Element Recycling Recorded by UHP Metasediments

Recycling of components from subducting sediments as recorded by UHP metasediments

In collaboration with Prof. Brad Hacker of UCSB, we are studying ultra-high pressure (UHP) metasediments to constrain the magnitude and P-T conditions of recycling of elements from subducting sediments into arc magmas. Preliminary data show that key tracers, such as Th, which are estimated to have high recycling efficiency (many tens of percent of the subducted Th and Be in sediments shows up in arc magmas), are not depleted in most UHP metasediments. Only rocks that record temperatures in excess of 1000°C at 2 to 4 GPa record significant loss of Th and other recycled elements. We are in the process of extending and documenting this observation.

1000°C is well above the water saturated solidus for metasediments at subduction zone pressures, and many of these samples were at or near saturated in aqueous fluids during peak metamorphism. We are studying trace element partitioning in natural and experimental samples to investigate the possibility that near-solidus melts do not fractionate Th and other recycled elements from metasedimentary residues. If so, either recycling is less efficient than proposed, or near-solidus melting (and sub-solidus loss of aqueous fluid) is not sufficient to recycle these elements.

1000°C is above even the hottest modeled geotherms for oceanic subduction, so our UHP samples that have measurably lost key trace elements are not on likely subduction geotherms. However, there is ample evidence for efficient recycling from subducted sediments in arcs. Thus, it may be that, when they become sufficiently ductile at > ~ 700 or 800°C, metasediments rise diapirically into the overlying mantle wedge.  We are working on dynamic constraints on this process, and considering phase equilibrium experiments on metasediments at mantle wedge conditions.