James P. Greenwood

Mass-independent isotope effects in Archean (2.5 to 3.8 Ga) sedimentary sulfides determined by ion microprobe analysis

We report sulfur isotope anomalies with 33 S, the deviation from a mass-dependent fractionation line for the three-isotope system ( 34 S/ 32 S vs. 33 S/ 32 S), ranging up to 2‰ within individual Archean sedimentary sulfides from a variety of localities. Our measurements, which are made in situ by multicollector secondary ion mass spectrometry, unequivocally corroborate prior bulk measurements of mass-independent fractionations (MIF) in sulfur and provide additional evidence for an anoxic atmosphere on the Earth before 2 Ga. This technique also offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record. The presence of MIF sulfur in sulfides from a 3.8-Ga Fe-rich quartzite from Akilia (island), West Greenland, is consistent with a marine sedimentary origin for this rock. Copyright

Los Angeles: The Most Differentiated Basaltic Martian Meteorite

Los Angeles is a new martian meteorite that expands the compositional range of basaltic shergottites. Compared to Shergotty, Zagami, QUE94201, and EET79001-B, Los Angeles is more differentiated, with higher concentrations of incompatible elements (e.g., La) and a higher abundance of late-stage phases such as phosphates and K-rich feldspathic glass. The pyroxene crystallization trend starts at compositions more ferroan than in other martian basalts. Trace elements indicate a greater similarity to Shergotty and Zagami than to QUE94201 or EET79001-B, but the Mg/Fe ratio is low even compared to postulated parent melts of Shergotty and Zagami. Pyroxene in Los Angeles has 0.7–4-µm-thick exsolution lamellae, ∼10 times thicker than those in Shergotty and Zagami. Opaque oxide compositions suggest a low equilibration temperature at an oxygen fugacity near the fayalite-magnetite-quartz buffer. Los Angeles cooled more slowly than Shergotty and Zagami. Slow cooling, coupled with the ferroan bulk composition, produced abundant fine-grained intergrowths of fayalite, hedenbergite, and silica, by the breakdown of pyroxferroite. Shock effects in Los Angeles include maskelynitized plagioclase, pyroxene with mosaic extinction, and rare fault zones. One such fault ruptured a previously decomposed zone of pyroxferroite. Although highly differentiated, the bulk composition of Los Angeles is not close to the low-Ca/Si composition of the globally wind-stirred soil of Mars.

The Lunar Apatite Paradox

Wetted Apatite The long-running story of the dry Moon was rewritten a few years ago when hydrogen-bearing glass spherules were discovered. The highest water contents are found in lunar apatite, at levels suspiciously comparable to the water content of Earth apatites. Boyce et al. (p. 400, published online 20 March; see the Perspective by Anand) now show that the water content of lunar apatite is not a reliable indicator of the abundance of water in mare basalts. The existence of apatite with high water content is an almost inevitable consequence of the loss of tiny amounts of fluorine-rich apatite from a melt and replacement by hydrogen and is thus no indication of a “wet” Moon. Hydrogen-rich apatite crystals in lunar volcanic rocks indicate self-inflicted loss of fluorine from basaltic melts. [Also see Perspective by Anand] Recent discoveries of water-rich lunar apatite are more consistent with the hydrous magmas of Earth than the otherwise volatile-depleted rocks of the Moon. Paradoxically, this requires H-rich minerals to form in rocks that are otherwise nearly anhydrous. We modeled existing data from the literature, finding that nominally anhydrous minerals do not sufficiently fractionate H from F and Cl to generate H-rich apatite. Hydrous apatites are explained as the products of apatite-induced low magmatic fluorine, which increases the H/F ratio in melt and apatite. Mare basalts may contain hydrogen-rich apatite, but lunar magmas were most likely poor in hydrogen, in agreement with the volatile depletion that is both observed in lunar rocks and required for canonical giant-impact models of the formation of the Moon.

Sulfur isotopic compositions of individual sulfides in Martian meteorites ALH84001 and Nakhla: implications for crust–regolith exchange on Mars

Atmospheric chemical reactions on Mars have been invoked to explain non-mass-dependent Δ33S anomalies (Δ33S=δ33S−0.516δ34S) reported from bulk analyses of Martian meteorites. To explore this signature in detail, a new ion microprobe multi-collector technique was developed to obtain precise in situ 32S, 33S and 34S measurements of individual sulfide grains from Martian meteorites ALH84001 (>4.0 Ga) and Nakhla (1.3 Ga). This technique permits high-precision simultaneous measurement of multiple isotopes to uniquely evaluate Δ33S at the grain scale (<30 μm). Our data reveal resolvable non-mass-dependent Δ33S anomalies in two separate ALH84001 pyrite grains (Δ33S=−0.74±0.39‰ and −0.51±0.38‰, 2σ); none were detectable in Nakhla pyrrhotite (total range in Δ33S=−0.4±0.5‰ to −0.07±0.5‰, 2σ). Our results might reflect a difference in how these meteorites exchanged sulfur with the Martian regolith and/or differences in their sources (atmospheric versus meteoritic) of anomalous sulfur. Nebular heterogeneities in sulfur isotope composition are indicated by Δ33S anomalies preserved in, for example, the ureilites. The Δ33S anomalies in ALH84001 pyrite could suggest that early (pre-4 Ga) additions of a meteoritic component carried isotopically anomalous sulfur to the Martian regolith, and was stored there as seen in the detection of Δ33S anomalies from bulk measurements of Nakhla. Therefore, meteoritic contributions should also be considered in addition to atmospheric effects when explaining the large non-mass-dependent anomalies seen in Martian meteorites. These studies provide insight into how hydrothermal systems have facilitated exchange between volatile reservoirs on Mars, a planet that lacks efficient crustal recycling mechanisms and preserves ancient (and anomalous) Δ33S signatures.

Ion microprobe measurements of 18O/16O ratios of phosphate minerals in the Martian meteorites ALH84001 and Los Angeles

Abstract Oxygen isotope ratios of merrillite and chlorapatite in the Martian meteorites ALH84001 and Los Angeles have been measured by ion microprobe in multicollector mode. δ18O values of phosphate minerals measured in situ range from ∼3 to 6‰, and are similar to Martian meteorite whole-rock values, as well as the δ18O of igneous phosphate on Earth. These results suggest that the primary, abiotic, igneous phosphate reservoir on Mars is similar in oxygen isotopic composition to the basaltic phosphate reservoir on Earth. This is an important first step in the characterization of Martian phosphate reservoirs for the use of δ18O of phosphate minerals as a biomarker for life on Mars. Cumulative textural, major-element, and isotopic evidence presented here suggest a primary, igneous origin for the phosphates in Los Angeles and ALH84001; textural and chemical evidence suggests that phosphates in ALH84001 were subsequently shock-melted in a later event.

Oxygen isotopes in R-chondrite magnetite and olivine: links between R chondrites and ordinary chondrites

Ion-microprobe studies yield Δ17O (=δ17O − 0.52 · δ18O) values in magnetite from the Rumurti chondrite (RC) PCA91241 (which is paired with PCA91002) of +3.1 to +3.9‰, slightly higher than that in O from whole-rock R samples. Despite Δ17O values in whole-rock RCs that are much (by ca. 1.6‰) higher than in whole-rock LL chondrites, the Δ17O in R magnetite is much lower (by ca. 2‰) than the values (+4 to +7‰) from LL3 Semarkona and Ngawi (Choi et al., 1998). The δ18O values in PCA magnetite (−15 to −10‰) are the lowest known in meteorites, well below the range in Semarkona (−4 to +9‰). On a δ17O–δ18O diagram both magnetite data sets form linear arrays with slopes of ca. 0.7, indicating mixing of O from different isotopic reservoirs; the slopes and intercepts of the two arrays are similar enough to permit them to be segments of a single array. This suggests that, in RCs and LL chondrites, magnetite formed from the same raw materials by the same processes, probably by aqueous alteration of metal in an asteroidal setting. We observed Δ17O values in olivines and pyroxene from RCs ranging from −1.2 to +2.9‰ and δ18O from +1.4 to +9.1‰. These compositions scatter in the same general range observed in chondrules from ordinary chondrites. The similarity in the O-isotopic composition of minerals that preserve a record of formation in the solar nebula supports a model in which RCs formed from nebular components similar to those in H chondrites, but with a matrix/chondrule ratio several times higher in the RCs, and with more extensive aqueous alteration in the RCs than in known H chondrites. We postulate that the matrix in R chondrites has Δ17O higher than whole-rock values. We suggest that the original Δ17O value of H2O in the RC body was similar to that incorporated into the ordinary chondrites, previously estimated by Choi et al. (1998) to be ca. +7‰ in the LL parent body.

The chlorine isotope fingerprint of the lunar magma ocean

The unusually heavy Cl of the Moon is related not to degassing of dry magmas but rather to the loss of Cl from the lunar magma ocean. The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free (“dry”) Moon. We have analyzed abundances and isotopic compositions of Cl and H in lunar mare basalts, and find little evidence that anhydrous lava outgassing was important in generating chlorine isotope anomalies, because 37Cl/35Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-outgassing hypothesis. Instead, 37Cl/35Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high 37Cl/35Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean. These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon’s history. Chlorine isotope variability is therefore an indicator of planetary magma ocean degassing, an important stage in the formation of terrestrial planets.

Evidence for an acidic ocean on Mars from phosphorus geochemistry of Martian soils and rocks

Recent analyses of elemental concentrations and mineralogy of iron-bearing compounds of Martian soils and rocks by the Mars Exploration Rovers at Meridiani Planum and Gusev Crater demonstrate that phosphorus concentration is correlated with sulfur and chlorine. The positive correlation of these three elements with each other in soils at both sites argues for a globally homogeneous soil component. Sulfur, and possibly chlorine, in Martian soils and Meridiani Planum outcrop rocks is likely derived from volcanic exhalations, but phosphorus must be derived from the weathering of igneous rocks. Here we show that the similar concentration of phosphorus in soils at the two Mars Exploration Rover sites, coupled with positive correlations to chlorine and sulfur, is best explained as resulting from mixing and homogenization of phosphate, sulfate, and chloride in a large acidic aqueous reservoir, such as an acidic ocean. Acidic thin-film or acid-fog weathering cannot readily produce the similar P/S and P/Cl ratios of soils measured on Mars, and more important, cannot explain the high phosphorus content of ancient (ca. 3–4 Ga) sulfate-rich rocks in outcrop at Meridiani. The existence of a global acidic hydrosphere or ocean at some time in early Martian history can also explain the lack of extensive carbonate deposits on the Martian surface.

Microbial diversity of boron-rich volcanic hot springs of St. Lucia, Lesser Antilles

The volcanic Sulphur Springs, St. Lucia, present an extreme environment due to high temperatures, low pH values, and high concentrations of sulfate and boron. St. Lucia offers some unique geochemical characteristics that may shape the microbial communities within the Sulphur Springs area. We chose six pools representing a range of geochemical characteristics for detailed microbial community analyses. Chemical concentrations varied greatly between sites. Microbial diversity was analyzed using 16S rRNA gene clone library analyses. With the exception of one pool with relatively low concentrations of dissolved ions, microbial diversity was very low, with Aquificales sequences dominating bacterial communities at most pools. The archaeal component of all pools was almost exclusively Acidianus spp. and did not vary between sites with different chemical characteristics. In the pool with the highest boron and sulfate concentrations, only archaeal sequences were detected. Compared with other sulfur springs such as those at Yellowstone, the microbial diversity at St. Lucia is very different, but it is similar to that at the nearby Lesser Antilles island of Montserrat. While high elemental concentrations seem to be related to differences in bacterial diversity here, similarities with other Lesser Antilles sites suggest that there may be a biogeographical component as well.

Age and temperature of shock metamorphism of Martian meteorite Los Angeles from (U-Th)/He thermochronometry

Mineralogic features attributed to impact-induced shock metamorphism are commonly observed in meteorites and terrestrial impact craters. Partly because the duration of shock metamorphism is very short, constraining the timing and temperature of shock events has been problematic. We measured (U-Th)/He ages of single grains of merrillite and chlorapatite from the Martian meteorite Los Angeles (LA). Merrillite and chlorapatite ages cluster at 3.28 6 0.15 Ma (2s) and 2.18 6 0.19 (2s) Ma, respectively. The mean age of the merrillites, which are larger than chlorapatites, is indistinguishable from cosmic-ray exposure ages (3.1 6 0.2 Ma), suggesting that impact-induced shock metamorphism was coeval with ejection of the LA precursor from Mars. To constrain the initial temperature of shock metamorphism in the LA precursor body, we modeled diffusive loss of He from merrillite as a function of diffusion domain size, LA precursor body size, and ablation depth. From these calculations, we suggest that the metamorphic temperature of the shock event was higher than 450 8C. These results support the idea that shock pressures of the Martian meteorite Shergotty were higher than 45 GPa, as inferred from the presence of post-stishovite SiO2 polymorphs. Single-grain (U-Th)/He dating of phosphates may provide unique constraints on the timing and pressure-temperature dynamics of shock metamorphism in a wide variety of extraterrestrial materials.