Microdiamonds — Frontier of ultrahigh-pressure metamorphism: A review

This is a comprehensive review paper devoted to microdiamonds from ultrahigh-pressure metamorphic (UHPM) terranes incorporated in orogenic belts formed at convergent plate boundaries in Paleozoic–Mesozoic–Alpine time. When in 1980 the first small diamonds were discovered within “amphibolite–granulate facies” metamorphic rocks, it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed. Since then, much progress has been made in understanding the mechanism of these diamonds' formation, and the number of new diamond-bearing UHPM terranes was significantly increased, especially within European orogenes. Moreover, new variations in tectonic settings in which UHP rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously “forbidden” for their formation—e.g., oceanic islands, ophiolites, and forearc environments. Over the past decade, the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature; that they contain nanometric, multiphase inclusions of crystalline and fluid phases; and that they keep a “crustal” signature of carbon isotopes. Scanning and transmission electron microscopy, focused-ion-beam techniques, synchrotron infrared spectroscopy, micro X-ray diffraction, and nano-secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs, and that they probably interacted with deep mantle plumes. Hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed. The paper also emphasizes that the discovery of these microdiamonds (as well as coesite) triggered a major revision in the understanding of deep subduction processes, leading to a clear realization of how continental materials can be recycled into the Earth's mantle and geochemically rejuvenate it.

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► This review paper highlights the recent studies of microdiamonds from UHPM terranes.
► The UHPM diamonds originate from a biogenic carbon dissolved into COH-fluid/melt.
► The nanoinclusions in UHPM diamonds record evidence of crust–mantle interactions.