ISOTOPIC DIFFERENCES BETWEEN PLAGIOCLASE AND MATRIX IN THE TUNGNÁRHRAUN, SOUTH ICELAND

The Tungnárhraun are a series of large volume (>1 km³) lava flows in southern Iceland that span nearly the entire Holocene period and include several plagioclase ultraphyric basalts (PUBs). Bulk rock geochemical data link the Tungnárhraun to the Bárðarbunga central volcano, the largest and most productive modern Icelandic magmatic system. Geochemical and petrologic features of Tungnárhraun basalts define three eruptive episodes: early and late periods of lavas with ~5.5-6 wt.% MgO and <10 vol.% plagioclase feldspar macrocrysts (>1 cm) and an intermediate period of more primitive lavas (~8 wt.% MgO) with 14-20 vol.% plagioclase macrocrysts. Plagioclase feldspar macrocrysts from all eruptive episodes have An_85-91 cores which are too primitive to have grown from the host lava, and An_55-80 rims that are in chemical equilibrium with the groundmass. MgO contents of the plagioclase feldspar macrocrysts are lower in crystal cores than in the rims, indicating early growth in the presence of an azeotrope and thereby constraining the pressure of nucleation to between 5-10 kbar. Plagioclase macrocrysts in lavas from each magmatic episode are isotopically distinct from the respective basaltic matrix with lower ⁸⁷Sr/ ⁸⁶ Sr values. Preliminary data show that the MgO-rich lavas have feldspars with the lowest Sr isotope values, suggesting multiple basaltic inputs derived from distinct mantle source domains. The Tungnárhraun basalts appear to entrain plagioclase feldspars from a complex lower crustal mush zone before transporting them to the surface.