U–Pb age determination of schorlomite garnet by laser ablation inductively coupled plasma mass spectrometry†
We report the first U–Pb geochronological investigation of schorlomite garnet from carbonatite and alkaline complexes and demonstrate its applicability for U–Pb age determination using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) due to its relatively high U and Th abundances and negligible common Pb content. The comparative matrix effects of laser ablation of zircon and schorlomite are investigated and demonstrate the necessity of a suitable matrix-matched reference material for schorlomite geochronology. Laser-induced elemental fractional and instrumental mass discrimination were externally-corrected using an in house schorlomite reference material (WS20) for U–Pb geochronology. In order to validate the effectiveness and robustness of our analytical protocol, we demonstrate the veracity of U–Pb age determination for five schorlomite samples from: the Magnet Cove complex, Arkansas (USA); the Fanshan ultrapotassic complex, Hebei (China); the Ozernaya alkaline ultramafic complex, Kola Peninsula (Russia); the Alnö alkaline–rock carbonatite complex (Sweden); and the Prairie Lake carbonatite complex, Ontario (Canada). The schorlomite U–Pb ages range from 96 Ma to 1160 Ma, and are almost identical to ages determined from other accessory minerals in these complexes and support the reliability of our analytical protocol. Schorlomite garnet U–Pb geochronology is considered to be a promising new technique for understanding the genesis of carbonatites, alkaline rocks, and related rare-metal deposits.
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