MK-1 Orthopyroxene—A New Potential Reference Material for In-Situ Microanalysis

Lihui Jia,Qian Mao,Yi Chen,Liangliang Huang,Shitou Wu,Di Zhang,Jiangyan Yuan,Bin Su

doi:10.3390/min11121321

Abstract

Orthopyroxene, an important phase in mantle-derived rocks, has become a powerful tool to unravel mantle nature and magma processes. However, the applications have been hindered by the lag in the development of analytical techniques, such as shortage of reference materials. Orthopyroxene grains derived from an ultramafic intrusion at the Mogok metamorphic belt (Myanmar) were evaluated for the potential use of orthopyroxene as a reference material for in-situ microanalysis. Approximately 20 g of 0.5–3 mm pure orthopyroxene grains were separated under binocular microscope and analyzed using EPMA, LA-ICPMS, and bulk analytical methods (XRD, XRF, and solution-ICPMS) for major and trace elements at four institutions. Eleven core-to-rim profiles carried out using EPMA and twelve core-to-rim profiles determined using LA-ICPMS suggest that MK-1 orthopyroxene grains are sufficiently homogeneous, with RSD < ±2% (1σ) for major elements (Mg, Si, and Fe) and RSD < ±10% (1σ) for trace elements (Na, Al, Ca, Ti, Cr, Co, Zn, Ni, Mn, Sc, and V). In addition, the composition of MK-1 orthopyroxene was also measured by XRF and solution-ICPMS measurements in two different laboratories, to compare with the results measured using EPMA and LA-ICPMS. The results indicated a good agreement with RSE < ±2% (1σ) for major elements and RSE < ±5% (1σ) for most trace elements, except for Na (±9.73%) and Ti (±6.80%). In an overall assessment of these data, MK-1 orthopyroxene can be considered as a reference material for in-situ microanalysis, which would provide solid trace elements data for a better understanding of mantle source and magmatic evolution.

Highlights

Orthopyroxene is the second most common phase in the upper mantle [1], and present as a phenocryst during crystallization of a wide range of melts, ranging from ultramafic boninite, to basalt, to andesite, rhyolite and granite [2,3,4]
The detection limits were 5 μg g−1 for Ti, 14 μg g−1 for Mn, 12 μg g−1 for Co, 23 μg g−1 for Ni, 41 μg g−1 for Zn, 6 μg g−1 for Ca, 6 μg g−1 for Al, and 11 μg g−1 for Cr based on a 3σ estimate of the measured background variance (Table 1)
Clean orthopyroxene grains were ground to powder and analyzed using X-Ray powder diffraction (XRD) for purity, X-ray fluorescence (XRF) spectrometry for major elements and solution ICP-MS for trace elements, respectively

Summary

Introduction

Orthopyroxene is the second most common phase in the upper mantle [1], and present as a phenocryst during crystallization of a wide range of melts, ranging from ultramafic boninite, to basalt, to andesite, rhyolite and granite [2,3,4]. Despite its simple major element composition, orthopyroxene contains a number of petrogenetically significant trace elements, such as Al, Ca, Na, Ti, Cr, Zn, Co, and Ni [7,8]. Despite its importance for petrological investigations, there is still a lack of wellcharacterized reference material available for validation of results obtained from high spatial resolution analysis of orthopyroxene. Trace element analysis using EPMA is becoming the backbone of numerous geochemical studies due to its high resolution, low-cost, and non-destructive characteristics [20,21,22], which requires a secondary reference for quality control and method validation purposes. MK-1 orthopyroxene is well suited for trace element validation because it is significantly homogeneous and contains relatively high mass fractions of trace elements. Our results would provide more solid data support for orthopyroxene compositions, and expand the applications of orthopyroxene in magmatic processes, temperature and pressure conditions, and lithospheric evolution

Sample Description and Preparation

Bulk Techniques

Comparison of Techniques and Recommended Values

Conclusions