Isotopic-heterogeneity of the Thjorsa lava—Implications for mantle sources and crustal processes within the Eastern Rift Zone, Iceland

Abstract

The Thjorsa lava in South Iceland is an early Holocene fissure lava within the Bardarbunga volcanic system—the most productive segment of the Eastern Rift Zone during the Quaternary and Holocene. The lava has a volume of 25 km 3 and is one of notably plagioclase-porphyritic early Holocene lava flows in Iceland. The possible genetic relationship between the large plagioclase phenocrysts and the host lava is investigated by analyzing carefully separated groundmass/plagioclase pairs. The groundmass was analyzed for major and trace elements and isotopic ratios of Sr, Nd and Pb. The large plagioclases were analyzed by microprobe and 87Sr/ 86Sr by mass spectrometry. The groundmass (host magma) of the host lava has a restricted range of elemental and isotopic composition. Both the groundmass and the separated plagioclases show a significant range of 87Sr/ 86Sr—but interestingly with only minor overlap between the two groups. Both are, however, within the range of values observed for the lavas of the Bardarbunga volcanic system. This lack of overlap suggests that the plagioclase and the host lava are genetically unrelated and derived from two distinct mantle sources. The suggested source for the large plagioclases is deep layered crustal gabbros formed in earlier intrusive events. The plagioclases are accompanied by minor amounts of Fo-rich olivine and occasional disintegrating Cr-diopside. There is a clear compositional gap between the An-rich (> An 80) plagioclases and the small groundmass plagioclase that appear to be in equilibrium with the groundmass composition. The parental magma of the anorthositic gabbro must have been relatively primitive, crystallizing additional olivine and Cr-diopside and with 87Sr/ 86Sr at the low end of the range observed in the Bardarbunga rift segment. During ascent of the host magma the gabbros were infiltrated and broken up by this more evolved host magma. With lowered pressures during ascent and lava effusion the Cr-diopside comes unstable with up to 5% being assimilated into the host magma mildly affecting isotopic ratios and those elements with clinopyroxene affinity. The two components—the host magma and the plagioclases are therefore genetically unrelated and with only minor mixing effect on the host magma. The results of the present study show that whole-rock chemical analyses of such rocks must be treated with caution as they may not represent actual magmatic liquids. At the beginning of the Holocene there was a large isostatic rebound in Iceland following the rapid ice melting. This was accompanied by a major pulse of basaltic eruptions and many of these eruptions, especially around Central Iceland, carry abundant large plagioclases. This is clearly not the only cause as there are a number of very plagioclase-porphyritic lavas in the Tertiary formations but may have facilitated plagioclase incorporation.