In situ rubidium–strontium geochronology of white mica in young metamafic and metasomatic rocks from Syros: testing the limits of laser-ablation triple-quadrupole inductively coupled plasma mass spectrometer mica dating using different anchoring appr

Abstract. The recent development of laser-ablation triple-quadrupole inductively coupled plasma mass spectrometry (LA-ICP-MS/MS) has revolutionized rubidium–strontium (Rb–Sr) mica dating, allowing us to obtain isotopic data within their microstructural context. While effective for old and felsic materials, this method presents challenges for young metamafic and metasomatic rocks due to limited radiogenic ingrowth associated with low Rb/Sr and young ages. We quantitatively address these limitations by combining laser-ablation ICP-MS/MS and MC-ICP-MS data for coexisting white mica and epidote, respectively, for 10 Cenozoic metamorphic rocks from Syros (Greece). White mica analyses from metamafic and metasomatic rocks yield limited Rb/Sr spread, which typically does not exceed an order of magnitude (87Rb/86Sr=14 to 231 for the combined dataset), and low radiogenic 87Sr/86Sr (generally <0.8), resulting in high age uncertainties of typically 10 to 50 % relative standard error (RSE), thus hampering robust geological interpretations. Epidote 87Sr/86Sr values range between ∼0.705 and 0.708. The former is typically expected for unaltered metamafic materials, whereas the latter is interpreted to reflect fluid–rock interaction along shear zones, with fluids derived from or having interacted with more radiogenic lithologies. These atypical values suggest that a commonly assumed value of 0.703 for mafic rocks may not always be representative. Anchoring white mica Rb–Sr to epidote 87Sr/86Sr data improves age accuracy and precision substantially (e.g., 29±17 Ma vs. 47.2±4.4 Ma for sample SYGR36). The new ages obtained in this study are consistent with multiple events previously recorded on Syros and the Cyclades blueschists unit including (i) metasomatism and metamorphism at near peak to epidote blueschist-facies conditions during early exhumation (ca. 47 to 41 Ma) and (ii) a late stage of high-pressure exhumation and metasomatism transitioning to blueschist and greenschist-facies conditions (ca. 21 to 20 Ma). Anchored white mica Rb–Sr ages in mafic rocks allow us to discriminate events of fluid–rock interactions and metasomatism associated with shear zone deformation at the subduction interface.