Morphology and Surface Features of Diamonds from Mengyin:Geological Implications for the Nature and Activity of Deep-Seated Melts and Fluids
Diamond undergoes intricate interactions with melts and fluids in the Subcontinental Lithospheric Mantle(SCLM)and kimberlite magma,resulting in a diverse array of crystal morphologies and surface features.These morphological attributes serve as proxies,reflecting the varying distinct properties of melts and fluids during mantle storage and kimberlite magma migration.This study meticulously analyzed the morphology and surface features of diamonds sourced from Mengyin.In addition,we systematically discuss the characteristics of mantle and kimberlite melt and fluids,as inferred from these diamonds,with reference to a comprehensive body of international dissolution experiments on natural diamonds that simulated the mantle and kimberlite temperature,pressure,and chemical conditions.This investigation yields the following key findings:① The morphology and surface features caused by mantle melts predominantly exhibit a combination of low-degree resorption of"octahedron+triangular/serrated laminae+joint pit/deep pit,"with the severe dissolution of"Dodecahedra+deep etch pit"being less common.Statistical analyses suggest that these morphologies indicate the occurrence of small-scale,low-degree peridotite-melt reactions within the ancient lithospheric mantle of the North China Craton(NCC),with melt compositions relatively enriched in CO2 and depleted in H2O.② The proportion of dissolution forming among diamonds in the three kimberlite belts in Mengyin indicates discernible differences in the activity of magmatic fluids,with the Changmazhuang belt showing relatively more intense magma fluid activity than Xiyu and Poli belt.③ The morphology and surface features of Mengyin diamonds,which result from magma fluid etching,primarily showcase a combination of"Tetrahexahedra(THH)+shallow depression+corrosion sculpture,"with the majority of samples displaying characteristics indicative of an overlay of shallow dissolution by magma fluids.Additionally,all shallow independent trigons were flat-bottomed and truncated,with no special types,indicating characteristics associated with fluids rich in H2O and poor in CO2 released by Hypabyssal Kimberlite(HK).These results underscore the heterogeneity in the properties of mantle melts and late-stage magmatic activity,even within relatively confined geographical areas.Moreover,the morphology and surface features of diamonds are crucial indicators of mantle melt and kimberlite magma fluid activity.They also offer insights into the underlying reasons for the pronounced variations in quality and grade observed among diamonds sourced from different rock masses and ore belts in numerous diamond-mining regions.
DiamondKimberliteMorphology and surface featuresDiamond mineralization
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