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.

Petrology of mafic and ultramafic intrusions from the Portneuf-Mauricie Domain, Grenville Province, Canada

The Portneuf-Mauricie Domain (PMD), located in the south-central part of the Grenville Province, comprises several mafic and ultramafic intrusions hosting Ni-Cu ± platinum-group element (PGE) prospects and a former small mining operation (Lac Édouard mine). These meter- to kilometer-scale, sulfide-bearing intrusions display diverse forms, such as layered and tabular bodies with no particular internal structure, and zoned plutons. They were injected ~ 1.40 Ga into a mature oceanic arc, before and during accretion of the arc to the Laurentian margin. The pressure-temperature conditions of the magmas at the beginning of their emplacement were 3 kbar and 1319-1200 °C (according to the petrologic modeling results from this study). The PMD mineralized intrusions are interpreted to represent former magma chambers or magma conduits in the roots of the oceanic arc. The parent magmas of the mineralized intrusions resulted mainly from the partial melting of a mantle source composed of spinel-bearing lherzolite. Petrologic modeling and the occurrence of primary amphibole in the plutonic rocks indicate that these parent melts were basaltic and hydrous. In addition, fractional crystallization modeling and Mg/Fe ratios suggest that most of the intrusions may have formed from evolved magmas, with Mg# = 60, resulting from the fractionation of more primitive magmas (primary magmas, with Mg# = 68). Petrologic modeling demonstrates that 30% fractional crystallization resulted in the primitive to evolved characteristics of the studied intrusive rocks (as indicated by the crystallization sequences and mineral chemistry). Exceptions are the Réservoir Blanc, Boivin, and Rochette West parent magmas, which may have undergone more extensive fractional crystallization, since these intrusions contain pyroxenes that are more iron rich and have lower Mg numbers than pyroxenes in the other PMD intrusions. The PMD mafic and ultramafic intrusions were intruded into an island arc located offshore from the Laurentian continent. Thus, their presence confirms the existence of a well-developed magmatic network (responsible of the fractionation processes) beneath the Proterozoic arc, which resulted in the wide range of compositions observed in the various plutons.

(Table 1) Concentrations of Os, Ir, Pt and 187Os/188Os ratio in ODP Hole 130-806C samples

New osmium (Os) isotope and platinum group element (PGE) concentration data are used in conjunction with published 3He and Th isotope data to determine the relative proportions of lithogenic, extraterrestrial and hydrogenous iridium (Ir) in a Pacific pelagic carbonate sequence from the Ocean Drilling Program (ODP) Site 806 on the Ontong Java Plateau (OJP). These calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary Ir budget are minor, while hydrogenous Ir accounts for roughly 85% of the total Ir. Application of analogous partitioning calculations to previously reported data from a North Pacific red clay sequence (LL44-GPC3) yields very similar results. Total Ir burial fluxes at Site 806 and LL44-GPC3 are also similar, 45 and 30 pg/cm**2/kyr, respectively. Average Ir/3He and Ir/xs230Th_initial ratios calculated from the entire Site 806 data set are similar to those reported earlier for Pacific sites. In general, down-core profiles of Ir, 3He and xs230Th_initial, are not well correlated with one another. However, all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two. While these results indicate that Ir concentration has potential as a point-paleoflux tracer in pelagic carbonates, Ir-based paleoflux estimates are likely subject to uncertainties that are similar to those associated with Co-based paleoflux estimates. Consequently, local calibration of Ir flux in space and time will be required to fully assess the potential of Ir as a point paleoflux tracer. Measured 187Os/188Os of the OJP sediments are systematically lower than the inferred 187Os/188Os of contemporaneous seawater and a clear glacial-interglacial 187Os/188Os variation is lacking. Mixing calculations suggest Os contributions from lithogenic sources are insufficient to explain the observed 187Os/188Os variations. The difference between the 187Os/188Os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic Os carried by particulate extraterrestrial material. Down-core variations of 187Os/188Os with Pt/Ir and Os/Ir also point to contributions from extraterrestrial particles. Mixing calculations for each set of several triplicate analyses suggest that the unradiogenic Os end member cannot be characterized by primary extraterrestrial particles of chondritic composition. It is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions, 187Os/188Os of pelagic carbonates has greater potential compared to abundances of PGE. An attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial Os in the OJP samples and previously reported extraterrestrial Os flux. Throughout most of the OJP record, Os isotope-based paleoflux estimates are within a factor of two of those derived using other constant flux tracers. Meaningful flux estimates cannot be made during glacial maxima because the OJP sediments do not record the low 187Os/188Os reported previously. We speculate that this discrepancy may be related to focusing of extraterrestrial particles at the OJP, as has been suggested to explain down-core 3He variations.

Chemical compositions of Fe-Mn nodules and host sediments from the Northwest Pacific

A number of regularities of ore element accumulation in iron-manganese nodules along the profile from the Tsugaru Strait to the Wake Atoll is identified in the paper. It is shown that the ore process is enhanced to the pelagic zone: in nodules content of ore material increases and content of mineral insoluble residue decreases from near-shore areas to central parts of the ocean. Diagenetic redistribution of the elements between host sediments and nodules resulting to enrichment of the latter increases from bottom sediments of the ocean periphery to fine grained pelagic muds. At absolute enrichment by Fe, Mn, Cu, Ni, Co, Mo, W, Ti, Zr, and V (as compared to host sediments) nodules are relatively enriched in Mn group elements (Cu, Ni, Co, Mo, W) and consequently depleted in Fe group elements (Ti, V, Zr) in the direction from the periphery to the center of the ocean. The ratio of reactive forms of Fe and Mn in host sediments is a factor determining the ratio of Fe group and Mn group elements in nodules.

Geochemical investigations of elements in sediment core CRP-1 from the Ross Sea, Antarctica

Geochemical data are presented for samples from strata, mainly of Miocene age, in the Cape Roberts-1 core (western McMurdo Sound, Antarctica) to assess the sediment provenance. Bulk (major and trace element) chemistry together with bulk mineralogy of fine-grained sandstones, siltstones, mudstones, and diamictites indicate that chemical alteration of source materials, fractionation due to sedimentary sorting, and diagenetic effects were not significant in the Cape Roberts sediment history. Relevant geochemical parameters are consistent with the Cape Roberts sediments being derived mainly from the crystalline basement and the Beacon Supergroup. On the basis of element distributions, an additional contribution from the Ferrar Dolerite and, mainly above about 60 m, influxes of detritus derived from basanitic to intermediate members of the McMurdo Volcanic Group are recognised.