Evidence for Coordination and Redox Changes of Iron in Shocked Feldspar from Synchrotron

Introduction: Shock modification of feldspar has been documented and experimentally reproduced in many studies since the recognition of maskelynite in Shergotty (1). Experimentally shocked feldspar samples have been well studied using chemical and crystallographic techniques (2-4). The crystallographic, site-specific characterization of major and minor elements is less well documented. We present early x-ray absorption (XAS) spectral data for a suite of albitite samples (5,6) that were experimentally shocked at pressures between 17 and 50 Gpa. Method: The experimental shock methods are described by [7]. The samples are about 98-99% fine grained albite with minor muscovite, zircon and Y-rich phosphate. Synchrotron microXANES spectra were acquired using the x-ray microprobe at the National Synchrotron Light Source beamline X26A using the methods of [8] and Fe and Fe feldspar end members for calibration. Several XANES spectra were acquired on 15x20 μm spots on each of 6 small chips of shocked albitite that were impregnated with epoxy and polished. Table 1: Sample pressures and selected spectra. Sample Gpa Fe/ΣFe NSLS ID 3315 17 0.75-0.99 629.032 3311 25.5 0.62-0.77 608.102 3312 29 0.65-0.70 608.104 3319 34.8 0.62-0.69 629.033 3320 44.6 0.62-0.65 629.035 3316 50 0.40-0.65 629.036 Results: The Fe content of the albite is very low, (400-1000 ppm), but there is sufficient Fe present to permit usable XANES spectra to be obtained with approximately 2 hour counting times for each spectrum. In Figure 1, XANES spectra of 5 of the 6 shocked samples are similar except for changes in their pre-edge features. Because the Fe content of this feldspar is very low, secondary fluorescence of adjacent muscovite by the scattered x-ray beam must be avoided. This was a problem in our earlier work on shocked anorthosites [9] where impurities were ubiquitous and such contamination was generably unavoidable. The main edge shapes of the spectra in Figure 1 provide a check for interference by muscovite, as the main edge shape of muscovite shows clear subpeaks that are absent in these spectra. The data in this study are therefore believed to be pure feldspar spectra. The main edge of the 50 Gpa spectrum is shifted ~1-2eV toward lower energy than the others.