Systematic data has recently been published on the compositions of major mineral phases in rocks of the tholeiitic, alkalic, and nephelinic suites from the island of Maui, Hawaii. The present study was initiated to provide similar data from the rocks of Molokai, Hawaii, from which the nature of interisland variations could be established. During the study, it became obvious that many of the rocks were unusual and so this study also provides information on the bulk rock, trace element, and mineral compositions of alkalic olivine basalts transitional to hawaiites and Sr-rich mugearites.
From a total of 14 basalts collected along the southern, coastal rim of the island of Molokai, eight were analyzed for bulk composition, and in selected samples the Sr and Rb contents and Sr isotopic ratios were determined. Quantitative electron microprobe analyses were made of phenocrysts, microphenocrysts, and groundmass grains of feldspar, pyroxene, olivine, opaques, interstitial material of feldspar composition, and Si02+k20 rich glass.
Results and conclusions: i) Alkalic olivine basalt transitional to hawaiite (AOB-H), as determined by whole rock and mineral compositions, is the common rock type of the southern, coastal rim of East Molokai Volcano and is associated, in places, with later mugearites. Tholeiites comprise most of West Molokai Volcano. ii) Unusual enrichment of Sr in one mugearite is thought to be the result of normal differentiation processes acting on a parent magma of slight Sr enrichment, possibly aided by some process of volatile transfer of Sr. iii) Compositions of the major mineral phases are usually excellent indicators of the composition of individual rocks and can be used to assign the rocks to the various suites, positions within suites, and determine the transitional nature of some samples. Silicates are more sensitive indicators than other groups with pyroxene being most sensitive followed by feldspar; olivine is not as useful because of limited chemical variation and extensive alteration. The presence or absence of certain mineral species can indicate the suite (orthopyroxene and low-Ca clinopyroxene in the tholeiites) or position within a suite (no ilmenite in mugearites) of a particular rock. iv) Uncommon minerals such as baddelyite, monazite (?), or pseudobrookite, are present in some of the samples studied. Their presence here indicates that these minerals may be common accessories of basaltic rocks which are apparently overlooked in the course of most studies because of their small size and scarcity.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Second Committee Member
Albert Masakiyo Kudo
Third Committee Member
Douglas Gridley Brookins
Hlava, Paul Frank. "Unusual Lavas From Molokai, Hawaii: Alkalic Olivine Basalts Transitional To Hawaiites And Strontium-Rich Mugearites." (1974). https://digitalrepository.unm.edu/eps_etds/343