Water release in subduction zones is not only an important part of the deep Earth’s water cycle, but also plays an essential role in the physical and chemical properties of rocks constituting the deep Earth. To understand water release processes, it is important to know properties of dehydration in hydrous phases of the downgoing slab. Although it is widely accepted that phengite can be stable to greater depth in subduction environment, behavior of hydroxyl and lattice of it at high temperature and high pressure are less investigated in contrast to other hydrous phases. Here, using IR and Raman spectroscopy, we characterize hydroxyl and lattice of ammonium-bearing and ammonium-free phengite at high temperature and high pressure. No proton transferring and structural phase transition in phengite were observed over the measured temperature and pressure range. Both pressure and temperature induce hydroxyl band shifting to lower frequencies, and pressure has a greater impact. The band width of hydroxyl increases with temperature and pressure. Hydroxyl bond weakening and hydrogen disordering at high temperature and high pressure should be responsible for the spectra variations. On the other hand, the lattice modes soften with increasing temperature whereas stiffen under compression, and ammonium plays an important role in the Grüneisen parameters of the lattice modes, especially the K-O mode. These features of hydroxyl and lattice at high temperature and high pressure could benefit for further understanding dehydration, thermodynamic properties and stability of phengite in subduction zones.

The Early Cretaceous Jehol Biota evolution has remarkable spatiotemporal correlation with the destruction of the North China craton though the coupling mechanism remains enigmatic. The craton destruction was accompanied by intense magmatic activity and the released volatiles and nutrients might have had climatic and environmental impacts on the biotic evolution. In this study, we investigated the mentioned hypothetical causal link by determining concentrations and total emissions of volatile elements (S, F, Cl) and bulk-rock P contents of volcanic rocks that were erupted during the pre-flourishing, flourishing and post-flourishing stages of the Jehol Biota. Our results show that the volcanism near the flourishing stage has lower S (1083-2370 ppm), Cl (1277-5608 ppm) and higher P2O5 contents (0.48-0.84 wt.%) than that in non-flourishing stages with S of 1991-3288 ppm, Cl of 7915-12315 ppm and P2O5 of 0.17-0.23 wt.%. Fluorine contents in the three stages vary from 893 to 3746 ppm. The total volatile emissions are minor in the flourishing stage (3.6-6.6 Gt S, 2.2-4.6 Gt Cl and 2.1-4.0 Gt F) but elevated in the non-flourishing stages (1-690 Gt S, 4-934 Gt Cl and 1-308 Gt F). Our data suggest that regional climatic and environmental impacts of volcanism in the non-flourishing stages probably hindered the species diversification. The high P flux released from lithospheric mantle-derived lavas during the peak time of craton destruction might enhance primary productivity and contribute to the flourishing of the Jehol Biota. Our study provides insights into the relationship between the biosphere and deep geodynamic processes driven by volcanism.

Global tomographic models have revealed the existence of two large low shear-wave velocity provinces (LLSVPs) underlying Africa and the Pacific, which are regarded as sources of most typical mantle plumes. Plume-induced basalts have the potential to imply the formation mechanisms and evolutional histories of the LLSVPs. In this study, we measured H2O contents in clinopyroxene and olivine phenocrysts from Cenozoic basalts produced by the Kerguelen and Crozet mantle plumes, which are deeply rooted in the African LLSVP. The results were used to constrain the H2O content in the source of basalts, yielding 1805 ± 579 ppm for the Kerguelen plume and 2144 ± 690 ppm for the Crozet plume. H2O contents in the mantle sources of basalts fed by other plumes rooted in these two LLSVPs were calculated from literature data. Combining these results together, we show that the African LLSVP seems to have higher H2O content and H2O/Ce (620-2144 ppm and 184-592, respectively) than the Pacific LLSVP (262-671 ppm and 89-306, respectively). These features could be ascribed to incorporation of subducted material, which had experienced variable degrees of dehydration during its downwelling, into the LLSVPs. Our results imply that the continuous incorporation of subducted oceanic crust modifies the compositions of LLSVPs and induces heterogeneous distribution of H2O within individual LLSVPs and distinct H2O contents between the African and Pacific LLSVPs. This suggests that the African and Pacific LLSVPs might have different formation and evolution histories.