Effects of superheating magnitude on olivine growth
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Magmatic superheating is a condition with relevance to natural systems as well as experimental studies of crystallization kinetics. Magmas on Earth and other planetary bodies may become superheated during adiabatic ascent from the mantle or as a consequence of meteorite impact-generated crustal melting. Experimental studies of igneous processes commonly employ superheating in the homogenization of synthetic starting materials. We performed 1-atmosphere dynamic crystallization experiments to study the effects of superliquidus thermal history on the morphologies and compositions of subsequently grown olivine crystals. An ultramafic volcanic rock with abundant olivine was fused above the experimentally determined liquidus temperature (1395 °C), held for 0, 3, or 12 h, cooled at 25 °C h−1, and quenched from 200 °C below the liquidus, all at constant fO2, corresponding to FMQ-2 ± 0.2 log units. An increase in olivine morphologic instability is correlated with superheating magnitude, parameterized as the integrated time the sample is held above the liquidus (“TtL”; °C h). We infer that a delay in nucleation, which intensifies monotonically with increasing TtL, causes crystal growth to be increasingly rapid. This result indicates that the structural relaxation time scale controlling the formation of crystal nuclei is (a) far longer than the time scale associated with viscous flow and (b) exceeds the liquidus dwell times typically imposed in crystallization experiments. The influence of magmatic superheating on crystal morphology is similar in sense and magnitude to that of subliquidus cooling rate and thus, both factors should be considered when interpreting the thermal history of a volcanic rock containing anhedral olivine.
KeywordsOlivine Crystal growth Superheating Textural analysis Kinetics Crystal nucleation
We gratefully thank M. Garcia for discussions and samples, E. Hellebrand, T. Shea, J. Boesenberg, S. Mallick, B. Chilson Parks, A. Charn, and I. Fendley for analytical assistance; B. Welsch, G. Libourel, M. Rutherford, and M. Davis for lively debates; N. Arndt, B. Lange, S. Mollo and anonymous reviewers for thoughtful comments. This work was supported by National Science Foundation (US) awards EAR1321890 and EAR1347887 and is SOEST publication #10803.
EF: Conceptualization, experimental methodology, chemical analysis, geochemical modeling. TL: Experimental investigation, chemical analysis, imaging and image processing methodology. JH: Geochemical modeling, manuscript preparation.
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