GEOLOGY | February 2013 | www.gsapubs.org 263 ABSTRACT Rock varnish is a thin dark coating best known from deserts, and is believed to grow extremely slowly. Varnish samples from near Socorro, New Mexico (United States), contain as much as 3.7% PbO, derived from nearby smelters operating from A.D. 1870 to 1931. Additional varnish, measuring as much as 4 μm beyond the Pb-rich layer, indicates continued growth from 1931 to 2003. Comparison with other varnish confirms that the Pb is not an artifact. Based on Pb layer thickness, and the period of smelter operation, these very young rock varnishes yield growth rates of 28–639 μm/k.y., substantially higher than previously documented fastest rates of 40 μm/k.y. These rates imply that the average 1–2 μm/k.y. rate for older varnish is not the active growth rate. Rather, it is a long-term value including peri- ods of nondeposition, erosion, and active growth. Therefore, models of rock varnish formation should be reevaluated with consideration of much faster maximum growth rates. INTRODUCTION Rock, or desert, varnish is a thin veneer (<200 μm) of manganese and/or iron oxides interlaminated with clays and often silica (Dorn, 1991, 2007; Liu and Broecker, 2000). Although exact mechanisms of rock var- nish formation remain controversial, most workers agree on some combi- nation of airborne dust supplying materials and microbial concentration of manganese, modified by considerable diagenesis (Liu et al., 2000; Garvie et al., 2008; Northup et al., 2010; Dorn and Krinsley, 2011). Additional background material can be found in the GSA Data Repository 1 . Archaeologists are interested in dating varnish in order to date petro- glyphs carved by ancient cultures (Stasack et al., 1996; Dietzel et al., 2008). Geomorphologists have attempted to date landforms and surfaces using rock varnish (Friend et al., 2000; French and Guglielmin, 2002). However, direct dating attempts have proven unsuccessful so far (Watchman, 2000). Using careful dating of the underlying surface, Liu and Broecker (2000) documented varnish growth rates of 1–40 μm/k.y. in the southwest- ern United States. The oldest samples have the slowest rates, while the youngest sample (1.5 ka) has the fastest rate, 40 μm/k.y. (Liu and Broecker, 2000). Here we document extremely young varnish that records anthro- pogenic Pb from historic smelters with substantially higher growth rates. These rapid rates have important implications for rock varnish formation. SAMPLE LOCATIONS Six samples of black varnish on rhyolite were collected near Socorro, New Mexico (United States; Fig. 1; Figs. DR1 and DR2 in the Data Repository); four of these were chosen for detailed analysis. This location is in the vicinity of historic smelters that operated from ca. A.D. 1870 to 1931 (Fig. 1; Evelith, 1983). The study site is ~11 km southwest of the Billing–Rio Grande Smelter site in Socorro, and ~25 km southeast of sev- eral smaller smelters in Kelly and Magdalena (Fig. 1). For comparison, two additional locations were analyzed (Fig. 1). A varnished granite from the Kelso Mountains, Mojave Desert (California), was collected 28 km from the nearest major highway, but the site is cen- trally located between two major metropolitan areas, the Los Angeles Basin and Las Vegas (Nevada). A siltstone from the Jurassic Morrison Formation, collected west of Hanksville in central Utah (Fig. 1), is the most remote site, 50 km from any major highway, with no known significant mining or smelting activity within 100 km, and no metropolitan area within 200 km. METHODS Samples were first examined using scanning electron microscopy (SEM) on varnish and host rock fragments coated with a 60:40 alloy of Au and Pd prior to imaging in a JEOL 5800LV SEM. Four samples, Socorro sites 2, 5, 6, and 7, were selected for electron microprobe analysis (EMP), along with one sample each from Hanksville and the Mojave Desert. Separate fragments from each sample were embed- ded in epoxy, cut with a diamond saw to expose undisturbed varnish, and polished. Elemental line profiles and continuous line scans across the var- nish were acquired on a fully automated JEOL 8200 electron microprobe equipped with five wavelength-dispersive X-ray spectrometers and backscat- tered electron imaging. For elemental line profiles, quantitative analyses were taken every 1.5–2 μm across the full varnish thickness. Operating parameters include a beam current of 30 nA, with a 20 s peak counting time for major elements (Al, Si, Mn, Fe), 30–40 s on minor elements (Na, Mg, K, Ca, Co, Ni, Cu, Zn, Ba), and 60 s on Pb. Calibration and precision and/or accuracy checks used natural mineral standards. The last point of each traverse was 1–2 μm from the edge, in order to contain the X-ray interaction volume. To refine Pb distribution, continuous stage-stepped elemental line scans utilized a reduced accelerating voltage of 10 kV and 20 nA to 1 GSA Data Repository item 2013065, supplemental background, Figures DR1 and DR2, and Tables DR1 and DR2, is available online at www.geosociety .org/pubs/ft2013.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. Anthropogenic lead as a tracer of rock varnish growth: Implications for rates of formation Michael N. Spilde 1 , Leslie A. Melim 2 , Diana E. Northup 3 , and Penelope J. Boston 4 1 Institute of Meteoritics, MSC03-2050, University of New Mexico, Albuquerque, New Mexico 87131, USA 2 Department of Geology, Western Illinois University, Macomb, Illinois 61455, USA 3 Biology Department, University of New Mexico, Albuquerque, New Mexico 87131, USA 4 Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA GEOLOGY, February 2013; v. 41; no. 2; p. 263–266; Data Repository item 2013065 | doi:10.1130/G33514.1 | Published online 4 January 2013 © 2013 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. I-25 Rio Grande River Socorro K S B 10 km US 60 Magdalena B Hanksville Socorro Mojave A 107°W 34°N N Figure 1. A: Location map for sample sites in southwestern United States. B: Detailed map of Socorro, New Mexico, region showing sample site (S), the Billing–Rio Grande Smelter (B), and the town of Kelly, New Mexico (K), where another smelter was located. Base im- age is from Google Earth™.