Current issue

Volume 3, Number 2

Comment on “A cometary origin for atmospheric martian methane” by Fries et al., 2016

Comment on “A cometary origin for atmospheric martian methane” by Fries et al., 2016:
Reports of transient plumes of martian atmospheric methane (Mumma et al., 2009; Webster et al., 2015) have led to suggestions of biologic or abiotic surface sources. Schuerger et al. (2012) examined the production of methane near the surface from interplanetary dust particles. They found this mechanism was capable of yielding the background value of methane, but could not reproduce plume densities by bolide, airburst or other meteor impact process. Fries et al. (2016) draw on the work of Schuerger et al. (2012) and propose that the methane plumes are sourced instead from intense meteor showers with conversion at high altitudes.

M.M.J. Crismani, N.M. Schneider, J.M.C. Plane

Geochem. Persp. Let. (2017) 3 | doi: 10.7185/geochemlet.1715 | Published 18 February 2017

Reply to Comment on “A cometary origin for martian atmospheric methane”
by Crismani et al., 2017

Reply to Comment on “A cometary origin for martian atmospheric methane”
by Crismani et al., 2017:

First, I would like to extend thanks to Crismani et al. for their commentary, which highlights an important uncertainty acknowledged in Fries et al. (2016), namely whether the total mass of infall material required to produce the observed methane is on par with that available from meteor showers. I would like to disagree on two points presented in the letter, and then discuss the implications of their findings.

M. Fries,

Geochem. Persp. Let. (2017) 3 | doi: 10.7185/geochemlet.1716 | Published 18 February 2017

Repulsion between calcite crystals and grain detachment during water–rock interaction

Weathering in carbonate rocks is often thought to be governed by chemical dissolution. However, recent studies have shown that mechanical detachment of tiny grains contributes significantly to the overall surface retreat. Whether this detachment is caused by shear forces acting at the surface, or repulsive forces acting between grains, was not known. In this study, we used atomic force microscopy to examine the mechanism of grain detachment and we demonstrate that it occurs even in the absence of shearing fluid flow. This suggests that the removal of grains from rock surfaces can be caused by repulsive forces between calcite grains. Although these repulsive forces are expected to be sensitive to the ionic strength of the solution, we did not find enough evidence to demonstrate a correlation between salinity and the frequency of grain detachment. Importantly, our findings suggest that grain detachment occurs during water-rock interaction under low flow conditions over a range of salinities, with potential impacts on geological carbon sequestration and enhanced oil recovery in carbonate formations.

Y. Levenson, S. Emmanuel

Geochem. Persp. Let. (2017) 3, 133-141 | doi: 10.7185/geochemlet.1714 | Published 31 January 2017

Evidence of sub-arc mantle oxidation by sulphur and carbon

The oxygen fugacity (ƒO2) of the Earth’s mantle at subduction zones exerts a primary control on the genesis of mineral deposits in the overlying magmatic arcs and on speciation of volcanic gases emitted into the atmosphere. However, the processes governing mantle ƒO2 such as the introduction of oxidised material by subduction are still unresolved. Here, we present evidence for the reduction of oxidised fluid-borne sulphur and carbon during alteration of depleted mantle by slab fluids at ultra-high pressure in the Bardane peridotite (Western Gneiss Region, Norway). Elevated ferric iron in metasomatic garnet, determined using synchrotron X-ray absorption near edge structure (XANES) spectroscopy, indicates that this process drove oxidation of the silicate assemblage. Our finding indicates that subduction oxidises the Earth’s mantle by cycling of sulphur and carbon.

A. Rielli, A.G. Tomkins, O. Nebel, J. Brugger, B. Etschmann, R. Zhong, G.M. Yaxley, D. Paterson

Geochem. Persp. Let. (2017) 3, 124-132 | doi: 10.7185/geochemlet.1713 | Published 27 January 2017

Water in alkali feldspar: The effect of rhyolite generation on the lunar hydrogen budget

Recent detection of indigenous hydrogen in a diversity of lunar materials, including volcanic glass (Saal et al., 2008), melt inclusions (Hauri et al., 2011), apatite (Boyce et al., 2010; McCubbin et al., 2010), and plagioclase (Hui et al., 2013) suggests water played a role in the chemical differentiation of the Moon. Water contents measured in plagioclase feldspar, a dominant mineral in the ancient crustal lunar highlands have been used to predict that 320 ppm water initially existed in the lunar magma ocean (Hui et al., 2013) whereas measurements in apatite, found as a minor mineral in lunar rocks, representing younger potassium-enriched melt predict a bulk Moon with <100 ppm water. Here we show that water in alkali feldspar, a common mineral in potassium-enriched rocks, can have ~20 ppm water, which implies magmatic water contents of ~1 wt. % in chemically evolved rhyolitic magmas. The source for these wet, potassium-rich magmas probably contained ~1000 ppm H2O. Thus, lunar granites with ages from 4.3–3.9 Ga (Meyer et al., 1996) likely crystallised from relatively wet melts that degassed upon crystallisation. Geochemical surveys by the Lunar Prospector (Jolliff et al., 2011) and Diviner Lunar Radiometer Experiment (Glotch et al., 2010; Jolliff et al., 2011) indicating the global significance of evolved igneous rocks suggest that the formation of these granites removed water from some mantle source regions, helping to explain the existence of mare basalts with <10 ppm water, but must have left regions of the interior relatively wet as seen by the water content in volcanic glass and melt inclusions. Although these early-formed evolved melts were water-rich, their petrogenesis supports the conclusion that the Moon’s mantle had <100 ppm water for most of its history.

R.D. Mills, J.I. Simon, C.M.O’D. Alexander, J. Wang, E.H. Hauri

Geochem. Persp. Let. (2017) 3, 115-123 | doi: 10.7185/geochemlet.1712 | Published 28 November 2016

Scandium speciation in a world-class lateritic deposit

Scandium (Sc) has unique properties, highly valued for many applications. Future supply is expected to rely on unusually high-grade (up to 1000 ppm) lateritic Sc ores discovered in Eastern Australia. To understand the origin of such exceptional concentrations, we investigated Sc speciation in one of these deposits. The major factors are unusually high concentrations in the parent rock together with lateritic weathering over long time scales in a stable tectonic context. At microscopic and atomic scales, by combining X-ray absorption near-edge structure spectroscopy, X-ray diffraction and microscopic and chemical analyses, we show that Sc-rich volumes are associated with iron oxides. In particular, Sc adsorbed on goethite accounts for ca. 80 % of the Sc budget in our samples. The remaining Sc is incorporated in the crystal structure of haematite, substituting for Fe3+. Scandium grades reflect the high capacity of goethite to adsorb this element. In contrast, the influence of haematite is limited by the low levels of Sc that its structure can incorporate. These crystal-chemical controls play a major role in lateritic Sc deposits developed over ultramafic–mafic rocks.

M. Chassé, W.L. Griffin, S.Y. O'Reilly, G. Calas

Geochem. Persp. Let. (2017) 3, 105-114 | doi: 10.7185/geochemlet.1711 | Published 23 November 2016