HR: 14:30h
AN: V43G-04    [Abstracts]
TI: How Many Hotspots are on Present-day Earth, and are all Plumes hot?
AU: * Herzberg, C
EM: herzberg@rci.rutgers.edu
AF: Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 United States
AB: The petrological characteristics of primary magmas that exit the melting regime are sensitive indicators of mantle potential temperature. However, most primary magmas partially crystallize some olivine during transit to the surface, and erupted lavas are typically hybrid mixtures of olivine and solidified liquid. Primitive glass on the surface can have an MgO content that is lower than a parental magma from which it was derived, and a parental magma can differ from its primary magma by partial crystallization of olivine in a crustal magma chamber. However, the parental magma composition can be restored using a simple petrological procedure when olivine is the sole phenocryst phase. On Kilauea the most primitive magnesian glass has been reported to contain 15% MgO, and the most magnesian olivines contain Fo 90-91. The exchange coefficients (Kd) for FeO and MgO between these olivine and glass compositions are 0.25-0.28, much lower than 0.33-0.34 for olivine equilibrated with liquid in melting experiments. The only way to obtain the correct Kd is by computing the effects of dissolving olivine into a 15% MgO liquid composition. This procedure results in a crustal parental magma with 17-19% MgO and a mantle primary magma with 18-20% MgO. The potential crystallization temperature for Kilauea is 1400C, an estimate that includes the effects of 0.34% H2O. Hawaii is therefore a hotspot. This is the most fundamental geological constraint that all models are required to satisfy. It is independent of ongoing questions concerning the role of subducted crust and pyroxenite in the melting regime. A primary magma with 18-20% MgO is successfully reproduced by decompression melting in a hot plume with potential temperatures in excess of 1550C. Hawaii is the only hotspot Earth at the present time. The mantle below Iceland is comparatively cooler, warmer than oceanic ridges, but it was hotter during the early Tertiary. A preliminary analysis of volcanics in and around the African and South Pacific superplumes indicates low extents of wet melting and potential temperatures that may be comparable to oceanic ridges (~1300-1400C). More work is needed for a quantitative petrological evaluation, but it is clear that these volcanoes cannot be hotspots even though they are associated with broad and narrow regions of slow seismic velocities that extend deep into the mantle. The implication is that most plumes or superplumes are buoyant for compositional reasons at or close to ambient mantle temperatures, and they are distinct from the Hawaiian hotspot.
DE: 8180 Tomography
DE: 8124 Earth's interior--composition and state (old 8105)
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