HR: 0800h
AN: V51B-0552 [Abstracts]
TI: Generation of Primary Kilauea Magmas: Constraints on Pressure, Temperature and Composition of
Melts
AU: * Gudfinnsson, G H
EM: g.gudfinnsson@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd, NW, Washington, DC 20015-1305
United States
AU: Presnall, D C
EM: presnall@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd, NW, Washington, DC 20015-1305
United States
AU: Presnall, D C
EM: presnall@gl.ciw.edu
AF: Dept of Geosciences, Univ of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083-0688
United States
AB:
Picrite glasses from the submarine extension of Kilauea, Puna Ridge, which contain up to 15.0 wt$%$ MgO, are the most
magnesian glass samples reported from Hawaii. Their compositions form a distinct olivine fractionation trend. A comparison of
this trend with phase relations of garnet lherzolite in the CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$ (CMAS) and
CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$-Na$_{2}$O-FeO (CMASNF) system indicates that melts parental to the Hawaiian picrites are
produced by melting of a garnet lherzolite source at a pressure of 5 $\pm$ 1 GPa. The primary melt composition for Kilauea
proposed by Clague {\it et al.} (1995), which has 18.4 wt$%$ MgO, is close to the expected 5 GPa melt composition. By using
the pressure-independent CMASNF geothermometer (Gudfinnsson and Presnall, 2001), we obtain a temperature of formation of
1450$\deg$C for the most magnesian Puna Ridge glass after correction for the presence of 0.4 wt$%$ H$_{2}$O and 0.7 wt$%$
CO$_{2}$. This assumes that the glass is not much modified after separation from the lherzolite source. However, comparison
with phase relations in the CMAS system strongly suggests that the most magnesian Puna Ridge glasses are the product of some
olivine fractionation, and therefore give temperature considerably lower than that of the source. When applied to the
proposed Kilauea primary melt composition of Clague {\it et al.} (1995), the CMASNF geothermometer gives a melting
temperature of 1596$\deg$C or about 1565$\deg$C after correction for the presence of volatiles. This compares well with the
anhydrous solidus temperature of 1600 $\pm$ 15$\deg$C at 5 GPa for the fertile KR4003 lherzolite (Lesher {\it et al.}, 2003),
which has the complete garnet lherzolite phase assemblage present at the solidus at this pressure. This consistency supports
use of phase relations from the CMAS system and the CMASNF geothermometer to the Puna Ridge picrite compositions. With the
pressure and temperature of melting known, one can calculate the potential temperature of the Hawaiian mantle, provided
certain conditions are met. The calculation assumes that the temperature at the point of melt segregation is close to the
temperature of the solid adiabat. If extensive melting has occurred prior to the segregation, this will be incorrect.
Secondly, it is assumed that the melting is occurring at the non-conducting part of the geotherm. Provided this is the case
and the Kilauea primary melt composition truly represents a near-primary melt composition, we derive a potential temperature
for the mantle beneath Kilauea of about 1500$\deg$C. The very high temperature and pressure conditions for magma generation
at Hawaii appear to be unmatched by any other currently active volcanism on the Earth. Thus, of all the candidates for plume
status, Hawaii appears to be the most robust.
DE: 8439 Physics and chemistry of magma bodies
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting