Silicate minerals

About: Silicate minerals is a research topic. Over the lifetime, 1794 publications have been published within this topic receiving 67064 citations.

U-Th-Pb partitioning behavior during partial melting in the upper mantle: Implications for the origin of high Mu Components and the “Pb Paradox”

Abstract: The partitioning behavior of U, Th, and Pb in the upper mantle has been investigated through the analysis of ultramafic nodules found in lava flows at two localities in the southwestern United States. Secondary (i.e., pedogenic) components were distinguished from primary (igneous) components on the basis of Nd and Sr isotope ratios. Most of the primary components have Nd, Pb, and Sr isotope ratios within the ranges observed in oceanic basalts. The 238U/204Pb (mu) ratios of residual (i.e., type IA) clinopyroxene separates analyzed in this study are generally high (60–120) and show an excellent positive correlation with 147Sm/144Nd ratios. Because the latter are consistently greater than the chondritic ratio and because clinopyroxene contains most of the U in the nodules, we suggest this mineral is a “high-mu” component in the upper mantle. Calculated “whole rock” U contents, based on analyses of purified separates of the major silicate minerals, are consistent with recent estimates for mid-ocean ridge basalt sources. Clinopyroxene/melt partition coefficients, based on the assumption that the lavas bearing the nodules are the melts with which the nodule minerals last equilibrated, decrease in the order DUs/l>DThs/l>DPbs/l, consistent with available experimental data. Sulfide is inferred to be a major Pb-bearing phase in the nodules. Bulk partition coefficients for Pb, Th, and U during melting are inferred to depend primarily on the abundance of clinopyroxene and sulfide in the residual mantle. On the basis of the model presented, the “Pb paradox” is explained as a direct consequence of melt depletion from the upper mantle.

Origin of the suppressed matrix effect for improved analytical performance in determination of major and trace elements in anhydrous silicate samples using 200 nm femtosecond laser ablation sector-field inductively coupled plasma mass spectrometry

Abstract: We have tested an ultraviolet (200 nm) femtosecond laser ablation (200FsLA) sector-field inductively coupled plasma mass spectrometry (SF-ICPMS) system for major and trace element analyses in anhydrous silicate glasses and minerals. Use of the 200FsLA minimized the matrix effect by 50% compared to that of a 193 nm nanosecond excimer LA. The origin of this improvement was identified as the suppression of ‘melting point (MP)-induced’ element fractionation at the LA site due to a decreased thermal effect of the 200FsLA. Sensitivity enhancement in elements with high first ionization energy remained for the basalt aerosols relative to silica-rich aerosols. This is interpreted as being due to the higher thermal conductivity of the basalt aerosols in the inductively coupled plasma enhanced ionization, which is essentially controlled by the first ionization energy of an element. This was confirmed by simulation using Saha's equation and by the analytical data after reduction of the MP-induced fractionation at the LA site. Accurate determination of trace elements (within 5% of accepted values) was achieved for MPI-DING glasses ranging from komatiite to rhyolite, using a single basalt glass BHVO-2G as the calibration standard. This method is also applicable to anhydrous silicate minerals such as plagioclase, pyroxenes, and garnet. However, SRM610 glass, which has a very different matrix compared to BHVO-2G, is preferred for zircon. Apart from this exception, the proposed method requires no external analytical techniques when the amounts of unmeasured elements in the materials, such as halogens or water, are negligibly low, which is the case for many geological materials.