HR: 1340h
AN: V13B-0527    [Abstracts]
TI: Light Elements in Plagioclase Phenocrysts of the Nea Kameni Dacites: What do they say about processes during and after crystallization, and just how fast crystallization can take place?
AU: * Cabato, M A
EM: jcabato@min.uni-heidelberg.de
AF: Mineralogisches Institut, Universität-Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Altherr, R
EM: raltherr@min.uni-heidelberg.de
AF: Mineralogisches Institut, Universität-Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Ludwig, T
EM: tl@min.uni-heidelberg.de
AF: Mineralogisches Institut, Universität-Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AB: Nea Kameni is the larger of two intra-caldera islands in Santorini Volcano, the most active volcanic center of the Hellenic Arc in Greece. Phenocrysts of samples collected from the 1939-1941 dacite lava flows were analyzed for major and trace element compositions using the EMP and SIMS, with particular focus on concentrations of Li, Be and B. Three types of plagioclase phenocrysts are highlighted in this study. Type N is normally-zoned in terms of An content, with values ranging from 62 in the core to 40 at the rim. A general increase in Li, Be, B from core to rim characterizes this plagioclase, with values of 6.6-7.6 μg/g for Li, 0.37-0.75 μg/g for Be, and 0.10-0.18 μg/g for B. Type O crystals exhibit only oscillatory variations in An content, which ranges from 40 to 57. In terms of Li, however, two subtypes have been identified for this population. Type O1 crystals show Li concentrations that markedly decrease from core to rim, with a maximum value of 23.4 μg/g in the core and a minimum of 6.5 μg/g at the rim. Type O2 crystals are more common, where Li patterns are rather flat and have values within 6.6-11.0 μg/g. Be and B in both subtypes do not vary systematically, except for the slight inverse correlation with An content, and have values of 0.54-0.96 μg/g and 0.10-0.24 μg/g, respectively. Type C phenocrysts have inherited cores with higher An content compared to the outer portions. Within the core, An is between 54 and 63 and Li, Be, B tend to increase from the central portion outwards, with values of 6.5-9.6 μg/g, 0.31-0.60 μg/g, and 0.10-0.16 μg/g, respectively. Towards the rim, the profiles are disrupted: An content drops to 40-54, whereas light elements, especially Be, are elevated at the boundary but decrease gradually toward the rim. Values lie within 6.8-13.9 μg/g for Li, 0.61-0.97 μg/g for Be, and 0.14-0.24 μg/g for B. The different plagioclase types exhibit the complexity of processes involved in the formation of these phenocrysts: from normal crystallization with its expected element concentration trends, to degassing of magma causing dramatic decreases in water-soluble elements, to diffusion of trace elements that modify primary compositions within the crystals. Also, magma mixing and entrainment of earlier-formed phases may have developed phenocrysts with distinct chemical disparities. Li profiles of certain crystals yield astonishing results about the residence time of plagioclase. Simple linear diffusion calculation gives spans of seconds to a few hours, compared to the hundred-year scale previously derived from diffusion modelling of other elements (e.g., Sr by Zellmer et. al, 1999; Mg by Costa et al., 2003). This suggests that crystallisation may have been driven by decompression and degassing during eruption, and that these combined processes occurred extremely fast. Although quite tentative, this method challenges our concept of maximum magma residence times and their implication on volcanic hazards. References: Costa F, Chakraborty S, Dohmen R (2003). GCA 67(12): 2189-2200. Zellmer GF, Blake S, Vance D, Hawkesworth C, Turner S (1999). CMP 136: 345-357.
DE: 1036 Magma chamber processes (3618)
DE: 1065 Major and trace element geochemistry
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3640 Igneous petrology
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005