Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.519h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 519
Mathematics
Logic
Chondrule, Chondrule Orientation, Chondrules, Flattening, Meteorites, Bovedy, Shock Effects, Shock Metamorphism
Scientific paper
Introduction: Chondrule flattening and distinct foliation are preserved in certain chondrites [1] and have been interpreted, by some, as evidence of shock-induced pressure through hypervelocity impacts on parent bodies [2]. Recently, mean aspect ratios of naturally and artificially shocked chondrules, in the Allende (CV3) chondrite, have been correlated with shock intensity [3] using established shock stage criteria [4]. Clearly, quantification of chondrule deformation and appropriate petrographic criteria can be useful tools for constraining parent body shock history and, possibly, post-shock heating [3]. Here, strain analysis techniques (R(sub)(f)/phi and Fry) normally employed in structural geology, have been adapted and evaluated [5], for measuring mean chondrule strain, and orientation. In addition, the possible use of such strain data for partial shock stage classification is considered. R(sub)(f)/phi and Fry Analysis: The relationship between displacement and shape changes in rocks is known as strain [6] and assumes that an initial circle with a unit radius is deformed to form an ellipse, the finite strain ellipse (Rf). The strain ratio (Rs) is an expression of the change of shape. The orientation of the strain ellipse (phi) is the angle subtended between the semi-major axes and the direction of a fixed point of reference. Generally, log mean Rf ~ Rs and, therefore, the approximation Rf = Rs is valid. For chondrules, this is reasonable as they were originally molten, or partially-molten, droplets [7]. Fry's 'center-to-center' geological strain analysis technique [8] is based on the principle that the distribution of particle centers in rocks can sometimes be used to determine the state of finite strain (Rf). Experimental Techniques: The Bovedy (L3) chondrite was chosen for investigation as it contains abundant, oriented, elliptical chondrules [5]. Hardware employed consisted of a Macintosh microcomputer and a flat-bed scanner. Chondrule outlines, obtained from macrophotographic tracings of four complete thin-sections (total area 8.2 cm2) and a sawn slab (49.45cm2), were digitally scanned using application Ofoto v. 1.0.0^(TM). Chondrule outline (pict) files were then exported to a fabric analysis program, Image v. 1.44, and Rf values obtained thereafter exported to a spreadsheet environment for manipulation. Fry analysis was undertaken with an interactive program, Fry v. 5.0 [9] using the same pict files as before. Chondrule central points were manually inserted and center-to-center distances, when calculated, were displayed on screen in a way which echoes mean chondrule strain and orientation. Results and Conclusion. 364 chondrule outlines (three thin-sections and a sawn slab) were analysed by R(sub)(f)/phi and Fry techniques. In its present form, the Fry technique was judged to be unsuited to chondrule shape analysis as it is too dependant on grain size, i.e. the smallest grain, and the need for a planar homogenous sample bearing several hundred grains [8]. Recent developments in the Fry technique [10] may make it more suitable for chondrule analysis. Representative strain (Rf) data obtained for parallel thin-sections Bovedy M5385b and M5385c (total of 158 chondrules) were 1.49 and 1.41 respectively. Corresponding phi values were 115.0 degrees and 114.6 degrees respectively (with respect to a fixed reference point). Rf data together with petrographic shock features noted, mostly in olivine (e.g. planar fractures, undulatory extinction and weak mosaicism), were suggestive of shock stage S3 [4]. The degree of chondrule flattening and the nature of the (S3) shock effects observed are comparable with artifically flattened chondrules belonging to the same shock stage [3, 11]. The R(sub)(f)/phi technique evaluated was found to be more precise and quantitative than other methods previously employed for measuring maximum and minimum chondrule axes and orientation. Furthermore, it can provide reliable strain (axial, orientation) data for material subjected to very low grades of shock which would otherwise be difficult to quantify. References: [1] Cain P. M. et al. (1986) EPSL, 77, 165-175. [2] Scott E. R. D. et al. (1992) GCA, 56, 4281-4293. [3] Nakamura T. et al. (1995) Meteoritics, 30, 344-347. [4] St"ffler D. et al. (1992) GCA, 55, 3485-3867. [5] Hill H. G. M. (1994) M.Sc. thesis, Univ. of Dublin. [6] Ramsay J. G. and Huber M. I. (1983) The Techniques of Modern Structural Geology. Volume 1: Strain Analysis, Academic, London. [7] Grossman J. N. et al. (1988) in Meteorites and the Early Solar System (J. F. Kerridge and M. F. Matthews, eds.), 619-659, Univ. of Arizona, Tucson. [8] Fry N. (1979) Tectonophys., 60, 89-105. [9] De Paor D. G. (1989) J. Geol. Educ., 37, 171-180. [10] Erslev E. A. and Ge H. (1990) J. Structural Geol., 12, 1047-59 [11] Schmitt R. T. et al. (1994) Meteoritics, 29, 529-530.
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