Pressure-temperature-volatile contents variability of magmas feeding caldera-forming eruptions at Toba volcano

Abstract

<p>Toba volcano in Sumatra, Indonesia is known for four large caldera-forming eruptions in</p><p>Quaternary, including the famous Youngest Toba Tuff (YTT) at ~74 ka and three older</p><p>eruptions, namely HDT (~1.42 Ma), OTT (~784 ka), and MTT (~504 ka) that were less studied.</p><p>Here we apply conventional and machine-learning based geothermobarometers and</p><p>hygrometers to the four Toba tuffs, to determine the P-T conditions and volatile budgets of the</p><p>sub-caldera magma. We found that the dacitic magma of the HDT had distinctly higher storage</p><p>and pre-eruptive P-Ts, as well as lower H2O contents than the rhyolitic magmas of the later</p><p>eruptions. Specifically, estimates for magma storage P-Ts from biotite-amphibole are similar</p><p>for the OTT, MTT and YTT (T=770--825 °C; P=200--320 MPa and 100--200 MPa from biotite and</p><p>amphibole respectively). Estimates of P-Ts from matrix glasses show two distinct populations</p><p>for the HDT (median: 783 °C and 200 MPa, and 867 °C and 330 MPa) but single populations of</p><p>lower values for the OTT, MTT and YTT (median: 750 ±20 °C and 120 ±30 MPa). Estimates for</p><p>melt H2O contents from apatite show generally lower values for the HDT (mostly 3--5 wt%,</p><p>indicating saturation P<140 MPa) than the OTT-MTT (mostly >5 wt%) and YTT (mostly >9 wt%,</p><p>indicating saturation P>320 MPa). These indicate that the magma reached volatile saturation</p><p>at different conditions before different eruptions, and the role that volatile saturation plays in</p><p>eruption triggering may vary between caldera cycles.</p>