Samples of Hawaiian tholeiitic basalt were collected from successive prehistoric flow and intrusive units, exposed on the walls of the summit caldera of Mauna Loa Volcano. Based on major, minor, and trace element analyses, a caldera stratigraphy divided into three separate lava suites is presented. Suite A, the oldest, consists of sparsely to moderately phyric lavas with generally low levels of MgO (6.6 $\pm$ 0.5%) and highly variable incompatible element ratios (K/Y 136 $\pm$ 16) that systematically decrease with time. Suite B is characterized by moderately to highly phyric lavas with highly variable major element chemistry (10.9 $\pm$ 3.2% MgO), predominantly controlled by the addition or subtraction of olivine. Incompatible element ratios are generally low (K/Y 129 $\pm$ 7) and progressively increase with time. The youngest Suite (C) consists of sparsely phyric lavas with low levels of MgO (6.6 $\pm$ 0.2%) and uniformly high levels of incompatible elements (K/Y 158 $\pm$ 5). A tentative correlation with radiocarbon dated flank eruptives constrain the age of the oldest examined lavas to approximately 1,500 yrs, and the youngest to roughly 590 yrs. Sill-like intrusive units were found to be compositionally indistinguishable from Suite B lavas, and probably the result of downward or lateral injection of dense, ponded Suite B lava. Correlations between incompatible element ratios, major element compositions, and flow morphologies indicate that Mauna Loan lava chemistry is linked with eruptive activity and magma supply rate. Periods of high magma supply rates are associated with MgO-rich, incompatible element depleted lavas, whereas infrequent, low-volume activity is linked with moderately evolved, incompatible element enriched eruptives. The observed systematic oscillation of incompatible element ratios with time is consistent with the view that the range of incompatible element abundances observed in Mauna Loan lavas has not significantly change over the last 30,000 years (Rhodes et al., 1982). A change in eruptive behavior probably occurs each time there is a significant shift in incompatible element ratios. This behavioral scenario is consistent with the open-system, shallow (3-5 km depth) magma reservoir model of Rhodes (1987; 1988).
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-6079 |
| Date | 01 January 1990 |
| Creators | Sparks, Joel Watson |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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