HR: 1330h
AN: V12A-0548    [PDF]
TI: Textural Constraints on Physical and Chemical Communication in Crystal Dominated Magmatic Systems
AU: * Wallace, G S
EM: gsw@u.washington.edu
AF: Univ. of Washington, Box 351310, Seattle, WA 98195 United States
AU: Bergantz, G W
EM: bergantz@u.washington.edu
AF: Univ. of Washington, Box 351310, Seattle, WA 98195 United States
AB: We have developed a set of statistical tools that identify shared characteristics in sets of crystals. This includes a means of addressing the geometric effects of sampling from randomly oriented sections through crystals. Rim-to-core compositional profiles of individual crystals provide a putative record for the evaluation of changes in environment during crystal growth. The profile segments in which chemical characteristics are the same in pairs of profiles imply shared growth history in a common environment. However, crystals sometimes record different patterns of shared history for different chemical tracers. This reflects independent variation of either chemical or physical parameters in the magmatic environment. Thus, comparison of the distribution of shared history between profiles provides a way to observe the relative length scales over which characteristics are homogeneous within magma chambers. Chemical tracers with more shared history are inferred to have a longer relative length scale. This framework provides a way to assess the efficiency of chemical communication between magma volumes- an important constraint in light of the increasing consensus that many magma chambers are dominated by crystal mushes. Application of the technique to Anorthite (An) profiles from plagioclase in mafic enclaves at Chaos Crags, California, show little shared history between crystals. $^{87}$Sr/$^{86}$Sr profiles from enclaves (Tepley, 1999) show more similar histories. This indicates that the length scale over which An content of plagioclase varies is shorter than that for $^{87}$Sr/$^{86}$Sr. If changes in An are compositionally controlled, the An and $^{87}$Sr/$^{86}$Sr should show similar shared histories. The shorter length scale for An is more likely due to passage of the crystals through steep thermal gradients. This is consistent with resorption and crystallization near interfaces between the commingling magmas. An profiles from crystals in the host rhyodacite also show little shared history. This implies a heterogeneous magma chamber prior to intrusion of the basaltic andesite.
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting