Silicic calderas can evacuate 100 to >1000 km3 of rhyolitic products in a matter of days to months, leading to questions on pre-eruptive melt generation and accumulation. Whereas silicic plutonic units may provide information on the igneous evolution of crystal-mush bodies, their connection with volcanic units remains enigmatic. In the Ivrea–Verbano Zone of the southern Alps, the plumbing system of a Permian rhyolitic caldera is exposed to a depth of about 25 km in tilted crustal blocks. The upper-crustal segment of this magmatic system (also known as the Sesia Magmatic System) is represented by the Valle Mosso pluton (VMP). The VMP is an ∼260 km3 composite silicic intrusion ranging from quartz-monzonite to high-silica leucogranite (∼67–77 wt% SiO2), which intrudes into roughly coeval rhyolitic products of the >15 km diameter Sesia Caldera. In the caldera field, the emplacement of a large, crystal-rich rhyolite ignimbrite(s) (>400 km3) is followed by eruption of minor volumes (1–10 km3) of crystal-poor rhyolite. Here, we compare silicic plutonic and volcanic units of the Sesia Magmatic System through a combination of geochemical (X-ray fluorescence, inductively coupled plasma mass spectrometry and electron microprobe analyses) and petrological (rhyolite-MELTS, trace element and diffusion modeling) tools to explore their connection. Textural and compositional features shared by both VMP and crystal-rich ignimbrites imply thermal rejuvenation of crystal-mush as the mechanism to create large volumes of eruptible rhyolitic magma. Bulk-rock composition of crystal-rich rhyolite erupted during the caldera collapse overlaps that of the bulk VMP. Quartz and plagioclase from these two units show resorbed cores and inverse zoning, with Ti- and anorthite-rich rims, respectively. This indicates crystallization temperatures in rims >60 °C higher than in cores (780–820 versus ∼720 °C), if temperature is the sole parameter responsible for zonation, suggesting heating and partial dissolution of the crystal-framework. Decrease in crystallinity associated with thermal energy input was calculated through rhyolite-MELTS and indicates lowering of the mush crystal fraction below the rheological lock-up threshold, which probably promoted eruptive activity. Also, after the climatic eruption, Si-rich melts in the Sesia Magmatic System were produced by extraction of interstitial melt from un-erupted, largely crystalline mush. Regarding both textures and chemical variations, we interpret the deep quartz-monzonite unit of the VMP as a compacted silicic cumulate. Fractionated melts extracted from this unit were emplaced as a leucogranite cupola atop the VMP, generating the final internal architecture of the silicic intrusion, or alternatively erupted as minor post-caldera, crystal-poor rhyolite. Ti-in-quartz diffusion profiles in thermally rejuvenated units of the Sesia Magmatic System demonstrate that the process of reheating, mobilization and eruption of crystal-mush took place rapidly (c. 101–102 years). A protracted cooling history is instead recorded in the diffusion timescales of quartz from the silicic cumulate units (c. 104–106 years). These longer timescales encompass the duration of evolved melt extraction from the cumulate residue. We argue that the VMP preserves a complex record of pre-eruptive processes, which span mechanisms and timescales universally identified in volcanic systems and are consistent with recently proposed numerical models.

Muong Nong-type (MN) tektites are a layered type of tektite associated to the Australasian strewn field, the youngest (790 kyr) and largest on Earth. In some MN tektites, coesite is observed in association with relict quartz and silica glass within inclusions surrounded by a froth layer. The formation of coesite-bearing frothy inclusions is here investigated through a 3D textural multiscale analysis of the vesicles contained in a MN tektite sample, combined with compositional and spectroscopic data. The vesicle size distribution testifies to a post-shock decompression that induced melting and extensive vesiculation in the tektite melt. Compared to free vesicles, nucleated homogeneously in the tektite melt, froth vesicles nucleated heterogeneously on relict quartz surfaces at the margins of coesite-bearing inclusions. The rapid detachment of the froth vesicles and prompt reactivation of the nucleation site favoured the packing of vesicles and the formation of the froth structure. Vesicle relaxation time scales suggest that the vesiculation process lasted few seconds. The formation of the froth layer was instrumental for the preservation of coesite, promoting quenching of the inclusion core through the subtraction of heat during froth expansion, thereby physically insulating the inclusion until the final quench of the tektite melt.

<p>Due to the complex geodynamic framework and the excellent state of preservation of the stratigraphic relationships towards the host metamorphic and sedimentary rocks, the Permo-Triassic magmatic sequences of the Southern Alps (Italy) are intensely studied. Throughout the Southalpine domain, the main peaks of the volcano-plutonic activity are both pre- and post-dated by the emplacement of small volume of magmas with variable chemical affinity. These magmas, preserved as dykes and veins intruded into the plutonic bodies and/or the overlying volcanites, are powerful tools for tracing the evolution of the magma source and reconstructing the temporal evolution of the magmatic episode. Here, we present a detailed geochemical and geochronological study of phonolitic dykes (SiO<sub>2</sub> from 56.8 to 57.8 wt.%; Na<sub>2</sub>O + K<sub>2</sub>O from 11.1 to 15.3 wt.%) cropping out near Predazzo (Southern Alps; Italy) and intruded into the basaltic to trachyandesitic Middle Triassic lavas. The phonolites are mostly aphyric with a porphyricity index <10%. The main mineral phases are concentric-zoned clinopyroxene, ranging in composition from diopside-hedenbergite, to aegirine (Wo<sub>13-51</sub>; En<sub>2-29</sub>; Fs<sub>20-85</sub>), K-feldspar and rare sodalite. Accessory phases are titanite, apatite and magnetite embedded in a aphyric matrix. Titanite has a highly variable U-Th concentration (U from 24 to 478 ppm and Th from 170 to 4328 ppm) and is characterized by a chondrite-normalized REE pattern with a convex-upward shape (La/Yb<sub>N</sub> from 18.9 to 41.5) with enrichment in LREE and depletion in HREE. Thermometry through Zr-in-Titanite calculations (Hayden et al., 2008) indicate crystallisation temperatures between 860.3 and 942.8 ± 57 °C. In-situ, U-Pb dating on titanite phenocrysts performed by laser ablation-inductively coupled-mass spectrometry (LA-ICP-MS) shows that the age of phonolite dykes is comprised between 240.4 ± 3.2 Ma and 242.0 ± 3.6 Ma, partially overlapping with the emplacement of the Middle-Triassic plutonic bodies of the Dolomites (238.190 ± 0.050 - 238.075 ± 0.087; Storck et al. 2019).</p> <p>These results provide new insights into the timing of the Middle Triassic magmatic event in the Southern Alps, fostering the debates about the temporal and chemical evolution of the magmatism in between the Variscan orogeny and the opening of the Alpine Tethys.</p> <p>References:</p> <p>Hayden, L. A., Watson, E. B., & Wark, D. A. (2008). A thermobarometer for sphene (titanite). Contributions to Mineralogy and Petrology, 155(4), 529-540.</p> <p>Storck, J. C., Brack, P., Wotzlaw, J. F., & Ulmer, P. (2019). Timing and evolution of Middle Triassic magmatism in the Southern Alps (northern Italy). Journal of the Geological Society, 176(2), 253-268.</p>

<p>Clustering of clinopyroxene (Cpx) and titanomagnetite (Tmt) is commonly observed in magmatic products and crystallisation experiments. The existence of crystallographic orientation relationships (CORs) between Cpx and Tmt is thought to indicate their formation by heterogeneous nucleation. Heterogeneous nucleation is promoted by high degrees of undercooling, and thus associated with disequilibrium microstructures and compositions. We studied the effect of isothermal annealing on Cpx-Tmt clusters exhibiting CORs, in order to examine whether information about cluster formation is preserved during re-equilibration at depth in a magmatic system.</p><p>We analysed samples synthesized in experiments of Pontesilli et al. (2019), which aimed to reproduce the crystallisation behaviour of an Etnean trachybasalt, under nominally anhydrous (0 wt.% H<sub>2</sub>O) and hydrous (2 wt.% H<sub>2</sub>O) conditions, at mid-crustal storage conditions (400 MPa, 1100°C, NNO+1 oxygen buffer), corresponding to a degree of undercooling of 120°C and 80°C, respectively. After superheating at 1300°C for 30 minutes, samples were cooled at 80°C/min to 1100°C and annealed for dwell times ranging from 0.5h to 8h.</p><p>We employed electron backscatter diffraction (EBSD) analysis to characterise microstructures and detect CORs. In hydrous samples, phase fraction, maximum crystal size, and perimeter/area ratio are unaffected by dwell time. In contrast, anhydrous samples exhibit decreasing crystal fraction with increasing dwell time. Although crystallinity falls overall in anhydrous samples, area fraction of Tmt increases slightly up to 2h dwell time. The increase in Tmt area correlates with an increase in maximum Tmt size and a decrease in Tmt perimeter/area ratio.</p><p>Tmt exhibits two closely related CORs to Cpx, COR1 ([-110]<sub>tmt</sub>//[010]<sub>cpx</sub>, [111]<sub>tmt</sub>//(100)*<sub>cpx</sub>, [-1-12]<sub>tmt</sub>//[001]<sub>cpx</sub>) and  COR 2 ([-110]<sub>tmt</sub>//[010]<sub>cpx</sub>, [-1-11]<sub>tmt</sub>//(-101)*<sub>cpx</sub>, [112]<sub>tmt</sub>//[101]<sub>cpx</sub>). The fraction of the total length of Cpx-Tmt boundaries that follow one of the two CORs (<em>F<sub>COR1+2</sub></em>) exceeds 60% in all samples. However, the relative frequencies of the two CORs vary. In hydrous samples with dwell times of 4h and below, <em>F</em><sub>COR2 </sub>(~55%) exceeds <em>F</em><sub>COR1</sub> (~10%). However, at 8h dwell time, the frequency of both CORs is ~30%. In anhydrous samples at dwell times of 1h and below, the pattern is reversed, with <em>F</em><sub>COR1</sub> (~40%) exceeding <em>F</em><sub>COR2 </sub>(~20%). The frequency of both CORs is once again ~30% for dwell times of 2h and above. The normalised abundance (total length/map area) of boundaries with a COR does not change in the hydrous samples, and only decreases slightly in anhydrous samples. After 8h, the total abundance of boundaries with a COR is similar, regardless of water content.</p><p>The different COR frequencies observed at short dwell times in hydrous and anhydrous samples imply that the Cpx-Tmt clustering mechanism is affected by degree of undercooling. Re-equilibration of COR frequencies progresses faster in the anhydrous samples, correlating with the greater intensity of microstructural re-equilibration observed. In rapidly cooled systems, relative frequencies of different Cpx-Tmt CORs could potentially be used to estimate degree of undercooling. Total abundance of boundaries associated with a COR remains constant, suggesting that Cpx-Tmt CORs preserve some information about heterogeneous nucleation on longer timescales.</p><p>Pontesilli et al. (2019), Chem Geol 510:113-129. 10.1016/j.chemgeo.2019.02.015</p><p>Funded by the Austrian Science Fund (FWF): P 33227-N</p>