Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii |
| |
| Authors: | Mark D Kurz William J Jenkins Stanley R Hart David Clague |
| |
| Institution: | 1. Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 U.S.A.;2. Center for Geoalchemy, Massachusetts Institute of Technology, Cambridge, MA 01239 U.S.A.;3. U.S. Geological Survey, Menlo Park, CA 94025 U.S.A. |
| |
| Abstract: | Helium isotopic ratios ranging from 20 to 32 times the atmospheric 3He/4He(RA) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the 3He/4He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower 3He/4He ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio (<5 RA), but a combination of low He concentration, high vesicularity, and presence of cracks lined with clay minerals suggests that the low ratio is due to gas loss and contamination by atmospheric helium.Crushing and melting experiments show that for modest vesicularities (<5% by volume) the Loihi glasses obey a MORB-type partitioning trend, but at higher vesicularities the data show considerably more scatter due to volatile mobilization. The high vesicularities, low extrusion pressure and generally low helium concentrations are consistent with a considerable degree of degassing. Analyses of dunites, plus a correlation between total helium concentrations with xenocryst abundances also suggest that xenocrysts are a significant carrier of contaminating (low 3He/4He) helium.3He/4He ratios from samples of other Hawaiian volcanoes (Kilauea, Mauna Loa, Hualalai, and Mauna Kea) show a smooth decrease in 3He/4He with increasing volcano age and volume. We interpret this to be a synoptic picture of the time evolution of a hot-spot diapir: the earliest stage is characterized by primitive (> 30 RA) helium with some (variable) component of lithospheric contamination added during “breakthrough”, while the later stages are characterized by a relaxation toward lithospheric 3He/4He ratios (～ 8 RA) due to isolation of the diapir from the mantle below (as the plate moves on), and subsequent mining of the inherited helium and contamination from the surrounding lithosphere. The abrupt contrast in 3He/4He ratios between Kilauea and Loihi, despite their close proximity, is indicative of the small lateral extent of the plume. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
