Rapid determination of the original boron isotopic composition from altered basaltic glass by in situ secondary ion mass spectrometry

Abstract

The determination of the original boron (B) isotopic composition in altered basaltic glass is important to characterize magma generation and evolution in the mantle. Traditional whole-rock analysis of the B isotopic composition of basaltic glass requires time-consuming pretreatment for removing the altered section. Here, a protocol of in situ B isotopic analysis of basaltic glass by secondary ion mass spectrometry was developed using an accessible reference material BCR-2G which was first ascertained as a standard material for in situ B-isotopic analysis, whose B isotopic value (δ¹¹B = −5.44‰; 2σ = 0.55‰) was determined by solution-based multi-collector-inductively coupled plasma-mass spectrometry in this study. Trans-sectional analysis of both slightly and extensively altered grains was performed from the cores outward to the rims (palagonite) by this in situ method to verify its ability in rapidly extracting information of the original (unaltered) B composition from natural samples. The δ¹¹B values of the pristine (unaltered) melt ranging from −8.99‰ to −12.00‰ could be rapidly determined from the slightly altered grain, which is distinguishable from those of its transition (−3.14‰ to +9.56‰) and palagonite (+5.04‰ to +8.56‰) sections. Compared with the slightly altered one, the extensively altered grain has just the transition and palagonite sections without the pristine section. Therefore, in situ B isotopic profiling analysis of basaltic glass is beneficial for rapidly obtaining original magma B information from a single natural altered grain.