Reappraisal of the applicability of MK-1 apatite as a reference standard for (UTh)/He geochronology

doi:10.1016/j.chemgeo.2021.120255

Chemical Geology

Volume 575, 5 August 2021, 120255

https://doi.org/10.1016/j.chemgeo.2021.120255 Get rights and content

Abstract

The (U single bond Th)/He dating technique is widely used in studying the shallow-crustal geological processes including exhumation of orogenic belts, surface weathering, fault motion, topographic evolution. A reference material with homogeneous composition and highly reproducible (UTh)/He dates is crucial to get accurate and precise date results for the unknown samples. As the most commonly used reference material, the Durango apatite has its limitations such as heterogeneous distribution of parent isotopes and large date dispersion for different fragments. In order to overcome the shortcomings of Durango apatite and meet the increasing demand for reference material, we developed a new reference material, MK-1 apatite, which was collected from the Mogok metamorphic belt (MMB) of Myanmar, for the (U single bond Th)/He community. In this study, we presented dating results of 191 apatite fragments in six different laboratories, which showed highly reproducible dates and Th/U ratios, with an average date of 18.01 ± 0.37 Ma (sd.) and Th/U ratio of 0.72 ± 0.06 (sd.) respectively. Combined with textural and compositional results in the previous study (Wu et al., 2019), we concluded that he MK-1 apatite could serve as a good new reference material for (U single bond Th)/He geochronology. We use the weighted mean dates of 17.99 ± 0.02 Ma that obtained from six different laboratories as the intercalibrated dates for the new reference material.

Introduction

The distinctive crystallographic structure and composition of apatite make it widely used in numerous research fields (Hughes and Rakovan, 2015). From the geological perspective, apatite is the most ubiquitous rock-forming mineral that residing in all kind of crustal rocks. In apatite structures, extensive substitution between trace elements and calcium makes it a good carrier for radioactive isotopes such as uranium and thorium. This characteristic of apatite forms the foundation of geochronological researches of apatite including U single bond Pb, fission track and (UTh)/He techniques (Chew and Spikings, 2015, David et al., 2019; Malusà and Fitzgerald, 2020).

The (U single bond Th)/He dating technique, which uses the decay of nuclides, ²³⁸U, ²³⁵U, ²³²Th, and ¹⁴⁷Sm, and the accumulation of their daughter isotope, ⁴He, to date minerals, is widely used in studies of shallow-crustal geological processes (Ehlers and Farley, 2003). (UTh)/He geochronology is an absolute dating method, meaning that the results do not depend upon a reference material. However, because date reference materials are used to inspect experimental procedures, it is still very important to get precise and accurate geological dates. At present, the most commonly used apatite reference material used by both the (U single bond Th)/He and fission track communities is the Durango apatite (McDowell et al., 2005). However, this gem-quality apatite is known to exhibit large (UTh)/He date dispersion between individual fragments partially due to the heterogeneous distribution of parent isotopes revealed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis (Boyce and Hodges, 2005). To overcome the limitations of Durango apatite as a reference material and meet increasing demand for more homogenous reference materials, Wu et al. developed a new potential reference standard: MK-1 gem-quality apatite from the Mogok metamorphic belt in Myanmar (Wu et al., 2019). As a reference material for (U single bond Th)/He geochronology, it should meet at least three criteria: (1) good (UTh)/He date reproducibility, (2) homogeneous both in structure and chemistry (such as uranium and thorium distribution), and (3) gem-quality crystals that have no inclusions and avoiding uncertainties resulted from alpha-ejection correction (Farley et al., 1996). Detailed structural, chemical and geochronological studies (Wu et al., 2019) suggest that the MK-1 apatite satisfied all the three criteria mentioned above. In this study, we summarized (U single bond Th)/He date results in six different laboratories and reemphasized that the MK-1 apatite is a good new reference material for (UTh)/He geochronology.

Section snippets

Sample description and analytical method

The MK-1 apatite, which was collected from the Mogok metamorphic belt (MMB) of Myanmar, is a centimeter-scale gem-quality megacryst developed in the ruby-bearing marble (Fig. 1 of Wu et al., 2019). This crystal of MK-1 is quite transparent with blue color. Detailed geological background of the MMB and the sample was described in Wu et al. (2019).

Apatite (U single bond Th)/He results were produced in six different laboratories: ⁴⁰Ar/³⁹Ar and (UTh)/He geochronology laboratory, Institute of Geology and

Results

(U single bond Th)/He date results of one hundred and ninety-one MK-1 fragments were shown in Table S1 and summarized in Table 1, Fig. 2. LA-ICP-MS trace element results were shown in Table S2.

One hundred and twelve fragments (including twenty-one fragments from Wu et al., 2019) obtained by IGGCAS got (U single bond Th)/He dates between 16.97 ± 0.32 Ma and 18.50 ± 0.35 Ma. The population forms a normal distribution with the weighted mean of 18.01 ± 0.03 Ma (Table S1, Fig. 2b). The Th/U ratios range between 0.61 and

Homogeneity and date reproducibility

The most important requirement for a good (U single bond Th)/He dating reference material is its chemical homogeneity. Chemical variation (such as uranium and thorium) revealed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis could partially lead to date dispersion that far exceeds analytical uncertainty, which was verified in most widely used Durango apatite (Boyce and Hodges, 2005). Chemical inhomogeneity (such as zoning, fluid and/or gas inclusions) could also

Conclusion

In this study, we reported 191 (U single bond Th)/He dates (including 30 previous published dates and 161 newly obtained dates) of MK-1 apatite fragments yielded from six different laboratories. All the dates were consistent with the published data. Moreover, statistical analysis of dates in each separate laboratory suggested that the MK-1 apatite could get highly reproducible (U single bond Th)/He dates in different laboratories. Weighted mean date of mean dates from all six laboratories of 17.99 ± 0.02 Ma (N = 6,

Declaration of Competing Interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was jointly founded by the National Natural Science Foundation of China (41503055, 41673015 and 41773045) and Ministry of Science and Technology of the People's Republic of China (2016YFC0600109). Desmond Patterson provided tremendous support during the establishment of the (U single bond Th)/He laboratory of IGGCAS. We also thank Noreen J. Evans (Curtin University) for her valuable suggestions that improved our standard procedure of helium dating. Dr. Peter Copeland, another anonymous reviewer

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Reappraisal of the applicability of MK-1 apatite as a reference standard for (UTh)/He geochronology

Abstract

Introduction

Section snippets

Sample description and analytical method

Results

Homogeneity and date reproducibility

Conclusion

Declaration of Competing Interest

Acknowledgements

Chem. Geol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Chem. Geol.

Earth Sci. Rev.

Chem. Geol.

Chem. Geol.

Chem. Geol.