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The recent development of LA-ICP-MS/MS has revolutionized Rb-Sr mica dating allowing 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 coexisiting white mica and epidote, respectively, for 10 Cenozoic metamorphic rocks from Syros Island (Greece). White mica analyses from metamafic and metasomatic rocks yield limited Rb/Sr spread, which typically does not exceed one 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 % RSE, and thus hampering robust geological interpretations. Epidote 87Sr/86Sr values range between ~0.705 and 0.708. The former (lower-end) is expected for typical, 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 in Syros and the Cyclades blueschists unit including: i) metasomatism at near-peak to epidote blueschist-facies during early exhumation (47.2 ± 3.8 Ma to 41.1 ± 3.1); ii) a late stage of high-pressure exhumation and metasomatism transitioning to blueschist-greenschist-facies (20.8 ± 3.1 Ma to 19.8 ± 5.2 Ma). Anchored white mica Rb/Sr dates thus allow us to discriminate events of fluid-rock interactions and metasomatism associated with shear zone deformation at the subduction interface. [ABSTRACT FROM AUTHOR]

Earth's carbon cycle is strongly influenced by subduction of sedimentary material into the mantle. The composition of the sedimentary subduction flux has changed considerably over Earth's history, but the impact of these changes on the mantle carbon cycle is unclear. Here, we show that the carbon isotopes of kimberlite magmas record a fundamental change in their deep-mantle source compositions during the Phanerozoic Eon. The 13 C/ 12 C of kimberlites before ~250 Ma preserves typical mantle values, whereas younger kimberlites exhibit lower and more variable ratios-a switch coincident with a recognized surge in kimberlite magmatism. We attribute these changes to increased deep subduction of organic carbon with low 13 C/ 12 C following the Cambrian Explosion when organic carbon deposition in marine sediments increased significantly. These observations demonstrate that biogeochemical processes at Earth's surface have a profound influence on the deep mantle, revealing an integral link between the deep and shallow carbon cycles.