PETROGRAPHY AND PETROLOGY OF THE HAWAII SCIENTIFIC DRILLING PROJECT LAVAS - INFERENCES FROM OLIVINE PHENOCRYST ABUNDANCES AND COMPOSITIONS

The Mauna Loa (ML) and Mauna Kea (MK) lavas recovered by the Hawaii Sc ientific Drilling Project (HSDP) include aphyric to highly olivine-phy ric basalts. The average olivine phenocryst abundance in the reference suite of ML flows is 14.5 vol % (vesicle-free and weighted by the flo w thickness), while the average abundances of olivine in the reference suites of the MK alkalic and tholeiitic basalts are 1.1 and 14.0 vol %, respectively. Plagioclase and augite phenocrysts are rare in the ML and MK tholeiites, but the MK alkalic basalts can have up to 4 vol % plagioclase phenocrysts. Strained olivine grains, thought to represent disaggregated dunite xenoliths from the cumulate pile within the magm a chamber(s), are ubiquitous in the drill core lavas. These deformed g rains can comprise up to 50 % of the modal olivine in a given rock. Ol ivine core compositions in the lavas span forsterite contents of 80.4- 90.7 (median 88.8, ML tholeiites), 75.8-86.6 (median 85.8, MK alkalic basalts), and 76.3-90.5 (median 88.0 mol %, MK tholeiites). Olivines w ith core compositions in the range Fo(89-90.5) are present in tholeiit ic lavas with a wide range of whole-rock MgO contents (9-30 wt %). Str ained and unstrained olivines completely overlap in composition as do the compositions of spinels (100Cr/(Cr+Al)similar to 59-72; Mg# = 100 Mg/(Mg+Fe2+)similar to 40-66) present as inclusions in the olivine ph enocrysts. The presence of Fo(90.5) olivine in the HSDP lavas requires magmas with similar to 16 wt % MgO in the ML and MK plumbing systems. Rare dunite xenoliths are also present in the drill core lavas. While compositionally homogeneous within a given xenolith, the six xenolith s contain olivines that span a wide range of forsterite contents (78.3 -89.2 mol %). Spinels in these xenoliths are chrome-rich, have Mg# bet ween 31 and 66, and define two populations on the basis of TiO2 conten ts. Whole-rock compositions for the ML and MK tholeiites define olivin e control lines on MgO-oxide diagrams, and the relationship between wh ole-rock MgO and olivine phenocryst abundance in these lavas suggests that the lavas with >12 wt % MgO have accumulated olivine. Comparing t he weighted bulk composition of all of the MK tholeiites in the drill core with a calculated parental magma suggests that, on average, the M K tholeiites entrained most of the olivine phenocrysts that crystalliz ed from their parental liquids. Although deformed olivines in Hawaiian lavas are widely thought to represent disaggregated dunite xenoliths, none of the major- or minor-element data on the strained or unstraine d olivine phenocrysts suggest that the strained olivines in the HSDP l avas are exotic. We suggest that most of the olivine phenocrysts in a given flow, whether strained or unstrained, are closely related to the evolved liquid that now forms the groundmass. This is consistent with observed correlations between isotopic systems measured on olivine se parates (e.g., O, He) and isotopic systems dominated by groundmass (e. g., Nd, Pb).