Plank, T. (2006) The volatile record in melt inclusions and phenocrysts. Eos. Trans. AGU, 87(52) Fall Meet. Suppl., Abstract V13D-04.
The volatile content of arc magmas critically controls many aspects of magmagenesis and evolution. We focus here on the water content of magmas from three Central American volcanoes: Irazu and Arenal volcanoes in Costa Rica, and Cerro Negro volcano in Nicaragua. Tephra samples from each of these volcanoes have yielded olivine-hosted melt inclusions that constrain the water contents of parental magmas to > 3 wt% H2O for Irazu, > 4 wt% H2O for Arenal, and > 5 wt% H2O for C.Negro [1,2,3]. These inclusion-based magmatic water contents are supported by recent measurements of H2O in nominally- anhydrous clinopyroxene phenocrysts by ion microprobe [4], using the Al(IV)-dependent partitioning relationship in [5].
This variation in H2O from volcano to volcano correlates with tracers of fluids from the subduction zone (e.g., Ba/La, Sr/Nd) as well as the extent of wet melting of the mantle (e.g., from Y and Ti). In addition to this information on magma source characteristics, volatile data within each volcano can be combined with measures of crystal fractionation (Mg#), pressure of entrapment (from H2O-CO2 solubility at vapor-saturation [6]), and crystallization temperature (from ol-liquid thermometry [7]) to infer the evolution, degassing, and ascent of magma in the crust. For example, the 1723 eruption of Irazu records coupled ascent (1.9-0.3 kb), degassing (3.2-1.7 wt% H2O), cooling (1080-1040 C) and crystal fractionation (Mg# of 87-79 of host olivines). On the other hand, the 930 yr old ET3 tephra of Arenal records nearly isobaric (1.2-1.9 kb) cooling (1080-1010 C) that maintains high water contents (> 3.2 wt%) during crystal fractionation (Mg# of 84-74 of cpx). The 1867 eruption of Cerro Negro is preceded by deep (3-5.5 kb) isothermal (1090 C) conditions with some degassing at high water contents (6-4.5 wt% H2O) leading to crystal fractionation (Mg# 82-70 in cpx).
These different evolutionary trends lead to very different conditions upon eruption (e.g., magma gas content, viscosity and ascent rate) that may ultimately relate to eruptive vigor. For example, the strombolian eruptions of Irazu may result from moderate initial water contents and degassing upon ascent; while the plinian/sub-plinian eruptions of Arenal and Cerro Negro may result from higher initial water contents, coupled with deep magma fractionation that retains dissolved water as the magma evolves.
[1] Benjamin, Plank, Wade, Kelley, Hauri, Alvarado, JVGR, submitted. [2] Wade, Plank, Melson, Soto, Hauri, JVGR, 2006. [3] Roggensack, EPSL, 2001. [4] Wade, this AGU. [5] Hauri, Green & Gaetani, EPSL