The Record Of Fluid-Rock Interaction In Oceanic Subduction Zones: Insights From The Compositions Of Lawsonite-Bearing Eclogite, Blueschist, And Metasomatite

Abstract

Various processes associated with the compositional evolution of oceanic lithosphere as well as the complex interplay of fluid flow and active deformation along the slab-mantle interface complicate the understanding of 1) the sources of subduction fluids; 2) the timing of fluid-rock interactions that mobilize certain groups of elements; and 3) the extent of fluid-rock interactions in more structurally-coherent terranes vs. mélanges. The results of this dissertation provide useful insights into these aspects based on the compositional record of lawsonite-bearing HP/LT rocks from exhumed subduction complexes. This study reports an extensive dataset of lawsonite (major- and trace-element, and oxygen isotope contents) and its host HP/LT rock (major and trace element contents) 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. Lawsonite compositions remarkably deviate from host rock compositions in terms of Cr/V and Sr/Pb, which are useful to evaluate interactions with serpentinite (high Cr/V) and sediment (low Sr/Pb). In general, both mélanges and the coherent Tavşanlı terrane record extensive interactions with sediment or sediment-sourced fluids prior to and/or at the earlier stage (core) of lawsonite growth. The additional influence of serpentinite or serpentinite-sourced fluids during the growth of lawsonite are recorded in Tavşanlı lawsonite grains, which show Cr-oscillatory zoning and positive deviations in Cr/V. Despite the different tectonic settings, the overall δ18O range of mélange and coherent Tavşanlı lawsonite is greater than that of average mid-ocean ridge basalt (5.7 ‰), suggesting significant contribution of isotopically heavier sediment relative to lighter serpentinite. In contrast, analyzed lawsonite grains from the Corsica coherent terrane are characterized by limited interactions with fluids, possibly sourced from Cr/V depleted, isotopically lighter metagabbro, although extensive metasomatism has been documented in other parts of this terrane. Therefore, the extent of fluid-rock interactions in coherent terranes may depend on the spatial association of efficient fluid pathways (e.g., slab-mantle, lithological interface) unlike in mélanges, which is extensive. Variable extents of fluid-rock interactions occur in oceanic lithosphere during seafloor metamorphism, resulting in compositional heterogeneities even within the same unit of oceanic crust. Mafic protoliths with different bulk-rock compositions likely develop different peak mineral assemblages at similar P-T conditions if factors, such as grain size and/or peak metamorphic H2O content, are sufficiently different and therefore simultaneously contribute to different reaction histories during subduction metamorphism.