THE EVOLUTION OF MAUNA-KEA VOLCANO, HAWAII - PETROGENESIS OF THOLEIITIC AND ALKALIC BASALTS

Mauna Kea Volcano has three exposed rock units. Submarine shield-building tholeites form the oldest unit. Subaerial, interbedded tholeiitic and alkalic basalts form the intermediate age unit (70-240 Ka), and they are partially covered by evolved alkalic lavas, hawaiites and mugearites (4-66 Ka). In contrast to other Hawaiian volcanoes, such as Haleakala and Kauai, lavas from Mauna Kea do not define systematic temporal variations in Pb, Sr or Nd isotopic ratios. However with decreasing age the tholeiitic basalts are increasingly enriched in incompatible elements; therefore the shield and postshield tholeiites were derived from compositionally distinct parental magmas. Submarine shield lavas from the east rift contain forsterite-rich olivine (up to Fo90.5) providing evidence for MgO-rich (14.4 to 17%) magmas. Postshield tholeiitic and alkalic basalts with similar isotopic ratios may have been derived from the same source composition by different degrees of partial melting. If a compositionally and isotopically homogeneous source and a batch melting model are assumed, inversion of incompatible element abundance data for the postshield basalts requires low degrees (< 2%) of melting of a garnet lherzolite source which had near-chondritic abundances of heavy rare-earth elements (REE) but less than chondritic abundances of highly incompatible elements such as Ba, Nb and light REE. As the volcano migrated away from the hotspot, eruption rates decreased enabling high Fe-Ti basalts to form by fractional crystallization in shallow crustal magma chambers. The associated and henocryst-rich, high-MgO postshield lavas (picrites and ankaramites) are products of phenocryst accumulation. Eventually basaltic eruptions ceased, and the youngest Mauna Kea lavas are exclusively hawaiites and mugearites which formed from alkalic basalt parental magmas by clinopyroxene-dominated fractionation at lower crustal pressures.