The Effect of emplacement-style on the assimilation of evaporite xenoliths in kimberlite melt

English Abstract:

The Udachnaya Kimberlite (367±3 Ma), located in the Yakutian Kimberlite province of Siberia, Russia, was emplaced through carbonate and evaporite sequences of the Siberian Platform. The Udachnaya kimberlite consists predominantly of pyroclastic (FPK) and lesser hypabyssal kimberlite (HK) textural varieties, consistent with a Fort-à-la-Corne style pyroclastic-fallout and crater-infill emplacement model. This study utilizes petrographic evidence, supported by microprobe and XRD data, combined with element distribution mapping, to provide insight into the interaction between crustal xenoliths, and the FPK and HK textural varieties of the Udachnaya Kimberlite. Udachnaya FPK’s contain very fresh, angular, olivine macrocrysts, and olivine phenocrysts, in an interclast matrix of irregularly distributed phlogopite, spinel, perovskite, and fragments of chlorides and alkali-carbonates derived from the disaggregation of crustal xenoliths. The interclast matrix is cemented by deuteric carbonate, serpentine, and halite. Udachnaya HK’s contain moderately to strongly serpentinized olivine macrocrysts and phenocrysts in a fine-grained coherent, crystalline groundmass of calcite, phlogopite, serpentine, spinel, and perovskite. Carbonate-evaporite xenoliths containing calcite, dolomite, gypsum, anhydrite, halite, and sylvite are common in both HK’s and FPK’s. They show strongly altered 1-3mm reaction-rims in HK’s, while showing no evidence of reaction in FPK’s, suggesting that the interaction temperature in HK’s is much higher than in FPK’s, and likely involved kimberlitic melt. Additionally, olivine macrocrysts and phenocrysts in HK’s, proximal to chloride-bearing crustal xenoliths, record a progressive enrichment of Cl in the serpentine, during olivine replacement from rim to core. The absence of alkali-carbonates and chlorides in the groundmass of HK’s and juvenile pyroclasts of FPK’s suggests that they were not a component of the primary kimberlite melt. The presence of alkali-carbonates and chlorides within the xenoliths and the interclast matrix of FPK’s, and the enrichment of Cl only within pseudomorphed olivine in HK’s, constrains the temperature and timing of late Cl, Na and K enrichment, to the late-magmatic assimilation of crustal material, and cementation by deuteric fluids.