Cycling of B, Li, and LILE (K, Cs, Rb, Ba, Sr) into subduction zones: SIMS evidence from micas in high-P/T metasedimentary rocks

The Catalina Schist (California) affords a unique opportunity to evaluate the effects of varying prograde P–T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting sediments. Analysis of a suite of trace elements (Li, B, Rb, Sr, Cs, Ba) in micas obtained by secondary ion mass spectrometry (SIMS) complements previously published whole-rock data for metasedimentary rocks in the Catalina Schist and circumvents problems associated with chemical/isotopic overprinting during retrogression, further elucidating trace element redistribution during devolatilization along prograde P–T paths.SIMS mineral analyses confirm the inference from the whole-rock data that white mica (and in a few higher-grade samples, also biotite) is the dominant reservoir for B, Rb, Ba, and Cs. The mineral residency of Li and Sr is more complex, as Li is also highly concentrated in chlorite (where present) and in some amphiboles, and Sr whole-rock budgets are strongly influenced by the presence or absence of Ca-rich phases (e.g., lawsonite, clinozoisite, minor carbonate and titanite). Changes of concentrations of B and Cs, in white micas, with increasing metamorphic grade parallel the whole-rock trends, further demonstrating the loss of these elements from recrystallizing white micas during prograde devolatilization reactions. Trace element data for other minerals in the metasedimentary samples (Ca–Al silicates, plagioclase, chlorite, biotite, garnet, kyanite) confirm expected concentrations as related to ionic radius and charge and demonstrate element redistribution during prograde (and retrograde) reactions. For amphibolite-facies metasedimentary rocks, multiple generations of white mica, including a finer-grained, higher-Si generation related to retrogradation, have distinct trace element compositions. In pegmatites produced by partial melting at amphibolite-facies conditions, B, Li, and the LILE are similarly concentrated in coarse-grained muscovite, indicating inheritance of trace element compositions from their metasedimentary sources.These SIMS data, combined with the whole-rock data for the same samples, demonstrate that the deep transfer of relatively “fluid-mobile” trace elements is strongly dependent upon the prograde P–T path experienced by the subducting sediments. Efficient retention of B, Cs, and N in micas, to depths of > 40 km, is evident from study of these and other forearc metasedimentary suites that experienced low-T prograde paths representative of most modern subduction zones, and is consistent with models invoking additions of these elements from the slab into arc source regions, perhaps to contribute to the across-arc chemical trends (Be/Be, B/La, Cs/Th, Ba/Th) observed in some margins.