Dating mantle metasomatism: LA‒MC‒ICPMS U‒Pb dating of garnet xenocrysts from the V. Grib kimberlite, Russia

地幔交代作用在很大程度上控制了岩石圈地幔的组成，从而控制了岩石圈地幔的物理和化学性质。因此，它控制着板块内碱性岩浆（如金伯利岩和碳酸岩）的岩石成因、钻石的形成以及稀土元素矿床的生成。随着时间的推移，成分多样的交代流体反复叠加，不同程度地耗尽了原始的地幔围岩。这产生了一个复杂的，基本上无处不在的交代岩石圈地幔，在微米到地面的尺度上是不均匀的。通过识别重要的事件和过程来解码这一复杂的记录，对于重建岩石圈演化、地球挥发性通量的性质和起源，以及深层岩石圈和地壳过程之间的联系至关重要。石榴石是表征次大陆岩石圈地幔交代事件最广泛使用的相，在深度达~250 km的厚克拉通岩石圈中占很大比例。然而，对于年龄测年研究，石榴石迄今为止只被用于繁琐、昂贵和耗时的传统同位素测年技术。法兰克福同位素和元素研究中心（FIERCE）在分析和数据还原技术方面的最新进展，首次使低u石榴石能够进行原位年龄测年，不仅可以快速通过，还可以检测矿物尺度的非均质性。在本项目中，将采用原位LA-MC-ICPMS U-Pb定年技术对V. Grib金伯利岩中一组特征良好的石榴石异晶进行测年，以研究特定类型的地幔交代作用与金伯利岩和钻石形成之间的时间和因果关系。V. Grib金伯利岩样品中的石榴石异晶在岩石圈柱中分布至~220 km深度，为硅酸盐和碳酸盐岩交代作用提供了证据。通过将深层岩石圈与年代久远的地壳过程联系起来，研究结果还将为了解波罗的海地盾下的岩石圈演化提供洞见。

Mantle metasomatism largely controls the composition, and thus the physical and chemical properties of the lithospheric mantle. It thereby exerts control over the petrogenesis of alkaline (e.g., kimberlitic and carbonatitic) within-plate magmas, diamond formation, and the generation of, e.g., rare-earth element deposits. Over time, compositionally diverse metasomatic fluids repeatedly overprint variably depleted original mantle wall-rocks. This produces a complex, essentially ubiquitously metasomatised lithospheric mantle, heterogeneous on scales of microns to terranes. Decoding this complex record by identifying significant episodes and processes is essential for reconstructing lithosphere evolution, the nature and origin of Earth’s volatile flux, and the link between deep lithospheric and crustal processes.Garnet is the most widely used phase for characterizing metasomatic events in the subcontinental lithospheric mantle, comprising a large proportion of thick cratonic lithosphere up to ~250 km depth. For age-dating studies, however, garnet has so far only been employed in tedious, costly and time-intensive conventional isotope dating techniques. Very recent advances in analytical and data reduction techniques at the Frankfurt Isotope and Element Research Center (FIERCE) make low-U garnet accessible to in situ age dating for the first time, allowing not only fast through-put but also the detection of mineral-scale heterogeneity. In this project, in situ LA-MC-ICPMS U‒Pb dating will be applied to a set of well-characterized garnet xenocrysts from the V. Grib kimberlite to investigate a temporal and causal link between specific types of mantle metasomatism and kimberlite and diamond formation. Garnet xenocrysts from the V. Grib kimberlite sample the lithospheric column to a depth of ~220 km and provide evidence for silicate and carbonatitic metasomatism. The results will also provide insights into the lithosphere evolution underneath the Baltic shield by linking deep lithospheric to well-dated crustal processes.