HR: 1340h
AN: V13B-0539    [Abstracts]
TI: Time scales of assimilation and mixing at Krafla volcano, Icleand
AU: * Cooper, K M
EM: kmcooper@u.washington.edu
AF: ESS Dept., University of Washington 310 Condon Hall, Seattle, WA 98195 United States
AU: Sims, K W
EM: ksims@whoi.edu
AF: Dept Geology & Geophysics, WHOI, Woods Hole, MA 02543 United States
AU: Eiler, J
EM: eiler@gps.caltech.edu
AF: Div Geological & Planetary Sci, Caltech 1200 East California Blvd, Pasadena, CA 91125 United States
AB: Assimilation of crustal material is an important process affecting the composition of magmas erupted from volcanoes in a wide variety of tectonic settings, yet the timescales over which assimilation occurs are difficult to measure directly. One particularly striking example of crustal assimilation is Krafla volcano in Iceland, where a systematic decrease in δ¹⁸O with decreasing MgO in erupted lavas has been previously interpreted as evidence for assimilation of hydrothermally-altered crustal rocks. We present ²³⁸U-²³⁰Th-²²⁶Ra and oxygen-isotope data for groundmass and plagioclase separates of two lavas representing different phases (Jan-Feb 1980 and Nov 1980) of the most recent eruption of Krafla in order to directly link crystal ages with variations in oxygen-isotope composition and by extension mass balance of assimilation. Multiple size fractions of plagioclase separates were analyzed for both U-series disequilibria and oxygen isotopes. Oxygen isotope measurements of 2-5 aliquots of each size fraction of plagioclase vary significantly outside of analytical uncertainty (standard deviation of replicate measurements is 0.14-0.27 permil compared to typical s.d. of 0.03-0.05 permil for homogeneous samples) and also differ between samples, with a total range in δ¹⁸O of over 1 permil. These variations are systematic with grain size and eruptive sequence, with δ¹⁸O increasing with decreasing grain size in plagioclase from the Jan-Feb 1980 lava, and higher but roughly constant δ¹⁸O in all size fractions of plagioclase from the Nov 1980 lava. Collectively these data suggest mixing of plagioclase (and presumably host liquids) with differing influence of assimilated crustal material. ²³⁸U-²³⁰Th disequilibria of plagioclase and groundmass yield zero-age isochrons for both samples, indicating that crystallization dominantly occurred within the past 10 kyr. This is consistent with ²²⁶Ra-excesses measured in all plagioclase separates [(²²⁶Ra)/(²³⁰Th) = 1.08-1.98]. A difference in age between large and small plagioclase fractions of the Jan-Feb 1980 samples may be suggested by the smaller ²²⁶Ra-excess in the large fraction. In-situ trace-element measurements in plagioclase will allow correction for impurities in the mineral separates, and when combined with U-series measurements currently in progress for corresponding whole-rock and groundmass separates, will quantify differences in average age between the different plagioclase separates.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1036 Magma chamber processes (3618)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005