The blueschist to eclogite transition is one of the major geochemicalmetamorphic processestypifying the subduction zone, which releasesfluids triggering earthquakes and arc volcan-ism. Although glaucophane is an index hydrous mineral for the blueschist facies, its stabilityat mantle depths in diverse subduction regimes of contemporary and early Earth has not beenexperimentally determined. Here, we show that the maximum depth of glaucophane stabilityincreases with decreasing thermal gradients of the subduction system. Along cold subductiongeotherm, glaucophane remains stable down ca. 240 km depth, whereas it dehydrates andbreaks down at as shallow as ca. 40 km depth under warm subduction geotherm or theProterozoic tectonic setting. Our results imply that secular cooling of the Earth has extendedthe stability of glaucophane and consequently enabled the transportation of water intodeeper interior of the Earth, suppressing arc magmatism, volcanism, and seismic activitiesalong subduction zones.

Yoonah Bang, Huijeong Hwang, Taehyun Kim, Hyunchae Cynn, Yong Park, Haemyeong Jung, Changyong Park, Dmitry Popov, Vitali B. Prakapenka, Lin Wang, Hanns-Peter Liermann, Tetsuo Irifune, Ho-Kwang Mao, Yongjae Lee, “The stability of subducted glaucophane with the Earth’s secular cooling,” Nat. Commun. 12, 1496 (2021). DOI: 10.1038/s41467-021-21746-8 abstract

Curved lines represent individual subduction geotherms fromSyracuse et al.19. Continuous and dashed curves denote the PT paths of subducting slab surfaces and corresponding slab Moho, respectively. The Tongaand Kermadec represent cold subducting slab, whereas the North Cascadia and South Chile represent warm subducting slabs. Black lines represent theupper- and lower-pressure stability of glaucophane from previous studies33,34,39, while black dashed lines represent related reactions40,6163. The coloredtrilateral regions from top to bottom represent high, intermediate, and low geothermal gradients (δT/δDepth), as defined by Brown and Johnson3. Theforbidden zone is a PT region of ultrahigh pressures, where many numerical models predict slab-top geotherms of <5 °C km1(ref.64). High-temperatureexperiments at ambient pressure were performed as a reference (Supplementary Fig. 9). Phase abbreviations: glaucophane (Gl), jadeite (Jd), enstatite(En), albite (Ab), talc (Tlc), quartz (Qtz), coesite (Cs), andfluid (F).