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Clinopyroxene in postshield Haleakala ankaramite: 1. Efficacy of thermobarometry

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Abstract

Magma storage depth is a fundamental aspect of a volcano’s magmatic plumbing system that may be resolved using mineral-melt thermobarometry, assuming crystal growth occurs at near-equilibrium conditions. We acquire major and minor element compositional analyses of whole rock, groundmass separates, and clinopyroxene in ankaramite erupted ca. 214 ka at Haleakala volcano to evaluate the efficacy of thermobarometry. Using various thermometer and barometer combinations, we obtain values of crystallization pressure (60–1500 MPa) that are generally consistent with those of previous studies, but find that the models most successful at recovering the conditions of relevant equilibrium experiments yield values at the low end of this range (≤950 MPa). We use quantitative EPMA spot analyses to transform X-ray element intensity maps into metal oxide concentrations maps and to produce qualitative pressure maps of whole crystals. The spatial context provided by this procedure reveals two compositionally distinct domain types not evident in the spot analysis data set, with median Na2O contents differing by up to 26 % between domains. Na-rich domains represent putative crystallization pressures that are up to 365 MPa higher than Na-poor domains, within individual crystals. The presence of Na-rich domains associated with euhedral facets in contact with matrix is not consistent with concentric growth at near-equilibrium conditions of decreasing pressure, but rather co-crystallization of both domains under conditions of partial disequilibrium. Conservatively assuming that low-Na regions are less prone to kinetic partitioning, crystallization pressures for the Haleakala ankaramite correspond to crustal levels. We conclude that the reservoir supplying postshield eruptions at Haleakala has not deepened into the mantle, as was reported in a previous application of clinopyroxene thermobarometry to Haleakala’s postshield magma (Chatterjee et al. 2005).

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Acknowledgments

JoAnn Sinton is gratefully acknowledged for the loan of many loose crystals and for thin section preparation. Olivier Odon is also thanked for his help with the Haleakala DEM. The manuscript was improved with comments from reviewers Keith Putirka and Matteo Masotta. This work was supported by NSF EAR 12-20084 to JEH and is SOEST publication number 9529.

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  1. Department of Geology and Geophysics, University of Hawaii, Honolulu, HI, USA

    Julia Hammer, Samantha Jacob, Benoit Welsch, Eric Hellebrand & John Sinton

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  1. Julia Hammer
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  2. Samantha Jacob
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  3. Benoit Welsch
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Correspondence to Benoit Welsch.

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Communicated by Gordon Moore.

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Fig. SM1

XP Photomicrograph of Ka02 (TIFF 3083 kb)

Fig. SM2

Photomosaic of loose crystal aug3, showing locations of analytical spots reported in Table 4, and various textural domains. Image types are as indicated: RL= reflected light; BSE= back-scattered electron, and RGB= X-ray intensity composites, with channels as defined in the legend. Width of top image is approximately 1 cm (TIFF 3767 kb)

Fig. SM3

Thermobarometry models applied to the experimental data sets of Baker and Eggler 1987, Bartels, et al. 1991, Kinzler and Grove 1992, and Putirka, et al. 1996. (a) The barometers of Putirka et al. (2003), Putirka et al. (1996) and Putirka (2008) yield significantly different results, particularly when combined with different input temperatures. The preferred (a) thermometer (Putirka, et al. 2003) and (b) barometer (Eq. 32b of Putirka 2008, used with temperature obtained using the thermometer of Putirka et al., 1996) are shown with error envelopes that indicate perfect recovery of experimental values ± 43 °C in temperature and 150 MPa in pressure, reflecting the average misfit of the preferred thermometer and barometer. (TIFF 5363 kb)

Fig. SM4

Conventional histograms showing distributions of Al2O3 and Na2O wt. % for crystal Ka02-04 obtained from calibration of X-ray intensity maps as described in the text (TIFF 9344 kb)

Fig. SM5

Compositional correlations with pressure for experimental (left) and natural (right) crystal-liquid pairings and application of the preferred thermobarometers. Solid and dashed lines indicate median values obtained from 2D frequency histograms of element maps of spongy and non-spongy regions of crystal Ka02-04, respectively (TIFF 8283 kb)

Fig. SM6

Linear correlations of X-ray counts (scaled to 0-255) obtained in EMP mapping of various elements (x-axes) against oxide weight percents obtained in quantitative EMP-WDS analysis (y-axes). The spots are marked on the BSE images collected concurrently using Photoshop image processing software. The coordinates of the marked spots are read into a MATLAB script that extracts and averages the intensity values for 9-12 pixels in the element map (TIFF 5134 kb)

Fig. SM7

2D frequency distributions of elemental covariations in three Qkuls clinopyroxene crystals. Linear correlations between oxide wt.% abundance and X-ray counts (Fig. SM6) are used to estimate oxide wt.% element for each of the ca. 300k pixels in each element map. The arithmetic means (white dots) differ from modal values in cases where the distributions are non-normal (e.g., SM4). The 2D frequency histograms reveal areally-dominant compositions in a manner that is not apparent in the sparser sampling afforded by spot analyses (Fig. 5). Although the compositions of distinct textural regions overlap substantially, spongy areas are compositionally distinct from non-spongy regions (TIFF 288 kb)

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Hammer, J., Jacob, S., Welsch, B. et al. Clinopyroxene in postshield Haleakala ankaramite: 1. Efficacy of thermobarometry. Contrib Mineral Petrol 171, 7 (2016). https://doi.org/10.1007/s00410-015-1212-x

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