Geochemistry of Serpentinite Muds and Metamorphic Rocks from the Mariana Forearc, ODP Sites 1200 and 778–779, South Chamorro and Conical Seamounts

Abstract

New geochemical data on serpentinite muds and metamorphic clasts recovered during Ocean Drilling Program Legs 195 (Holes 1200A– 1200E) and 125 (Holes 778A and 779A) provide insights into the proportions of rock types of various sources that compose the serpentinite mudflows and the fluid-rock interactions that predominate in these muds. We interpret the metamorphic rock fragments as derivatives of mostly metamorphosed mafic rocks from the descending Pacific oceanic crust. Based on their mid-ocean-ridge basalt (MORB)-like Al2O3, TiO2, CaO, Si/Mg, and rare earth element (REE) systematics, these metamorphic rocks are classified as metabasalts/metagabbros and, therefore, ~30-km depths represent an active subduction zone setting. The serpentinite muds from Holes 1200A and 1200E have slightly lower REE when compared to Hole 1200D, but overall the REE abundance levels range between 0.1–1 × chondrite (CI) levels. The chondrite-normalized patterns have [La/Sm]N ~ 2.3 and [Sm/Yb]N ~ 2. With the exception of one sample, the analyzed metamorphic clasts show flat to slightly depleted light REE patterns with 1.0–15 × CI levels, resembling MORBs. Visually, ~6 vol% of the serpentinized muds are composed of “exotic” materials 1Savov, I.P., Guggino, S., Ryan, J.G., Fryer, P., and Mottl, M.J., 2005. Geochemistry of serpentinite muds and metamorphic rocks from the Mariana forearc, ODP Sites 1200 and 778–779, South Chamorro and Conical Seamounts. In Shinohara, M., Salisbury, M.H., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 195, 1–49 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Department of Geology, University of South Florida—Tampa, 4202 East Fowler Avenue, SCA 528, Tampa FL 33620-5201, USA. Correspondence author: savovi@si.edu 3Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Road, POST 602B, Honolulu HI 96822, USA. 4Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Marine Sciences Building, Honolulu HI 96822, USA. Initial receipt: 10 January 2004 Acceptance: 12 July 2004 Web publication: 24 February 2005 Ms 195SR-103 I.P. SAVOV ET AL. SERPENTINITE MUD AND METAMORPHIC ROCK GEOCHEMISTRY 2 (metamorphic clasts [schists]). Our mixing calculations confirm this result and show that the serpentinite muds are produced by additions of ~5% metamafic materials (with flat and up to 10 × CI REE levels) to serpentinized peridotite clast material (with very low REE abundances and U-shaped chondrite-normalized patterns). The preferential incorporation of B, Cs, Rb, Li, As, Sb, and Ba into the structure of H2O-bearing sheet silicates (different than serpentine) in the Leg 125 and Leg 195 metamorphic clasts (chlorite, amphibole, and micas) have little effect on the overall fluid-mobile element (FME) enrichments in the serpentinite muds (average B = ~13 ppm; average Cs = ~0.05 ppm; average As = ~1.25 ppm). The extent of FME enrichment in the serpentinized muds is similar to that described for the serpentinized peridotites, both recording interaction with fluids very rich in B, Cs, and As originating from the subducting Pacific slab.