HR: 17:30h
AN: T44C-07    [Abstracts]
TI: Temperature of mantle melts beneath Central America: Integrating petrologic and seismic observations
AU: * Plank, T
EM: tplank@bu.edu
AF: Boston University, Dept Earth Sci, Boston, MA 02466,
AU: Rychert, C
EM: crychert@ucsd.edu
AF: UC San Diego, Scripps Inst Oceang, La Jolla, CA 92037,
AU: Fischer, K
EM: Karen_Fischer@brown.edu
AF: Brown University, Dept Geol Sci, Providence, RI 02912,
AU: Abers, G
EM: abers@bu.edu
AF: Boston University, Dept Earth Sci, Boston, MA 02466,
AU: Syracuse, E
EM: syracuse@bu.edu
AF: Boston University, Dept Earth Sci, Boston, MA 02466,
AB: Mantle melting beneath volcanic arcs is likely polybaric and polythermal, driven by a combination of decompression and hydration. Here we integrate petrologic and seismic observations to constrain the temperature and water content of the bulk melt and the hot zone within the mantle wedge. Results from the TUCAN seismic experiment in Central America show a vertical region, from 150-50 km, of high Vp/Vs beneath the volcanic front of Nicaragua, which may correspond to a melting column. Seismic attenuation tomography shows a wedge-shaped region of high attenuation (1/Q) centered at 75 km depth, which may correspond to the region of maximum temperature in the mantle wedge. The intersection of these regions predicts a zone of maximum melt generation at around 75 km, and melts from this region might dominate the bulk magma that erupts from the volcanoes. Maximum values of 1/Q are higher for the Nicaragua wedge than the Costa Rica wedge, and correspond to higher peak temperatures, assuming dry olivine with uniform grain size. To test this view, we calculate the olivine-liquid temperatures for the most primitive basaltic compositions (Mg# > 50) erupted from Nicaragua and Costa Rica volcanoes, using the thermometer of Sugawara (JGR 2000). Equilibrium olivine is added until whole rock compositions are in equilibrium with mantle olivine (Fo90), assuming 15% Fe3+/FeT. Temperatures calculated from anhydrous melt compositions at 2.5 GPa (75 km) are 1419 +/- 14 C (1 sd) and 1392 +/- 13 C for Nicaragua and Costa Rica magmas, respectively. The small regional difference in mean temperature derives from significant differences in the Fe90 of Nicaragua and Costa Rican volcanics. The sense of the temperature difference is in agreement with the predictions of the seismic attenuation, but the magnitude is small. Central American magmas, however, contain substantial dissolved water as measured in olivine-hosted melt inclusions (c.f., Benjamin, JVGR 2007), with Nicaragua magmas (4-6 wt% H2O) being wetter than Costa Rican magmas (2-4 wt%). Such high water contents in melts and equilibrium mantle olivines will affect both the olivine-liquid temperatures and attenuation-based temperatures. Assuming 4 and 2.5 wt% H2O in primary Nicaragua and Costa Rica magmas, respectively, the olivine-liquid temperatures drop to 1314 +/- 14 C (Nicaragua) and 1321 +/- 13C (Costa Rica). Thus, the temperature of the mantle wedge might actually be similar regionally, with the higher attenuation observed beneath Nicaragua due to higher water contents in the mantle. Peak wedge temperatures of 1315-1320 C beneath Central America are well below the dry mantle solidus at 2.5 GPa (1420 C; Hirschmann G3 2003), and would require water-fluxed melting throughout the mantle wedge.
DE: 1038 Mantle processes (3621)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting