Katherine Fornash
Dr. Katherine Fornash, Assistant Professor of Geological Sciences at Ohio University, recently published “Lawsonite-rich layers as records of fluid and element mobility in subducted crust (Sivrihisar Massif, Turkey)” in the journal Chemical Geology.
For this study, Fornash studied metamorphic veins and metasomatic layers from northwest Turkey that contain a rarely preserved hydrous mineral, lawsonite. By employing a suite of geochemical techniques, the authors were able to document the fluid sources and the behavior of trace elements during the formation of these layers. This work contributes to our understanding of the behavior of subduction zone fluids and fluid-rock interaction during high-pressure/low-temperature metamorphism.
Abstract:
Lawsonite is an abundant hydrous mineral in subducted oceanic crust and sediments and is an important carrier of water (∼11.5 wt%) and trace elements (REE, U, Th, Sr, Pb) into the mantle. Despite its widespread stability at the high-pressure/low-temperature conditions of subduction, it is rarely preserved in the geologic record, particularly in eclogite. Therefore, scarce fresh lawsonite eclogites in which the composition of lawsonite can be directly determined are important geochemical archives of fluid-mediated processes during subduction. Of particular interest are lawsonite-rich veins and layers that may represent former fluid pathways or metasomatic zones and that therefore may record element mobility related to fluid-rock interaction during subduction and/or incipient exhumation. In the Sivrihisar (Turkey) subduction complex, lawsonite-rich veins and layers comprised of ∼30–50 % lawsonite occur at pod margins where lawsonite eclogite has been partially transformed to lawsonite blueschist. To understand the petrogenesis of these lawsonite-rich assemblages, we conducted a petrographic and geochemical study of a representative lawsonite-rich layer at the margin of a meter-scale lawsonite eclogite pod. Bulk rock and mineral major and trace element analyses were conducted along a transect consisting of the lawsonite-rich layer (Lws + Grt + Ph), its glaucophane-rich margin (Gln + Grt + Lws + Ph + Rt), and the lawsonite eclogite host at varying distances from the layer (∼1–2 cm away and >10 cm away). The bulk-rock composition of the lawsonite eclogite indicates a basaltic protolith that experienced interactions with (meta-)sedimentary rocks before or during the crystallization of high-pressure phases such as garnet and rutile. Integrated major and trace element composition and zoning in high-pressure phases indicate that the lawsonite-rich layer and its associated glaucophane-rich margin likely formed at or near peak eclogite-facies conditions (2.2–2.4 GPa, 520 °C) as a result of fluid-mediated processes that scavenged Al2O3, MnO, Y, Th, HFSE, and REE from the eclogite immediately adjacent to the vein (∼1–2 cm away). Mass balance calculations also suggest the addition of LILE and transition metals (Ni, Cr, Zn) to the lithologic layers at the pod margin; these elements were likely supplied from an external source, such as serpentinites and/or sediments, both of which occur interspersed with mafic rocks in the Sivrihisar complex. Fluid-rock interaction may have also driven changes in fO2, as omphacite, glaucophane, lawsonite, and phengite from the pod margin record core-to-rim increases in Fe3+. These changes in bulk composition preferentially stabilized glaucophane-rich (blueschist) and LREE- and LILE-enriched lawsonite-rich assemblages at the pod margin.
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