MKED1: A new titanite standard for in situ analysis of Sm–Nd isotopes and U–Pb geochronology
Introduction
Due to advances in in situ microanalysis techniques and understanding of phase petrology, the geochemical and isotopic compositions of accessory minerals are now widely used to unravel complex, and often cryptic, petrological processes and geological histories. Titanite is a common accessory mineral in a range of igneous and metamorphic rocks (e.g. Frost et al., 2000), and is often associated with hydrothermal alteration and ore formation processes (Li et al., 2010, Che et al., 2013). The crystal structure of titanite can accommodate high levels of trace elements including rare earth elements (REE), F, Nb, Ta, Zr, Hf, U, Th, Sr, Sn and Pb (Tiepolo et al., 2002), so it has potential to be used for geochemical and isotopic fingerprinting of geological processes, and as a U–Pb geochronometer (Frost et al., 2000, Amelin, 2009). Titanite is stable over a large range of P–T conditions (to > 1000 °C in mafic rocks, Romer and Rötzler, 2003; to > 3.5 GPa in calc-silicate rocks, Rubatto and Hermann, 2001), and has a high closure temperature for the U–Pb system (~ 700 °C or higher; Cherniak, 1993, Frost et al., 2000, Spencer et al., 2013, Stearns et al., 2015), but it is relatively reactive during metamorphism, hydrothermal alteration, and weathering and erosion. These characteristics offer advantages over other mineral geochronometers such as zircon, in that titanite may be particularly useful for dating metamorphism and hydrothermal alteration, in addition to igneous crystallisation (Frost et al., 2000). Titanite trace element chemistry can also inform on the temperature and pressure of crystallisation (Hayden et al., 2008), oxygen fugacity conditions (King et al., 2013) and fractionation processes (Piccoli et al., 2000), while Sm–Nd and Rb–Sr isotope systematics of titanite may be used to track fluid, melt or rock sources (Lucassen et al., 2011, Hammerli et al., 2014a).
New analytical developments, such as laser ablation split stream (LASS) analysis (Yuan et al., 2008, Kylander-Clark et al., 2013, Fisher et al., 2014), now make it possible to simultaneously collect multiple isotopic and elemental datasets from the same micro-analytical volume of a phase. These techniques have been used to effectively couple in situ U–Pb dating and trace element analysis, with Lu–Hf isotope analysis of zircon (Fisher et al., 2014), and Sm–Nd isotope analysis of monazite (Goudie et al., 2014). In situ Sm–Nd isotope analysis of titanite also holds great promise as an isotopic tracer (Fisher et al., 2011, Hammerli et al., 2014a), but so far has not been directly coupled with U–Pb dating and trace element analysis.
A conventional requirement of these analytical methods is concurrent analysis of matrix-matched mineral standards that have been well characterised for their elemental and isotopic composition. For analysis of zircon and monazite, suitable standards are widely available (e.g. 91500 and Plesovice zircons, and Manangoutry and Namaqualand monazites), whereas there is presently no well characterised titanite standard that is suitable for combined trace element, Sm–Nd isotope and U–Pb dating analysis. Titanite standards currently employed for U–Pb dating include Khan (ca. 520 Ma; Kinny et al., 1994, Heaman, 2009), OLT1 (1014.8 ± 2.0; Kennedy et al., 2010), and BLR-1 (1047.1 ± 0.4 Ma; Aleinikoff et al., 2007) but like many natural titanites, these grains contain appreciable contents of common lead and/or mineral inclusions, which can complicate their use for microanalysis, particularly by laser ablation techniques that sample a large volume of material relative to SIMS. Fisher et al. (2011) have shown the Hondo Canyon titanite to be a promising reference material for in situ Sm–Nd isotope analysis, but the young age (ca. 30 Ma) and relatively heterogeneous trace element composition (Fisher et al., 2011) make it less suitable as a general standard for U–Pb dating or trace element analysis. Synthetic glasses of titanite composition are well suited to calibration of trace element concentrations (Klemme et al., 2008), but are yet to be developed for use as radiogenic isotope standards. Clearly, the availability of a single, high quality titanite reference standard for trace element and isotopic analysis would greatly assist researchers to take advantage of the chemical and isotopic archives of geological processes and events that are recorded by titanite.
Here we describe a new titanite standard, labeled MKED1, that may be used to calibrate U–Pb isotope ages, Sm–Nd isotope compositions and some trace element concentrations of titanite in situ. Our analysis demonstrates that MKED1 is almost completely free of mineral inclusions and has a high degree of elemental and isotopic homogeneity, including very low levels of common Pb. We suggest that MKED1 is fit for purpose for microanalysis techniques, as demonstrated through cross-referencing with established titanite standards, and application to specific case studies.
Section snippets
Geological occurrence of MKED1
MKED1 titanite derives from euhedral, double terminated crystals measuring up to 8 cm in length (Fig. 1B) associated with coarse pink calcite and minor diopside in a vein that cuts banded diopside-K-feldspar-scapolite skarn rocks of the Elaine Dorothy Cu–Au–REE prospect of the Mount Isa Inlier, Queensland, Australia (Fig. 1A; Page, 1983, Oliver et al., 1999; see Section 5.2.1, below). Both the Elaine Dorothy prospect and nearby Mary Kathleen REE-U deposit (Fig. 1A) are hosted within
Electronprobe microanalysis
Backscattered electron imaging (BSE) and quantitative major element analyses of polished fragments of MKED1 and OLT1 titanite were completed using a JEOL JXA8200 superprobe housed at the Advanced Analytical Centre (AAC), James Cook University (JCU), Townsville. Quantitative major element analysis of minerals was conducted by wavelength dispersive spectrometry (WDS), using a 20 nA beam defocused to 5 μm, with acceleration voltage set to 15 kV. All analyses were standardised using well-characterised
BSE imaging and major element composition
Investigation of multiple mm-sized chips of MKED1 crystals using BSE imaging and qualitative energy dispersive spectrometry reveals that most of the material is homogenous and free of inclusions (Fig. 3). Rare sub-micrometer grains that are likely to be REE-carbonates and allanite were only found within the outer ~ 200 μm rim zone of the crystals. No regular zoning of the crystals was found, although irregular zones with significantly brighter BSE intensity (Fig. 3b) were recognised in many of
A potential new standard for in situ analysis of titanite
Our analyses indicate that the MKED1 titanite crystals have a high degree of homogeneity in major element and Sm–Nd and U–Pb isotope composition. Extensive BSE imaging and laser ablation analysis reveal that MKED1 is largely free of mineral inclusions and lacks significant zoning, with the exception of some BSE bright zones with relatively high trace element contents (MKED1-TEEZ). Nonetheless, the respective trace element compositions of regular MKED1 and MKED1-TEEZ are also remarkably uniform (
Conclusions
Through comprehensive bulk sample and in situ microanalysis, we demonstrate that MKED1 titanite has a high degree of chemical and isotopic homogeneity, lacks mineral inclusions, and has relatively high concentrations of REE, Th, U, and radiogenic Pb, and low levels of common Pb. We suggest that MKED1 is suitable for use as a primary calibration standard for U–Pb dating of titanite and can be used as a standard for in situ trace element microanalysis and Sm–Nd isotope analysis and, hence, may be
Acknowledgements
We thank Allen Kennedy and Daniela Rubatto for providing us with samples of OLT1 and Khan titanite standards, respectively, and Kevin Blake for assistance with the EPMA analyses. The manuscript was improved thanks to the comments of two reviewers, and editorial handling of Klaus Mezger. This work was funded through an ARC Future Fellowship (FT 120100198) to Spandler.
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