Geochemistry and Mineral Deposits, Department of Earth Sciences
University of Ottawa, Canada
Current Research in Earth Science Seminar, "Role of Serpentinites in Recycling of Elements"
Thursday, October 10, 2013
to 5:00 PM
100 Keith-Wiess Geological Laboratories
6100 Main St
Houston, Texas, USA
Serpentinites consist of serpentine-group minerals that are hydration products of olivine-rich ultramafic rocks (peridotites). These rocks are not abundant at the surface but the geophysical data suggest their presence in mantle wedges and at depth along major faults. Their high water contents, ~ 13 %, and wide pressure-temperature stability field means that they are a significant reservoir of water in the mantle; in addition, they play a major role in the recycling of elements from the surface to the mantle, and back to the surface via arc volcanoes.
Serpentinites form in three settings. Abyssal peridotites hydrate near oceanic ridges (type 1) and outer-rises near trenches (type 2). Forearc mantle peridotites are hydrated near the base of mantle wedge (type 3). Abyssal peridotites near oceanic ridges (type 1) take up halogens, B, S, U, Sr and carbonate from seawater during their hydration. Such abyssal serpentinites are well exposed on the ocean floor formed at slow spreading ridges, such as the Atlantic Ocean. Plate motion transports serpentinites with fluid-mobile elements from the ridge toward the subduction zone. When the oceanic plate approaches trenches, fractures around outer rises allow deep penetration of sea water through sediment cover. Soluble elements in the sediments, such as F, I and As, are transferred to these hydrating peridotites (type 2). As sediments and slabs are subducted and heated, they release water to the overlying mantle wedge, causing a serpentinite layer to form near the base of the mantle wedge (type 3). These mantle-wedge serpentinites show a wide compositional range reflecting the nature of subducted material, but overall are enriched in I, As, Sb, Li, Pb, and LREE. After formation they are dragged to deeper levels by mantle corner flow; eventual dehydration of the mantle-wedge serpentinites to anhydrous peridotites at ~ 100 km depth transfers their water and fluid-mobile elements to the interior of the mantle wedge. The water, in particular, acts as a flux to generate partial melts that lead to arc magmatism. This intermediate reservoir of water in serpentine minerals explains the lack of volcanism in forearcs, despite dehydration of sediments and slab starting at shallow depths.
Biography of Keiko Hattori: http://www.science.uottawa.ca/fac/professor_details.html?en/59