RECYCLING OF NOBLE GASES BY CONTINENTAL DEEP SUBDUCTION

Noble gases are useful in tracing the geochemical evolution of Earth' mantle, crust and atmosphere. Recycling of noble gas into the mantle is recored by oceanic basalts and mantle xenoliths, which also indicate the recycling of oceanic crust into the mantle. It is intriguing whether continental crust with noble gas can be recycled into the mantle by continental deep subduction to result ing ultrahigh-pressure metamorphism. If such recycling did occur, the mechanism by which the noble gas isotopic signature of supracrustal materials were incorporated into the mantle remain enigmatic.

Postcollisional mafic igneous rocks are common in collisional orogens, which may have bearing on the recycling of subducted contiental crust into the mantle. The noble gas extracted from minerals in the postcollisional mafic igneous rocks may provide important information about the recycling during continental deep subduction. In this contribution, we report for the first time the mineral noble gas isotope compositions of postcollisional mafic igneous rocks from the Dabie orogeny in east-central China. The He and Ar isotope analyses of pyroxene from these postcollisional mafic rocks yield extremely low ³He/⁴He of 0.002 to 1.8 Ra and air-like ⁴⁰Ar/³⁶Ar ratios of 393.6 to 1599.8. There are also correlations between mineral noble gas isotope and whole-rock geochemical compositions. These observations demonstrate the mass transfer from the deeply subducted continental crust to the overlying mantle wedge, witnessing the source mixing between the crust-derived melt and the mantle peridotite in the continental subduction channel. A direct addition of the crustal He via crust-derived melt to the mantle leads to the extremely low ³He/⁴He ratios in orogenic lithospheric mantle, and the dissolved atmospheric Ar in the subducted supracrustal rocks results in the air-like ⁴⁰Ar/³⁶Ar ratios. It is demonstrated that the recycling of noble gases into the mantle by the deep subduction of continental crust. Our findings also establish the slab-mantle interaction model for the recycling of noble gases in the continental subduction zone.