HR: 1330h
AN: V12A-0549    [PDF]
TI: Windows Into an Open-System Magma Chamber: Cognate Xenoliths From the Kameni Islands, Santorini, Greece
AU: * Martin, V M
EM: vmar01@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Holness, M B
EM: marian@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Pyle, D M
EM: dmp11@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: The composition of the dacite forming the Kameni Islands has been remarkably uniform over the last 2200 years, attesting to significant replenishment and mixing in the shallow open-system chamber. Insights into chamber evolution can be gained from the abundant cognate xenoliths in the dacite. The xenoliths include both cumulates from the chamber floor and quenched fragments of replenishing magma. They fall into 7 groups distinguishable by differences in chemistry, texture and mechanical behaviour. Quench xenoliths with linear CSDs are thought to have formed on the injection of aphyric magma into the chamber. Since these xenoliths commonly have an andesitic composition, this suggests the expulsion of an aphyric melt from a crystal mush in a lower chamber by filter-pressing during compaction. Those with strongly bi-modal grain size distributions indicate injection of phenocryst-bearing replenishing magma. Quench xenoliths with evolved compositions are typically found as large isolated blocks randomly dispersed throughout the host lava. In contrast, the more mafic xenoliths are distributed in tight, elongate, clusters aligned in the direction of flow, and comprising up to 100 individuals. The contrasting spatial distributions of the xenoliths reflects the different rheological properties of the xenolith types, with implications for the ease of mixing and assimilation of batches of replenishing melt in the magma chamber. The rare cumulate xenoliths are small, and are characterised by coarse grain-size, low glass and vesicle content, and are predominantly found in lava from the last (1950) eruption. They are generally found enveloped by less dense and highly vesicular quench-type xenoliths, which probably provided the buoyancy forces necessary for entrainment. Mapping of the xenolith types shows that each flow forming the Kameni Islands has a distinct xenolith population, reflecting the changing state of the emptying magma chamber. Seismic activity has preceded most recent eruptions on Santorini by approximately 20 months. This activity may signify the movement of magma at depth and the injection of new magma into the dacite chamber. Plagioclase crystal sizes in the quench xenoliths are consistent with growth timescales of the order of a few years, suggesting that these xenoliths derive from the replenishing magma. We suggest that replenishing material exists as a series of sills at the base of the chamber, with later pulses of magma intruding into, and disrupting, the pre-existing crystal pile. The concentration of cumulate xenoliths in the 1950 flow suggests that the replenishment event prior to this eruption intruded through a mature layer of crystal mush.
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting