Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones

A new record of deeper and colder subduction in the Acatlán complex, Mexico: Evidence from phase equilibrium modelling and Zr-in-rutile thermometry

Variable pressure–temperature (P–T) conditions and paths have previously been reported from eclogites and blueschists from the Acatlán complex, Mexico; yet, the large range of P–T estimates, and uncertainties related with conventional thermobarometry suggest that the P–T evolution of the Acatlán complex remains incompletely understood hindering reliable tectonic interpretations. In this paper, we focus on two high-pressure samples: a phengite eclogite and a garnet-epidote blueschist. We couple phase equilibrium modelling and Zr-in-rutile thermometry to re-evaluate the P–T conditions from these rocks and discuss their implications for the Acatlán complex. The phengite eclogite displays a granoblastic texture with a peak mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and an early retrograde mineral assemblage of amphibole + zoisite + plagioclase + titanite. The garnet-epidote blueschist display a nematoblastic texture with a peak mineral assemblage glaucophane + phengite + epidote + quartz + plagioclase + chlorite + rutile + garnet. Our results indicate that the phengite eclogite followed a clockwise P–T path from peak conditions at ~22 kbar and ~690 °C to re-equilibration conditions at ~14.5 kbar and ~660 °C. Similarly, the garnet-epidote blueschist followed a clockwise P–T path. Phengite zoning indicates two different possibilities for the P–T evolution: (1) from peak conditions at ~19 kbar and ~505 °C or (2) from ~13 kbar and ~480 °C, to re-equilibration conditions at ~8.5 kbar and ~487 °C. Epidote + phengite + albite aggregates (pseudomorphs after lawsonite) and phengite zoning constrain the prograde path within the lawsonite stability field, indicating that the complex evolved through a colder subduction zone (gradient of ~6 °C km−1) than pre-existing estimates. Our results indicate that the eclogite and blueschist reached depths higher than previously thought, ~70 and ~50 km respectively, and experienced a near-isothermal exhumation (~0.15–0.60 °C km−1 for the blueschist, and ~0.9 °C km−1 for the eclogite). These new results, combined with published geochronology, result in a burial rate of ~5.4 km Myr−1, and an exhumation rate of ~3.4 km Myr−1

Mesozoic and Cenozoic Plate Evolution of the Caribbean Region

The reconstruction of Caribbean plate history is an uncertain task, but a task that has intrigued generations of geologists. Each worker has turned to the task of historical interpretation influenced by a particular set of experiences or a special approach, and the results have been accordingly varied. A complete history of interpretations would form the subject of a fascinating chapter in the history of geological philosophy, but such is not the purpose of this chapter. Instead, I will dwell on a set of data that call for what I believe to be a relatively conservative view of Cretaceous and Tertiary plate history. My own interpretation is based heavily on my own or my students’ field experiences in the northeastern West Indies, Guatemala, Belize, and Venezuela, as well as extensive field excursions in Hispaniola, Jamaica, the Lesser Antilles, Central America, and the Dutch Antilles. I am further heavily influenced by the results of the Deep Sea Drilling Project (DSDP), Leg 15, which produced information of fundamental interest in the Venezuelan and Colombian Basins, and by several dissertations of the Princeton University group in northern Venezuela