NEWS & EVENTS

Taal volcano, an active and well-monitored volcano in Luzon, Philippines, erupted on 12^th January 2020 with a Volcano Explosivity Index (VEI) of 4 after 43 years’ quiescence, and then continued to produce smaller phreatomagmatic eruptions till now. The build-up and triggering of the 2020 explosive eruption and the contribution of the shallow hydrothermal system remain unconstrained. Here we investigate this based on petrological-geochemical analyses of andesitic pumices from the 2020 eruption with a focus on a volatile-bearing mineral – apatite. We conducted petrography observation of the pumices and determined apatite compositions using an electron probe microanalyzer (EPMA). The major minerals (plagioclase + clinopyroxene + olivine + Fe-Ti oxide) exhibit mainly normal zoning in olivine and rarely oscillatory/reverse zoning in plagioclase and clinopyroxene. Apatite shows a low modal abundance (<0.01%) and occurs as inclusions in clinopyroxenes and Fe-Ti oxides, as well as microphenocrysts in the groundmass. Some apatite crystals exhibit zoning in Mn, S, Na, and Cl in X-ray maps. Our EPMA data show that the apatite crystals characterized by low S (0.01-0.36 SO₃ wt%) and low Mn (0.08-0.27 wt%) have relatively high Cl (0.73-1.36 wt%) contents, while the rest of the crystals have higher SO₃ (0.32-2.75 wt%), MnO (0.36-2.72 wt%), and F (2.07-4.50 wt%). The latter population contains much higher Mn-S contents than the syn-caldera eruption products reported by previous studies. To testify if these different apatite compositions can be produced by Rayleigh fractionation, we calculate them using varying initial melt compositions and partition coefficients (K_d) for S and Mn determined by previous experiments. We find that the high-Mn apatites can be reproduced by an initial melt of high Mn (1400 ppm, similar to our whole-rock data) and low S (200 ppm), and a high K_d(Mn) for rhyolite. In comparison, the high-S apatites can be produced by an initial melt of comparable or lower Mn but higher S (500-800 ppm) contents and a high K_d(S) for rhyolite. The high-S and high-Mn populations may indicate additional supply of sulfur by magma/volatile flux from greater depths and that of Mn (e.g., by fluids-melt interaction), respectively. Further geochemical analyses and thermodynamic modelling will be conducted to assess the effect of magma recharge and fluids on the eruption triggering and ultimately enhance monitoring and eruption forecasting at Taal volcano.

For more information, please contact: Mr. AUSEJO Mike, mike.ausejo@connect.hku.hk