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>4Petrology and trace element geochemistry of eclogite xenoliths from the Colorado PlateauiI#i( : :I i ;;;,*>4f*4)

Tomohiro Usui (Department of Earth and Planetary Sciences, Tokyo Institute of Technology):etAtL tltf:S (a : ( FJzllffSf )

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It is very important to characterize the trace element behavior in subducted oceanic crust for better understanding

ofmantle/crust recycling. Eclogite xenoliths from the kimber]ite diatreme breccias on the Colorado Plateau have beenthought to be fragments of subducted oceanic crust, because of their basaltic bulk compositions and mineraiogicaltextures similar to ophiolitic eclogites from blueschist terranes (He]mstaedt and Doig, 1 975). The aims ofthis st}Idy areto verify the hypothesis that the eclogite xenoliths are the consequence of subduction zone metamorphism, and to
determine the trace element behavior during subduction of oceanic crust using the eclogite samples.

Eclogite xenoliths examined in this study mainly consist of almandine‑rich gamet, Na‑clinopyroxene, Iawsonite,phengite, zoisite, rutile, pyrite and zircon. Additionall)

coesite, which was identified by using a laser Raman

microscopy, occurs as minute (<20um) inclusion in garnet. The maximum temperature deduced by the method of Ellis

and Green (1979) for neighboring rims of garnet and clinopyroxene ranges from 550'C to 650'C at 2.5GPa. Garnet
crystals are extremely zoned in Mn content from core (‑1.5wto/o) to rim (‑0.3wiolo), with euhedral zoning texture

preserving growih during prograde metamorphism. The major element compositions of clinopyroxene and lawsonite
occurring as inclusions in gamet indicate that the inclusions were crystallized at lower‑grade metamorphic conditions
than the same minerals in the matrix.

Trace element compositions of constituent minerals in the eclogite xenoliths were analyzed by an ion microprobe,

Cameca ims‑5f. Combining the ion microprobe data with the mineralogical observations, we evaluated the traceelement behavior in the eclogite xenolith, which represents subducted oceanic crust. Based on the mass balancecalculations, it is suggested that most of HREE and Zr have been preserved in garnet and zircon, both of which are stablein the eclogite facies. Moreover, approximately 970/0 of the LREE and 890/. of the Sr in the bulk rock are retained bylawsonite, which decomposes during progressive metamorphism in the lawsonite eclogite facies. Prom theseobservations, it is inferred that: (i) migration of the HREE and Zr to the mantle wedge is significantly small; and (ii)migration of the LREE to the mantle wedge is significantly smaller than that of Sr, because LREE contents in lawsonite

increase with nearly constant Sr content during progressive metamorphism.
In addition, we preliminary determined ages of the zircons in garnet of the lawsonite eclogite by a newly installed

ion microprobe. Cameca ims‑1270, resulting in approximately 80Ma. Throughout the course of these observations, it isfinally concluded that the lawsonite eclogite xenolith is a fragment of oceanic crust subducted into more than 90km
depth along a cold geotherm and with a subduction rate >2cm/y, underneath the North American continent.

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