Current Research Projects
Maui ankaramite
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Crystals weathered from Maui ankaramite displaying polyhedral growth forms, plateaued faces, and buds. |
An ankaramite lava (194-230 k.a.) representing Kula stage postshield volcanism (Macdonald 1942) typical of summit lavas at East Maui (Haleakala) Volcano. Circled crystal (#26) was selected for detailed analysis. |
Volcanic rocks are envisioned as probes of the magmatic plumbing system that created them, with reservoir temperatures and pressures revealed by well-calibrated mineral-melt thermobarometers. Because Ca-clinopyroxene (cpx) is a dominant phenocryst phase in mildly alkalic magmas and cation exchange equilibria are sensitive to temperature and pressure, cpx-melt thermobarometry is a promising tool for inferring the depth and thermal structure of post-shield magma reservoirs. Do cpx crystals in Hawaiian post-shield magmas grow at near-equilibrium conditions, as suggested by faceted surfaces and inferred in assessment of magma chamber pressure (Chatterjee 2005, JVGR 145, 1-22)?
In collaboration with Eric Hellebrand, Samantha Jacob and Benoit Welsch are conducting a morphological, petrographic, and microanalytical study of Ca-clinopyroxene (cpx) in terrestrial ankaramite lava flows from the post-shield eruptive stage of East Maui Volcano, Hawaii.
Funding: EAR-1220084
Heterogeneous nucleation and epitaxial crystal growth of magmatic minerals
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Clinopyroxene dendrite exhibits lattice rotation about {010} as well as curvature. Whereas intra-segment rotation is modest (<1°), segments are separated by large (10–45°) jumps. |
(A) BSE image of titanomagnetite (timt) crystals decorating curving clinopyroxene (cpx) dendrites in a glass matrix. Cpx growth direction (blue arrows) inferred from bifurcation orientation. (B) Cpx phase orientation map of inset area overlayed on band contrast image (similar to BSE). Color scheme matches {001}cpx pole figure, (C). Curved and straight crystals exhibit rotation about {010}cpx (D). {010}cpx poles are coincident with {110}timt poles, as shown in frequency-contoured face pole plots (E). |
A topic central to igneous rock texture is the tendency of major rock-forming minerals to intergrow. A strong inclination for crystals to either share interfaces or to avoid contact has important implications for evolution of mineral compositions and texture during magma crystallization, as it would control grain size, shape, and the spatial distribution of minerals.
In the case of mafic magmas, pyroxene and magnetite occur in contact more frequently than is expected from a random distribution of these phases. We quantify this contiguity and examine synthetic and natural rocks using a variety of electron beam techniques in order to investigate the possibility that interface energetics acting at the nano-scale during crystal nucleation compel pyroxene and magnetite to form in mutual contact.
Electron backscatter diffraction analysis of dendritic clinopyroxene forming in rapidly-cooled basalt reveals two features that are unexpected for phases growing from a liquid: (a) helical growth about the crystallographic b-axis of clinopyroxene, and (b) strong crystallographic preferred orientation (CPO) between clinopyroxene (cpx) and titanomagnetite (timt) decorating branch tips.
These findings are somewhat provocative because epitaxial relationship between pyroxene and magnetite has never been reported for these phases forming from the melt, having been discussed only in the case of subsolidus exsolution of magnetite from pyroxene host crystals.
Funding: NSF EAR-0449888