ALBERT

Description: We have applied a combination of fluid inclusion and amphibole thermobarometry to felsic tephras from highly explosive volcanic eruptions along the Central American volcanic arc (CAVA) from Guatemala through Nicaragua in order to constrain pre-eruptive magma ascent and storage conditions. We note that this is the first time a combination of pressure estimates from fluid inclusions and amphibole chemistry have been used to quantify multi-stage magma chamber processes and magma ascent velocities of large eruptions. Our data document a stepwise ascent of magmas through the crust, typically involving at least two levels of stagnation. Amphibole and fluid inclusion thermobarometry both indicate a shallow preeruptive magma storage level at 80 to 200 MPa (3-8 km depth) along the entire arc. The deeper levels of magma storage vary along-arc, with a tendency to greater maximum depths of up to 25 km in Guatemala and El Salvador, compared to maximum depths of 15 km in Nicaragua. We assume that the continental crust of about 45 km thickness in Guatemala, compared to the 30km thickness of the largely oceanic crust of Nicaragua, allowed for deeper positions of the magma chambers. Thus the observed along-arc changes in mid-crustal magma storage depths indicate a dependence between magma chamber formation and the composition and probably density of the local crust. The average composition of the pre-eruptive fluid phase for highly explosive eruptions in Central America amounts to 90% water, 5% CO2 and 5% NaCl equivalents, and show no systematic alongarc variations. The pressures obtained from the earliest fluid inclusions were taken as the pressures of fluid oversaturation and thus for the beginning of degassing. They range between 150 and 400 MPa, and do not show systematic along-arc variations. Such fluid oversaturation pressures correspond to water contents between 4-8 wt% in the felsic melts. Our results show that the depths of fluid saturation are mostly independent of crustal properties. Degassing typically started at pressures 150 to 300 MPa higher that those corresponding to the last stagnation level, providing evidence for the pre-eruptive criticality of the systems.

Description: The Lonquimay Volcanic Complex (LVC) in South Central Chile (38.38°S, 71.58°W) is part of the Southern Volcanic Zone of the Andes, which formed in response to the subduction of the Nazca Plate beneath the South American Plate. During the last 10200+-70 years of its magmatic evolution, the LVC produced 23 explosive eruptions documented in the succession of widespread tephra deposits. We investigated this stratigraphic sequence for matrix glass, mineral and bulk rock compositions of the juvenile components. Furthermore, melt inclusions were analyzed for their major element and volatile contents. The tephra succession reflects six mafic replenishments of the LVC magma reservoir followed by progressive magmatic differentiation. Each cycle has been successively tapped by several eruptions. Compositionally zoned tephras were typically deposited early in a cycle, whereas late eruptions discharged more evolved magmas. Intermediate compositions typically contain mixed disequilibrium mineral assemblages. The maximum degree of fractionation reached during a cycle increases with younger ages. Our investigations of melt inclusions, in order to reconstruct the pre-eruptive volatile inventories of the LVC magma chamber, reveal the exsolution of two separate fluid phases. One S-rich fluid phase released from mafic melts in the middle crust and one Cl-rich aqueous phase, released from more ifferentiated melts that resided in the upper part of LVC´s plumbing system. The pre-eruptive saturation state of the LVC melts indicates that felsic eruptions may have been triggered by H2O-supersaturation whereas mafic melts seem to have experienced a complex replenishment history potentially exciting LVC´s mafic eruptions.

Description: The Lonquimay Volcanic Complex (LVC) in the high Southern Andes comprises a stratocone and NEtrending flank-cone alignments. Numerous effusive and explosive volcanic eruptions characterize its post-glacial magmatic activity. Our tephrostratigraphic record, pre-dating the four historically documented eruptions, comprises 22 dated pyroclastic deposits that are used to constrain repose time distribution and eruption probability of the LVC magmatic system. Statistical examination of the stratigraphy-based eruption time series yields probabilities of 20-50% for at least one explosive (VEI"3) eruption within the next 100 years as of 2011. The tephra deposits are subdivided into three petrographic groups: a felsic group (Lonquimay Colored Pumice Tephra, LCPT), an intermediate population (Lonquimay Grey Pumice Tephra, LGPT) and a mafic member (Lonquimay Dark Scoria Tephra, LDST). The distribution of these petrographic groups through the LVC tephrostratigraphy is linked to the observed changes in repose times. LDST deposits as well as deposits compositionally zoned from LCPT to LGPT dominate the lower part of the stratigraphy for which recurrence times are short (RTmean=417±169a). Deposits younger than 6000 b2k (years before 2000 AD) have dominantly LCPT and minor LDST compositions, no longer contain LGPT, and repose times are significantly longer (RTmean=1350±310a). We interpret the change in eruption regime to result from a rearrangement in the magma storage and plumbing system. Thermobarometric calculations based on cpx liquid equilibria and amphibole compositions reveal three distinct magma storage levels: the mafic LDST derive from mid crustal storage (Pmean= 476±95 MPa, Tmean=1073±24°C), felsic LCPT mainly erupted from upper-crustal level (Pmean= 86±49 MPa , Tmean=936±24°C), whereas LGPT samples yield intermediate storage depths (Pmean= 239±100 MPa, Tmean=1013±17°C). Magma contributions from this intermediate reservoir are restricted to 〉6000 b2k when the Lonquimay plumbing system was in a regime of short repose times; disappearance of the intermediate reservoir coincides with the change to longer repose times between eruptions.