The Origins of Glass and Amphibole in Spined—Wehrlite Xenoliths from Foster Crater, McMurdo Volcanic Group, Antarctica

Abstract

Vesicular interstitial glass in four kaersutite-bearing spinel–wehrlite xenoliths from Foster Crater, Antarctica has reacted with host olivine (Fo_75–79) and clinopyroxene (Ca₄₇ Mg₄₅ Fe₈) and contains a microphenocryst assemblage of spinel, olivine, and clinopyroxene together with later rhonite and plagioclase. Electron microprobe analyses of the glasses have low SiO₂ (46–49 wt. per cent) and MgO (2.2–3.7) contents and high contents of alkalis, TiO₂ (3.4–4.8), Al₂O₃ (18.1–20.6) and P₂O₅ (1.1–1.3). Olivine microphenocryst cores in glass are magnesian (up to Fo₈₈) and must have precipitated from more primitive liquids; rim compositions are Fe-rich (Fo₇₅) and in equilibrium with glass. Continuous core to rim zonation in the olivine microphenocrysts indicate that glass compositions have fractionated due to crystallization of the enclosed mineral assemblage. Mass balance addition calculations, using the compositions and proportions of the crystals in glass, produce melt compositions appropriate to primary alkali basaltic magmas. Glasses show light rare earth element (REE) enrichment relative to chondrites (Ce/YbN = 10.5) and, together with Ba, Rb, Cr, Hf, Ta, and Th are similar to many of the basanites from the Erebus Volcanic Province.

Textural relationships of the kaersutite are complex owing to the instability of kaersutite in the presence of melt. However, in the association with glass we observe textural evidence such as olivine and clinopyroxene microphenocrysts, identical to the liquidus phases of the glasses, enclosed by kaersutite crystals. We believe that relationships such as this link the crystallization of kaersutite to mafic melt which infiltrated and reacted with the host wehrlite. Thus, the melt did not form in situ within the xenolith but originated elsewhere in the upper mantle.