HR: 16:30h
AN: V52F-03    [PDF]
TI: On the Thermo-Kinetic Consequences of Slab Melting
AU: * Liang, Y
EM: Yan_Liang@brown.edu
AF: Brown University, 324 Brook St, Providence, RI 02912
AB: Melting of subducted oceanic crust occurs when its {\it P-T} trajectory intercepts the wet basalt solidus ($700-800\deg$C at 2-4 GPa). Thermomechanic calculations have shown that the temperature near the top of a subducted plate is around $500-800\deg$C at 100 km depth, depending on the age of the slab and the rate of subduction. Given the relatively cold environment one might expect that kinetics play an important role in slab dehydration melting, as diffusion rates of many incompatible trace elements in eclogitic minerals are very slow under these conditions. The purpose of this study is to explore the thermo-kinetics of slab melting. A key parameter that measures the role of kinetics during slab melting is the rate of mass transfer ($R$) for a given element between a residual mineral and the melt relative to the rate of melting ($M$), designated as $Dm$. Disequilibrium melting is important when $Dm < 1$. A lower bound for $M$ during slab melting can be estimated from a mass balance calculation ($10^{-15}$ s$^{-1}$). $R$ depends on (1) the rate of cation diffusion in the solid, (2) the extent of deformation in the slab, and (3) the degree of dissolution-reprecipitation during melting. Given that (2) and (3) are poorly constrained for eclogitic minerals, we estimate $R$ using the solid diffusive time scale but with a small effective grain radius, to account for those complicated processes (10 $\mu$m for cpx and 100 $\mu$m for garnet). Using diffusivities extrapolated from Van Orman et al. (2002), we found that $Dm$ is around $10^{-3}\sim10^{-1}$ for LREE in the cpx and $\sim 1$ for REE in the garnet at $800\deg$C and 3 GPa, whereas $Dm > 1$ for REE in the cpx when $T > 1000\deg$C. Hence during slow melting of an effectively fine-grained eclogite, the the LREE in the cpx and the melt are likely controlled by the disequilibrium melting processes at near solidus temperatures. Diffusivities of the alkali elements Li and Na in the cpx are expected to be much larger than those of REE in the same mineral. Hence some extends of decoupling between the alkali elements and LREE are also expected during slab melting. To better understand the thermo-kinetic consequences of slab melting we calculate the incompatible trace element abundance in melts produced by dynamic disequilibrium partial melting of a hornblende-bearing eclogite, following a {\it P-T} path similar to that of a young subducting slab. Model calculations show that the LREE abundance in melts produced at small degrees of melting is more likely affected by the sluggish kinetics and hence is lower (not higher) than the LREE abundance in melts generated at larger degrees of melting. The LREE abundance in the larger degree melts is broadly similar to that in adakites and in melts produced at smaller degrees of equilibrium melting. If adakites are generated by partial melting of residual eclogites that underwent small degrees of disequilibrium partial melting at low temperatures, the actual degree of melting needed to generate these magmas is likely larger than previous equilibrium melting models suggested. The small-degree melts produced in the colder parts of the slab are more strongly affected by the sluggish kinetics and thus have lower La/Yb ratios, and higher Li$_2$O and Na$_2$O than melts produced by large degrees of melting at higher temperatures. Such Na$_2$O-rich, hydrous melts may be easily segregated from the slab. If these small-degree melts are sampled, their slab-origin may remain unrecognized because their trace element compositions are different than typical adakites. More likely, the small-degree melts may be dispersed in the mantle wedge, leaving a unique trace element and isotopic fingerprint in the peridotite that itself may undergo partial melting at a later time during island arc magmatism.
DE: 1065 Trace elements (3670)
DE: 3210 Modeling
DE: 3640 Igneous petrology
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting