Squeezing out the slab — modelling the release of Li, Be and B during progressive high-pressure metamorphism

A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab

“…A plausible explanation for the Li decrease during prograde garnet growth is a coupled substitution of Li + and HREE 3+ with 2 Mg,Fe 2+ . Garnet rims that grew during peakmetasomatic conditions are relatively Li-poor due to co-crystallization of omphacite, which strongly prefers Li relative to garnet (Marschall et al, 2007a). Comparing the different zoisite eclogite samples, Li contents in omphacite are relatively homogenous, suggesting a more penetrative metasomatism than in the MORB-type eclogites and/or re-equilibration in omphacite aided by the presence of the metasomatic fluid.…”

“…4). This is surprising as Li is usually considered relatively incompatible in garnet, with Li partition coefficients D grt-fluid in the range 0.005-0.008 (Brenan et al, 1998;Marschall et al, 2007a). A plausible explanation for the Li decrease during prograde garnet growth is a coupled substitution of Li + and HREE 3+ with 2 Mg,Fe 2+ .…”

“…Only the highly mobile and fast diffusing Li, which diffuses 2-3 orders of magnitude faster than most other elements (Richter et al, 2003) and which is enriched in the fluid relative to the rock (e.g. Marschall et al, 2007a), was able to leave a geochemical imprint of the high-pressure fluid flow in the surrounding rocks.…”

“…The isotopic evidence for localized fluid flow and the Li enrichment is most consistent with a MORB dehydration fluid affecting the MORB-type eclogites. Lithium enrichment in the MORB-type eclogites clearly points towards a Li-rich fluid, and MORBdehydration fluids constitute a potential Li source because they can be significantly enriched in Li (up to 200 ppm) due to rock-fluid partition coefficients of 0.06-0.13 (Marschall et al, 2007a). To maintain the high Li contents from the MORB-fluid in the rock, the fluid needs to be at least partly consumed during recrystallization in the eclogite facies.…”

“…During exhumation, HP subducted rocks can be incorporated into the subduction channel, i.e. the intensely serpentinized part of the mantle wedge above the surface of the subducted slab (e.g., Gerya et al, 2002), and re-hydration and addition of fluidmobile elements from slab-derived fluids can be expected (Bebout, 1991;Horodyskyj et al, A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 5 2009; Marschall et al, 2007a;Sorensen and Grossman, 1989;Sorensen et al, 1997;van der Straaten et al, 2008).…”