Limited underthrusting of India below Tibet: He/He analysis of thermal springs locates the mantle suture in continental collision.

During continent–continent collision, does the downgoing continental plate underplate far inboard of the collisional boundary or does it subduct steeply into the mantle, and how is this geometry manifested in the mantle flow field? We test conflicting models for these questions for Earth’s archetypal continental collision forming the Himalaya and Tibetan Plateau. Air-corrected helium isotope data (3He/4He) from 225 geothermal springs (196 from our group, 29 from the literature) delineate a boundary separating a Himalayan domain of only crustal helium from a Tibetan domain with significant mantle helium. This 1,000-km-long boundary is located close to the Yarlung-Zangbo Suture (YZS) in southern Tibet from 80 to 92°E and is interpreted to overlie the “mantle suture” where cold underplated Indian lithosphere is juxtaposed at >80 km depth against a sub-Tibetan incipiently molten asthenospheric mantle wedge.

Differential Diffusion of Helium Isotopes in Glass, Quantum-tunneling He Enrichment, and Portable He/He Monitoring of Mantle Processes.

While studying the scientific and engineering aspects of a field-portable He/He ratio detector, we found elevated ratios at comparatively lower temperatures that appear to result from differential diffusion of these isotopes in pure quartz glass. The He enrichment relative to He in lab air, expressed as the ratio R and normalized to the accepted He/He ratio of 1.40 E-06 (R), ranges from peak values of about 200 to 600 in dry static samples.

Multi-level magma plumbing at Agung and Batur volcanoes increases risk of hazardous eruptions.

The island of Bali in Indonesia is home to two active stratovolcanoes, Agung and Batur, but relatively little is known of their underlying magma plumbing systems. Here we define magma storage depths and isotopic evolution of the 1963 and 1974 eruptions using mineral-melt equilibrium thermobarometry and oxygen and helium isotopes in mineral separates. Olivine crystallised from a primitive magma and has average δO values of 4.

Magma reservoir dynamics at Toba caldera, Indonesia, recorded by oxygen isotope zoning in quartz.

Quartz is a common phase in high-silica igneous rocks and is resistant to post-eruptive alteration, thus offering a reliable record of magmatic processes in silicic magma systems. Here we employ the 75 ka Toba super-eruption as a case study to show that quartz can resolve late-stage temporal changes in magmatic δO values. Overall, Toba quartz crystals exhibit comparatively high δO values, up to 10.

Subduction and recycling of nitrogen along the Central American margin.

We report N and He isotopic and relative abundance characteristics of volatiles emitted from two segments of the Central American volcanic arc. In Guatemala, delta15N values are positive (i.e.