Abstract
The presence of xenolithic accumulate rocks that contain combinations of clinopyroxene, olivine, biotitic phlogopite, and apatite in the ejecta of Somma-Vesuvius volcano verify that crystal fractionation was an operative process in the pretrogenesis of the leucite-bearing lavas. Many of the cumulates contain small quantities of pervasive interstitial glass that leads us to interpret these rocks as fragments of ejected crystal mush which was quenched upon rapid upward transport and ejection. Petrographic and chemical data indicate that the glasses represent liquids in equilibrium with the associated crystallizing phases and probably are representative of larger volumes of magma. Despite the fact that our glasses are in cumulate rocks mainly from the 1944 eruption, the compositions generally overlap the entire compositional range exhibited by lava compositions and tephra reported for the 2000-year life span of Vesuvius activity. Generally similar glass compositions also are exhibited by three samples that stratigraphically occur between two lava flows thought to be erupted, respectively, in 1440 and 1631, but the limited number of samples studied for this eruption does not allow us to determine if the range of compositions are as diverse as the 1944 suite. For the glasses (intercumulus liquids) of single eruptive episodes to exhibit such compositional ranges is strong evidence that the Vesuvius magma reservoir was heterogeneous. It may have existed as either a chemically zoned magma chamber, or a series of local magma pockets that evolved independently to different degrees, but were simultaneously traversed or tapped during these two eruptive episodes.
Published Version
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