Death of a back-arc magmatic system: Temporal variations of melt-stagnation in peridotites from the Godzilla Megamullion

Sampling of the Godzilla Megamullion (GM) Oceanic Core Complex located on the extinct Parece Vela Rift within the Philippine Sea has returned a dominantly ultramafic lithology comprised of distinct petrographically depleted, fertile, and melt-percolated groups of samples (1). Petrographic observations of the ultramafic rocks from GM coupled with major element geochemistry of spinels have exhibited systematic, temporal trends in the degree of melt-stagnation along the length of the OCC (2,3,4,5). Notable trends in the percentage of peridotite samples containing evidence of plagioclase impregnation as well as trends in spinel Cr# (Cr / Cr + Al), Mg# (Mg / Mg + Fe), and TiO2 are exhibited along the length of the structure. The distal portion of the mullion (the region closest to the breakaway) exhibits characteristics attributed to normal mid-ocean ridge spreading. The percentage of plagioclase impregnation is ∼25% (7 out of 28 samples), which compares to the roughly 20% average for abyssal peridotites (6). The average Cr# is 0.35 with variation from 0.3 to 0.6 in melt-reacted samples and the TiO2 follows suit with values lower than 0.12 wt. % with the exception of 1.6 wt. % for a single reacted sample. The Medial region exhibits a decrease in plagioclase percentage to 12% (2 out of 17 samples) and fertile Cr#s as low as 0.12 with low TiO2 (values <0.1 wt. %). The proximal region (the region closest to the termination of spreading) exhibits a large increase in the degree of plagioclase impregnation to 53% (62 out of 116 samples). Cr#s in the proximal region range from very fertile (0.12) to depleted (0.65) with an overall increase in the variation and maximum TiO2 content (0 to 0.8 wt. %). These length scale variations are interpreted to indicate a change in overall character for the magmatic system of the GM ridge segment with time. The distal region may represent a robust mantle section related to normal mid-oceanic ridge spreading that rapidly gives way to the fertile medial region. The medial region may then represent a portion of mantle rock which was present in the zone of melting beneath the ridge during the formation of the breakaway. The proximal region exhibits a large increase in melt-stagnation which can be attributed to the runaway process of entrapment of transient melt within the thickening lithosphere beneath the ridge. (1) Ohara, et al., (2003) G3. 4 (7), 8611, 10.1029/2002GC000469. (2) Ohara, et al., (2009), Eos Trans. AGU, 90(52), Fall Meet. Suppl. Abst.Num. T33D-06 (3) Loocke, et al., (2009), Eos Trans. AGU, 90(52), Fall Meet. Suppl. Abst.Num. T21A-1776 (4) Snow, et al., (2009), Eos Trans. AGU, 90(52), Fall Meet. Suppl. Abst.Num. T33D-07 (5)Loocke, M., Snow, J.E., and Ohara, Y. (2010), Systematics of melt stagnation in peridotites from the Godzilla Megamullion, Abstract T23A-2227 presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 Dec. (6) Dick (1989) Geol Soc. Lond. Spec. Pub. 42:71-105.