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Zoning of phosphorus in igneous olivine

Milman-Barris, Margaret S. and Beckett, John R. and Baker, Michael B. and Hofmann, Amy E. and Morgan, Zachary and Crowley, Meghan R. and Vielzeuf, Daniel and Stolper, Edward (2008) Zoning of phosphorus in igneous olivine. Contributions to Mineralogy and Petrology, 155 (6). pp. 739-765. ISSN 0010-7999. https://resolver.caltech.edu/CaltechAUTHORS:20120823-103423279

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Abstract

We describe P zoning in olivines from terrestrial basalts, andesites, dacites, and komatiites and from a martian meteorite. P_(2)O_5 contents of olivines vary from below the detection limit (≤0.01 wt%) to 0.2–0.4 wt% over a few microns, with no correlated variations in Fo content. Zoning patterns include P-rich crystal cores with skeletal, hopper, or euhedral shapes; oscillatory zoning; structures suggesting replacement of P-rich zones by P-poor olivine; and sector zoning. Melt inclusions in olivines are usually located near P-rich regions but in direct contact with low-P olivine. Crystallization experiments on basaltic compositions at constant cooling rates (15–30°C/h) reproduce many of these features. We infer that P-rich zones in experimental and natural olivines reflect incorporation of P in excess of equilibrium partitioning during rapid growth, and zoning patterns primarily record crystal-growth-rate variations. Occurrences of high-P phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling. Most cases of oscillatory zoning in P likely reflect internal factors whereby oscillating growth rates occur without external forcings, but some P zoning in natural olivines may reflect external forcings (e.g., magma mixing events, eruption) that result in variable crystal growth rates and/or P contents in the magma. In experimental and some natural olivines, Al, Cr, and P concentrations are roughly linearly and positively correlated, suggesting coupled substitutions, but in natural phenocrysts, Cr zoning is usually less intense than P zoning, and Al zoning weak to absent. We propose that olivines grow from basic and ultrabasic magmas with correlated zoning in P, Cr, and Al superimposed on normal zoning in Fe/Mg; rapidly diffusing divalent cations homogenize during residence in hot magma; Al and Cr only partially homogenize; and delicate P zoning is preserved because P diffuses very slowly. This interpretation is consistent with the fact that zoning is largely preserved not only in P but also in Al, Cr, and divalent cations in olivines with short residence times at high temperature (e.g., experimentally grown olivines, komatiitic olivines, groundmass olivines, and the rims of olivine phenocrysts grown during eruption). P zoning is widespread in magmatic olivine, revealing details of crystal growth and intra-crystal stratigraphy in what otherwise appear to be relatively featureless crystals. Since it is preserved in early-formed olivines with prolonged residence times in magmas at high temperatures, P zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma’s history. Study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks, especially because these features can be observed with standard electron microprobe techniques.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1007/s00410-007-0268-7DOIArticle
http://www.springerlink.com/content/g22m571768n77265/PublisherArticle
ORCID:
AuthorORCID
Stolper, Edward0000-0001-8008-8804
Additional Information:© 2007 Springer-Verlag. Received: 20 April 2007; Accepted: 19 November 2007; Published online: 3 January 2008. We thank Ma Chi and J.L. Devidal for guidance with electron microprobe work and L.V. Bleacher, P. Boivin, F. Costa, J. Eiler, L. Folco, T. L. Grove, A. Kerr, A. Saal, and the NASA Meteorite Working Group for the loan of samples. The authors also thank J. Boesenberg, P.R. Buseck, F. Brunet, K.V. Cashman, C. Donaldson, C.A. Goodrich, T.L. Grove, A. Saal, and P. Schiano for critical insights and suggestions over the course of this study and C.A. Goodrich and an anonymous reviewer for comments that led to significant improvements in the manuscript. Funding was provided by DOE grant DE-FG02-06ER15773, NASA grant NNG04GG14G, and NSF grant EAR-9528594.
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG02-06ER15773
NASANNG04GG14G
NSFEAR-9528594
Issue or Number:6
Record Number:CaltechAUTHORS:20120823-103423279
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20120823-103423279
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:33474
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:23 Aug 2012 20:12
Last Modified:03 Oct 2019 04:10

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