Rachel C. F. Lentz

Geochemical Evidence for Magmatic Water Within Mars from Pyroxenes in the Shergotty Meteorite

Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites. Here, however, we report the distributions of trace elements within pyroxenes of the Shergotty meteorite—a basalt body ejected 175 million years ago from Mars—as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars.

A combined remote Raman and LIBS instrument for characterizing minerals with 532 nm laser excitation.

The authors have developed an integrated remote Raman and laser-induced breakdown spectroscopy (LIBS) system for measuring both the Raman and LIBS spectra of minerals with a single 532 nm laser line of 35 mJ/pulse and 20 Hz. The instrument has been used for analyzing both Raman and LIBS spectra of carbonates, sulfates, hydrous and anhydrous silicates, and iron oxide minerals in air. These experiments demonstrate that by focusing a frequency-doubled 532 nm Nd:YAG pulsed laser beam with a 10x beam expander to a 529-microm diameter spot on a mineral surface located at 9 m, it is possible to measure simultaneously both the remote Raman and LIBS spectra of calcite, gypsum and olivine by adjusting the laser power electronically. The spectra of calcite, gypsum, and olivine contain fingerprint Raman lines; however, it was not possible to measure the remote Raman spectra of magnetite and hematite at 9 m because of strong absorption of 532 nm laser radiation and low intensities of Raman lines from these minerals. The remote LIBS spectra of both magnetite and hematite contain common iron emission lines but show difference in the minor amount of Li present in these two minerals. Remote Raman and LIBS spectra of a number of carbonates, sulfates, feldspars and phyllosilicates at a distance of 9 m were measured with a 532-nm laser operating at 35 mJ/pulse and by changing photon flux density at the sample by varying the spot diameter from 10 mm for Raman to 530 microm for LIBS measurements. The complementary nature of these spectra is highlighted and discussed. The combined Raman and LIBS system can also be re-configured to perform micro-Raman and micro-LIBS analyses, which have applications in trace/residue analysis and analysis of very small samples in the nano-gram range.

Water in martian magmas: clues from light lithophile elements in shergottite and nakhlite pyroxenes

Abstract There is abundant geomorphic evidence that Mars once had potentially significant amounts of water on its surface. Bulk martian meteorites are curiously dry, and hydrated minerals found in some of these rocks are also surprisingly low in water content. We look for evidence of pre-eruptive magmatic water by analyzing the abundances of Li, Be, and B, light lithophile elements that have proven useful in tracking water-magma interactions in terrestrial studies because of their solubility differences. We performed secondary ionization mass spectrometer (SIMS) analysis of these incompatible elements in pyroxenes of two nakhlites and two basaltic shergottites, with quite different results. In Nakhla and Lafayette, all three elements behave as incompatible elements, with increasing abundance with magma evolution from pyroxene cores to rims. In Shergotty and Zagami, Be increases, but both B and Li decrease from pyroxene cores to rims. From terrestrial studies, it is known that Be is virtually insoluble in aqueous hydrothermal fluids, whereas B and Li are quite soluble. We suggest, therefore, that the elemental decreases in the shergottite pyroxenes reflect dissolution and loss of B and Li in a hot, aqueous fluid exsolved from the magma. Consistent with our results, recent experimental work proposes that the shergottite parent magmas contained 1.8 wt% water (Dann et al., 2001) . We suggest that the pyroxene cores grew at depth (>4 km) where the water would remain dissolved in the magma. Once the magma began to ascend, the volatile component could gradually exsolve, removing the soluble species from the melt in the process. Upon eruption, the volatile component might then be lost through degassing, leaving a B- and Li-depleted magma to crystallize pyroxene rims and plagioclase. This magmatic water might have derived from the martian mantle or resulted from deep crustal contamination. If the water contents proposed for the shergottite magmas, and implied by our results, are typical of basaltic magmas on Mars, this mechanism could provide an efficient method of delivering substantial amounts of water to the martian surface at later times in martian history.

Daytime rapid detection of minerals and organics from 50 and 100 m distances using a remote Raman system

We have developed a remote Raman system, using an 8-in telescope and a 532-nm pulse laser (20 Hz and 20 mJ/pulse), which is capable of operating in daylight. From distances of 50 and 100 m and with an integration time of just 1 second (equivalent to 20 laser pulses at 20 Hz), good quality Raman spectra with high signal-to-noise ratios were readily obtained. The Raman system was also tested using only single-laser-pulse excitation (8 ns pulse width) with an integration time of 2 μs. The spectra obtained from single-laser-pulse excitation also show clear Raman features and can be used for rapid, unambiguous identification of various chemical substances. We successfully identified a number of substances, including organic chemicals (acetone, naphthalene, nitro-methane, nitro-benzene and cyclohexane); inorganic chemicals and minerals (nitric acids, sulfuric acid, potassium perchlorate, gypsum, ammonium nitrate, epsomite, melanterite, calcite and sulfur); and amino acids. The remote Raman system has a range of applications, such as environmental monitoring (e.g., detection of hazardous chemicals and chemical spills from a safe distance in real time) or homeland security (e.g., rapid identification of chemicals on a conveyor belt or from a fast-moving object).