HR: 0800h
AN: U11A-0012    [Abstracts]
TI: Micron-Scale Correlations Among Ti, P, Ce, and Y in Hadean Jack Hills Zircons
AU: * Hofmann, A E
EM: hofmann@caltech.edu
AF: Caltech, GPS Division, Pasadena, CA 91125, United States
AU: Cavosie, A J
EM: acavosie@uprm.edu
AF: University of Puerto Rico, Department of Geology, Mayaguez, PR 00681, United States
AU: Valley, J W
EM: valley@geology.wisc.edu
AF: University of Wisconsin, Department of Geology & Geophysics, Madison, WI 53706, United States
AU: Eiler, J M
EM: eiler@gps.caltech.edu
AF: Caltech, GPS Division, Pasadena, CA 91125, United States
AB: Detrital zircons and the inclusions found therein are our only mineralogical constraints on geologic events that occurred on the Hadean Earth. These zircons are commonly small (ca. <100 μm in the longest dimension) and preserve micron to sub-micron chemical zonations indicative of a dynamic petrological history. Trace elements within zircon are of particular interest because concentrations and ratios of these elements can provide information regarding chemical and physical conditions during zircon growth. In this study, we analyzed Hadean-age detrital zircons from Archean metasediment in the Jack Hills (Australia) using the Caltech Microanalysis Center Cameca NanoSIMS 50L. Trace elements analyzed included Ti, P, Ce, and Y. Ti- thermometry [1,2,3] can potentially constrain growth and/or re-equilibration temperatures of zircons; P, Ce, and Y are known to enter the zircon lattice by the coupled xenotime-type substitution mechanism: (Y, REE)³⁺ + P⁵⁺ = Zr⁴⁺ + Si⁴⁺ [5]. The ⁸⁹Y/²⁸Si ratio was observed to correlate with, and was used as a proxy for, cathodoluminescence (CL) banding. Growth features manifested in CL (e.g., sector, oscillatory zoning) were observed in all zircons analyzed. CL zones vary from <1 μm to several microns in width; therefore, the NanoSIMS---with a beam diameter resolved to ca. 250 nm on the sample surface when operating with an O^- primary beam---is uniquely suited for this scale of analysis. Regions displaying CL banding were imaged as 20 x 20 μm areas. All elements were normalized to ²⁸Si; ⁴⁹Ti/²⁸Si ratios were converted to [Ti] via calibration based on analyses of synthetic, high-Ti zircons (provided by B. Watson) that were independently analyzed on Caltech's JEOL JXA-8200 electron microprobe. We observe three types of relationships between trace element distribution and CL banding in the zircons imaged: 1) strong positive correlations between CL banding, P, Ce, and Ti; 2) subtle positive correlations between CL banding, P, Ce, and Ti; 3) no correlation between minor/trace elements and CL banding. Positive correlations between CL banding, 3+ cations, and [Ti] have previously been reported by Holden et al. [4]. In this study, gradients at least as sharp as a factor of ~3 in [Ti] are observed between adjacent CL bands in the strongly correlated images. These images also have the highest absolute concentrations of trace elements and display both sector and oscillatory zoning in CL. The correlations observed may be due to: temperature-dependent equilibrium partitioning of all trace elements during rapid cycles in growth temperature; episodic diffusion-limited enrichment of incompatible trace elements in the boundary layer melt adjacent to growing crystals; and/or kinetically controlled, non- equilibrium crystal-melt partitioning caused by trace element enrichments in the boundary layer melt surrounding fast-growing grains (e.g., [6]). We will discriminate between these alternatives based on quantitative relationships between relative enrichments of [Ti] and other trace elements. [1]Watson, E.B. & Harrison, T.M. (2005) Science 308, 841-844. [2]Watson, E.B., Wark, D.A., & Thomas, J.B. (2006) CMP 151, 413-433. [3]Ferry, J.M. & Watson, E.B. (2007) CMP 154, 429-437. [4] Holden, P. et al. (2005) Eos Trans. AGU 86 (52) Fall Meet. Suppl., Abstract V41F-1539. [5] Speer, J.A. (1982) Zircon. In Rev. Min. 5 (ed. P.H. Ribbe), 67-112. [6] Watson, E.B. (2004) GCA 68, 1473-1488.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
SC: Union [U]
MN: 2007 Fall Meeting