Chromite-induced magnesium isotope fractionation during mafic magma differentiation

To better understand the mechanism of Mg isotopic variation in magma systems, here we report high precision Mg isotopic data of 17 bulk rock samples including dunite, clinopyroxenite, hornblendite and gabbro and 10 pairs of dunite-hosted olivine and chromite separates from the well-characterized Alaskan-type Xiadong intrusion in NW China, which formed by continuous and high degree of lithological differentiation from mafic magmas. Chromite separates have highly variable δ^(26)Mg values from -0.10‰ to 0.40‰, and are consistently heavier than coexisting olivine separates(-0.39‰ to -0.15 T‰). Both mineral δ^(26)Mg values and the degrees of inter-mineral fractionation are well correlated with geochemical indicators of magma differentiation, indicating that these inter-sample and inter-mineral Mg isotope fractionations are caused by magma evolution. The δ^(26)Mg values range from -0.20‰ to -0.02‰ in the dunite,-043‰ in the clinopyroxenite,-043‰ to -0.28‰ in the hornblendite, 0.18 T‰ in the chromite-bearing hornblendite, and -0.56 T‰ to -0.16‰ in the gabbro. The Mg isotopic variations in different types of rocks are closely related to fractional crystallization and accumulation of different proportions of oxides vs. silicates. Chromite crystallization and accumulation is the most important factor in controlling Mg isotope fractionation during the formation of the Xiadong intrusion. Compared to basaltic and granitic magmas, differentiation of the Alaskan-type intrusions occurs at a relatively high oxygen fugacity, which favors chromite crystallization and consequently significant Mg isotope fractionations at both mineral and whole-rock scales. Therefore, Mg isotope systematics can be used to trace the degree of magma differentiation and related-mineralization.

Siderophile Element Compositions of Pyroxenes in HED Meteorites: Implications for the Differentiation of Magma Ocean on Vesta

1 Introduction The howardite,eucrite and diogenite(HED)meteorites are ultramafic and mafic igneous rocks and impact-engendered breccias derived from a thoroughly differentiated asteroid 4 Vesta.Diogenites include dunites,

Generation of andesite through partial melting of basaltic metasomatites in the mantle wedge:Insight from quantitative study of Andean andesites

Continental crust in average exhibits a similar composition in both major and trace elements to andesites along active continental margins.For this reason,andesitic magmatism above oceanic subduction zones is considered to have played a key role in the generation of continental crust along convergent plate boundaries.With respect to the origin of andesites themselves,however,there is still a hot debate on how they have acquired their geochemcial compositions.The debate is mainly centralized on the relative contributions of crustal contamination,magma differentiation and source mixing,which reaches an impasse in the past decades.The essential reason for this kind of debates is that these three mechanisms only can account for some of the geochemical observations for andesites,leading to insufficient discrimination among them.Nevertheless,the geochemical features of andesites are primarily controled from early to late by the composition of their source rocks in addition to partial melting and magma differentiation processes.If source mixing and partial melting processes in the early stage of andesite magmatism can account for the first-order geochemical features of andesites,there is no need to invoke the late processes of magma differentiation and crustal contamination for andesite petrogenesis.This is illustrated by quantitative forward modeling of the geochemical data for Quaternary andesites from the Andean arc in South America based on an integrated interpretation of these data.The modeling has run with four steps from early to late:(1)dehydration of the subducting oceanic crust at forearc depths;(2)partial melting of the subducting terrigenous sediment and altered oceanic basalt at subarc depths to produce hydrous felsic melts;(3)the generation of basaltic metasomatites(e.g.,Si-excess pyroxenite)in the mantle wedge through reaction of the mantle wedge peridotite with large amounts of the hydrous felsic melts;(4)the production of andesitic melts by partial melting of the basaltic metasomatites.The results not only testify the hypothesis that the trace element and radiogenic isotope compositions of andesites can be directly produced by the source mixing and mantle melting but also demonstrate that partial melting of the basaltic metasomatites can reproduce the lithochemical composition of andesites.The compositional variations of Andean andesites within a single volcanic zone and among different volcanic zones can be explained by incorporating different amounts of heterogeneous hydrous felsic melts into their mantle sources,followed by different degree of partial melting under different pressures and temperatures.Therefore,the source mixing and partial melting processes at subarc depths can account for the first-order geochemical features of Andean andesites.In this regard,it may be not necessary for andesite petrogenesis to invoke the significant contributions from the processes of magma differentiation and crustal contamination.

Lunar surface mineralogy using hyperspectral data： Implications for primordial crust in the Earth-Moon system

Mineralogy of the Lunar surface provides important clues for understanding the composition and evo- lution of the primordial crust in the Earth-Moon system. The primary rock forming minerals on the Moon such as pyroxene, olivine and plagioclase are potential tools to evaluate the Lunar Magma Ocean （LMO） hypothesis. Here we use the data from Moon Mineralogy Mapper （M3） onboard the Chandrayaan- 1 project of India, which provides Visible/Near Infra Red （NIR） spectral data （hyperspectral data） of the Lunar surface to gain insights on the surface mineralogy. Band shaping and spectral profiling methods are used for identifying minerals in five sites： the Moscoviense basin, Orientale basin, Apollo basin, Wegener crater-highland, and Hertzsprung basin. The common presence of plagioclase in these sites is in conformity with the anorthositic composition of the Lunar crust. Pyroxenes, olivine and Fe-Mg-spinel from the sample sites indicate the presence of gabbroic and basaltic components. The compositional difference in pyroxenes suggests magmatic differentiation on the Lunar surface. Olivine contains OH/H20 band, indicating hydrous phase in the primordial magmas.

Redox processes in subduction zones:Progress and prospect

Oxygen fugacity(fO_(2))is an intensive variable that describes the redox state of a system.By controlling the valence state of multivalent elements,fO_(2)affects the stability of iron-bearing minerals,dominants the species of volatile elements(e.g.,carbon and sulfur),and controls the partitioning behaviors of multivalent elements(e.g.,iron,vanadium,cerium,europium).Thus,fO_(2)plays a key role in understanding the generation and differentiation of arc magmas,the formation of magmatic-hydrothermal deposits,and the nature of magmatic volatiles.Subduction zones are an important site for arc magmatism and fluid action,and the study of redox processes is indispensable in subduction zone geochemistry.In this paper,we first introduce the concept,expression,and estimation methods of fO_(2).Then we retrospect the history and progress about the oxidation state of the metasomatized mantle wedge,summarize the redox property of slab-derived fluids,and review the latest progress on redox evolution of arc magmas during magma generation and differentiation.The main conclusions include:(1)despite its wide variation range,fO_(2)of the mantle wedge is generally higher than that of the oceanic mantle;(2)the redox property of the subducting slab-derived fluids is still controversial and the mechanism for the oxidization of the mantle wedge remains unclear;(3)how the fO_(2)varies during the generation and differentiation of the arc magmas is debated.We propose that the crux in deciphering the oxidization mechanism of the mantle wedge is to determine the mobility of iron,carbon and sulfur in subducting slab-derived fluids(especially solute-rich fluid or supercritical fluid);the key in understanding the redox evolution during arc magma generation and differentiation is to determine the partition coefficients of Fe^(3+)and Fe^(2+)between ferromagnesian minerals and silicate melts.