Hydrothermal Alteration Of Oceanic Crust Research Articles

Alteration enrichment of nitrogen in the gabbroic oceanic crust: Implications for global subducting nitrogen budget and subduction-zone nitrogen recycling

Uptake of nitrogen (N) during hydrothermal alteration of oceanic crust makes the altered oceanic crust (AOC) a significant N reservoir in parallel to seafloor sediments. While N enrichment in the upper (basaltic) oceanic crust during low-temperature alteration has been widely observed, the N characteristics of moderate- to high-temperature altered gabbroic oceanic crust, which accounts for ∼70 vol% of the crustal material in subducting slabs, have been rarely studied. The lack of these data resulted in large uncertainties in calculating the N input flux of global subducting slabs and modeling the N quantities released through the arc and subducted into the deep mantle. To fill this gap, we examined the N concentrations and isotope compositions of 38 altered gabbroic rocks recovered by ODP/IODP drillings from three oceans, i.e., Hole 735B at the Atlantis Bank in the Southwest Indian Ridge, Hole 1309D at the Atlantis massif in the Mid-Atlantic Ridge, and Hole 1415P at the Hess Deep in the East Pacific Rise. All the gabbroic samples show significant N enrichment with mostly positive δ15N values (in average: 7.4±3.8 ppm and +0.6±2.0‰ for Hole 735B, 5.3±2.1 ppm and +1.4±1.3‰ for Hole 1309D and 7.9±1.9 ppm and +2.0±1.8‰ for Hole 1415P), which are comparable to those of altered basalts from global oceanic crust. The N concentrations and δ15N values of these gabbroic rocks can be readily explained by mixing between minor inherited mantle N and mainly secondary N derived from seawater. The secondary N-hosting minerals in altered gabbroic rocks are likely plagioclase, amphibole and chlorite formed during moderate- to high-temperature alteration stages. Using these new data, we obtained a global nitrogen input flux of 20.6-1.3+0.9 × 109 mol·yr−1 to 29.9±2.2 × 109 mol·yr−1 for subducting AOC, in which 50% – 64% is contributed by the altered gabbroic oceanic crust. Integrating with previous estimate of N input flux from seafloor sediments, a total N input flux of 74.9-1.3+0.9 – 84.2±2.2 mol·yr−1 was estimated for global subducting slabs. Mass balance between this input N flux and various estimates of N output flux in global arcs gave a large range (from 47% – 53% to 10% – 20%) for the fraction of slab N to be subducted beyond the sub-arc depth to the deeper mantle. The upper end (47% – 53%) of this range is more consistent with the results from other constraints, such as thermal structural control and comparison between altered gabbros and meta-gabbros.

The effect of high-temperature alteration of oceanic crust on the potassium isotopic composition of seawater

High-temperature hydrothermal alteration of oceanic crust is one of the two main sources of potassium (K) to the oceans, with modern flux estimates ranging roughly from ∼8 to 30 % of the K flux from rivers. Despite the role of high-temperature hydrothermal fluids in the global seawater K budget, little is known about its effect on the K isotopic composition of the oceans. Here we present stable K isotope measurements (δ41KSRM3141a) of globally distributed high-temperature hydrothermal fluids from three mid-ocean ridge systems: the East Pacific Rise (n = 21), the Juan de Fuca Ridge (n = 34), and the Mid-Atlantic Ridge (n = 18). We find a strong correlation between δ41K and Mg/K ratios, consistent with conservative mixing between a high-temperature hydrothermal fluid (i.e., Mg = 0) and seawater as fluids ascend to the seafloor and/or due to seawater entrainment during sampling. The δ41K of end-member hydrothermal fluids is found to range between −0.80 ‰ and 0.07 ‰, with an average value of −0.36 ± 0.30 ‰ (2σ, n = 38). Most (∼76 %) of the variability in end-member fluid δ41K compositions observed here can be explained by high-temperature fluid-rock K exchange, with little (∼0.2 ‰) to no K isotope fractionation between hydrothermal fluid and altered crust. Larger deviations from the average end-member hydrothermal fluid value are likely to result from processes other than high-temperature fluid-rock exchange, such as (1) low-temperature hydrothermal reactions during fluid recharge, (2) reaction of fluids with local sedimentary sources, and (3) phase separation. The K contents of end-member fluids vary considerably, from ∼1 to 38 mM, thus a K-weighted average of −0.37 ± 0.24 ‰ (2σ, n = 38) is estimated to represent the δ41K composition of the global hydrothermal K flux. Our results suggest that K sourced from axial hydrothermal alteration does not contribute to the elevated 41K/39K of seawater compared to bulk silicate Earth (BSE). In addition, subduction of oceanic crust altered under high-temperature conditions is unlikely to be a significant source of K isotopic heterogeneity to Earth’s mantle.

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge, Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.

A possibility of 18O-depleted oceans in the Precambrian inferred from triple oxygen isotope of shales and oceanic crust

Oxygen-18/16 ratio recorded in geological media has been utilized to reconstruct surface temperature and discuss associated climatological forcing through Earth’s history. The accuracy of the estimated paleo-temperature through the application of the δ18O record, however, is limited by our understanding of the oceanic 18O/16O ratio in the past. As a longstanding issue, sedimentary rocks in the Precambrian, which are generally depleted in 18O relative to the modern sediments, can be interpreted either to have reflected higher surface temperature when assuming relatively constant oceanic δ18O values, or more 18O-depleted oceans relative to the present day with limited global temperature (climate) deviations. Additional use of a third isotope, 17O, can help constrain the isotopic composition of the porewater during the formation of a rock record as it provides an additional fractionation equation within the same isotopic system. This study utilizes reactive-transport models of triple oxygen isotope (16O, 17O, 18O) for continental weathering and hydrothermal alteration of oceanic crust, built upon the δ18O modeling framework developed by Kanzaki (2020a, b). Comprehensive oxygen isotope exchanges in dynamically moving media, both rocks and porewaters, are realized in these models, reflecting spatially-variable and process-dependent alteration conditions (e.g., spreading rate of midocean ridges, uplift rate of continents, porewater advection rate, and isotope exchange kinetics). The models were confirmed to be able to reproduce the signatures of triple oxygen isotope in Phanerozoic shales and oceanic crust. The validated models were then applied to Precambrian shales, which suggests that the Precambrian oceans could have been depleted in 18O, e.g., as low as −12‰ at >2.5 Ga and − 4‰ around 0.5 Ga, relative to the present-day seawater. General absence of positive and near-zero Δ′17O signatures in oceanic crust also suggests that the buffering of oceanic 18O through hydrothermal alteration of the oceanic crust could have been weak. The 18O flux from continental weathering thus could have played a significant role in controlling the 18O/16O ratio of ancient sea and together with climatic forcing could have enabled a secular transition of oceanic δ18O during the Precambrian.

Provenance of metasiliciclastic rocks at the northwestern margin of the East Gabonian Block: Implications for deposition of BIFs and crustal evolution in southwestern Cameroon

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

Reply to comment on “How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology’’ by Hubert Mvondo and Thierry Bineli Betsi

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

Interpretation of Oxygen Isotopes in Phanerozoic Ophiolites and Sedimentary Rocks

AbstractTwo reactive transport models were developed to integrally simulate fluxes of oxygen isotopes from hydrothermal alteration of oceanic crust and weathering of continental rocks for a better understanding of the regulation of oxygen isotopes in the ocean. The hydrothermal alteration model consists of three boxes to represent pillow basalt, sheeted dike, and gabbro sections and simulates the steady‐state δ18O of solid rocks and porewaters in three individual sections along the spreading direction. The continental weathering model similarly calculates steady‐state δ18O profiles along the uplift direction. The two models were run in concert to simulate the evolution of seawater δ18O during the Phanerozoic, reflecting the relevant forcing factors to the corresponding parameter values of the models as functions of time. The seawater δ18O was calculated to have evolved from less than −5‰ in the early Paleozoic to between −2 and 0‰ for the Mesozoic and Cenozoic, as a result of declines in atmospheric CO2 and land area of igneous and high‐grade metamorphic rocks. The transition of seawater δ18O is similar to the δ18O trend that is commonly recorded in authigenic sedimentary rocks such as carbonates and cherts. The simultaneously simulated δ18O values for altered continental and oceanic rocks are decoupled from seawater δ18O because of kinetic inhibition and solid‐rock buffering at respective low and high temperatures and are similar to the shale and ophiolite δ18O records. Overall consistency between the simulation and observations supports the dynamic behavior of oceanic δ18O over the Phanerozoic eon.

Seawater residence times of some elements of geochemical interest and the salinity of the oceans

Residence times of several dissolved chemical elements in seawater are revisited on the basis of updated river and groundwater chemical fluxes. A classification of the considered elements of geochemical interest (B, C, N, Na, Mg, Si, P, S, Cl, Ca, K, Li and Sr) is proposed accordingly to the solubility products of their main host minerals as well as their biological reactivity in the seawater column. The origin of seawater salinity is also discussed through both processes of early mantle degassing and continental growth. Finally, changes in seawater chemistry as well as in the total concentration of dissolved elements are investigated in the light of geological and biological processes such as continental erosion, hydrothermal activity, closure and opening of marine basins, climate forcing, marine productivity, and the Devonian rising of vascular plants. Taking into account groundwater fluxes and chemistry, and a revised lower mass of the oceans, residence times of chemical elements in the ocean have been calculated lower than commonly accepted in the literature for several decades. Both changes in salinity and chemistry of seawater took place throughout the Earth’s history at the m.y. timescale. Continents did not emerge before 3 Gy ago, thus excluding the weathering of their surfaces and the transport of alkali and calc-alkali elements by the rivers to the oceans. The only available sources, albeit to a limited extent, were possibly volcanic islands. As sodium is the main cation responsible for seawater salinity, calcium could have contributed to the charge balance of seawater to preclude the development of highly acidic waters. Indeed, the high-temperature hydrothermal alteration of oceanic crust is an important source of calcium that could have accumulated in seawater in the absence of its removal by calcifying organisms. It means that the earliest oceans could have been ‘CaCl2 dominated oceans’ instead of the ‘NaCl dominated oceans’ that prevailed during the Proterozoic and Phanerozoic.

Influence of continental growth on mid‐ocean ridge depth

AbstractThe interconnectedness of life, water, and plate tectonics is strikingly apparent along mid‐ocean ridges (MOR) where communities of organisms flourish off the disequilibrium of chemical potentials created by circulation of hydrothermal fluids driven by Earth's heat. Moreover, submarine hydrothermal environments may be critical for the emergence of life on Earth. Oceans were likely present in the Hadean but questions remain about early Earth's global tectonics, including when seafloor spreading began and whether mid‐oceanic ridges were deep enough for maximum hydrothermal activities. For example, plate tectonics influences global sea level by driving secular variations in the volume of ocean basins due to continental growth. Similarly, variations in the distribution of seafloor age and associated subsidence, due to assembly and dispersal of supercontinents, explain the largest sea level variation over the past 140 Myr. Using synthetic plate configurations derived from numerical models of mantle convection appropriate for early Earth, we show that MOR have remained submerged and their depths potentially constant over geologic time. Thus, conditions in the early Earth existed for hydrothermal vents at similar depths as today, providing environments conducive for the development of life and allowing for processes such as hydrothermal alteration of oceanic crust to influence the mantle's geochemical evolution.

Study on hydrothermal alteration of oceanic crust in the Oman ophiolite based on isotope geochemistry

Study on hydrothermal alteration of oceanic crust in the Oman ophiolite based on isotope geochemistry

The origin of early Archean banded iron formations and of continental crust, Isua, southern West Greenland

A long-standing geological debate is whether the Fe component of Precambrian banded iron formations (BIFs) was derived from continental sources, or had its origin in hydrothermal alteration of the oceanic crust. Recent REE, and Nd and Fe isotope studies of iron-rich layers in BIFs have provided strong supporting evidence for the hydrothermal origin of the Fe component. Pillow basalts from the 3800–3700 Ma Isua greenstone belt, West Greenland, underwent significant enrichment of Fe (up to 21 wt.% Fe 2O 3) and some chalcophile elements (e.g., Pb = 5.4–7.2 ppm) prior to a late Archean tectonothermal metamorphic event recorded in the region. Published and new Pb-Pb isotopic data from the Isua pillow basalts and the spatially associated BIFs suggest that the enrichment of Fe and Pb and deposition of the BIFs took place between 3800 and 3600 Ma. Similar Pb isotopic compositions in pillow basalts and BIFs are consistent with a common source of Pb for both supracrustal rocks. Iron and Pb were probably scavenged from the oceanic crust and were re-deposited on the ocean floor by hydrothermal fluids, suggesting that high-temperature hydrothermal alteration of early Archean oceanic crust played an important role in the deposition of the Isua BIFs. Geochemical, lithological, and structural data from the Isua greenstone belt are all collectively consistent with a convergent margin geodynamic setting. The Isua BIFs are spatially and temporally associated with boninitic and island arc picritic pillow basalts. Given the observations that Cenozoic boninites and picrites tend to form in an intra-oceanic arc-forearc setting, we suggest that the Isua BIF-boninite and BIF-picrite associations were also deposited in a similar geodynamic setting. The original pillow basalt-BIF stratigraphy was dismembered and transposed throughout the belt during early to late Archean tectonothermal events. The high-temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of Fe necessary to contribute to the deposition of the Isua BIFs. The hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early Archean arc-forearc region. The ridge subduction model can also explain the origin of the contemporaneous tonalite-trondhjemite-granodiorite (TTG) intrusions in the Isua region. Partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced TTG melts. The diapirically upwelling TTG melts formed the early Archean continental crust and the complementary eclogitic residues contributed to the sub-continental lithospheric mantle.

Hydrothermal alteration of oceanic crust in the West Philippine Sea Basin (Ocean Drilling Program Leg 195, Site 1201): inferences from a mineral chemistry investigation

Summary Secondary minerals of a 91 meters-thick sequence of pillow basalts cored during ODP Leg 195 (Site 1201, West Philippine Basin) were investigated to reconstruct the hydrothermal alteration history and regime. The basement was first buried by red clays, and then by a thick turbidite sequence, thereby isolating it from seawater. The basalts are primitive to moderately fractionated, texturally variable from hypocrystalline and spherulitic to intersertal, sub-ophitic and intergranular. Relic primary minerals are plagioclase, clinopyroxene and opaques. Hydrothermal alteration pervasively affected the basalts, generating secondary clay minerals (mostly glauconite, minor Al-saponite and Fe-beidellite), ‘‘iddingsite’’, Ca–Na-zeolites, minor alkali-feldspar and calcite. The secondary mineral paragenesis and mutual relationships suggest that the hydrothermal alteration occurred under zeolite-facies conditions, at temperatures <100–150 � C. The main phase of alteration occurred under oxidizing conditions, with a high seawater=rock ratio, in an open-circulation regime, at temperatures of 30–60 � C, with precipitation of abundant glauconite and ‘‘iddingsite’’. A later stage of alteration occurred at ca. 70 � C, with precipitation of abundant Ca–Na-zeolites and minor calcite, in a more restricted circulation regime as a consequence of basement burial under the sedimentary cover, which supplied an altered, Ca-rich and Mg–K-sulfate-poor water causing precipitation of almost pure calcite.

Carbon isotope cycle and mantle structure

Despite that no change is observed in the δ13C of sediments and upper mantle rocks over the Earth's history, the carbon isotope cycle is probably not at steady state. The δ13C of −5‰ of the upper mantle is different from that of the subducted carbon, which is close to −1‰. Indeed, most of the subducted carbon is made of carbonates: 85% of subducted sedimentary carbon plus carbonates formed by low temperature hydrothermal alteration of oceanic crust. Since no long term evolution is observed for the mantle and sedimentary rocks, the net subduction flux of heavy carbon requires to be compensated by a deep mantle process. End‐member models for the carbon isotope cycle at the Gyr scale suggest that it could be a flux from a deep primitive reservoir or the segregation of subducted oceanic crust at the core‐mantle boundary.

Osmium isotope variations in the oceans recorded by Fe [sbnd]Mn crusts

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya-Tibet region is not a major source for Os. Recently formed Fe Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (≥4 km) possess a remarkably unradiogenic Os isotope composition ( 187Os/ 186Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of mantle or meteoritic material, or simply indicate that the continental source itself is not uniform.

Precise Re–Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater

Rhenium and osmium isotope and abundance data have been obtained on precisely-located samples from three suites of immature, organic-rich mudrocks from Jurassic coastal outcrops in England. The data provide accurate whole-rock ages of 207±12 Ma, 181±13 Ma and 155±4.3 Ma for suites of Hettangian, Toarcian ( exaratum Subzone) and Kimmeridgian (sensu anglico, wheatleyensis Subzone) samples. These new Re–Os ages are indistinguishable, within the assigned analytical uncertainties, from interpolated depositional ages estimated from published geological timescales, and establish the importance of the Re–Os dating technique for chronostratigraphic studies. Early-diagenetic pyrite nodules possess levels of Re and Os which are ∼1–2 orders of magnitude lower than in the enclosing organic-rich mudrocks, indicating that these elements had already been removed from sediment pore waters at the time of nodule formation. Thus the Re–Os isotope system in these organic-rich mudrocks has been closed since, or from very soon after, the time of sediment deposition. Because most of the Re (98+%) and Os (95–99.8+%) in the mudrocks is shown to be hydrogenous, the 187Os/ 188Os (i) of the samples is interpreted to be that of contemporaneous seawater. The data thereby provide the first estimates of the Os isotope composition of Jurassic seawater. During the earliest Jurassic (Hettangian), the seawater 187Os/ 188Os ratio was extremely unradiogenic (∼0.15); it had increased to ∼0.8 at the end of the Early Jurassic (Toarcian) ∼20 Ma later, while in the Late Jurassic (Kimmeridgian) the seawater 187Os/ 188Os ratio was ∼0.59. The most likely explanation for the unradiogenic Os isotope composition of Hettangian seawater is that the contribution of unradiogenic Os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic Os from the continents at that time. This interpretation is in line with observations suggesting that global weathering rates were low in the Hettangian, and that increased hydrothermal and volcanic activity preceded the break-up of Pangea. The Re/Os ratios of Hettangian mudrocks (and by inference, of contemporaneous seawater) are similar to those of mudrocks deposited at later times during the Jurassic, and argues against the unradiogenic Os in Hettangian seawater being derived from extraterrestrial meteoritic sources.

CO 2-depleted fluids from mid-ocean ridge-flank hydrothermal springs

Hydrothermal spring fluids were collected from Baby Bare, a basement outcrop on the sediment-covered eastern flank of the Juan de Fuca Ridge overlying 3.5 Ma-old crust. These waters are venting at 25°C but have cooled from 64°C within the upper 200–300 m of the upflow zone during their ascent. Unlike ridge-axis hydrothermal vent fluids previously sampled, the total CO 2 content of the endmember spring fluids (0.85 μmol kg −1) is depleted with respect to bottom seawater. The very low alkalinities (0.43 meq L −1) and high Ca 2+ concentrations (55 mmol kg −1) of endmember spring fluids suggest that the removal of C in the spring fluids is associated with carbonate precipitation in the igneous basement. Assuming that 8–20% of the total ridge-flank heat loss rate of 50 × 10 18 cal yr −1 (6.6 TW) is removed by porewater advecting from the sediment at ≥25°C (an upper limit dictated by the global input rate of Mg +2 to the oceans; Mottl and Wheat, 1994), a maximum global carbon sink of 1.0–2.6 × 10 11 mol yr −1 ( 1.2–3.1 Mton-C yr −1) and a maximum global alkalinity sink of 140–340 × 10 9 eq yr −1 are calculated for warm (≥25°C) ridge-flank hydrothermal circulation. This carbon flux is only 2–5% of current estimates of subaerial and submarine volcanic CO 2 emissions (∼50 × 10 11 mol yr −1), indicating that hydrothermal alteration of oceanic crust on young mid-ocean ridge flanks at temperatures of a few tens of degrees Celsius is only a relatively minor sink for carbon on a global basis. It is still possible, however, that ridge-flank alteration at lower temperatures (<25°C) is an important component of the global carbon budget.

Rhenium-Osmium Isotopic Systematics of an Ancient Laterite Profile

The objectives of this study were 1) to investigate the Re-Os isotopic systematics of a well-characterized laterite profile, including the unaltered bedrock, 2) to utilize the Re-Os isotopic data to address the issue of laterite genesis, and 3) to speculate on the extent to which Os derived from laterite soils may affect the Os budget of the oceans. Approximately 80% of Os dissolved in the world oceans is derived from continental weathering, the rest being from hydrothermal alteration of oceanic crust and from dissolution of cosmic dust (Sharma et al., 1997). At present, there is a large uncertainty in our understanding of the behaviour of Os during weathering and transport into deep oceans, which needs to be addressed to evaluate properly the marine Os isotopic record. Laterites cover more than onethird of the exposed continental surface with lateritecapped landforms occurring extensively in the humid tropical regions (30 ~ N to 30 ~ S). Outside the tropics, the laterite occurrences are thought to represent climate change and/or continental drift (e.g. Schwarz, 1997). Given that the tropics supply approximately 50% of the annual water input to the oceans, an understanding of the laterite genesis and evolution is important in evaluating the fluxes of elements from the continents into the oceans. In this study we are concerned with investigating the behaviour of Re and Os during tropical weathering of a granodiorite. Despite the potential of many laterites as sources of precious metals, high quality Re and Os concentration data on laterites are non-existent. We present ReOs isotopic and concentration data on a laterite profile about 18 m deep developed on a Birrimian granodiorite in the Republic of Burkina Faso, West Africa. This profile is thought to have developed in situ in a tectonically stable and slowly eroding environment with possibly no relief inversion and no extensive lateral translocation of material. These observations make this profile especially suitable to study the vertical changes of elemental concentration, as a result of weathering. Sample description

Sources of Rare-Earth Elements and Iron in Paleoproterozoic Iron-Formations from the Transvaal Supergroup, South Africa: Evidence from Neodymium Isotopes

Neodymium isotopic data are reported for pure chemical sediments from the Siderian Kuruman and Penge Iron-Formations (IFs) in the Transvaal Supergroup, South Africa. From a large sample set for which petrographical and chemical data was available, we selected samples which best represent the pure chemical sediment and still carry the primary rare-earth element (REE) distribution for isotope analyses. Both the Kuruman IF ( + 1.9 and +1.1) and the Penge IF (+ 0.2 and -0.2) show values more radiogenic than published values for epiclastic sediments from the slightly older Black Reef Formation (-5.1 to -3.2). This suggests that REE and iron in the Transvaal IFs were predominantly, though not exclusively, derived from hydrothermal alteration of oceanic crust. The value of the Kuruman IF is similar to that proposed for the Hamersley IF in western Australia, suggesting that were typical of IF-depositing open ocean waters at the Archean/Proterozoic boundary.

Sulfur isotopic profile through the oceanic crust: Sulfur mobility and seawater-crustal sulfur exchange during hydrothermal alteration

Measurements of the abundance of anhydrite and pyrite veins and the distribution of alteration types in oceanic basement enable estimates of global mass and isotopic fluxes of sulfur during hydrothermal alteration of oceanic crust. Fluxes of 2.5 × 10 12 g/yr S (δ-34 S = 2.7%) from the volcanic section to seawater and 2.1 × 10 12 g/yr of seawater S (21%) into the crust result in an insignificant change in the S content of altered ocean crust. These are clearly second-order fluxes compared to the riverine input of S to the oceans and the output of sedimentary pyrite. Most of the anhydrite that forms in the high-temperature axial convection cell must later be dissolved at lower temperatures during off-axis circulation. The mean δ-34 S of altered oceanic basement is 0.9%, only slightly changed from the primary value of 0.1%. Unless S is preferentially mobilized from 34 S-rich sulfide deposits or mineralized zones, subduction of altered basaltic crust is unlikely to be the source of high δ 34 S values of arc volcanic rocks (∼4%), but could contribute to isotopic heterogeneities of S in the mantle.