Isotopic constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle xenoliths, Washington, USA

Spinel harzburgite and websterite mantle xenoliths from Simcoe volcano in s outhern Washington represent fragments of mantle lithosphere from the back- are side of the Cascade are front. Previous studies have shown that metasom atism by either silica-rich fluids or hydrous melts crystallized phlogopite , imparted high oxygen fugacities (0.3 to 1.4 log units above QFM), and mor e radiogenic Os isotopic compositions on these peridotites. These features are consistent with part or all of the metasomatic agent being derived from the Juan de Fuca slab. New Re-Os, Sm-Nd, Sr, and U-Th-Pb isotopic data she d further light on the origin and composition of the metasomatic agent. The clinopyroxenes from the xenoliths have correlated Pb isotopic compositions (Pb-206/Pb-204 = 18.63-19.55, Pb-207/Pb-204 = 15.56-15.63, Pb-208/Pb-204 = 38.22-38.87). The most radiogenic Pb isotopic compositions extend beyond t he most radiogenic Pb isotopic compositions for the Cascade are lavas and d isplay a shallower trend. Mixtures between Juan de Fuca basalts and pelagic or terrigenous sediments would result in Pb isotopic compositions that are not radiogenic enough in Pb-207/Pb-204 and Pb-208/Pb-204 at the high Pb-20 6/Pb-204 end of this array. Therefore, models for rapid transfer of compone nts from the slab to the mantle lithosphere are not viable in this case. In stead, a multi-stage model is preferred. In the first stage, the slab compo nent is transferred via fluid or melt into, and reacts with the hanging wal l mantle. This results in a residual slab depleted in Pb relative to U and Th, and consequent high U/Pb and Th/Pb. Additional dehydration or melting o f the slab imparts this chemical signature to the peridotite in the hanging wall. In the second stage, the hybridized hanging wall peridotite evolves for tens of million years until corner flow drags it down to deeper levels in the mantle wedge where melting occurs in response to higher temperatures . In the third stage, this melt migrates upward where it metasomatizes the mantle lithosphere represented by the Simcoe xenoliths. Trace element compo sitions of the clinopyroxenes, and the presence of high alkali glasses in t he xenoliths, are consistent with the metasomatic agent derived from the hy bridized hanging wall being alkali-rich, and possibly similar to potassic-r ich lavas found in are and back-are settings. These data therefore demonstr ate the importance of the hybridized hanging wall mantle above slabs as a s ource for melts which can be metasomatic agents in the upper mantle, and as a site for storage of material derived from the slab for periods of at lea st tens of million years. (C) 1999 Elsevier Science B.V. All rights reserve d.