Syn Eruptive Crystallisation In Silicic Magmas

Author : Clara Waelkens
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (125 download)

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Download or read book Magmatic Processes in a Silicic Caldera-forming System written by Clara Waelkens and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "Large, silicic, caldera-forming eruptions are infrequent, but their impact on humans and their environment can be dramatic. This emphasises the importance of a good understanding of these so-called “supervolcanic” systems and of the processes that take place in their magma chambers. Valles caldera is an ideal system in which to study these processes; its continuous sequence of eruptions provides a window into both the magma chamber primed for eruption, and into the re-establishment of the magma chamber between two super-eruptions.Volatiles play a key role in magma chambers, both as dissolved species in the melt and in exsolved vapour phases. In this thesis, I measured halogen, H2O and CO2 concentrations in melt inclusions from the Lower Bandelier Tuff (LBT), Upper Bandelier Tuff (UBT), the Valle Toledo Member (VTM) rhyolites in between, and the Deer Canyon Rhyolites at the end of the UBT sequence, in order to reconstruct volatile evolution in the Bandelier magmatic system. By comparing the behaviour of fluorine, chlorine and bromine in the eruption sequence, I concluded that large masses of Cl and Br were fractionated from the magma to a separate exsolved fluid phase. While syn-eruptive degassing of halogens was negligible in both the LBT and the UBT eruptions, the fluid phase released 4800 Tg of Cl and 5.7 Tg of Br to the atmosphere during the LBT eruption, and 2090 Tg Cl and 3.6 Tg Br in the UBT eruption. This implies the Bandelier eruptions had a severe impact on the chemistry of the paleo-atmosphere, particularly on the ozone layer.Three sharp increases to higher CO2 concentrations are recorded in the Bandelier sequence: (1) in the LBT plinian, where dissolved CO2 concentrations increase from a mean value of 27 ppm at the base of the plinian to 173 ppm in the mid-plinian; (2) in VTM group III, which at a mean of 197 ppm exhibit the highest CO2 concentrations of the entire sequence; and (3) in the UBT plinian,where CO2 concentrations increase from a mean of 54 ppm at the base of the plinian to 101 ppm in the mid-plinian. I interpret these sharp increases as the result of the injection of fresh magma into the chamber, with CO2-rich vapours that exsolved from the recharge magma percolating through the magma chamber and increasing CO2 concentrations.In a departure from volatiles, I finally focused on the importance of recharge events into the Valles caldera system. Prompted by the suggestion in previous studies that a series of hornblende-rich dacite pumices in the early-erupted UBT are related to the pre-caldera, intermediate composition Tschicoma Formation, I performed a geochemical study of the Tschicoma lava dome and lava flow complexes. Through major, trace element and isotopic analyses, I identified different evolutionary trends, which through different combinations of fractional crystallisation, assimilation of crustal material and magma mixing led to the geochemical diversity observed today. By relating the geochemistry of the Tschicoma domes to that of the dacite pumices in the UBT, I propose that a more mafic, possibly andesitic, parental magma to the Tschicoma dacites was injected into the UBT magma chamber, where it mixed with the resident UBT rhyolite magma to form the hornblende-rich dacite pumices"--