Dissolved chemical elements in river waters of the Wrangel Island

Pioneer information about chemical composition of river waters in the Wrangel Island has been obtained. It is shown that water composition reflects the lithogeochemical specifics of primary rocks and ore mineralization. In contrast to many areas of the Russian Far North river waters of the island are characterized by elevated background value of total mineralization (i.e., total dissolved solids, TDS) (0.3-2 g/l) and specific chemical type (SO4-Ca-Mg). This is related to abundance of Late Carboniferous gypsiferous and dolomitic sequences in the mountainous area of the island. It has also been established that salt composition of some streams is appreciably governed by supergene alterations of sulfide mineralization associated with quartz-carbonate vein systems. They make up natural centers of surface water contamination. Waters in such streams are characterized by low pH values (2.4-5.5), high TDS (up to 6-23 g/l) and SO4-Mg composition. These waters are also marked by anomalously high concentrations of heavy and non-ferrous metals, as well as REE, U, and Th.

Platinum group elements and geochemistry of altered oceanic crust of ODP Hole 140-504B

Primary chemical heterogeneity in the sheeted dike complex in Deep Sea Drilling Project Hole 504B makes these rocks unsuitable for conventional mass balance calculations in determining element mobility associated with hydrothermal alteration. Due to the original heterogeneity and variable degrees of fractionation in the dikes, an appropriate reference sample on which calculations can be based is difficult to find. Therefore, the use of incompatible element ratios is developed to evaluate geochemical changes during alteration(s). For example, on a Zr/Yb-La/Yb plot, scatter along a straight line suggests tapping of a variably depleted mantle source and deviation from the line suggests element mobility (gain or loss). Using this method, our data indicates that the hydrothermal evolution of the sheeted dike complex was accompanied by significant loss of Cu, Zn, and Ti and some loss of La. The sheeted dike complex has low platinum group element (PGE) concentrations and steep PGE patterns, typical of mid-ocean ridge basalts (MORBs) on the global scale. We propose that the unusual PGE patterns of MORBs cannot be entirely generated by a partial melting and sulfide segregation model; instead, these patterns in part must have been inherited from their mantle source. The Au data show no evidence for mobilization during hydrothermal alteration of the dikes.

Mineralogy and geochemistry of sulfide minerals and bulk rocks from ODP Hole 176-735B

Ocean Drilling Program (ODP) Leg 176 built upon the work of ODP Leg 118 wherein the 500-m section that was sampled represented the most complete recovery of an intact portion of lower oceanic crust ever described. During Leg 176, we deepened Hole 735B to >1500 m below seafloor in an environment where gabbroic rocks have been tectonically exposed at the Southwest Indian Ridge. This new expedition extended the remarkable recovery (>85%) that allowed unprecedented investigations into the nature of the lower oceanic crust as a result of Leg 118. Sulfide mineral and bulk rock compositions were determined from samples in the 1000-m section of oceanic gabbros recovered during Leg 176. The sulfide assemblage of pyrrhotite, chalcopyrite, pentlandite, and troilite is present throughout this section, as it is throughout the 500-m gabbroic section above that was sampled during Leg 118. Troilite is commonly present as lamellae, and the only interval where troilite was not observed is from the uppermost 150 m of the section sampled during Leg 118, which is intensely metamorphosed. The common presence of troilite indicates that much of the sulfide assemblage from Hole 735B precipitated from a magmatic system and subsequently underwent low-temperature reequilibration. Evaluation of geochemical trends in bulk rock and sulfides indicates that the combined effects of olivine accumulation in troctolites and high pentlandite to pyrrhotite ratios account for the sporadic bulk rock compositions high in Ni. Bulk rock and sulfide mineral geochemical indicators that are spatially coincident with structural and physical properties anomalies indicate a heretofore unrecognized lithologic unit boundary in this section. Platinum-group element (PGE) compositions were also determined for 36 samples from throughout the section that were recovered during Leg 176. Whereas most samples had low (<0.4 ppb) PGE concentrations, rare samples had elevated PGE values, but no unique common trend between these samples is evident.

Element concentrations and Osmium, Platinum and Rhenium isotope ratios of ODP Site 153-920 abyssal peridotites

Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190Pt-186Os and 187Re-187Os systematics. They have uniform 186Os/188Os ratios with a mean of 0.1198353 +/- 7, identical to the mean of 0.1198340 +/-12 for Os-Ir alloys and chromitites believed to be representative of the upper mantle. While the Pt/Os ratios of the upper mantle may be affected locally by magmatic processes, these data show that the Pt/Os ratio for the bulk upper mantle has not deviated by more than about +/- 30% from a chondritic Pt/Os ratio over 4.5 billion years. These observations are consistent with the addition of a chondritic late veneer after core separation as the primary control on the highly siderophile element budget of the terrestrial upper mantle. The 187Os/188Os of the samples range from 0.12267 to 0.12760 and correlate well with Pt and Pt/Os, but not Re/Os. These relationships may be explained by variable amounts of partial melting with changing D(Re), reflecting in part garnet in the residue, with a model-dependent melting age between about 600 and 1700 Ma. A model where the correlation between Pt/Os and 187Os/188Os results from multiple ancient melting events, in mantle peridotites that were later juxtaposed by convection, is also consistent with these data. This melting event or events are evidently unrelated to recent melting under mid-ocean ridges, because recent melting would have disturbed the relationship between Pt/Os and 187Os/188Os. Instead, this section of abyssal peridotite may be a block of refractory mantle that remained isolated from the convecting portions of the upper mantle for 600 Ma to >1 Ga. Alternatively, Pt and Os may have been sequestered during more recent melting and possibly melt/rock reaction processes, thereby preserving an ancient melting history. If representative of other abyssal peridotites, then the rocks from this suite with subchondritic 187Os/188Os are not simple residues of recent MORB source melting at ridges, but instead have a more complex history. This suite of variably depleted samples projects to an undepleted present-day Pt/Os of about 2.2 and 187Os/188Os of about 0.128-0.129, consistent with estimates for the primitive upper mantle.

Platinum-group element abundances in background samples and the K-T boundary interval from DSDP Hole 91-596 (Table 2)

New data on Ru/Ir abundance ratios are presented for nonmarine (Hell Creek, Montana; Frenchman River, Saskatchewan) and marine Cretaceous-Tertiary boundary sites (Brazos River, Texas; Beloc, Haiti; DSDP 577 and DSDP 596). The Ru/Ir ratio varies from 0.5 to 1 within 4000 km of Chicxulub and increases to 2-3 at paleodistances (65 Ma) of up to 12,000 km from the impact site. For CI chondrites, Ru/Ir = 1.5. A ballistic model of ejecta cloud cooling and expansion, which employs the available vapor-pressure versus temperature data for Ru and It, predicts qualitatively similar global variation in the Ru/Ir ratio but by only a factor of 1.5. We infer that several other factors, such as remobilization of PGE during diagenesis, preferential oxidation of Ru, condensation kinetics and atmospheric chemical and circulation processes, may account for the observed larger Ru/Ir variation.

Precious metals and volatiles in rocks of the Mid-Atlantic Ridge near 15°20'N

Distribution patterns of gold, platinum group metals (PGE), and volatile components are studied in the main rock types of the Mid-Atlantic Ridge lithosphere (ultrabasites, gabbro, and basalts) from the transform fracture zone at 15°20'N. It is ascertained that PGE content depends on the reduction degree of fluids, on rock types, and on their formation conditions. It is noted that concentrations of refractory elements (Os, Ir, Rh) decrease, while those of fusible elements (Pt, Pd, Au) increase with depth. The chondrite type of distribution is only noted in the ultrabasite rocks. Increase in water and CO2 contents and in oxidation degree of fluids occurs with transition from basalts to ultrabasites, as well as from normal magmatic systems to fluid-enriched anomalous systems.

Rhenium and platinum-group element concentrations and Os isotope data for Philippine Sea Plate basalts from DSDP Legs 31, 58, and 59

Platinum-group elements (PGE), rhenium and osmium isotope data are reported for basalts from Deep Sea Drilling Project cores in the Philippine Sea Plate (PSP). Lithophile trace element and isotopic characteristics indicate a range of source components including DMM, EMII and subduction-enriched mantle. MORB-like basalts possess smooth, inclined chondrite-normalised PGE patterns with high palladium-PGE/iridium-PGE ratios, consistent with previously published data for MORB, and with the inferred compatibility of PGE. In contrast, while basalts with EMII-type lithophile element chemistry possess high Pt/Ir ratios, many have much lower Pd/Ir and unusually high Ru/Ir of >10. Similarly, back-arc samples from the Shikoku and Parece-Vela basins have very high Ru/Ir ratios (>30) and Pd/Ir as low as 1.1. Such extreme Pd/Ir and Ru/Ir ratios have not been previously reported in mafic volcanic suites and cannot be easily explained by variable degrees of melting, fractional crystallisation or by a shallow-level process such as alteration or degassing. The data appear most consistent with sampling of at least two mantle components with distinct PGE compositions. Peridotites with the required PGE characteristics (i.e. low Pd, but relatively high Ru and Re) have not been documented in oceanic mantle, but have been found in sub-continental mantle lithosphere and are the result of considerable melt depletion and selective metasomatic enrichment (mainly Re). The long-term presence of subduction zones surrounding the Philippine Sea Plate makes this a prime location for metasomatic enrichment of mantle, either through fluid enrichment or infiltration by small melt fractions. The Re-Os isotope data are difficult to interpret with confidence due to low Os concentrations in most samples and the uncertainty in sample age. Data for Site 444A (Shikoku Basin) give an age of 17.7+/-1.3 Ma (MSWD = 14), consistent with the proposed age of basement at the site and thus provides the first robust radiometric age for these samples. The initial 187Os/188Os of 0.1298+/-0.0069 is consistent with global MORB, and precludes significant metasomatic enrichment of Os by radiogenic slab fluids. Re-Os data for Sites 446A (two suites, Daito Basin) and 450 (Parece-Vela Basin) indicate ages of 73, 68 and 43 Ma, which are respectively, 30, 17 and >12 Ma older than previously proposed ages. The alkalic and tholeiitic suites from Site 446A define regression lines with different 187Os/188Osinitial (0.170+/-0.033 and 0.112+/-0.024, respectively) which could perhaps be explained by preferential sampling of interstitial, metasomatic sulphides (with higher time-integrated Re/Os ratios) by smaller percentage alkalic melts. One sample, with lithophile elements indistinguishable from MORB, is Os-rich (146 pg/g) and has an initial 187Os/188Os of 0.1594, which is at the upper limit of the accepted OIB range. Given the Os-rich nature of this sample and the lack of evidence for subduction or recycled crust inputs, this osmium isotope ratio likely reflects heterogeneity in the DMM. The dataset as a whole is a striking indication of the possible PGE and Os isotope variability within a region of mantle that has experienced a complex tectonic history.