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The Loma Baya complex in south-western Mexico is a volume of chromitite-bearing oceanic mantle that records a complex metamorphic history, defined by a first stage of hydrous metamorphism overprinted by a short-lived thermal event associated with an Eocene granite intrusion. During the hydrous metamorphism, the primary magmatic chromite-olivine assemblage was replaced by a secondary, porous intergrowth of Fe.sup.2+-rich chromite and chlorite. The heat supplied by an Eocene-age granite intrusion reversed the hydration reaction, producing chromite rims with perfectly developed crystal faces. This third-generation chromite is in equilibrium with highly magnesian (neoformed) olivine and defines a chemical trend analogous to the original magmatic one. The preservation of both reactions in the Loma Baya chromitite provides compelling evidence that the hydration of chromite can be reversed by either prograde metamorphism or any heating event, confirming previous thermodynamic predictions. Understanding these complex features is of particular interest due to the fact that changes in temperature and variable degrees of fluid/rock interaction during metamorphism and intrusion have also significantly affected the chromite-hosted IPGE carrier phases. Here, we propose that the metamorphic fluids involved in the hydrous metamorphism have caused the desulphurization of laurite RuS.sub.2, releasing minute particles of Ru-Os-Ir alloys Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1169-9) contains supplementary material, which is available to authorized users., Author(s): Jose M. Gonzalez-Jimenez[sup.1] [sup.2 ] , Martin Reich[sup.1] , Antoni Camprubi[sup.3] , Fernando Gervilla[sup.4] , William L. Griffin[sup.2] , Vanessa Colas[sup.5] , Suzanne Y. O'Reilly[sup.2] , Joaquin A. Proenza[sup.6] [...]

The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton is comprised of chromitite-bearing sill-like ultramafic-mafic rocks that are surrounded by metavolcanic schists (compositionally komatiitic to komatiitic basalts) and a suite of tonalite-trondhjemite-granodiorite gneissic rocks. The sill-like plutonic unit consists of a succession of serpentinite (after dunite)-peridotite-pyroxenite and gabbro with bands of titaniferous magnetite ore. The chromitite ore-bodies (length [almost equal to]30-500 m; width [almost equal to]2-15 m) are hosted by the serpentinite-peridotite unit. Unaltered chromites from massive chromitites (>80 % modal chromite) of the Byrapur and Bhaktarhalli chromite mines in the greenstone belt are characterized by high Cr# (100Cr/(Cr + Al)) of 78-86 and moderate Mg# (100 Mg/(Mg + Fe.sup.2+)) of 45-55. In situ trace-element analysis (LA-ICPMS) of unaltered chromites indicates that the parental magma of the chromitite ore-bodies was a komatiite lacking nickel-sulfide mineralization. In the Ga/Fe.sup.3+# versus Ti/Fe.sup.3+# diagram, the Byrapur chromites plot in the field of suprasubduction zone (SSZ) chromites while those from Bhaktarhalli lie in the MOR field. The above results corroborate our previous results based on major-element characteristics of the chromites, where the calculated parental melt of the Byrapur chromites was komatiitic to komatiitic basalt, and the Bhaktarhalli chromite was derived from Archean high-Mg basalt. The major-element chromite data hinted at the possibility of a SSZ environment existing in the Archean. Altered and compositionally zoned chromite grains in our study show a decrease in Ga, V, Co, Zn, Mn and enrichments of Ni and Ti in the ferritchromit rims. Trace-element heterogeneity in the altered chromites is attributed to serpentinization. The trace-element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic Fe-Ti-V-rich magnetite bands in layered intrusions, and magnetites from andesitic melts, suggesting that magnetite crystallized from an evolved gabbroic melt. Enrichments of Ni, Co, Te, As and Bi in disseminated millerite and niccolite occurring within chromitites, and in disseminated bravoite within magnetites, reflect element mobility during serpentinization. Monosulfide solid solution inclusions within pyroxenes (altered to actinolite) in pyroxenite, and interstitial pyrites and chalcopyrites in magnetite, retain primary characteristics except for Fe-enrichment in chalcopyrite, probably due to sub-solidus re-equilibration with magnetite. Disseminated sulfides are depleted in platinum-group elements (PGE) due to late sulfide saturation and the PGE-depleted nature of the mantle source of the sill-like ultramafic-mafic plutonic rocks in the Nuggihalli greenstone belt. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1148-1) contains supplementary material, which is available to authorized users., Author(s): Ria Mukherjee[sup.1] [sup.2] , Sisir K. Mondal[sup.1] [sup.4] , Jose M. Gonzalez-Jimenez[sup.3] [sup.5] , William L. Griffin[sup.3] , Norman J. Pearson[sup.3] , Suzanne Y. O'Reilly[sup.3] Author Affiliations: (1) PGE [...]

Episodic refertilization and metasomatism of depleted subcontinental lithospheric mantle (SCLM) is rarely revealed within a single orogenic peridotite. We have investigated the Shenglikou peridotite massif in the North Qaidam Orogen (NE Tibet, China) to understand its origin and secular re-enrichment history and possible links with global tectonism, using multiple chemical and isotopic techniques. The massif contains two main groups of rocks, one rich in olivine (Ol) and the other rich in pyroxene (Pyro). In situ Re-Os isotope analyses on sulfides and previous reported whole-rock Os data show the Shenglikou peridotite massif has an Archean origin. The Ol-rich group has Mg# up to 0.923, Al.sub.2O.sub.3 contents down to 0.55 wt%, low FeO contents, whole-rock HREE ~0.01-0.1 times chondritic values and high spinel Cr# (up to 0.71). Modeling suggests that the Ol-rich peridotites are residues after ~30-40 % melt extraction at ~1600-1500 °C and [less than or equal to]5 GPa. Mineral compositions, oxygen-isotope compositions ([delta].sup.18O.sub.V-SMOW = 5.7 ± 0.1 â°) and attainment of Hf-isotope internal equilibration at ~1.5-1.4 Ga with initial ratios similar to that of depleted convective mantle indicate that the Ol-rich group was refertilized by silicate melts derived from the depleted asthenosphere at ~1.5-1.4 Ga, corresponding to one phase in the breakup of the Columbia supercontinent. On the other hand, chemical compositions and Hf-O isotopic signatures of the pyroxene-rich rocks suggest a second refertilization superimposed on the Archean lithospheric mantle at ~700 Ma, coeval with the global extension that affected Rodinia. Both groups show metasomatism by fluids, sourced from subducting slabs during the early Paleozoic convergence between the Qaidam and Qilian blocks. This study shows that the episodic refertilization and metasomatism of depleted Archean SCLM, temporally linked to supercontinental cycles, can be revealed in a single orogenic peridotite, and suggests that continental extension and convergence play an important role in the destruction of cratonic keels. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1126-7) contains supplementary material, which is available to authorized users., Author(s): Qing Xiong[sup.1] [sup.2] , William L. Griffin[sup.2] , Jian-Ping Zheng[sup.1] , Suzanne Y. O'Reilly[sup.2] , Norman J. Pearson[sup.2] Author Affiliations: (1) State Key Laboratory of Geological Processes and Mineral [...]