MKED1: A new titanite standard for in situ analysis of Sm–Nd isotopes and U–Pb geochronology

doi:10.1016/j.chemgeo.2016.01.002

Chemical Geology

Volume 425, 1 May 2016, Pages 110-126

https://doi.org/10.1016/j.chemgeo.2016.01.002 Get rights and content

Highlights

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We describe a new titanite standard for in situ U–Pb dating, and trace element and Sm–Nd isotope analysis.
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MKED1 titanite is 1518 Ma, has a high degree of chemical and isotopic homogeneity, and lacks mineral inclusions.
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We use MKED1 as a standard to date titanite intergrown with sulfides from a Cu-mineralised skarn from the Mount Isa Inlier.
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MKED1 is also used as a standard to date titanites from zircon-deficient Oligiocene tuffs from the East African Rift System.

Abstract

Titanite (sphene) has great potential as a petrogenetic indicator and mineral geochronometer, as it can host high concentrations of a range of trace elements and it occurs in diverse rock types. In this paper we describe a natural titanite standard material that may be used to calibrate chemical and isotopic analysis of titanite of varying age and origin. Through comprehensive bulk analysis of mm-size crystal fragments and in situ microanalysis, we show that the titanite, named MKED1, is largely free of inclusions and is homogenous at the level of analytical precision for major element, U–Pb isotope and Sm–Nd isotope composition. There is some minor zoning in trace element composition, but these zones are easily recognised using backscattered electron imaging and the trace element concentrations of each of these zones are also very homogenous. MKED1 has relatively high levels of high field strength elements (Nb, Ta, Zr, Hf, Sn), rare earth elements (REE), Th, U, and radiogenic Pb, but very low levels of common Pb. Uranium–Pb isotope data show MKED1 to be concordant with ²⁰⁶Pb/²⁰⁷Pb, ²⁰⁷Pb/²³⁵U, and ²⁰⁶Pb/²³⁸U ages (ID-TIMS) of 1521.02 ± 0.55 Ma, 1518.87 ± 0.31 Ma, and 1517.32 ± 0.32 Ma, respectively. Cross calibration with other titanite standards demonstrates that MKED1 can be used as a primary standard for determining U–Pb ages of titanite ranging in age from Precambrian to Neogene.

We show that MKED1 is also suitable as a Sm–Nd isotope reference material due to its relatively high REE concentrations and homogenous ¹⁴⁷Sm/¹⁴⁴Nd (0.127) and ¹⁴³Nd/¹⁴⁴Nd (0.5116303 ± 0.0000025) content. Isotopic homogeneity is demonstrated through extensive in situ microanalysis, which returned an average initial εNd value of − 6.30 ± 0.65 that is comparable to the value obtained by TIMS of − 6.13 ± 0.05.

We suggest that MKED1 can be employed as a U–Pb isotope and Sm–Nd isotope reference material for in situ or bulk analytical methods, including techniques that allow simultaneous collection of multiple elemental and/or isotopic data sets in situ. MKED1 may also be used as a standard for in situ trace element microanalysis on the provision that locations for analytical sampling are selected with consideration to grain-scale elemental zoning. To demonstrate the potential value of titanite analysis for resolving geological problems we present case studies from two very different geological settings; the first examines the timing and origin of Cu–Au-REE skarn mineralisation from the Mount Isa Inlier, Australia, and the second study investigates the timing and origin of rift related volcanism and sedimentation in the western branch of the East African Rift System.

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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Monazite and titanite U[sbnd]Pb ages of granitoids from the North Qinling Terrane: Implications for the subduction of the Shangdan Ocean and accretionary processes
2023, Lithos
The early Paleozoic granitoids of the North Qinling Terrane have been widely viewed as the products of an Andean-type margin related to the long-lasting subduction of the Proto-Tethys (Shangdan) ocean. However, the spatial and temporal distributions of the arc granitoids coupled with subduction-accretion processes are still a subject of debate. Here, we report LA-ICP-MS UPb monazite and titanite ages and trace element concentrations for granites and a paragneiss from the North Qinling Terrane. The results, together with data in the literatures, are used to reconstruct the magmatic evolution related to the subduction of the Shangdan ocean during the early Paleozoic. As revealed by UPb geochronology, subduction-related magmatism in the southern belt of the North Qinling magmatic arc (NQMA) was most active between 423 and 404 Ma, and therefore younger and shorter than previously speculated. The ages are in agreement with a metamorphic monazite UPb age (415 Ma) obtained from the paragneiss, suggesting that crustal anatexis of the NQMA was contemporaneous with the magmatism. Overall, magmatism in the NQMA moved progressively from north to south. This southward propagation is consistent with roll-back of the Shangdan oceanic slab and transition from an oceanic arc into a back-arc basin setting. The timing of magmatism and migmatization as well as high-temperature granulite metamorphism in the NQMA signifies that the Shangdan ocean retreated to the North Qinling Unit before c. 423 Ma. Monazites from NQMA generally show HREE depletion and negative Eu anomalies, and some display the REE tetrad effect. These geochemical features indicated that the NQMA granites were highly fractionated. In addition, the abrupt changes in REE compositions displayed by titanites probably record a mixing event with a mafic magma along the Shangdan suture zone.

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MKED1: A new titanite standard for in situ analysis of Sm–Nd isotopes and U–Pb geochronology

Highlights

Abstract

Introduction

Section snippets

Geological occurrence of MKED1

Electronprobe microanalysis

BSE imaging and major element composition

A potential new standard for in situ analysis of titanite

Conclusions

Acknowledgements

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