The record of magma storage conditions in mineral compositions below a glacial to ice-free Askja volcano, Iceland

Abstract

Askja is a caldera volcano located in central Iceland that was glaciated approximately 70,000 to 10,500 years ago. Askja’s basaltic lavas have been classified into one preglacial, six glacial, one Early Holocene and thirteen historical eruptive units. The purpose of this study was to use mineral compositions and textures from basaltic lavas to reconstruct magma chamber conditions before, during and after glaciation. In this study, 36 lava flow and gabbro xenolith samples were collected from seven separate units: two glacial, the Early Holocene, and four historical. These samples were analyzed along with a select number of pre-existing samples. The data collected from this study were combined with previous data to create larger data sets of X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry, and electron probe mineral composition data for Askja throughout the preglacial to historical time periods. The composition of data collected displays a number of trends that indicate crystal cargo and magma composition have changed as a result of magma residence time and magma production rates at Askja. Evidence of change in the crystal cargo and magma storage over the deglaciation at Askja includes the glacial units containing larger phenocrysts called megacrysts and many more phenocrysts than any other time period. This classifies the lavas as plagioclase ultraphyric basalts. There is also evidence of magma recharge into a storage area that occurred to produce the compositional characteristics of Askja’s basalts. Mineral chemistry diversity including bimodal plagioclase compositions reflect multistage crystal growth histories. The diversity is also supported by two populations of gabbro xenoliths in the glacial deposits. Clinopyroxene and olivine crystals are less abundant but also display variability within individual eruptive units. Pressure from 900 m of ice on top of Askja may have contributed to some of these changes. Plagioclase ultraphyric basalts are created in magma chambers at greater depths with longer residence times that are favorable to crystal growth. In future projects, further investigation could involve melt inclusion analysis and geothermobarometry to constrain specific depths and pressures at which the crystals in the lavas formed. By investigating mineral compositions and textures and analyzing the implications on crystal cargo and magma storage throughout a period of glaciation, the full impact of a glacier on a volcanic system can continue to be uncovered.