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2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 58
Presentation Time: 8:00 AM-12:00 PM

JIMTHOMPSONITE, CHESTERITE, AND CHAIN-WIDTH DISORDER IN ARCHAEAN ULTRAMAFIC ROCKS FROM WEST GREENLAND: HIGH-RESOLUTION TEM INVESTIGATION


KONISHI, Hiromi, Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, XU, Huifang, Department of Geology and Geophysics, Univesity of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, DYMEK, Robert F., Department of Earth and Planetary Sciences, Washington University, 1 Brookings Drive, St. Louis, MO 63130 and LI, Xiaochun, Department of Mechanical Engineering, Center for Structurally Integrated Micro/Nano-Systems, University of Wisconsin-Madison, 1513 University Avenue, Madison, WI 53706, hfxu@geology.wisc.edu

Biopyribole minerals provide excellent examples of polysomatism and polytypism. The exact species of biopyribole can be identified only by characterizing its polysome (chain-widths along the b-axis), polytype (stacking sequences along the a-axis), and chemical composition. As such, High-resolution Transmission Electron Microscopy (HRTEM) is essential for identifying the wide-chain silicates jimthompsonite and chesterite, because their optical properties and chemical compositions are very similar, and for recognizing chain-width disorder.

In this study, we report HRTEM results of wide-chain silicate minerals found in talc-magnesite schist from the ~3800 Ma Isua Supracrustal Belt, West Greenland. The samples (chlorite + talc + magnesite + anthophyllite + dolomite + biopyribole -- ~4 modal%) were collected from schist horizons developed via retrograde metamorphism of tectonically entrained bodies of dunite and peridotite. Both jimthompsonite and chesterite were discovered in these samples based on their optical properties, cleavage angles, and results of microprobe analysis (O'Leary and Dymek, 1987, EOS, p. 452). Our HRTEM studies indicate that optically observed (010) intermediate lamellae in these silicates are indeed jimthompsonite (triple-chain silicate with ortho-stacking) and chesterite (mixed double- and triple-chain silicate with ortho-stacking), with additional chain-width disordered areas, which are intergrown with anthophyllite at the submicrometer scale.

The jimthompsonite and chesterite from Isua formed under conditions similar to those at Chester, Vermont. However, our samples are unique because the host rocks form mappable horiznons (rather than restricted blackwall zones), and the biopyroboles occur at several locations in the area. Combining phase-petrographical analyses and HRTEM characterization, it is possible to clarify the reactions and P-T conditions needed for the formation of jimthompsonite and chesterite.

Session No. 143--Booth# 73

Mineralogy/Crystallography; Petrology; Volcanology (Posters)
Tuesday, 30 October 2007: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 39, No. 6, p.395
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