Volatiles in pillow rim glasses from Loihi and Kilauea volcanoes, Hawaii

Abstract

Volatiles and major elements in submarine glasses from Loihi seamount and Kilauea volcano. Hawaii were analyzed by high temperature mass spectrometry and the electron microprobe. Loihi glasses are subdivided into three groups: tholeiitic, transitional and alkali basalts. The glasses are evolved: Mg numbers range from 48–58. The alkalic lavas are the most evolved. Total volatiles range from 0.73 to 1.40 wt.%. H 2O shows a positive linear correlation with K 2O content [ H 2 O = 0.83 (± .09) K 2 O + 0.08 (± .06)]. Concentrations of H 2O are higher in the alkalic lavas, but Cl and F abundances are highly variable. Variations in ratios of incompatible elements (K 2O, P 2O 5, H 2O) indicate that each group was derived from a distinct source. CO 2 contents range from 0.05 to 0.19 wt.% but show no systematic correlation with rock type or Mg #. A well-defined decrease in glass CO 2 content with increasing vesicularity is shown by the alkalic lavas. CO 2 may have been outgassed from the tholeiitic and transitional magmas prior to eruption during storage in a shallow magma chamber. Reduced carbon species (CO and CH 4) were found in small amounts in most of the alkalic samples. Although the redox histories of Hawaiian lavas are poorly known, these new data indicate the presence of a reduced source for Loihi magmas. The Kilauea tholeiitic glasses are evolved (Mg # 48.3 to 55) and have higher H 2O contents (av. 0.54 wt.%) than Loihi tholeiites (av. 0.42 wt.%) at the same Mg # (~55). Cl is distinctly lower in Kilauea glasses (0.01 wt.%) compared to Loihi glasses (0.09 wt.%). The data indicate significant source differences for the two volcanoes, consistent with results of other geochemical studies. Loihi tholeiites have distinctly higher 3He/ 4He ratios than Kilauea tholeiites and are the highest measured in submarine basalts (KURZ et al., 1983). These high ratios have been used to invoke a primitive source for Loihi basalts. The high Cl content of these basalts, the highest we have ever measured in submarine basalts, may be a fingerprint of this primitive source, as previously noted for Icelandic basalts ( Schilling et al. 1980).