ADVANCED MATERIALS CHARACTERIZATION OF P-RICH AND P-POOR REGIONS WITHIN SINGLE-CRYSTAL OLIVINE. J. E. Hammer1, H. A. Ishii1,2, J. P. Bradley2

, T. Shea1, B. Welsch1, and E. Helle-

brand1, 1Dept. Geology and Geophysics, Univ. Hawaii; Honolulu, HI, USA (jhammer@hawaii.edu), 2Hawaii Insti-tute of Geophysics and Planetology, Univ. Hawaii, Honolulu, HI USA.

Introduction: Although high valence cations such

as P5+ are relatively incompatible in olivine, crystals containing spatially coherent variations in the concen-tration of phosphorous occur in virtually every point along the forsterite-fayalite solid solution spectrum and in every mafic igneous rock, including komatiites, mid-ocean ridge basalts, ocean island basalts, arc la-vas, layered mafic intrusions, mantle xenoliths, H-chondrites, pallasites, as well as meteorites from Mars and the moon[1][2][3]. Otherwise compositionally homogeneous and euhedral, these so-called “zoned-in-P” crystals contain feathery or concentric areas that contain 10-fold or more P than P-poor areas. The co-herency of the enriched areas with growth surfaces in hopper crystals suggests that phosphorus uptake occurs during primary growth of this early-crystallizing mag-matic phase[4,5]. Unlike other trace elements, phos-phorus diffuses relatively slowly in olivine[6][7]. The preservation of heterogeneities over geologic time thus provides a window into earliest magmatic crystalliza-tion processes.

P-enrichment and the crystal growth process: While P-poor zones are regarded as domains that crys-tallized at conditions near chemical equilibrium, inter-preting the P-rich areas is problematic. These areas may represent disequilibrium “solute trapping” (dise-quilibrium uptake incompatible elements with the min-eral) or equilibrium partitioning at the solid-melt inter-face occuring in the presence of P-enriched boundary layers[8,9]. A key unresolved question is whether P is hosted within nanophase inclusions or incorporated within the crystal lattice. Local charge balance re-quires that incorporation of P5+ be balanced by a near-by substitution of a lower-valence cation (e.g., Al3+ or Na+) or a vacancy. Such substitutions, occurring at the frequencies consistent with extreme P-enrichments, are anticipated to cause lattice strain and potentially induce development of defects. Our investigation aims to de-termine how P is incorporated, chemically and struc-turally, by resolving the abundance, geometry (dimen-sionality), and dimensions of defects within P-rich olivine.

Methods. We used EPMA X-ray element maps to locate regions of high (1600 ppm) and low-P (<100 ppm) concentration within a single olivine crystal in a Kilauea basalt erupted July, 1983, used in a previous study of magmatic processes[10], (Fig 1). Electron-transparent thin sections from each area (“foils”) were

prepared with a Helios NanoLab 650 Focused Ion Beam (Oregon State U.). High spatial resolution high-angle annular dark field (HAADF) and bright field imaging was performed on an aberration-corrected Titan (scanning) transmission electron microscope at 200 and 300 kV at Univ. Hawaii. Energy-dispersive x-ray fluorescence maps were collected at 200 kV with pixels 4.6 nm on a side on a TitanX with quad-EDX detectors at the Molecular Foundry, Lawrence Berke-ley Laboratory and processed using Bruker’s Esprit 1.9 software. The (S)TEM data were used to evaluate de-fects in P-rich and P-poor areas in order to test the null hypothesis that there is no difference in the concentra-tion or type of defects present in P-rich areas as com-pared with P-poor areas. Since the crystal we are stud-ying has not undergone extrinsic deformation (e.g., caused by crystal accumulation and compaction), re-jecting this hypothesis would support the idea that P enrichment is controlled by crystal growth processes[4] and permit evaluation growth-related crystal defects associated specifically with P-uptake.

Results. Our TEM investigation resolved planar defects within the P-rich foil, all oriented parallel to the c-axis (Fig. 2). The dimensions of the foil (5 x 3.8 µm area) suggest defect density of the order 107 cm-2 within the plane of section. No defects were detected within the P-poor foil (12.5 x 5.4 µm), indicating de-fect density lower than 106 cm-2 in this region. X-ray maps revealed an even distribution of P within the high-P regions (Fig. 3), obviating the possibility that P is accommodated as discrete inclusions of other phases (e.g., phosphate nanoparticles). The results suggest instead that the defects accommodate lattice strain im-posed by P5+ atoms, but are not themselves regions wherein P is concentrated or partitioned.

References: [1] M.S. Milman-Barris, et al., Contrib. to Mineral. Petrol. 155 (2008) 739–765. [2] J.S. Boesenberg, et al., Lunar Planet. Sci. XXXV (2004) 1366. [3] C.A. Goodrich, GCA 48 (1984) 1115–1126. [4] B. Welsch, et al., Geology (2014) 1–4. [5] B. Welsch, et al., J. Petrol. 54 (2013) 539–574. [6] S. Chakraborty, Rev. Min. Geochem. 72 (2010) 603–639. [7] E.B. Watson, et al., Am. Min. 100 (2015) 2053–2065. [8] T.B. Grant and S.C. Kohn, Am. Min. 98 (2013) 1860–1869. [9] E.B. Watson, et al., Am. Min. 100 (2015) 2053–2065. [10] T. Shea, et al., Geology 43 (2015) 935–938.

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Figure. 1. X-ray intensity maps of skeletal olivine crystal at full-crystal (a) and detail (b) scale, showing location of positions of foils with respect to heterogeneous phosphorus distribution

Figure 2. High-P foil viewed in an [100] projection with 300 keV STEM. (a) Low magnification brightfield STEM image showing linear defects (arrows) running parallel to (001). (b) Lattice-fringe image of one linear defect.

Figure 3. High-angle annual darkfield (HAADF) image of a defect-rich region in the high-P specimen. Boxes 1-3 bracket three defects, box 4 brackets a defect-free region. (b). Red trace is the summed spectrum from the defect re-gions 1-3, black trace is the spectrum from region 4. There is no indication of enrichment or depletion of P or any of the other elements detected (Mg, Al, Si, Ca, Cr, Mn, Fe and Ni) associated with the defects.

2375.pdfLunar and Planetary Science XLVIII (2017)