The transcrustal plumbing system of Large Igneous Provinces: Insights from glomerocrysts and melt inclusions

The magma plumbing system throughout the entire transcrustal section of Large Igneous Provinces (LIPs) is still poorly understood. Among the most voluminous LIPs, the Central Atlantic Magmatic Province (CAMP [1], ca. 201 Ma) and the Deccan Traps (DT [2], ca. 66 Ma) coincided in time with end-Triassic and end-Cretaceous mass extinctions, respectively. Glomerocrysts containing abundant primary melt inclusions from both CAMP and DT basaltic lava flows were investigated via a multi-analytical approach (confocal Raman microspectroscopy for volatile species within bubbles, electron microprobe for major element compositions, secondary ion mass spectrometry for oxygen isotope compositions, Synchrotron X-ray microtomography and optical microscopy for microstructural analysis). The analysed glomerocrysts are dominated by augitic clinopyroxenes, and represent portions of crystal mushes. The analysed melt inclusions consist of an intermediate to felsic composition glass and CO2-bearing bubbles, and represent relics of interstitial melts and fluids entrapped during the evolution of these crystal mushes. The different volume proportions in terms of bubbles within melt inclusions indicate a heterogeneous entrapment, implying that melts were entrapped along with already exsolved fluids. The MgO-rich composition of glomerocrysts and whole rocks is in contrast with the SiO2-rich composition of melt inclusions, unveiling disequilibrium conditions of entrapment, as supported by thermodynamic modelling too. The oxygen isotope compositions of clinopyroxene in glomerocrysts indicate that they crystallized from mafic melts with normal (i.e., mantle-like) to mildly low δ18O values. However, the oxygen isotope compositions of glass in melt inclusions indicate that they entrapped distinct, intermediate to felsic melts with normal to extremely high δ18O values, which may be explained by variable degrees of crustal assimilation and partial mixing in an open system. Hence, the oxygen isotope compositions of glass in melt inclusions also suggest that the CO2 within their coexisting bubbles may be derived partly from the mantle and partly from assimilated crustal melts as well. Overall, geochemical data and microstructural observations reveal the presence of multiphase (i.e., solid + liquid + gaseous phases) crystal mushes within the magma plumbing system of both CAMP and DT, and shed light on the origin of carbon and its transfer from the mantle to Earth’s surface. [1] Marzoli et al. (1999), Science 284, 616–618.[2] Sprain et al. (2019), Science 363, 866–870.