HR: 1340h
AN: T53B-1296    [Abstracts]
TI: The use of Mid-Ocean Ridge Basalts to Infer Subsurface Processes Along Slow and Ultraslow Spreading Ridges: a Cautionary Tale
AU: * Lissenberg, C
EM: lissenberg@ipgp.jussieu.fr
AF: Equipe de Géosciences Marines, Institut de Physique du Globe de Paris, 4 Place Jussieu Tour 14, Paris, 75252, France
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road MS#8, Woods Hole, MA 02543, United States
AU: Mével, C
EM: mevel@ipgp.jussieu.fr
AF: Equipe de Géosciences Marines, Institut de Physique du Globe de Paris, 4 Place Jussieu Tour 14, Paris, 75252, France
AB: Mid-ocean ridge basalts (MORB) are commonly used to infer melting and crystallization processes in the underlying subsurface. Here, we draw attention to two caveats, which may be particularly important at the lower end of the global range in spreading rate. First, in areas where crustal accretion is focused, as appears to be the case along most slow- and ultraslow- spreading ridge segments, MORB may not originate from the immediately underlying crust and/or mantle. This is demonstrated by gabbroic rocks exposed in the Kane Core Complex (Mid-Atlantic Ridge), which are primitive in the ancient segment center, compatible with being residues of MORB generation, but too evolved along most of the segment to have been fractionated from MORB. Along most of the segment, then, the basalts are unrelated to the underlying lower crust. The fractionation process appears to have been restricted to the segment center, followed by extensive along-axis intrusion of melts. Second, it is often assumed that most crystallization of melts generated by mantle melting occurs in deep parts of the lithosphere at slow and ultraslow spreading ridges. This is generally based on MORB liquid lines of descent (LLD), which are offset to lower CaO and higher Al₂O₃ at a given MgO from low-pressure LLD. This can be explained by earlier onset of clinopyroxene crystallization, combined with a smaller role for plagioclase fractionation, both of which are expected at higher pressures because the clinopyroxene stability field increases, and that of plagioclase decreases, with increasing pressure. In addition, gabbros often contain high Mg# clinopyroxene, also taken as evidence for pressure-induced early clinopyroxene crystallization. However, the offset of MORB LLD, as well as high-Mg# clinopyroxene, can be explained by reactions between ascending melts and lower crustal primitive cumulates. Thus if melt-rock reaction is an important process in the lower crust, as is suggested by many gabbro suites, MORB compositions cannot be used to infer fractionation pressure.
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Tectonophysics [T]
MN: 2007 Fall Meeting