PRE-ERUPTIVE HEATING MAY AID MAGMA OUTGASSING AND REDUCE THE RISK OF EXPLOSIVE VOLCANIC ERUPTIONS

A fundamental problem in the assessment of volcanic hazards is understanding how and why volcanoes erupt either effusively or explosively. Although eruptive style largely depends on whether gas escapes or remains trapped in the magma during shallow ascent, a clear understanding of the factors governing outgassing remains elusive. To investigate the potential influence of pre-eruptive conditions on gas escape during effusive eruptions, we are studying the texture and chemical composition of crystals from the Nimbin Rhyolite in eastern Australia. The Nimbin Rhyolite is a complex of lava domes and flows covering an area of 400 km² on the southern flank of the Tweed shield volcano. Eruptive units range from crystal-poor (< 5 vol. %) to moderately crystal-rich (10-30 vol. %) flow-banded rhyolite comprised primarily of quartz, sanidine, and plagioclase with minor opx, cpx and ilmenite.

Plagioclase phenocrysts in crystal-rich lava display euhedral to subhedral morphologies and uniform compositions (e.g., anorthite (An) contents of 23±2 mole % for both core and rim analyses). In contrast, although most phenocrysts in crystal-poor lava yield core compositions (An=22±3) similar to those in crystal-rich lava, plagioclase crystals are highly resorbed, and, where new rim growth is present, anorthite contents extend up to 50 mole %. The calcium-rich rims are similar to micro-phenocrysts with anorthite contents of 52±3 mole %. These textural and chemical relationships are consistent with late-stage heating of crystal-rich magma to yield crystal-poor lava. Application of two-feldspar thermometry indicates an increase on the order of 100°C. Pre-eruptive heating was likely achieved by chamber recharge involving hotter, more primitive magma. While recharge is a commonly accepted eruption trigger, our results support the suggestion that, in cases where reheating is substantial, it may also reduce the risk of explosive eruptions. Late, pre-eruptive heating lowers melt viscosity and accelerates volatile diffusion, and is therefore expected to promote outgassing through enhanced bubble nucleation, growth, and coalescence during ascent.