Pavel KepezhinskasNikolai BerdnikovNikita Kepezhinskas
17页
查看更多>>摘要：Peridotite xenoliths from the Avachinsky volcano in southern Kamchatka contain diverse assemblages of native metals and intermetallic compounds associated with primary olivine and orthopyroxene and metasomatic amphibole. Mineral compositions in Avachinsky xenoliths (Mg-rich olivine and low-Al enstatite associated with Cr-spinel) coupled with common absence of primary clinopyroxene and low bulk Al2O3 and CaO contents, suggest a depleted mantle wedge beneath the southern segment of the Kamchatka volcanic arc.Metallic phases in Avachinsky xenoliths occur as euhedral to anhedral grains in fractures andirregularly shaped voids in Fe-Mg silicates as well as micro-particles localized along boundaries between oivine and orthopyroxene. Siderophile metal assemblage is composed of platinum, iron and gold along with Cr-Fe and W-Fe-Mn alloys. Chalcophile metals include native copper, bismuth, lead and zinc along with Cu-Ag, Cu-Sn, Cu-Sn-Au, Ag-Au, Cu-Ag-Au and Ni-Cu-Ag-Au intermetallic compounds. Metals in Avachinsky xenoliths are associated, both spatially and texturally, with magnetite, rutile, galena, cassiterite, Cu-Ag-bearing Sb-Bi-Pb sulfide phase, AgS and non-stoichiometric Cl-bearing silver compound. Siderophile metals were introduced by primitive metasomatizing melts into low fO2 domains in the mantle wedge below the Kamchatka arc. Phase relations in the Cu-Ag-Au system suggest high-temperature (>900 °C) origin for Cu-Ag-Au intermetallic compounds in Avachinsky xenoliths. While Cu-Ag-Au alloys precipitated from metal-rich, mantle-derived silicate melts, low-melting point chalcophile metals and their minerals (Ag, Bi, Sb, Zn, Pb, Sn) were deposited in the sub-Kamchatkan mantle from hydrous slab fluids under reduced conditions. Metal assemblages in Avachinsky xenoliths further attest to reducing conditions in some subduction systems and potentially indicate existence of extreme redox gradients at sufficiently small scales in sub-arc mantle wedge environments.
查看更多>>摘要：The Rio Ceara-Mirim magmatism produced Mesozoic giant arcuate dike swarms during the initial stage of the West Gondwana breakup near the Equatorial Atlantic Ocean (NE Brazil), where they assemble tholeiites and subordinate intermediate rocks (SiO2 > 57 wt%). A multidata approach based on petrography, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb isotopes enables the discrimination of four geochemical groups (two high-Ti and two low-Ti). High-Ti groups are composed of (1) basalts and basaltic andesites with TiO2 generally higher than 2.0 wt%, MgO > ~3 wt% (named the HT group) and rich in incompatible elements and (2) (trachy)andesites and trachytes with lower TiO2 (<2.5 wt%; Mg < 3 wt%) interpreted as fractionated magmas from the HT group, therefore named evolved HT. Both these groups have similar ~(87)Sr/~(86)Sr_((27)) (-0.706) and εNd_((127)) (-2.90 to -2.56) and moderately radiogenic ~(206)Pb/~(204)Pb_((m)) ratios between 18.66 and 18.21. Conversely, low-Ti tholeiites (TiO2 < 2 wt%) are (3) LT1 composed of evolved basaltic andesites, with ~Ti/Zr < 70, MgO <6 wt%, and Al2O3 > 15 wt%; and (4) LT2 composed of subalkaline (to transitional) basalts and basaltic andesites but ~Ti/Zr >70, and MgO > 6 wt%. These two low-Ti groups are, respectively, less radiogenic (LT1 εNd_((127))) -4.65 to -4.40) and slightly more radiogenic (LT2 εNd_((127))) -1.44 to -1.08) in Nd relative to the broad HT group. The geochemical groups present different degrees of enrichment in large ion lithophile and light rare-earth elements coupled with depletion in high-field-strength elements (Nb-Ta) combined with isotopic (Sr-Nd-Pb) signatures compatible with enriched mantle sources. We showed that the EMI (Gough-like) isotope compositions of Rio Ceara-Mirim magmatism may be explained by mixing DMM with enriched SCLM-derived melts plus minor crustal assimilation or by the involvement of OIBs derived from mantle plumes of the South Atlantic area mixed with lithospheric sources. In a geodynamical perspective, given the narrow similarity of HT Rio Ceara-Mirim isotope compositions with EMI-Gough type and high-Ti magmas of Transminas swarm, a genetic connection between Equatorial Atlantic and Parana-Etendeka provinces must be considered,
查看更多>>摘要：Chromite is among the first minerals to crystallize from mantle derived magmas and one of the last to be consumed during partial melting of the mantle. Chromite is also an important mineral in major ore deposits of Cr and Pt. The composition of chromite could, therefore be of use in interpreting the petrogenetic conditions during partial melting of the mantle, crystallization of primitive magmas and formation of Pt and Cr ore deposits. However, most mafic rocks contain very little chromite and post-crystallization processes such as re-equilibration during cooling, metamorphism and weathering could change the composition. The composition of chromites from high- and low-Ti picrites from the Emeishan large igneous province have been determined to assess the degree to which the chromite compositions reflect the melt compositions. Aluminium, Sc, Ti, Ga, Nb, Sn, Hf and Ta concentrations in the chromites do appear to reflect the melt compositions in that they correlate with the whole rock compositions and have empirical partition coefficients similar to those determined in experiments. The V contents of both types of picrite are similar, but concentrations of chromites from high-Ti picrites are higher than those of low-Ti picrites. This can be explained if, in the high-Ti picrite more of the V was in the V3+ state (which can more readily substitute into chromite than V4+ or V5+) than in the low-Ti picrite. This implies that fO2 was lower for high-Ti picrites than low-Ti picrites. Concentrations of elements with a 2+ charge, Mg, Mn, Fe, Co, Ni and Zn are different in chromites included in olivine and chromites in the matrix and appear to have re-equilbrated. The compositions of the UG-2 chromites from the World's largest Pt deposit show some similarities with the chromites from the high-Ti picrite, but are depleted in Sc and Ti and enriched in Ga and Al.
查看更多>>摘要：Intra-oceanic arcs are typically associated with intermediate (andesitic) cone volcanoes. However, caldera volcanoes may also form in these settings from very large eruptions, resulting in sudden changes to the magma reservoir. These reservoirs can then produce either semi-continuous or intermittent low-intensity volcanism between major caldera-producing or caldera-deepening eruptions, providing insights into the post-caldera evolution of the system. Hunga volcano (Kingdom of Tonga, Southwest Pacific) is a large mainly submarine edifice that produced a series of caldera-forming eruptions -900 years ago. Since then, numerous smaller-scale subaerial and submarine eruptions occurred, the most recent forming new islands in 2009 and 2014/15. Pyro-clastic deposits associated with these latest eruptions have identical (range - 0.1 wt% of all major oxides) andesitic composition that overlap with the primitive end of the slightly wider compositional range of the caldera-forming episodes. Texturally simple plagioclase, clinopyroxene and orthopyroxene phenocrysts in pre-, syn- and post-caldera pyroclasts point to a single shallow storage reservoir at 5-8 Ian depth. Lack of complex zonation indicates that this reservoir is constantly resupplied by low-flux inputs of basaltic andesite magma and is large enough that convective mixing rapidly homogenises new inputs. The reservoir feeds intermittent, low-intensity, post-caldera volcanism with constant andesite composition, driven possibly by magmatic overpressure and "leakage" of gas-rich magma pockets around the edges of the caldera. More primitive and compositionally variable basaltic andesites formed a lava-dominated edifice prior to the caldera-forming event. This suggests a causal link between magma supply dynamics and caldera priming relating to the maturing of the plumbing system and formation of a sustained subvolcanic andesite magma reservoir.
查看更多>>摘要：It was recently shown that partial melting of carbonated metapelites, subducted to a depth of 200 km, yields the formation of two immiscible melts, CO2-bearing phonolitic and K-rich carbonate. These melts resemble silicic and low-Mg carbonatitic melt inclusions in diamonds from kimberlites and placers worldwide. Here we studied the interaction of these melts with natural garnet lherzolite at 6 GPa. We found that the CO2-bearing phonolite melt reacts with peridotite consuming olivine to produce orthopyroxene and garnet, while K2O and CO2 enter carbonate melt. The latter has Ca# 24-29 and appears in equilibrium with garnet lherzolite. The SiO2 content in the carbonate melt varies from 2 to 18 wt% as temperature increases from 1200 to 1500 °C. Our results imply that the slab-derived immiscible silicic and low-Mg carbonatitic melts react with peridotitic mantle producing the high-Mg carbonatitic melt, which makes up the majority of carbonatitic inclusions in diamonds. Thus, the melt entrapped by diamonds may decipher genetic signatures of different mantle lithologies: silicic and low-Mg carbonatitic inclusions correspond to eclogite or recycled pelite, while high-Mg carbonatitic inclusions correspond to peridotite.
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