Micro-windows to deep COH-rich planetary fluids and associated mantle processes

Metasomatism, a process whereby infiltrating fluids and melts rich in trace elements and volatiles interact and alter mantle rocks is a common mantle process, as reflected by the mineralogy and chemical composition of mantle-derived samples from all continents. However, the metasomatic agent itself is only rarely available as fluid/melt inclusions for direct analyses; and thus, in many cases, the nature of metasomatism remains elusive.

High-density fluid (HDF) microinclusions in diamonds provide a unique record of the compositions and nature of deep mantle carbon- and water-bearing (COH) fluids. Their high volatile content: 8-20 wt.% H₂O and 4-32 wt.% CO₂, and their enrichment in incompatible elements make them a key player in mantle metasomatism. The most common HDFs vary in compositions between four major types: silicic, rich in Si, Al, K and water; low- and high-Mg carbonatitic, rich in Ca, Mg, Fe, K and carbonate; and saline, rich in Cl, K, Na and water. Few lines of evidence indicate the relation of saline HDFs and subducted surface material: their K/Cl ratio overlaps the range of altered oceanic crust; pronounced positive Eu and Sr anomalies reflect the involvement of plagioclase during low-pressure crustal processes of protolith formation; and, low ³He/⁴He isotope ratios (2.7–4.4 Ra) further strengthen a connection with recycled surface material. In addition, recent radiogenic isotope data points to the involvement of distinct mantle sources and subducted components in the formation of varying HDF types. For example, the isotopic composition of silicic to low-Mg carbonatitic HDFs in a suite of diamonds from Canada indicate the contribution of two distinct sources within the continental lithosphere: one with relatively primitive isotopic compositions characterized by εNd of −0.2, ⁸⁷Sr/⁸⁶Sr of 0.7044 and ²⁰⁶Pb/²⁰⁴Pb of 17.52, and another with more unradiogenic εNd < −16 and radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb > 0.713 and 18.3, respectively. The latter reflects an old metasomatic event in the continental lithosphere involving fluid addition from a subducting slab, most probably in the Paleoproterozoic. In comparison, isotopic compositions of HDFs in South African diamonds suggest that saline HDFs record the involvement of metasomatized Archaean lithosphere and subducting surface material that includes recent sediments.

HDFs are the deepest mantle fluids we have at hand. Due to their high mobility, they can migrate and react with different mantle lithologies over a range of depths. Such HDFs-rock interaction leads to the formation of new metasomatic phases and enriches depleted mantle rocks in volatile and incompatible elements, thereby impacting their density, rheology and melting behavior. Indeed, the mineralogy and chemical composition of xenoliths/xenocrysts and some alkaline magmas suggest a prevalent role of HDFs in mantle metasomatism and deep Earth processes.