Dynamic Evolution of the Transcrustal Plumbing System in Large Igneous Provinces: Geochemical and Microstructural Insights from Glomerocrysts and Melt Inclusions

• Published in: Journal of petrology 2024, Vol.65 (10)
• Main Authors: Capriolo, Manfredo, Callegaro, Sara, Deegan, Frances M, Merle, Renaud, Jeon, Heejin, Whitehouse, Martin J, Aradi, László E, Storm, Malte, Renne, Paul R, Baker, Don R, Dal Corso, Jacopo, Newton, Robert J, SzabÓ, Csaba, Carvalho, Bruna B, Youbi, Nasrrddine, Marzoli, Andrea
• Format: Article
• Language: English
• Subjects: Central Atlantic Magmatic Province (CAMP); Confocal Raman microspectroscopy; Deccan Traps (Deccan); Oxygen isotopes; X-ray microtomography
• ISSN: 0022-3530; 1460-2415; 1460-2415
• DOI: 10.1093/petrology/egae103

Abstract The nature of the magma plumbing system of Large Igneous Provinces is still poorly understood. Among these exceptional magmatic events from Earth's past, the end-Triassic Central Atlantic Magmatic Province (CAMP) and the end-Cretaceous Deccan Traps (Deccan) coincided in time with two of the most catastrophic biotic crises during the Phanerozoic. In order to constrain the architecture of their magma plumbing system, glomerocrysts containing abundant bubble-bearing melt inclusions from basaltic lava flows of both CAMP and Deccan were investigated via in situ geochemical and microstructural analyses. The analysed glomerocrysts, dominated by augitic clinopyroxene crystals, represent fragments of a crystal mush entrained by basaltic magmas before eruption. The analysed melt inclusions, consisting of an intermediate to felsic composition glass and CO2-bearing bubbles, represent relics of interstitial melts and fluids within a porous crystal framework forming the crystal mush. The different volume proportions between bubbles and whole inclusions reveal that melt entrapment occurred after volatile exsolution. The minimum observed bubble/inclusion fraction indicates that the CO2 concentration in CAMP and Deccan melts was at least 0.3 wt.%, consistent with a maximum entrapment pressure of about 0.5 GPa at CO2–H2O fluid-saturated conditions. The MgO-rich composition of host clinopyroxene crystals and whole rocks is in contrast with the SiO2-rich composition of (trachy-) andesitic to rhyolitic glass of melt inclusions, pointing to disequilibrium conditions. Thermodynamic and geochemical modelling shows that fractional crystallization alone cannot explain the evolved composition of glass in melt inclusions starting from their whole rock composition. On one side, the oxygen isotope composition of clinopyroxene crystals in glomerocrysts ranges from +3.9 (± 0.3) to +5.8 (± 0.3) ‰ and their sample-averaged oxygen isotope composition spans from +4.4 (N = 10) to +5.6 (N = 10) ‰, implying that glomerocrysts crystallized from mafic melts with normal (i.e., mantle-like) to slightly low δ18O values. On the other side, the oxygen isotope composition of glass in melt inclusions ranges from +5.5 (± 0.4) to +22.1 (± 0.4) ‰, implying that melt inclusions entrapped intermediate to felsic melts with normal (i.e., mantle-like) to extremely high δ18O values, typical of (meta-) sedimentary rocks. Some melt inclusions are compatible with fractionation from the same mafic melts tha