Does the "mantle" helium signature provide useful information about lithospheric architecture of Tibet/Himalaya?
Abstract:Measurements of 3He/4He > 0.1*Ra (where Ra = 3He/4He in Earth's atmosphere) in geothermal fluids are conventionally taken to represent derivation from a mantle source. 3He/4He values < 0.1*Ra are taken to represent only radiogenic helium with no modern mantle input (the canonical 3He/4He ratio for the crust is 0.02*Ra). Upward transport rates are hard to constrain, but transit times of 3He through the crust in a CO2-rich carrier fluid may be as short as a few years, so 3He/4He measurements offer a proxy for mantle temperature on geologically short time-scales. In Tibet, enhanced 3He/4He ratios could in principle represent (1) incipient partial melt of Indian lithospheric mantle; (2) of Asian lithospheric mantle; (3) upwelling asthenosphere north of underthrust India or along tears in the subducting Indian plate; and/or (4) high-T prograde metamorphism releasing previously trapped 3He from older, voluminous mafic/ultramafic rocks in the crust.
We present data from our recent field campaigns and our compilations from the western and Chinese literature. Any individual observation of 3He/4He > 0.1*Ra may still be argued to result from mantle-derived 3He previously stored in the crust. However, our growing regional database of widely spaced observations of 3He/4He > 0.1*Ra, from the Karakoram Fault in the west to the Sangri-Cona rift and Yalaxiangbo Dome in the east, and from south of the Yarlung-Zangbo suture (YZS) to north of the Banggong-Nujiang suture, makes such special pleading increasingly implausible. The observation of 3He/4He > 0.1*Ra at the YZS and even within the Tethyan Himalaya south of the YZS cannot represent melting of Indian mantle close to the Moho unless existing thermal models are grossly in error. The source of 3He close to the YZS is likely either asthenosphere accessed by faults and shear zones that cut through subducting Indian lithospheric mantle; or incipient melt of Asian lithospheric mantle at the Moho north of the northern edge of underthrust India (the “mantle suture”) which must therefore lie close to the YZS. Thus far we have barely tapped the rich potential that helium-isotope data offer for understanding transit of mantle volatiles through some of Earth's thickest (and ductilely flowing) crust.