Earth and Planetary Sciences ETDs
Twelve samples from Kohala volcano, Hawaii were selected for chemical analyses and electron microprobe study. Seven of the samples are from the Pololu volcanic series. Three of these samples are tholeiitic basalts, two are oceanites, and two are alkali olivine basalts transitional to hawaiite. The five samples from the Hawi volcanic series include one hawaiite, two mugearites, and two benmoreites. Electron microprobe study of the minerals in the basaltic matrix of the two oceanites reveal that one of the oceanites was derived from a tholeiitic magma while the other was derived from an alkalic magma. The alkalic oceanite matrix is characterized by the presence of high Ca clinopyroxene, low forsterite groundmass olivine, and interstitial material of anorthoclase composition. The matrix of the tholeiitic oceanite is characterized by the presence of low Ca clinopyroxenes and groundmass grains of plagioclase high in anorthite content.
The phases with the highest strontium content are the feldspars and apatite. The phenocryst-matrix partition coefficient for SrO in apatite varies from 2.3 to 2.5 while the Sr0 partition coefficients for plagieclase varies with the anorthite content of the plagioclase. The plagioclase-matrix distribution coefficients for SrO reported here are low (3.0 to 4.0) in the anorthite rich plagioclase and rise with decreasing An content until a maximum coefficient of 7.0 is reached at a plagioclase composition of An30. The plagioclase-matrix distribution coefficient for SrO then falls off rapidly to approximately 2.0 for anorthoclase (An10_20). The increase in strontium in the intermediate members of the alkalic suite is thought to be due to the fractional crystallization of pyroxene and olivine outweighing the effects of the depletion of strontium produced by the fractional crystallization of plagioclase with high anorthite content. Later fractional crystallization of apatite and more sodic plagioclase (with higher Sr partition coefficients) depletes Sr in the later residual liquids (benmorei tes and trachytes).
The presence of hydrous minerals such as biotites, hornblende, and kaersutite in the later members of the alkalic suite indicates that hydrous conditions did not develop until later stages of volcanic activity when the vapor pressure of water in the magma chamber might be expected to be higher due to previous extensive fractional crystallization of anhydrous minerals. Limited K/Rb and REE data from the published work of Lessing et al. (1963) and Schilling and Winchester (1969) indicate that amphibole fractional crystallization may have been important in the petrogenesis of the ben.moreites and trachytes of the Hawi volcanic series (Kohala volcano) and the Puu WaaWaa volcanic series (Hualalai volcano). The solutions to several petrologic mixing problems have shown the subtraction of amphibole, particularly those lower in silica and alkalies such as kaersutite, can be used in fractional crystallization models to derive magmas close in composition to the amphibole bearing late differentiates of the Hawi series.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Second Committee Member
Douglas G. Brookins
Third Committee Member
Albert Masakiyo Kudo
Sibray, Steven Sherman. "Mineralogy, Petrology, and Geochemistry of Some Lavas from Kohala Volcano, Hawaii." (1977). https://digitalrepository.unm.edu/eps_etds/268