Pre-eruptive conditions of basaltic andesite at Arenal Volcano, Costa Rica
Final Report Abstract
Arenal volcano is nearly unique with its 45 year long continuous activity erupting remarkably monotonous basaltic andesites but mineral zoning records indicate complex open-system processes including the episodic injections of basalt. The condition of the mafic input as well as the evolution to the currently erupting basaltic andesite were addressed by an experimental study on a phenocryst-poor (~3%) Arenal-type basalt (50.5 wt% SiO2) from a nearby scoria cone containing olivine (ol, Fo92), plagioclase (pl, An86), clinopyroxene (cpx, mg#=82) and magnetite (mag, Xulvo=0.13). Our experiments (200 MPa, 900-1050°C, oxidizing and fluid-saturated conditions with various water activities, XH2Ofluid=0.3 to 1) produced at 1050°C and watersaturated conditions (H2Omelt=5 wt%), only magnetite, whereas mag+cpx+ol crystallized at low water activity. Plagioclase and cpx crystallized at water-saturated conditions with olivine at 1000°C and with amphibole at 950-900°C, whereas they crystallized with opx at low water activity (for T=1000-900°C). The mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1000°C with basaltic andesitic melt (55 wt% SiO2) with 5 wt% H2O. Our results can be used to substantiate previously envisioned crystallization environments beneath Arenal. Basaltic magma inputs are water-rich at relatively high pressure and high temperature. Evolution to more evolved magmas would have happened under fluid-saturated conditions but variably water activities. A decrease of water activity (degassing?) induces the change of the mineral assemblage pl+ol+cpx to pl+cpx+opx observed in basalt (e.g. ET3-1) to basaltic andesitic tephras (e.g. Ar7/68). On the other hand, the lack of opx and the presence of amphibole also in basaltic andesitic tephra units of earlier eruptive phases (e.g. ET3-2) indicate water-saturated conditions at temperature lower than 950°C. Continuous degassing may contribute to persistent small-scale explosive activity - so characteristic for the current eruption-, likely prevailed during crystallization of the water-rich magmas at Arenal. Mixing of magma batches (with subtle but distinct compositional and mineralogical differences) plus the occasional process of concentrating mineral contents by melt expulsion will do the rest to make crystal-rich basaltic andesites of the current eruption.
