Showing papers by "Paul W. Layer published in 1994"
TL;DR: In this paper, the chemical evolution of mafic lava flows in the Long Valley Caldera has been investigated, showing a trend toward more evolved compositions with time, and the chemical contrasts between adjacent adjacent groups were dominated by assimilation.
Abstract: Continuous core from research drill hole Inyo-4 through a thick 300 m thick sequence of mafic lava flows in the west moat of the Long Valley Caldera has provided an unusual opportunity to investigate the chemical evolution of this exceptionally complete record of postcaldera mafic magmatism. Lavas are mainly basalts and trachyandesites ranging from 48 to 58% SiO{sub 2} having a nearly fourfold range in MgO contents. The lavas fall into five distinct chemical groups with little or no compositional overlap. These groups correlate remarkably well with stratigraphic position, and they define a trend toward more evolved compositions with time. The groups appear to represent periodic eruptions from a continuously evolving magma body. Preliminary {sup 40}Ar/{sup 39}Ar dates indicate that these lavas erupted over a span of at least 0.264 m.y. between 0.415 Ma and 0.151 Ma. Except for the most evolved group, the chemical variation within a group was dominated by crystal fractionation. Except from the least evolved groups, the chemical contrasts between adjacent groups were dominated by assimilation. Warming the crustal environmental facilitated increasing assimilation. A zoned lava flow (more mafic upward) followed by a mafic flow in an otherwise progressively evolved sequence of flows provides evidence formore » eruption from a zoned magma reservoir. Deeper, more mafic portions of this zoned magma body were drawn up to shallower levels in the chamber during a period of high eruption rates. The heterogeneity of mafic clasts in the vent breccia dike beneath the 600-year-old South Inyo phreatic explosion crater indicates that these breccia clasts dropped > 300 m down the vent from the overlying lava sequence during waning stages of the phreatic activity, rather than forming by brecciation of an older feeder dike, as previously proposed. 39 refs., 13 figs., 5 tabs.« less
16 citations
TL;DR: The Rainier Mesa ash-flow is a large (1200 km3), 116 My old, chemically zoned unit that ranges in composition from 55 to 76% SiO2 as discussed by the authors.
Abstract: The Rainier Mesa ash-flow is a large (1200 km3), 116 My old, chemically zoned unit that ranges in composition from 55 to 76% SiO2 — one of the largest chemical ranges ever observed in a large volume ash-flow sheet Two chemical trends occur in this sheet, a low silica (55–66% SiO2) and a high silica (>66% SiO2) trend Ninety per cent of the Rainier Mesa sheet occurs in the high silica trend Immediately beneath the Rainier Mesa sheet is a thick tephra sequence The chemical variation of this sequence is nearly equivalent to the high silica portion of the Rainier Mesa ash-flow sheet (about 66–78% SiO2) Throughout the tephra sequence numerous small ash-flow layers occur, and each ash-flow layer is chemically zoned from more evolved at the base to less evolved at the top This is consistent with having been erupted from a zoned magma body The lowest silica tephra units are at the base of the sequence and the highest silica units are at the top — that is, the large-scale chemical trend of the entire sequence is opposite to that of the individual ash-flow layers These ash-flow layers are of very small volume The tephra sequence provides a unique record of the incremental development of the zoned, high silica portion of the Rainier Mesa magma body
11 citations