Preliminary analysis on the mobility of trace incompatible elements during the basalt and peridotite reaction under uppermost mantle conditions

Reaction experiments were conducted between basalt (JB-1) and peridotite (JP-1) at 3 GPa and 1100 °C to investigate the mobility of trace elements migrated from basalt to peridotite and the behavior of incompatible trace elements under upper mantle conditions was studied. The stable phase assemblage of basalt is garnet + clinopyroxene + melt, while olivine + minor orthopyroxene, clinopyroxene and spinel recrystallize in peridotite under this experimental condition. A reaction zone consisting of garnet and orthopyroxene was formed at the interface between the basalt and peridotite layers. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICPMS) was used to determine the concentrations of the major and trace elements as a function of the distance from the reaction zone in the peridotite layer. The migration of the major elements (Mg, Fe, Ca and Al) was restricted to less than the size of a few grains (∼15 μm), from the interface of basalt and peridotite on both sides. In addition, the water content of the peridotite layer was not increased by migration from the hydrous basalt in the sample. On the other hand, the trace incompatible elements derived from the basalt penetrated deep into the peridotite layer beyond the reaction zone to some extent, and the relative abundance of each trace incompatible element showed a distinct dependence on the distance from the reaction zone. Therefore, the transportation is not explained by a small degree of fluid and/or melt infiltration. Elemental depth profiling data obtained by LA-ICPMS are successfully fitted to a semi-infinite diffusant model equation. The effective diffusion coefficients (D) show the following order: DNa ≈ DK ≈ DRb > DSr ≈ DZr ≈ DNb ≈ DBa ≈ DLa ≈ DCe ≈ DTh. DNa in peridotite is 3.3 ± 0.4 × 10−12 m2/s at 3 GPa and 1100 °C, which is about four times larger than D(Sr,Zr,Nb,Ba,La,Ce,Th). We suggest that the fundamental mechanism of mobility in the peridotite layer is likely to be grain boundary diffusion, and the diffusion coefficients of trace elements are shown to be related to their stability in the olivine crystal lattice. We further discuss the formation of the unique chemical heterogeneities in the mantle around the basaltic crust on the basis of the observed mobility difference in the trace incompatible elements.