater into a high temperature hydrothermal fluid within the oceanic crust. We use ‘hydrothermal variation diagrams’ of Mg/Ca, Sr/Ca, 87Sr/86Sr and δ44/40Ca versus calculated temperature (δ18O) to trace this fluid evolution within the Troodos oceanic crust. We find that successive fluid-crust-interaction, the precipitation of Mg- and Ca-bearing minerals and the early formation of anhydrite (>44 °C) gradually transform Cretaceous seawater into a Troodos hydrothermal fluid. Comparison of the Troodos data with a global dataset of abiogenic carbonates from in-situ oceanic crust shows that the chemical pathways of low-temperature fluid evolution are similar for all Cretaceous sites. These different sites represent varied geotectonic settings (mid-ocean ridge vs. suprasubduction zone), with different basement composition (basalt, basaltic andesite/boninite) and situated in different ocean basins (Atlantic, Pacific, Mediterranean [Tethys]). The similarity in the carbonate record indicates that these differences do not significantly influence seafloor weathering and hydrothermal alteration at low temperatures. However, abiogenic carbonates from younger oceanic crust differ from the Cretaceous trends and follow different fluid evolution pathways. This indicates, that temporal variations in the composition of seawater may control the nature and the extent of seafloor weathering and hydrothermal alteration at low temperatures. A thermodynamic model of fluid-crust interaction, in which modern and Cretaceous seawater are heated to 200 °C while an average Troodos basaltic andesite is successively added under otherwise identical conditions predicts that fluid evolution and alteration of the oceanic crust were different in the Cretaceous than they are today, and that initial seawater chemistry affects the nature and the extent of seafloor alteration up to moderate fluid temperatures. For example, twice the amount of carbonate formed during alteration of the oceanic crust in the Cretaceous compared to modern times, indicating that the flux of CO2 from the hydrosphere-atmosphere system into the oceanic crust was greater in the Cretaceous than it is nowadays, and that it probably varied throughout geologic time.

In Situ Oxygen Isotope Determination in Serpentine Minerals by Ion Microprobe: Reference Materials and Applications to Ultrahigh‐Pressure Serpentinites

We present the first investigation of in situ oxygen isotopes in serpentine minerals by sensitive high‐resolution ion microprobe (SHRIMP). Chemically homogeneous samples of antigorite (δ18O = 8.30 ± 0.12‰), chrysotile (δ18O = 4.37 ± 0.02‰) and lizardite (δ18O = 5.26 ± 0.20‰) analysed by laser fluorination are identified as potential reference materials. They were analysed by SHRIMP to assess their homogeneity compared with the San Carlos olivine, as well as for potential matrix bias and crystal orientation effects. The reproducibility achieved for all samples was ± 0.30–0.55‰ (95% confidence level). Matrix bias between antigorite/olivine and antigorite/lizardite was up to ~ +3‰ and −1‰, respectively. Crystal orientation effects were identified only in chrysotile, and no matrix bias was observed over the investigated compositional range within each serpentine mineral. The new reference materials were used to measure the oxygen isotope composition of serpentines in an ultrahigh‐pressure metamorphic belt from Tianshan (China). By combining oxygen isotopes and trace element microanalyses, several stages of serpentinisation were recognised: (a) seafloor alteration, (b) recycling of internal metamorphic fluids during isothermal decompression and (c) shallow interaction with meteoric water during exhumation. No interaction with fluids derived from the surrounding metapelites during subduction was identified.

Subduction and atmospheric escape of Earth's seawater constrained by hydrogen isotopes

The hydrogen isotopic (D/H) ratio reflects the global cycling and evolution of water on Earth as it fractionates through planetary processes. We model the water cycle taking seafloor hydrothermal alteration, chemical alteration of continental crust, slab subduction, hydrogen escape from the early Earth, and degassing at mid-ocean ridges, hot spots, and arcs into account. The differences in D/H ratios between present-day oceans, oceanic and continental crust, and mantle are thought to reflect isotopic fractionation through seafloor alteration, chemical alteration, and slab dehydration. However, if the speed of plate tectonics has been nearly constant throughout Earth's history, the degassing and regassing rates are too small to reach the present-day D/H ratios. We show that (a) hydrogen escape from reduced early atmosphere, (b) secular net regassing, or (c) faster plate tectonics on early Earth is needed to reproduce the present-day D/H ratios of the water reservoirs. The low D/H ratio of Archean seawater at 3.8 Ga has previously been interpreted as a signature of (a) hydrogen escape, but we find it can also be explained either by (b) secular net degassing or by (c) faster plate tectonics on early Earth. The rates of hydrogen escape from early Earth and secular regassing on present-day Earth are constrained to be lower than 2.1×1011 kg/yr and 3.9×1011 kg/yr. Consequently, the volume of water in the present-day mantle could result entirely from the regassing through Earth's history. In that case, the volume of initial oceans could be 2 to 3 times larger than that of current Earth. We suggest that, in addition to the D/H ratio of Archean seawater, identifying the D/H ratios of both seawater and mantle throughout Earth's history would allow to distinguish these evolutionary scenarios.

Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon

Ophiolitic serpentinites and secondary peridotites formed by serpentinite dehydration were investigated to improve constraints on the fates of noble gases and halogens during subduction zone metamorphism. The work extends previous studies to encompass F and four stages of serpentinization and serpentinite dehydration including: (i) oceanic serpentinites preserving the features of seafloor serpentinization; (ii) subducted high grade (olivine bearing) antigorite-serpentinites; (iii) spinifex and granofels textured chlorite harzburgites; and (iv) a garnet peridotite.Serpentinites and secondary peridotites from different ophiolites are shown to have characteristic ranges of 40Ar/36Ar: chrysotile and antigorite serpentinites from Erro Tobbio (Western Alps) have 40Ar/36Ar of ∼ 296–390; antigorite serpentinites and chlorite harzburgites from Cerro del Almirez (Betic Cordillera) have 40Ar/36Ar of ∼ 340–600, and chlorite harzburgites and garnet peridotites from Cima di Gagnone (Swiss Alps) have 40Ar/36Ar of ∼ 600–1100. The variation of 40Ar/36Ar is unrelated to metamorphic grade at each locality but is broadly correlated with variation in other radiogenic isotopes (206Pb/204Pb and 87Sr/86Sr) between localities. This suggests excess 40Ar was derived from terrigenous sediments with characteristic ranges of 40Ar/36Ar and 87Sr/86Sr in different subduction zones.The secondary chlorite harzburgites have 20Ne/36Ar ratios of greater than seawater, contain parentless (or excess) 4He, and have higher F concentrations than any of the serpentinites investigated. The 20Ne/36Ar is broadly correlated with 40Ar/36Ar in samples from Cerro del Almirez suggesting derivation of excess 40Ar and atmospheric 20Ne from a common source. The chlorite is shown to have higher concentrations of F, Ne and other noble gases than coexisting olivine and enstatite, which contain fluid-related inclusions. The high F content and high 20Ne/36Ar ratios of the chlorite harzburgites are ascribed to fluxing of dehydrating serpentinites with F-, 40Ar-, 4He- and 20Ne-rich fluids derived from metasediments in the subducting slab, and an inferred high compatibility of F and Ne in chlorite.The garnet peridotite from Cima di Gagnone records the final and complete dehydration of serpentinite. Based on the analysis of mineral separates minimally affected by retrogression (marked by garnet breakdown and the appearance of Cl-rich hornblende), nominally anhydrous garnet peridotite retains Cl, Br, I and non-radiogenic noble gas concentrations up to an order of magnitude higher than average depleted mantle.The data are consistent with serpentinised lithosphere and related secondary peridotites as major sources of deeply subducted seawater-derived volatiles in the Earth’s mantle. The data also demonstrate that the relative abundances of volatiles subducted into the mantle are controlled by multiple factors including: original seafloor alteration, the relative compatibilities of different noble gases and halogens in minerals forming during different stages of subduction and chemical exchange between different lithologies during subduction. The combination of these processes has produced elevated 20Ne/36Ar in chlorite harzburgites from two unrelated localities. This suggests that subduction of atmospheric Ne could be significantly more efficient than previously realised, which has implications for interpretation of the mantles primordial 20Ne/22Ne ratio and how the Earth accreted.

Chemical alteration and preservation of sedimentary/organic nitrogen isotope signatures in a 2.7 Ga seafloor volcanic sequence

AbstractMassive to lobate volcanic flows and brecciated hyaloclastite units in the Abitibi greenstone belt allow investigation of Late Archæan seafloor alteration and associated incorporation into these rocks of nitrogen (N) biogeochemical signatures. In this suite (the Blake River Group), hyaloclastite units containing putative microbial ichnofossils are particularly enriched in large-ion lithophile elements (K, Rb, Ba, Cs), B, and Li, consistent with their having experienced the greatest fluid–rock interaction during subseafloor hydrothermal alteration. Similarly, silicate-δ18O and δ15N values for samples from the hyaloclastites show the greatest shifts from plausible magmatic values. The chemical and isotopic patterns in these tholeiitic igneous rocks greatly resemble those in modern altered seafloor basalts, consistent with the preservation of an Archæan seafloor alteration signature. The N enrichments and shifts in δ15N appear to reflect stabilization of illite and interaction with fluids carrying sedimentary/organic signatures. Enrichments of N (and the δ15N of this N) in altered glass volcanic rocks on Earth's modern and ancient seafloor point to the potential utility of N for tracing past and present biogeochemical processes in similar rocks at/near the Mars surface.

Elemental and isotopic (C, O, Sr, Nd) compositions of Late Paleozoic carbonated eclogite and marble from the SW Tianshan UHP belt, NW China: Implications for deep carbon cycle

Subduction zones are important for understanding of the global carbon cycle from the surface to deep part of the mantle. The processes involved the metamorphism of carbonate-bearing rocks largely control the fate of carbon and contribute to local carbon isotopic heterogeneities of the mantle. In this study, we present petrological and geochemical results for marbles and carbonated eclogites in the Southwestern Tianshan UHP belt, NW China. Marbles are interlayered with coesite-bearing pelitic schists, and have Sr-Nd isotopic values (εNd (T=320Ma) = −3.7 to −8.9, 87Sr/86Sr (i) = 0.7084–0.7089), typical of marine carbonates. The marbles have dispersed low δ18OVSMOW values (ranging from 14 to 29‰) and unaffected carbon isotope (δ13CVPDB = −0.2–3.6‰), possibly due to infiltration of external H2O-rich fluids. Recycling of these marbles into mantle may play a key role in the carbon budget and contributed to the mantle carbon isotope heterogeneity.The carbonated eclogites have high Sr isotopic compositions (87Sr/86Sr (i) = 0.7077–0.7082) and positive εNd (T = 320 Ma) values (from 7.6 to 8.2), indicative of strong seafloor alteration of their protolith. The carbonates in the carbonated eclogites are mainly dolomite (Fe# = 12–43, Fe# = Fe2+/(Fe2+ + Mg)) that were added into oceanic basalts during seafloor alteration and experienced calcite – dolomite – magnesite transformation during the subduction metamorphic process. The uniformly low δ18O values (∼11.44‰) of carbonates in the carbontaed eclogites can be explained by closed-system equilibrium between carbonate and silicate minerals. The low δ13C values (from −3.3 to −7.7‰) of the carbonated eclogites most likely reflect contribution from organic carbon. Recycling of these carbonated eclogites with C isotope similar to typical mantle reservoirs into mantle may have little effect on the mantle carbon isotope heterogeneity.