Abstract
ABSTRACT: This contribution describes the successful implementation of in situ Sr isotope analyses by LA-MC-ICP-MS at the CPGeo-USP. The choice for an analytical configuration using measurements of half-masses allows the accurate assessment of lanthanide interferences, permitting the determination of Sr isotopes in important REE-rich accessory phases, such as apatite. Likewise, the on-peak-zero method effectively corrects the background contribution (both from Kr and residual Sr contributions from previous ablations) to the signals of the unknown samples. The analytical campaigns resulted in an accuracy, in respect to reference TIMS values, better than 57 ppm (~ ±0.000057 2σ SD) for a modern coral and the Batjberg clinopyroxene which impart significant quality to our data. Similarly, the majority of the stable Sr isotope ratios are close to the accepted values, which also confirms the effectiveness of the method. The achieved accuracy allows the identification and investigation of spatially-controlled isotopic heterogeneities on the micrometric scale in several Sr-rich minerals (apatite, carbonates, plagioclase, and clinopyroxene) with important implications to the understanding of relevant geochemical processes, particularly AFC, source geochemical heterogeneities and magma-mixing.
Highlights
The in situ analyses of Sr isotopes have experienced a systematic improvement over the last two decades
Best accuracy levels obtained via LA-MC-ICP-MS are around tens of Sr ppm and, under optimal analytical conditions, resultant Sr isotope data are comparable to those obtained by thermal ionization mass spectrometry (TIMS)
We present detailed data reduction strategies and petrological applications based on two analytical campaigns dedicated to the determination of Sr isotopes in widely used standards and on apatite, plagioclase and carbonate crystals from alkaline rocks, granites and basaltic samples exposed in southeastern Brazil and Central Patagonia, Argentina
Summary
The in situ analyses of Sr isotopes have experienced a systematic improvement over the last two decades. A continuous cast for analytical improvement has aimed the identification of physical factors controlling data quality Among such factors the (a) isobaric and molecular interferences, (b) instrument mass fractionation, (c) background levels, and (d) counting statistics are the main concerns associated to the acquisition of accurate Sr isotope data (Vroon et al 2008). An unfortunate consequence of such rare isotope homogeneity on the mineral scale is the scarcity of matrix-matching natural standards, especially for plagioclase In this contribution, we adopt the same strategy of several reference studies dedicated to in situ Sr isotopes in different minerals and use a REE- and Rb- poor present-day marine carbonate (Sr: 1000 – 2000 ppm) as an “in-house” standard to check for external reproducibility (Bizarro et al 2003, Schmidberger et al 2003, Ramos et al 2004). The chosen materials allow tracking of the contribution from different sources (enriched mantle, depleted mantle, and crust) and processes (equilibrium fractional crystallization, AFC and magma mixing), and, selected applications are intented to give an idea about the problems that can be addressed using the described technique
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