HR: 14:25h
AN: V22F-04    [PDF]
TI: Heterogeneous melting of intermediate/fast spreading backarc peridotites, Parece-Vela Rift, Marianas backarc
AU: * Snow, J E
EM: jesnow@mpch-mainz.mpg.de
AF: Max-Planck-Institut f\"{u}r Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Ohara, Y
EM: ohara@jodc.go.jp
AF: Hydrographic and Oceanographic Department of Japan, 5-3-1 Tsukiji, Chuo-ku, Tokyo, 104-0045 Japan
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max-Planck-Institut f\"{u}r Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Okino, K
EM: okino@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, Tokyo, 164-8369 Japan
AU: Ishii, T
EM: ishii@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, Tokyo, 164-8369 Japan
AB: The Marianas backarc region is unique in being the only one in which peridotites are extensively exposed. It is also paradoxically one of the fastest-spreading mantle peridotite localities on the ocean floor, second only to Hess Deep. The dredging cruise Kairei KR03-01 in January 2003 added considerably to the sample set available for study from this region. The samples range from fertile lherzolite to harzburgite and dunite. The level of alteration was quite high: Since the Parece Vela Rift (PVR) failed, these samples have been exposed on the sea floor a minimum of 10 million years. The oldest section of the rift sampled was a megamullion structure in chaotic terrain west of the PVR, on crust that is about 25 million years old. Peridotites from a single dredge haul (dredge 3) on this megamullion structure range from residual to highly melt impregnated, with spinel TiO2 contents from 0.01 to 0.9 wt.%. The degree of melting is moderate to high, judging from Cr\# (Cr/(Cr+Al)) between 0.4 and 0.5 in residual (i.e. low TiO2) samples. This is in keeping with an estimated full spreading rate of 8cm/yr during the time the PVR was spreading E-W. In the Parece Vela Rift proper, the new dredging was about evenly divided between fertile compositions (dredges 7, 9 and 15) and more depleted ones (dredges 6, 10, and 12). The dredges without significant TiO2 enrichment are quite homogeneous. In the PVR, the long fracture zones would be expected to have some effect on the melting of the peridotites, but they apparently do not. Fertile dredges 7 and 9 are relatively close to the northern Godzilla Mullion bounding fracture zone, but depleted dredge 12 is on an outside corner (where it should be fertile) and depleted dredge 10 is no farther from the fracture zone than fertile dredge 7. We thus cannot yet draw firm conclusions about the relationship between fracture zone proximity and melting in this region. The intermediate to fast spreading rate (7-8 cm/yr) seems to have resulted in a preponderance of the sites having about 15-17% melting, and is in line with data from Hess Deep (10cm/yr, Cr\# ~ .55, F=18%). In contrast to Hess Deep, many of the samples are quite fertile (Cr\# 15), thus either the degree of melting or the efficiency of melt extraction was not as great as at the East Pacific Rise during the formation of the Hess Deep mantle section. This may be due generally to the many fracture zones in the region even if individual dredge hauls fail to show a clear transform fault effect.
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting