Geochemistry of volcanic glass from Mahanadi offshore region,eastern continental margin of India:Constraints on the contribution of latest Toba super-eruption

The tephra layers in multiple sediment cores from the offshore region of the Mahanadi basin in the northern Bay of Bengal were investigated for possible volcanic sources. The glass shards from those tephra layers were studied for size distribution, texture, and elemental geochemistry to establish chronostratigraphic markers for regional and global Quaternary correlation. The textural features of fine-grained(silty) volcanic glasses suggest the distal source of these tephra deposits. Major element composition with elevated SiO_(2) contents ranging between75%–76% and dominance of K_(2)O(> 4.5%) over CaO(< 0.9%) suggest ashes have originated from siliceous rhyolitic melts, similar to the petrographic composition of tephra from the Toba volcano. The bulk trace element compositions of the same glass shards were comparable with those reported in the youngest Toba tephra reported elsewhere. Likewise, the LREE-dominated chondrite normalized REE profiles of tephra from the Mahanadi basin closely resemble the characteristic REE patterns in Toba ash from other parts of the Indian Ocean and thus confirmed the contribution of the youngest Toba super-eruption for this ash layers.

Petrogenesis of the Lunar volcanic glasses and Mg-suite:constraints on a post-magma-ocean cumulate overturn

The lunar volcanic glasses and Mg-suite rocks represent the early enigmatic episodes of lunar magmatism.Due to the gravitational instability of the Fe-Ti enriched(±KREEP)layer,which is formed at the later stage of fractional crystallization,a post-magma-ocean cumulate overturn occurred contemporaneously or near-contemporaneously with the lunar magma ocean(LMO)solidification.The radioactive elements within the KREEP layer were transferred downward and provided continuous energy for the partial melting of the Moon’s interior.The melt from the Moon’s interior and those from decompression melting,in turn,provide source magma for the origin of lunar volcanic glasses and Mg-suite.However,experimental and theoretical studies on the formation process of lunar volcanic glasses and Mg-suite show that the origin of their parental magma is poorly constrained,which largely depends on the initial depth and composition of the LMO.This review examines the mineralogy,petrogenesis,and distribution of lunar volcanic glasses and Mg-suite.Combining with existing models,we constrain the degree,distribution,and timescale of lunar mantle overturn and explore their relationship with later stages of LMO differentiation.We propose an updated chemical composition of the lunar interior,which provides a useful reference for estimating the bulk composition and early differentiation of the Moon and the early Earth.

Phillipsite in deep-sea sediments

A phillipsite as one of oceanic authigenic zeolites gradually forms from alteration of volcanic glass under low temperature and pressure. In this paper physical property and chemical and mineralogical compositions ofsome zeolite grains collected from the study area in the North Pacific Ocean are first comprehensively studied, andthey are defined as phillipsite. Then, statistics of grain percentage of the phillipsite widely-distributed at 35 sitesin the study area in the deep-sea sediments were made ; it has been found that they mainly occur in the two sediments: red clay and calcareous ooze, and are rich in the former. Finally, the authors preliminarily discuss the formation process of the phillipsite on the basis of this study results, together with the view points of predecessors.