216-1 Mica Rb-Sr dating by laser ablation ICP-MS/MS using an isochronous calibration material and application to the REE-mineralized Twihinate carbonatite complex

Session: Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals

Presenting Author:

Andrea Giuliani

Authors:

Tatnell, Lucas¹, Giuliani, Andrea², Fiorentini, Marco³, Bouabdellah, Mohammed⁴, Boukirou, Wissale⁵, Fisher, Chris⁶, ezad, Isra⁷, Sapor, Laurent⁸

(1) Centre for Exploration Targeting and ARC Centre in Critical Resources for the Future, School of Earth and Oceans, The University of Western Australia, Perth, Australia, (2) Carnegie Institution for Science, Washington, DC, USA, (3) Centre for Exploration Targeting and ARC Centre in Critical Resources for the Future, School of Earth and Oceans, The University of Western Australia, Perth, Australia, (4) Département de Géologie, Faculté des Sciences, Université Mohammed Premier, Oujda, Morocco; Geology and Sustainable Mining Institute, Mohammed VI Polytechnic University, Benguerir, Morocco, (5) Département de Géologie, Faculté des Sciences, Université Mohammed Premier, Oujda, Morocco, (6) Centre for Exploration Targeting and ARC Centre in Critical Resources for the Future, School of Earth and Oceans, The University of Western Australia, Perth, Australia, (7) Centre for Exploration Targeting and ARC Centre in Critical Resources for the Future, School of Earth and Oceans, The University of Western Australia, Perth, Australia, (8) Groupe Managem, Casablanca, Morocco,

Abstract:

In-situ Rb-Sr dating of mica by laser ablation ICP-MS/MS is increasingly employed to date several geological processes including magmatic intrusions and hydrothermal mineralization. This analytical method overcomes the isobaric interference of ⁸⁷Rb on ⁸⁷Sr by reaction of Sr⁺ ions with a gas reagent (SF₆ or N₂O) in a collision cell. Rb⁺ ions are not affected by this reaction while the isotopes of Sr are quantified as reacted species (e.g., SrO⁺). The majority of available age results are based on Rb/Sr calibration using the MicaMG pressed-powder pellet. Strontium isotope ratios are calibrated using the synthetic glass NIST610. However, various studies have reported low accuracy associated with mica Rb-Sr ages using this method or large variations in calculated Rb/Sr for the same mica attributed to the different ablation properties of MicaMG and natural mica. In this presentation we first show the results of a recently developed strategy to obtain in-situ mica Rb/Sr ages where NIST610 is initially employed to estimate both ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr, thus producing very stable splines for drift correction. ⁸⁷Rb/⁸⁶Sr in the mica unknowns is then corrected based on the age of an isochronous in-house mica standard. Comparison of in-situ Rb/Sr ages with those obtained by isotope dilution for the same micas from kimberlites in southern and western Africa testifies to the improved accuracy of this procedure compared to those utilizing MicaMG. In-situ mica Rb/Sr dating is then combined with U/Pb dating of perovskite and apatite to constrain the emplacement ages of multiple lithological units (carbonatites, syenites, ultramafic lamprophyres) within the REE-endowed Twihinate complex in Morocco. Age variations between 89.0±1.3 Ma and 82.6±3.1 Ma (2s) indicate a protracted history of magma emplacement including an apparent inward younging which has not been previously inferred in carbonatite complexes elsewhere. This pattern probably resulted from repeated pulses of melt injected into the same lithospheric pathway over ~7 million years, and implies melt channeling towards the center of annular carbonatite structures. This process can foster the re-dissolution, extraction and concentration of critical elements including REE in late-stage melts and fluids hence explaining why REE are distributed in the center of the complex at Twihinate and in other major carbonatite complexes such as Mt Weld in Australia.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4505

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