TY - JOUR
T1 - Fractionation of trace elements by subduction-zone metamorphism - effect of convergent-margin thermal evolution
AU - Bebout, Gray E.
AU - Ryan, Jeffrey G.
AU - Leeman, William P.
AU - Bebout, Ann E.
N1 - Funding Information:
We acknowledge support by NSF grants (EAR92-06679 and EAR94-05625 to GEB; EAR90-18996, EAR91-19110, and EAR94-19018 to WPL; EAR94-05404 to AEB). Special thanks are extended to the Oregon State University Reactor Sharing Program, through which we obtained the INAA data, and to Julie Morris and Fouad Tera, in whose laboratories (Department of Terrestrial Magnetism and Geophysical Laboratory, Carnegie Institution of Washington) the B-Be-Li analyses were performed. We thank Colin Graham and John Craven at the University of Edinburgh for their help in acquiring the ion microprobe data and Simon Peacock and James Brenan for their reviews. \[CL\]
PY - 1999/8/15
Y1 - 1999/8/15
N2 - Differential chemical/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at ≤45 km depths) of rock packets metamorphosed along successively lower-T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, affords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks. In the Catalina Schist, the most extensive devolatilization occurred in metasedimentary rocks which experienced prograde P-T paths encountering the epidote-blueschist facies (>350°C at 9 to 12 kbar) or higher-T conditions; such rocks are depleted in 'fluid-mobile' elements such as N, B, Cs, As, and Sb relative to protoliths. Removal of these elements resulted in changes in B/(Be, Li, La, Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and C(reduced)/N, and increases in δ15N and δ13C. The relative susceptibilities of the 'fluid-mobile' elements to loss along increasingly higher-T P-T paths can be categorized. Boron and Cs show the greatest susceptability to low-T removal by fluids, showing >50% depletion in even lawsonite-blueschist-facies metasedimentary rocks which experienced relatively low-T prograde metamorphic paths. In rocks which experienced higher-T paths, As and Sb (likely in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at ~25% of protolith contents in even partially melted amphibolite-facies rocks. Variations in B/Be, Cs/Th, As/Ce, and Sb/Ce among arcs from differing convergent-margin thermal regimes, and conceivably some cross-arc declines in these ratios, are compatible with evidence from the Catalina Schist for varying degrees of element removal as a function of prograde thermal history. In relatively cool subduction zones (e.g., Kuriles, Marianas, Aleutians, southern Alaska) with thermal regimes similar to that which formed the low-grade units of the Catalina Schist (and blueschist-facies rocks in the Franciscan Complex), forearc devolatilization is less profound, B, Cs, As, Sb, and N are more likely to be deeply subducted, and enriched in arc lavas, and significant devolatilization occurs at the blueschist-to-eclogite transition. High-grade units could reflect thermal evolution analogous to that of relatively warm subduction zones (e.g., Cascadia) and back-arcs in which arc lavas are depleted in B, Cs, As, and Sb due to prior removal by forearc devolatilization. The results of this study also imply less efficient recycling of these elements during the warmer Archean subduction which resulted in greater slab melting and production of abundant trondhjemite-tonalite magmatic suites.
AB - Differential chemical/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at ≤45 km depths) of rock packets metamorphosed along successively lower-T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, affords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks. In the Catalina Schist, the most extensive devolatilization occurred in metasedimentary rocks which experienced prograde P-T paths encountering the epidote-blueschist facies (>350°C at 9 to 12 kbar) or higher-T conditions; such rocks are depleted in 'fluid-mobile' elements such as N, B, Cs, As, and Sb relative to protoliths. Removal of these elements resulted in changes in B/(Be, Li, La, Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and C(reduced)/N, and increases in δ15N and δ13C. The relative susceptibilities of the 'fluid-mobile' elements to loss along increasingly higher-T P-T paths can be categorized. Boron and Cs show the greatest susceptability to low-T removal by fluids, showing >50% depletion in even lawsonite-blueschist-facies metasedimentary rocks which experienced relatively low-T prograde metamorphic paths. In rocks which experienced higher-T paths, As and Sb (likely in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at ~25% of protolith contents in even partially melted amphibolite-facies rocks. Variations in B/Be, Cs/Th, As/Ce, and Sb/Ce among arcs from differing convergent-margin thermal regimes, and conceivably some cross-arc declines in these ratios, are compatible with evidence from the Catalina Schist for varying degrees of element removal as a function of prograde thermal history. In relatively cool subduction zones (e.g., Kuriles, Marianas, Aleutians, southern Alaska) with thermal regimes similar to that which formed the low-grade units of the Catalina Schist (and blueschist-facies rocks in the Franciscan Complex), forearc devolatilization is less profound, B, Cs, As, Sb, and N are more likely to be deeply subducted, and enriched in arc lavas, and significant devolatilization occurs at the blueschist-to-eclogite transition. High-grade units could reflect thermal evolution analogous to that of relatively warm subduction zones (e.g., Cascadia) and back-arcs in which arc lavas are depleted in B, Cs, As, and Sb due to prior removal by forearc devolatilization. The results of this study also imply less efficient recycling of these elements during the warmer Archean subduction which resulted in greater slab melting and production of abundant trondhjemite-tonalite magmatic suites.
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U2 - 10.1016/S0012-821X(99)00135-1
DO - 10.1016/S0012-821X(99)00135-1
M3 - Article
AN - SCOPUS:0032727608
SN - 0012-821X
VL - 171
SP - 63
EP - 81
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
IS - 1
ER -