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Lawsonite geochemistry and stability – implication for trace element and water cycles in subduction zones
Author(s) -
Martin L. A. J.,
Hermann J.,
GauthiezPutallaz L.,
Whitney D. L.,
Vitale Brovarone A.,
Fornash K. F.,
Evans N. J.
Publication year - 2014
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/jmg.12093
Subject(s) - lawsonite , geology , allanite , geochemistry , trace element , metamorphic rock , titanite , solidus , subduction , monazite , paleontology , eclogite , chemistry , zircon , alloy , organic chemistry , tectonics
This contribution reviews the existing data on lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE , Sr, U, Th and Pb in basaltic compositions. Trace element‐rich lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high‐ P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction‐zone metamorphism. The lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the lawsonite‐breakdown reaction is within 50 °C of the solidus at sub‐arc pressures of 3–4 GPa.