Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ‹orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning pro®les. However, there are signi®cant di€erences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 ́ Mg/(Mg+Fe)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more ma®c magma saturated only with olivine. The di€erences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations re ̄ect magmas that were bu€ered near the olivine + melt ˆ)Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of ma®c olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges re ̄ect magmas that evolved beyond this reaction point when there was no replenishment with olivinesaturated magma. Two of these samples contain augite cores with Mg# of 71, corresponding to Mg# of 36±40 in equilibrium melts, and augite in another sample has Mg# of 63±65 which is in equilibrium with a very evolved melt with a Mg# of 30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2±3 kbar and 1130± 1160 °C. However, in one sample, augite and glass rind compositions re ̄ect crystallization at higher pressures (4±5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Contrib Mineral Petrol (1999) 135: 355±372 Ó Springer-Verlag 1999 H.-J. Yang (&) Department of Geology, Florida International University, Miami, FL 33199, USA F.A. Frey Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA D.A. Clague Monterey Bay Aquarium Research Institute, PO BOX 628, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, USA M.O. Garcia Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA Present address: 1 SciLab Boston Inc., Weymouth, MA 02189, USA Supplementary Material Full set of compositional data for the minerals has been deposited in electronic form and can be obtained from http://link.springer.de/link/service/journals/00410 Editorial responsibility: T.L. Grove