Petrology and geochemistry of the 1991 and 1998^1999 lava £ows from Volca¤n de Colima, Me¤xico: implications for the end of the current eruptive cycle

Point-counted modes, representative mineral analyses, and whole-rock majorand trace-element compositions determined by X-ray fluorescence spectroscopy and inductively coupled plasma mass spectroscopy (ICP-MS) are presented for 11 samples of the hornblende^andesite block-lava flows erupted from Volca¤n de Colima in 1991 and 1998^1999. New ICP-MS trace-element data are also presented for 37 andesites erupted between 1869 and 1982, described in previous publications. These data are used to evaluate mineralogical and whole-rock compositional changes during two well-defined historical eruption cycles: 1818^1913 and 1913^present. The eruptive cycles of Volca¤n de Colima are dominated by andesitic lavas with V61 wt% SiO2, but terminate with major explosive eruptions, as occurred in 1818 and 1913, involving relatively mafic andesites with V58% SiO2. Following eruptions of andesitic block-lava flows with V61% SiO2 in 1961^1962 and 1975^1976, a trend toward lower SiO2 contents began in 1976 and peaked in 1981, probably as a small volume of deeper, more mafic magma ascended into the shallower andesitic reservoir beneath the volcano. Since then, however, andesitic lavas have become progressively richer in SiO2 through the 1991 and 1998^1999 eruptions. Andesites erupted between 1961 and 1999 show many subtle but important differences compared to those erupted between 1869 and 1913. Based on rocks of similar SiO2 content, the 1961^1999 andesites are richer in modal plagioclase and in the elements Y, Nb, Tb, Ho, Er, Yb, and Ta, and they are poorer in modal hornblende and in the elements Ba and Sr. All of these observations are consistent with the interpretation that the magmas of 1961^1999 had significantly lower water contents compared to those erupted in 1869^1913, which diminished the role of hornblende crystallization and enhanced the role of plagioclase crystallization. The relatively lower magmatic water contents for the 1961^1999 andesites imply that the explosive eruption anticipated to terminate the current eruptive cycle will be less powerful than the 1913 eruption. Of equal importance to this question, however, is the role of magmatic degassing. The relatively higher viscosities of andesitic magmas with V61% SiO2 likely lead to relatively slow ascent rates and more thorough degassing prior to arrival of the magma at the summit crater and its eruption as block lava. In contrast, the lower viscosities of more mafic andesitic magmas with V58% SiO2 result in faster ascent and greater retention of volatiles, so that they erupt explosively upon reaching the summit crater. Published by Elsevier Science B.V.