Chemical composition of hydrous and other minerals and their chromian spinel hosts in chromitite and troctolite from the Hess Deep

Primary hydrous and other minerals enriched with incompatible components were found in ocean-floor ultramafic-mafic plutonic rock suites recovered from two contrasting ridge systems, i.e., the East Pacific Rise (Hess Deep, equatorial Pacific), a typical fast-spreading system, and Mid-Cayman Trough, a typical slow-spreading system. They are characteristically associated with chromian spinel, enriched with Cr, one of compatible elements. The hydrous minerals can be formed through interaction between depleted mid-ocean ridge basalts (MORBs) and oceanic peridotite. Primary MORB produced in the deeper part inevitably react with shallower mantle peridotite; the magma selectively dissolves orthopyroxene with simultaneous olivine precipitation. Chromium is supplied to the melt from orthopyroxene, which is enriched with Cr over Al relative to ordinary basaltic melts. The effects of zone refining are also important for concentrations of the incompatible components, especially H20, Na, and Ti, in the modified magma, which in the extreme case is able to precipitate hydrous minerals. This mechanism is common to both fast- and slow-spreading ridges, and is more effective in stagnant or failed melt conduits. Some ultramafic rocks from upper mantle or transition-zone members of ophiolites have primary hydrous minerals, usually included by chromian spinel. Despite the often a priori assumption that slab-derived components are necessary for the formation of the chromitite with inclusions of primary hydrous minerals, this is clearly not necessary

Chemical composition of ultrabasites and composing minerals from the Tonga Trench

A petrologic-geochemical study (petrochemistry, contents of siderophile and certain lithophile elements, composition of rock-forming silicates and accessory chrome spinels) of ultrabasic rocks dredged from the arc side in the northern end of the Tonga deep-sea trench has been carried out. The ultrabasites included harzburgites and dunites. Peridotites show clearly manifested material characteristics of ultrabasic relicts strongly depleted in low-temperature basaltic components. It is suggested that they have arose in the high degree of partial melting (about 30%) of a matrix mantle source of the lherzolite type. Great similarity of the rocks studied with ultrabasites of many ophiolites that are widespread in folded belts indicates that young island arcs are among the most likely geodynamic environments of ophiolite generation

Multiple Sulfur Isotope Evidence for Bacterial Sulfate Reduction and Sulfate Disproportionation Operated in Mesoarchaean Rocks of the Karelian Craton

Sulfur isotope in sulfides from the Paleoarchean and the Neoarchean sedimentary rocks evidence microbial sulfur metabolism in Archean sulfur cycle. However, sulfur metabolism for the Mesoarchean interval is less obvious since evidence for a large range in sulfur isotope values has not yet been observed in Mesoarchean samples. We report the results of multiple sulfur isotope measurements for sulfide minerals from ~2.8 Ga sedimentary rocks in the southeastern part of the Karelian Craton. In situ isotope analysis of sulfide grains have been performed using a femtosecond laser-ablation fluorination method. Sulfide samples studied here yielded Δ33S values between −0.3 and +2.7‰ and δ34S values between −10 and +33‰. The Δ33S dataset was interpreted to indicate the incorporation of sulfur from two coexisting sulfur pools, photolytic sulfate and photolytically derived elemental sulfur. We suggest that the relative contributions of these Δ33S different pools to the pyritic sulfur could be controlled by the metabolic activity of coexisting sulfate-reducing and sulfur-disproportionating bacteria during pyrite formation. We therefore suggest the operation of different metabolic pathways of sulfur in Mesoarchean sedimentary environments