EarthRef.org Reference Database (ERR) -- Hawkins et al. 1983

Loihi Seamount is the southeasternmost active volcano of the Emperor-Hawaii linear volcanic chain. It comprises a spectrum of basalt compositional varieties including basanite, alkali basalt, transitional basalt and tholeiite. Samples from four dredge collections made on Scripps institution of Oceanography Benthic Expedition in October 1982 are tholeiite. The samples include highly vesicular, olivine-rich basalt and dense glass-rich pillow fragments containing olivine and augite phenocrysts. 80th quartz-normative and olivine-normative tholeiites are present. Minor and trace element data indicate relatively high abundances of low partition coefficient elements (e.g., Ti, K, P. Rb, Ba, Zr) and suggest that the samples were derived by relatively small to moderate extent of partial melting, of an undepleted mantle source. Olivine composition, MgO, Cr and Ni abundances. and Mg/(Mg+Fe), are typical of moderately fractionated to relatively unfractionated 'primary', magmas. The variations in chemistry between samples cannot be adequately explained by low-pressure fractional crystallization but can be satisfied by minor variations in extent of melting if a homogeneous source is postulated. Alternatively, a heterogeneous source with variable abundances of certain trace elements, or mixing of liquids, may have been involved. Data for (super 3) He/ (super 4) He, presented in a separate paper implies a mantle plume origin for the helium composition of the Loihi samples. There is little variation in the helium isotope ratio for samples having different compositions and textures. The helium data are not distinctive enough to unequivocally separate the magma sources for the tholeiitic rocks from the other rock types such as Loihi alkalic basalts and the whole source region for Loihi may have a nearly uniform helium compositions even though other element abundances may be variable. Complex petrologic processes including variable melting fractional crystallization and magma mixing may have blurred original helium isotopic signatures.

Keywords
basalts; chemical composition; composition; East Pacific; East, Pacific Ocean Islands; geochemistry; Hawaii; igneous rocks; lava;, Loihi Seamount; magmas; major elements; mineral composition; North, Pacific; Northeast Pacific; ocean floors; Oceania; oceanography;, Pacific Ocean; petrology; Polynesia; seamounts; trace elements;, United States; volcanic rocks; volcanism; volcanology, 05A Igneous and metamorphic petrology; 02C Geochemistry of rocks,, soils, and sediments