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fractional crystallization, glass, Karoo, mantle source, picrite
Abstract:
Magmatic products of the Karoo Large Igneous Province can be divided into a volumetrically dominant, compositionally uniform low‐Ti tholeiitic suite, and a subordinate, geographically restricted, compositionally diverse, incompatible‐rich high‐Ti suite. High‐Ti picrites contain up to 2,400 ppm Sr, 1,900 ppm Ba, and 550 ppm Zr, which seems unusual for olivine‐enriched rocks. We studied six Karoo picrites to determine the phase(s) in which Sr resides. Samples consist of 10–30% olivine phenocrysts in a groundmass of brown glass, augite, feldspar, ilmenite, and apatite. Glass compositions vary, but are generally evolved, ranging from basaltic trachyandesite to dacite. X‐ray intensity maps demonstrate that most of the Sr resides in the glasses, and to a lesser extent, in feldspars, if present. Some samples contain two texturally and compositionally distinct glasses, best modeled in terms of magma hybridization: variably accumulated olivine phenocrysts surrounded by evolved Sr‐rich (to 9,470 ppm) Type 2 melts formed by extensive olivine fractionation were infiltrated by chemically distinct Type 1 melts. Upon eruption, Type 2 melt quenched to minor glass around olivine phenocrysts, and the dominant Type 1 glass acquired its evolved composition by quench crystallization of groundmass mineral phases. Both glasses are rich in H2O (up to 3.8 wt. %) but are nearly devoid of CO2. Calculated parental melts have much higher K2O and incompatible trace elements (e.g., Sr or Ba >1,200 ppm) relative to low‐Ti tholeiites. The unusual parental melt compositions imply derivation by small degrees of partial melting from SCLM mantle sources enriched in Sr and other incompatibles.
