Crustal evolution of island-arc ultramafic magma: Galmoenan pyroxenite-dunite plutonic complex, Koryak highland (Far East Russia)

Alaskan-type platinum-bearing plutons and potassium-enriched mafic to ultramafic volcanic rocks are temporally and spatially associated within the Late Cretaceous-Paleocene Achalvayam-Valaginskii intra-oceanic palaeo-arc system, allochthonousy present in the Kogak Highland and Kamchatka Peninsula (Far East Russia). The compositions of the parental magmas to the Alaskan-type complexes are estimated using the Galmoenan platonic complex as an example. This complex, composed of dunites, pyroxenites and minor gabbros, is the largest (~ 20 km3) in the system and the best studied owing to associated platinum placer deposits. The compositions of the principal mineral phases in the Galmoenan intrusive rocks [olivine (Fo(79-92)), clinopyroxene (1-3.5 wt % Al2O3, 0.1-0.5 wt % TiO2), and Cr-spinel (5-15 wt % Al2O3 and 0.3-0.7 wt % TiO2)] are typical of liquidus assemblages in primitive island-arc magmas in intra-oceanic settings, and closely resemble the mineral compositions in the Achaivayam-Valaginskii ultramafic volcanic rocks. The temporal and spatial association of intrusive and extrusive units, and the similarity of their mineral compositions, suggest that both suites were formed from similar parental magmas. ne composition of the parental magma for the Galmoenan platonic rocks is estimated using previously reported data for the Achaivayam-Valaginskii ultramafic volcanic rocks and phenocryst-hosted melt inclusions. Quantitative simulation of crystallization of the parental magma in the Galmoenan magma chamber shows that the compositions of the cumulate units are best modelled by fractional cryallization with periodic magma replenishment. The model calculations reproduce well the observed mineral assemblages and the trace element abundances in clinopyroxene. Based upon the estimated composition of the parental magmas and their mantle source, we consider that fluxing of a highly refractory mantle wedge (similar to the source of boninites) by chlorine-rich aqueous fluids is primarily responsible for both high degrees of partial melting and the geochemical characteristics of the magmas, including their enrichment in platinum-group elements

Platinum-group element abundances and Os isotope composition of mantle peridotites from the Mamonia complex, Cyprus

Platinum-group element (Os, Ir, Ru, Pt and Pd) abundances and Re–Os isotopic composition of fifteen peridotites (eleven spinel lherzolites and four spinel harzburgites) from the Mamonia complex, Cyprus, were determined as well as major and trace element compositions of bulk-rocks and minerals. Spinel lherzolites show excellent correlation between parameters indicating the degree of melting — e.g. Fo content in olivine, Cr# (Cr/(Cr+Al)) of Cr-spinel, Al and Yb concentrations in clinopyroxene. The degree of partial melting, calculated using Cr-spinel compositions, range from 1% to 9%. The PGE contents in spinel lherzolites show correlation with each other and with the spinel compositions, and thus can be related to the partial melting of the mantle source. The PGE abundances of the mantle source are estimated using the least depleted spinel lherzolite samples: Os 3.6±0.5, Ir 3.4±0.5, Ru 6.5±0.9, Pt 6.1±0.2, Pd 3.9±0.2 (in ppb). Spinel harzburgites, despite a good correlation between the whole rock major element abundances and mineral compositions (e.g., Yb in clinopyroxene and Cr# of spinel), indicate no relationship between the Fo content of olivine and the Cr# of spinel. Hence, the harzburgites cannot be the residuum of simple partial melting, but have a more complex origin (e.g. melt percolation). Pt/Ir ratios increase in the harzburgites as Pt increases. Similar behavior of Pt and Pd is observed in abyssal and SCLM harzburgites, and explained by sulfide precipitation during melt percolation. Rhenium concentrations in most Mamonia peridotites are significantly higher than in the primitive mantle and does not show correlations with PGE. Indeed Re concentrations tend to increase with the Cr# of spinel. Thus its distribution is not governed by partial melting and we suggest that Re addition to the peridotites of Mamonia occurred during serpentinization. The Re–Os model ages of peridotites form three age clusters at 250 Ma, 600–800 Ma and >1000 Ma. The youngest age is consistent with the age of magmatic rocks in the Mamonia Complex, whereas the “oldest” peridotites may belong to remnants of the subcontinental mantle

Komatiites reveal a hydrous Archaean deep-mantle reservoir

Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source1, 2. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes3, 4, 5, 6 while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes7, 8, 9. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone10. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history