Crystals within erupted volcanic rocks record geochemical and textural signatures during magmatic evolution prior to the onset of eruptions. Growth times of microlites can be derived through Crystal Size Distribution (CSD) analysis combined with well-constrained microlite growth rates, yielding petrologically-determined magma ascent timescales. Our newly developed, machine learning image processing scheme allows for the rapid generation of CSD, saving many hours of processing time, which previously involved hand-drawing the outer margins of crystals. For the present study, we examined a range of andesitic tephras from the Tongariro Volcanic Centre, New Zealand. A total of 228 plagioclase and pyroxene microlites CSDs were generated from individual tephra shards. All combined pyroxene and plagioclase microlite CSDs exhibit concave-up shapes, and similar intercepts and slopes at the smallest sizes. This implies similar growth durations of the smallest microlites of 15±9 to 28±15 (2σ) hours, regardless of the eruptive style or source, using an orthopyroxene microlite growth rate constrained from one of the samples. The orthopyroxene thermometer and the plagioclase hygrometer reveal the magmas were erupted at ~ 1079 to 1149 (±39 SEE), and H2O contents ranging from 0-0.4 to 0-1.7 wt.% (95% confidence maxima). In the absence of CO2, these results indicate shallow H2O exsolution pressures of < 240 bars, using a recent H2O-CO2 solubility model. Given the microlite residence times, shallow H2O exsolution driving microlite growth is inconsistent with the explosivity of the eruptions. Instead, our data suggest that the melts either carried large amounts of CO2, triggering earlier degassing of volatiles including H2O, or that microlite crystallisation began prior to degassing. Ongoing work investigates the H2O and CO2 contents hosted by melt inclusions in phenocrysts and microphenocrysts in these tephras to provide constraints on magma ascent rates, with implications for hazard characterization and mitigation.

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.