HR: 0830h
AN: B21C-0719    [PDF]
TI: Desert Varnish: Relative and Absolute Dating Using Portable X-Ray Fluorescence
AU: * Pingitore, N E
EM: nick@geo.utep.edu
AF: The University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968-0555 United States
AU: Lytle, F W
EM: fwlytle@icturbonet.com
AF: The EXAFS Company, HC 74 Box 236 Eagle Valley, Pioche, NV 89043-9521 United States
AB: Levels of manganese and iron measured {\it in situ} with a portable x-ray fluorescence instrument permit relative and absolute dating of desert varnish. This novel technique may have wide potential application to dating Pleistocene and Holocene events and geomorphic surfaces in dry climate settings. Desert varnish is a thin biogenic coating, enriched in Mn and Fe, found on rock surfaces in arid and semi-arid regions. The accumulation of varnish marks the passage of time since a fresh rock surface was created or exposed. Thus the varnish thickness reflects the age of the event that created the fresh surface, whatever the agent was, e.g., a rock fall, a fault movement, or an aboriginal artist. Past attempts to date rock varnish have been marked more by ambiguity or outright failure than by success. Our recent research suggests a practical and rapid method for dating varnish using a portable x-ray fluorescence instrument (PXRF). Varnish thickness encodes two distinct signals, metal and clay. The biogenic Mn and Fe record the passage of time, whereas the accumulation of clay particles is a more time-random process. X-ray fluorescence (XRF) can measure just the "metal thickness" of Mn and Fe in varnish. Earlier tedious microscope techniques focused on physical thickness that includes the noise associated with the clay component. XRF integrates the metal thickness of a broad area of varnish, which is seen to vary significantly in a thin-section traverse. Thus XRF provides a meaningful average thickness over a surface. A portable x-ray fluorescence unit provides rapid, non-destructive, {\it in situ} measurements. On outcrop a single analysis takes about 2 minutes and the varnish is not consumed or even disturbed. The hand-held PXRF instrument is simple to operate and relatively inexpensive (\$ 30,000). PXRF analysis of varnish on independently dated materials yielded a substantive correlation between age and metal (Mn + Fe) thickness. This provided an initial validation of the technique for relative dating, and a cautious optimism for reliable absolute dating. Using that initial correlation as a thickness vs. age calibration has generated what appear to be reasonable ages on unrelated varnish "unknowns." Obvious applications of the technique include dating of petroglyphs by degree of repatination; assessment of rock-fall risks by dating old falls; dating motion from varnish development on fault scarps; and finding ages of geomorphic surfaces.
DE: 0400 Biogeosciences
DE: 1035 Geochronology
DE: 1600 GLOBAL CHANGE (New category)
DE: 1615 Biogeochemical processes (4805)
DE: 1694 Instruments and techniques
SC: Biogeosciences [B]
MN: 2003 Fall Meeting