Abstract
The oxygen (O) and neodymium (Nd) isotopic composition of monazite provides an ideal tracer of metamorphism and hydrothermal activity. Calibration of the matrix effect and monitoring of the external precision of monazite O–Nd isotopes with microbeam techniques, such as secondary ion mass spectrometry (SIMS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICPMS), require well-characterized natural monazite standards for precise microbeam measurements. However, the limited number of standards available is impeding the application of monazite O–Nd isotopes. Here, we report on the RW-1 monazite as a potential new working reference material for microbeam analysis of O–Nd isotopes. Microbeam measurements by electron probe microanalysis (EPMA), SIMS, and LA-MC-ICPMS at 10–24 µm scales have confirmed that it is homogeneous in both elemental and O–Nd isotopic compositions. SIMS measurements yield δ18O values consistent, within errors, with those obtained by laser fluorination techniques. Precise analyses of Nd isotope by thermal ionization mass spectrometry (TIMS) are consistent with mean results of LA-MC-ICPMS analyses. We recommend δ18O = 6.30‰ ± 0.16‰ (2SD) and 143Nd/144Nd = 0.512282 ± 0.000011 (2SD) as being the reference values for the RW-1 monazite.
Highlights
Monazite is a light rare earth element (LREE)-bearing phosphate mineral and has an extremely variable composition, e.g., monazite-(Ce), monazite-(La), monazite-(Nd), and monazite-(Sm) [1].It is a ubiquitous accessory mineral in peraluminous granite, pegmatite, metapelite, metapsammite, carbonatite, and hydrothermal veins [1]
This study reports a comprehensive analysis of the elemental and O–Nd isotopic compositions of the RW-1 monazite, which has been reported as a reference material for U–Th–Pb microbeam dating [17]
The major element composition of RW-1 fragments, including P, La, Ce, Pr, Nd, Sm, Gd, Dy, Y, Si, Ca, Th, U, and Pb, was determined using a CAMECA SXFiveFE electron probe microanalysis (EPMA) equipped with thallium acid phthalate (TAP), and large LiF and polyethylene terephthalate (PET) diffraction crystals, with an accelerating voltage of 10 kV, beam current of 200 nA, defocused beam diameter of 10 μm, 20–30 s counting time per element (120 s for Pb), and a total acquisition time of ~6 min
Summary
Monazite is a light rare earth element (LREE)-bearing phosphate mineral and has an extremely variable composition, e.g., monazite-(Ce), monazite-(La), monazite-(Nd), and monazite-(Sm) [1]. It is a ubiquitous accessory mineral in peraluminous granite, pegmatite, metapelite, metapsammite, carbonatite, and hydrothermal veins [1]. It occurs as a detrital mineral in many clastic sedimentary rocks. Monazite is sensitive to relatively low-grade metamorphism (down to greenschist facies) [2].
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