March GEOLOGY and GSA TODAY media highlights

Nanometer-scale complexity, growth, and diagenesis in desert varnish

Laurence Garvie et al., Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. Pages 215-218.

Finely layered coatings, rich in manganese and iron and commonly called desert varnish, are common on rocks in desert environments worldwide. These coatings have been the subject of intense scientific debate and extensive research, owing to their potential for indicating past climates, for dating geological surfaces, and, via artwork carved in varnish, for providing information about ancient cultures. The full scientific potential of desert varnish can only be realized through a rigorous probing of the physico-chemical variables and fundamental properties of varnish components, especially its mineralogical components. Determining the mineralogy of the manganese- and iron-bearing materials is challenging because the minerals are extremely fine grained, generally down to nanometer-sized, and often poorly crystalline. In addition, the thin film-like nature of varnish on rock makes separating and studying it difficult. Garvie et al. used novel sample preparation methods, high-resolution electron microscopy, and spectroscopic imaging to provide novel insights into desert varnish structure, mineralogy, and chemistry. The spectroscopic imaging shows nanometer-scale separation of manganese- and iron-bearing phases, possibly reflecting differing degrees of chemical oxidation. A suite of late-grown manganese and iron phases commonly occur also, together with sparse barium and strontium sulfates, and rare, entrained, carbonaceous particles. These data demonstrate that varnish remains a mineralogically and structurally active system. They furthermore suggest that there must be strict climate controls on varnish growth.

Forearc diamond from Japan

Simon Wallis et al., Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan. Pages 219-222.

Most diamonds are brought to the Earth's surface in explosive volcanic eruptions in geologically old continental regions. Volcanism in convergent margins is generally thought to be too shallow in origin to be a host for diamond. Micro-diamonds found in xenoliths within a lamprophyre dike in southwest Japan show this assumption is incorrect, and, furthermore, that diamonds occur in a wider range of geological settings than previously realized. This may help explain the origin of some known anomalous diamond deposits. Studies of associated minerals show the newly recognized diamond-bearing rocks rose from depths of around 160 km and cooled from temperatures of ~1500 C. This result implies that mantle flow in convergent plate boundaries occurs on a larger scale that previously recognized.

First exposure ages from the Amundsen Sea Embayment, West Antarctica: The Late Quaternary context for recent thinning of Pine Island, Smith, and Pope Glaciers

Joanne Johnson et al., British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Pages 223-226.