Sources of Metasomatic Liquids in the Mantle Wedge beneath Batan Island, Philippines
Abstract
The compositions of mantle wedges are complicated by interactions between mantle peridotite and liquids released from subducted materials, mainly altered oceanic crust and sediments. The chemical consequences of such "metasomatism" process are mostly inferred from the compositions of arc lavas because mantle wedge samples are rare. In this study, seven ultramafic xenoliths sampled from Batan Island, northern Luzon arc, were analyzed for trace element abundances as well as Sr and Nd isotopic ratios to trace the sources of metasomatic agents. All these xenoliths are characterized by U-shape REE patterns reflecting interactions with liquid phases. They also have higher 87Sr/86Sr (0.704808-0.706194) and lower 143Nd/144Nd (0.512483-0.512656) and Sr/Ce ratios than host lavas indicating that xenoliths and host lavas are not directly related by partial melting and the interactions between them are limited. These xenoliths, however, contain silicic glass inclusions. The abundances of trace elements in five of these xenoliths are consistent with addition of ~1% silicic melt into peridotites that experienced 20-30% melt extraction. The budgets of highly incompatible elements, such as LILE and LREE, in xenoliths are dominated by silicic melt whereas those of the HREE are controlled by mantle peridotite. Subudcted altered oceanic crust is not a suitable source for the silicic melts for its high Sr/Ce, U/Th and Ba/La ratios. In contrast, the similarities in these abundance ratios between subducted sediments and analyzed xenoliths imply that sediments can be proper sources for the silicic melts. If so, large extents (> 50%) of melting are required to generate silicic melts from subducted sediments. Two of the studied xenoliths are distinct from others for the absence of significant depletions in HFSE and low abundances of highly incompatible elements. In a primitive mantle normalized diagram, their trace element variation patterns are explained by addition of aqueous fluids in equilibrium with eclogites containing no Fe-Ti oxides. In the 87Sr/86Sr-1/Sr and 143Nd/144Nd-1/Nd plots, these two samples form trends extending to low Sr and Nd contents and relatively high 87Sr/86Sr and 143Nd/144Nd ratios, which are interpreted as characteristics of metasomatic aqueous fluids. Consequently, it is proposed that the mantle wedge beneath Batan Island has, at least, undergone two stages of metasomatism; interaction with aqueous fluids at relatively low temperatures, then, at higher temperatures, fluxed by silicic melts derived from melting of subducted sediments.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V51D..03Y
- Keywords:
-
- 8434 Magma migration;
- 8439 Physics and chemistry of magma bodies