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Several hydrothermal deposits associated with ultramafic rocks have recently been found along slow spreading ridges with a low magmatic budget. Three preferential settings are identified: (1) rift valley walls near the amagmatic ends of ridge segments; (2) nontransform offsets; and (3) ultramafic domes at inside corners of ridge transform-fault intersections. The exposed mantle at these sites is often interpreted to be a detachment fault. Hydrothermal cells in ultramafic rocks may be driven by regional heat flow, cooling gabbroic intrusions, and exothermic heat produced during serpentinization. Along the Mid-Atlantic Ridge (MAR), hydrothermal deposits in ultramafic rocks include the following: (1) sulfide mounds related to high-temperature low-pH fluids (Logatchev, Rainbow, and Ashadze); (2) carbonate chimneys related to low-temperature, high-pH fluids (Lost City); (3) low-temperature diffuse venting and high-methane discharge associated with silica, minor sulfides, manganese oxides, and pervasive alteration (Saldanha); and (4) stockwork quartz veins with sulfides at the base of detachment faults (15°05′N). These settings are closely linked to preferential circulation of fluid along permeable detachment faults. Compared to mineralization in basaltic environments, sulfide deposits associated with ultramafic rocks are enriched in Cu, Zn, Co, Au, and Ni. Gold has a bimodal distribution in low-temperature Zn-rich and in hightemperature Cu-rich mineral assemblages. The Cu-Zn-Co-Au deposits along the MAR seem to be more abundant than in ophiolites on land. This may be because ultramafic-hosted volcanogenic massive sulfide deposits on slow spreading ridges are usually not accreted to continental margins during obduction and may constitute a specific marine type of mineralization.

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Abstract-We report on the major and trace element abundances of 18 diogenites, and O-isotopes for 3 of them. Our analyses extend significantly the diogenite compositional range, both in respect of Mg-rich (e.g., Meteorite Hills [MET] 00425, MgO = 31.5 wt%) and Mg-poor varieties (e.g., Dhofar 700, MgO = 23 wt%). The wide ranges of siderophile and chalcophile element abundances are well explained by the presence of inhomogeneously distributed sulfide or metal grains within the analyzed chips. The behavior of incompatible elements in diogenites is more complex, as exemplified by the diversity of their REE patterns. Apart from a few diogenite samples that contain minute amounts of phosphate, and whose incompatible element abundances are unlike the orthopyroxene ones, the range of incompatible element abundances, and particularly the range of Dy/Yb ratios in diogenites is best explained by the diversity of their parental melts. We estimate that the FeO/MgO ratios of the diogenite parental melts range from about 1.4 to 3.5 and therefore largely overlap the values obtained for non-cumulate eucrites. Our results rule out the often accepted view that all the diogenites formed from parental melts more primitive than eucrites during the crystallization of a magma ocean. Instead, they point to a more complex history, and suggest that diogenites were derived from liquids produced by the remelting of cumulates formed from the magma ocean.

International audienceThe eucrites and diogenites are meteorites that probably originate from asteroid 4-Vesta. The upper part of the crust of this body is certainly composed of eucrites which are basaltic or gabbroic rocks. Diogenites are ultramafic cumulates whose relationships with eucritic lithologies are unknown. Here, we show that the orthopyroxenes of some diogenites display very deep negative Eu anomalies (Eu/Eu* close to 0.1 or lower). The contamination of the parental magmas of diogenites by melts derived by partial melting of the eucritic crust can satisfactorily explain the range of the Eu anomalies displayed by diogenites. Thus, these anomalies are the first firm indication that parental melts of diogenites have intruded the eucritic crust, and consequently are younger than eucrites

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International audienceWe have performed a mineralogical and geochemical study of eight metamorphosed basaltic eucrites. These are classified into granulitic eucrites and type 4–7 eucrites on the basis of their textures and pyroxene mineralogy, and display mineralogical evidence for high temperature metamorphism, including partial melting. In particular, rare earth element (REE) patterns of a number of the eucrites studied show varying degrees of light REE depletion due to partial melting, with subsequent melt extraction. A simple correlation between metamorphic grade, as deduced from pyroxene mineralogy, and the degree of light REE depletion was not detected. This can be explained by the fact that homogenization, exsolution and inversion of pigeonite would have required prolonged heating at moderate temperatures (not, vert, similar800–1000 °C), whereas partial melting would have taken place over a short time interval where temperatures exceeded that of the solidus. The eucrites studied therefore record a two stage thermal regime consisting of short, high temperature reheating events superimposed on long duration global crustal metamorphism. The short reheating events may have been caused by impact events and/or intrusions of hot magmas. The results of this study demonstrate that the thermal history of eucritic crust was more complex than can be explained by a simple burial model alone. In particular, the origin of Stannern trend eucrites requires contamination of Main-Group magmas by partial melts extracted from residual eucrites

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