Spandler, Carl, Hammerli, Johannes, Sha, Peng, Hilbert-Wolf, Hannah, Hu, Yi, Roberts, Eric, and Schmitz, Mark (2016) MKED1: a new titanite standard for in situ analysis of Sm–Nd isotopes and U–Pb geochronology. Chemical Geology, 425. pp. 110-126.

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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 paperwe 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 showMKED1 to be concordant with ²⁰⁶Pb/²⁰⁷Pb, ²⁰⁷Pb/²³⁵U, and ²⁰⁶Pb/²³⁸U ages (ID-TIMS) of 1521.02± 0.55 Ma, 1518.87 ± 0.31Ma, and 1517.32 ± 0.32Ma, 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 orbulk 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 skarnmineralisation 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.

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