Microbeam analyses of glasses and minerals in Karoo picrites

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 2400 ppm Sr, 1900 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 modelled in terms of magma hybridization: variably accumulated olivine phenocrysts surrounded by evolved Sr-rich (to 9470 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 >1200 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.