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Abstract Lawsonite is a major host mineral of trace elements (TEs; e.g. REE, Sr, Pb, U, Th) and H2O in various rock types (metabasite, metasediment, metasomatite) over a wide range of depths in subduction zones. Consequently, the composition of lawsonite is a useful archive to track chemical exchanges that occurred during subduction and/or exhumation, as recorded in high-pressure/low-temperature (HP/LT) terranes. This study provides an extensive dataset of major element and TE compositions of lawsonite in HP/LT rocks from two mélanges (Franciscan/USA; Rio San Juan/Dominican Republic), two structurally coherent terranes (Tavşanlı/Turkey; Alpine Corsica/France), and the eclogite blocks of the Pinchi Lake/Canada complex. Bulk major and TE compositions were also determined for lawsonite-bearing host rocks to understand petrogenesis and assess compositional evolution. Most analyzed mélange and coherent-terrane metabasalts have normal mid-ocean ridge/back-arc basin basalt signatures and they preserve compositional evidence supporting interactions with (meta)sediment ± metagabbro/serpentinite (e.g. LILE/LREE enrichments; Ni/Cr enrichments). Most lawsonite grains analyzed are compositionally zoned in transition-metal elements (Fe, Ti, Cr), other TEs (e.g. Sr, Pb), and/or REE, with some grains showing compositional variations that correlate with zoning patterns (e.g. Ti-sector zoning, core-to-rim zoning in Fe, Cr-oscillatory zoning). Our results suggest that compositional variations in lawsonite formed in response to crystallographic control (in Ti-sector zoning), fluid–host rock interactions, modal changes in minerals, and/or element fractionation with coexisting minerals that compete for TEs (e.g. epidote, titanite). The Cr/V and Sr/Pb ratios of lawsonite are useful to track the compositional influence of serpentinite/metagabbro (high Cr/V) and quartz-rich (meta)sediment (low Sr/Pb). Therefore, lawsonite trace and rare earth element compositions effectively record element redistribution driven by metamorphic reactions and fluid–rock interactions that occurred in subduction systems.
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This content will become publicly available on January 1, 2026
Fe 3+ ∕ΣFe variation in lawsonite and epidote in subducted oceanic crust
Abstract. The hydrous Ca–Al silicates lawsonite and epidote group minerals (EGMs) are key phases in subduction-zone H2O and element cycling. In high-pressure–low-temperature metamorphic rocks, Fe in both minerals is typically assumed to be entirely Fe3+, which substitutes for Al in octahedral sites as a major component in most EGMs and as a minor component in lawsonite and zoisite. New Fe micro-X-ray absorption near-edge spectroscopy (μ-XANES) analyses show substantial Fe2+ in lawsonite in blueschist from New Caledonia and zoisite from an unknown locality. Analysed Fe-rich EGMs (epidote, clinozoisite) contain primarily Fe3+. Lawsonite and some EGMs in subducted oceanic crust may contain more Fe2+ than is currently known, with possible implications for understanding subduction redox processes and conditions and why they vary in different subduction zones.
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- PAR ID:
- 10576904
- Publisher / Repository:
- Copernicus
- Date Published:
- Journal Name:
- European Journal of Mineralogy
- Volume:
- 37
- Issue:
- 2
- ISSN:
- 1617-4011
- Page Range / eLocation ID:
- 143 to 149
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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