Monica M. Grady

Evaporite mineral assemblages in the nakhlite (martian) meteorites

Abstract A mineralogical study of the three nakhlite (martian) meteorites has revealed that they contain evaporite mineral assemblages. Lafayette has Ca-siderite and clay minerals (smectite/illite) along fractures within olivine; Governador Valadares contains clay mineral veins in olivine, with siderite, gypsum and anhydrite in interstitial areas; Nakhla has clay and gypsum veins in olivine, with Mg-, Mn-rich siderite, anhydrite and halite in interstitial sites. Minor goethite is also present in the three meteorites. Lafayette siderite has the range of compositions (mol%) CaCO 3 21.6–36.8, MnCO 3 4.2–35.3, MgCO 3 0.1–1.6, FeCO 3 27.4–67.0; Governador Valadares has CaCO 3 3.6–11.1, MnCO 3 1.1–2.1, MgCO 3 9.0–29.2, FeCO 3 64.3–77.8; Nakhla has CaCO 3 0.1–5.7, MnCO 3 1.0–39.9, MgCO 3 2.0–40.9, FeCO 3 23.2–87.0. Trace element abundances for clay, siderite and gypsum are all similar with LREE, Y>HREE, Zr, Nb and La 0.9–95×CI; Y 0.2–2.4×CI. This pattern of abundances reflects the trace element contents of the parent fluid, which in turn were derived through dissolution of LREE-enriched feldspathic mesostasis. The close similarities in silicate petrography and radiometric ages determined by other workers for these olivine clinopyroxenites suggests that the parent rocks were close to one another on Mars and therefore the same fluid may have been responsible for the precipitation of the evaporite mineral assemblages. Lafayette contains the mineral assemblage and siderite composition which are least soluble in water and Nakhla contains the most soluble minerals and carbonate composition. On the basis of our new data we consider a new model of progressive evaporation from a Na–Mg–Fe–Ca–SO 4 –Cl–H 2 O–HCO 3 − acidic brine in an area of enclosed drainage (e.g. crater or low-lying flood plain) on Mars. Partial dissolution of near-surface rocks by the acidic brine released Fe, Mg and trace elements from mesostasis and olivine into the fluid. The Lafayette assemblage was formed where >25% volume of the water remained following evaporation, Governador Valadares 20% and Nakhla 0°C and a thicker atmosphere (pCO 2 =30–100 mbar) at least over brief periods of time ≤1.3 Ga.

CM CHONDRITES EXHIBIT THE COMPLETE PETROLOGIC RANGE FROM TYPE 2 TO 1

We have characterized the most phyllosilicate-rich members of the CM chondrite group. Based upon petrographic and compositional factors, we conclude that these particular meteorites have experienced pervasive aqueous alteration far beyond that witnessed by typical CMs. The lack of anhydrous silicates, CAI and (except as relicts) chondrules merits the classification of type CMI for three meteorites (EET 83334, ALH 88045, and one Kaidun lithology). Still other CMs, notably ALH 83100, EET 90047, and Yamato 82042, are clearly intermediate between types 1 and 2. Brecciated CM chondrites like Cold Bokkeveld contain all of these varied lithologies. The CM chondrites thus exhibit the complete petrologic range from 2 through 1. Our results show that progressive aqueous alteration on the parent CM asteroid(s) was, locally, accompanied by significant increases in temperature (to a peak of ∼450°C for the Kaidun lithology), fO2, and (locally) degree of chemical leaching, all well beyond the conditions recorded by typical CM2s. The most altered CMs also are commonly deformed, displaying a distinct lineation probably due to static rather than dynamic forces.

Infrared Spectroscopy of Comet 81P/Wild 2 Samples Returned by Stardust

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Selection of the landing site in Isidis Planitia of Mars probe Beagle 2

This paper describes selection and characterization of the landing site for the Mars 2004 Beagle 2 mission. The site is within Isidis Planitia between 10°–12°N, 266°–274°W, centered at 11.6°N, 269.5°W. This is at low elevation (-3600 to -3900 m MOLA), is flat (MOLA RMS slope = 0.57°), radar data suggest a smoother surface at decimeter to meter scales than the Pathfinder site and it has a moderate rock abundance (2–17%, mean 11%). In addition to this, Isidis shows evidence for concentration and remobilization of volatiles. In particular, the basin contains conical landforms. We favor models involving the formation of tuff cones during magma-ice interaction. Structures identified as dykes in MOC images may be remnants of magma conduits. The pattern of bulk thermal inertia in Isidis (higher values of 500 Jm-2s-0.5K-1 around the SW-S-E margin decreasing toward the center and north) suggests that an influx of sediment spread from the Noachian areas around the southern half of the basin over the basin floor. The coarse, higher thermal inertia material was deposited closest to the sediment source. The variable state of erosion of the tuff cones suggests that they formed intermittently over a long period of time during Amazonian and possibly Hesperian epochs. Geologically recent resurfacing of Isidis has also occurred by aeolian processes, and this is shown by a deficit in impact craters <120 m diameter. The proportion of rocky material is predicted to be slightly less than the Viking and Pathfinder sites, but there will probably be more duricrust.

Paired Renazzo-type (CR) carbonaceous chondrites from the Sahara

Ten chondrites with chemical and mineralogical similarities to the carbonaceous chondrite Renazzo were recovered at two locations of the Sahara: Acfer 059, 087, 097, 114, 139, 186, 187, 209, 270 and El Djouf 001. Although the El Djouf location is more than 500 km away from the Acfer location, all samples appear to result from a single fall based on chemical and petrographic similarities and supported by light element stable isotope geochemistry, noble gas record, and similar 26Al contents. The Acfer-El Djouf meteorite is classified as a CR (Renazzo-type) carbonaceous chondrite. This group presently comprises three non-Antarctic members (Al Rais, Renazzo, Acfer-El Djouf) and five Antarctic meteorites. The major lithological components of the Acfer-El Djouf meteorite are large chondrules (up to 1 cm in size; mean diameter: 1.0 ± 0.6 mm), chondrule and mineral fragments, Ca,Al-rich inclusions, FeNi-metal (about 8–10 vol%) and dark inclusions embedded in a fine-grained fragment-bearing groundmass. Mineral compositions of the ten Acfer-El Djouf samples are similar to those of other CR chondrites. Most of the Ca,Al-rich inclusions are below 300 μm in size and rich in melilite and spinel. In some CAIs the rare phase CaAl4O7 is dominant. Fo-rich, Cr-bearing olivine (Fa0–4) and enstatite (Fs0–4) are the major phases of the chondrite. The meteorite is mildly shocked with a shock stage of S2 indicating a peak shock pressure of 5–10 GPa for the bulk meteorite. The oxygen isotopic compositions and carbon and nitrogen stable isotope geochemistry of the Acfer-El Djouf samples are very similar to those of the other CR-type chondrites. The major element composition of the Acfer-El Djouf meteorite is indistinguishable from CR chondrites. When compared to Renazzo the Acfer-El Djouf samples, however, have systematically lower contents of the moderately volatile elements Zn, Ga, As, Au, Sb, and Se and the highly volatile elements Br, C, and N. This is thought to reflect primary differences between Renazzo and the Acfer-El Djouf meteorite.

Investigating the variations in carbon and nitrogen isotopes in carbonaceous chondrites

The carbonaceous chondrites contain significant amounts of carbon- and nitrogen-bearing components, the most abundant of which is organic matter. Stepped combustion data of whole rock and HF/HCl residues of carbonaceous chondrites reveal that the organic material can be subdivided operationally into three components: (1) free organic matter (FOM), which is readily extractable from whole-rock meteorites and is enriched in 13C and 15N; (2) labile organic matter (LOM), which has a macromolecular structure but is liberated by hydrous pyrolysis; LOM is the parent structure for some FOM and is also enriched in 13C and 15N; and (3) refractory organic matter (ROM), which is also macromolecular but is virtually unaffected by hydrous pyrolysis and is relatively depleted in 13C and 15N. The macromolecular entities (LOM and ROM) are by far the most abundant organic components present, and as such, the relative abundances of the 13C- and 15N-enriched LOM and the 13C- and 15N-depleted ROM will have a major influence on the overall isotopic composition of the whole-rock meteorite. Laboratory experiments designed to simulate the effects of parent body aqueous alteration indicate that this form of processing removes LOM from the macromolecular material, allowing ROM to exert a stronger influence on the overall isotopic compositions. Hence, aqueous alteration of macromolecular materials on the meteorite parent body may have a significant control on the stable isotopic compositions of whole-rock carbonaceous chondrites. The enstatite chondrites are also carbon rich but have been subjected to high levels of thermal metamorphism on their parent body. Stepped combustion data of HF/HCl residues of enstatite chondrites indicate, that if they and carbonaceous chondrites inherited a common organic progenitor, metamorphism under reducing conditions appears to incorporate and preserve some of the 13C enrichments in LOM during graphitisation. However, when metamorphism is at its most extreme, the 15N enrichments in LOM are lost.

The textures and compositions of fine-grained Antarctic micrometeorites: Implications for comparisons with meteorites

Abstract Micrometeorites recovered from the Earths surface constitute the most abundant interplanetary dust now falling to Earth. We studied eighty-nine fine-grained Antarctic micrometeorites (fg-AMMs) to evaluate their state of alteration and to identify the nature of their precursor materials. Fine-grained AMMs are divided into melted and unmelted groups and subdivided on the basis of textures related to atmospheric entry heating. The textures of melted particles reflect crystallisation after entry heating and those of unmelted AMMs broadly resemble Cl, CM, and CR chondrite matrix. The matrix compositions of the fg-AMMs closely resemble those of CM2 chondrites. Matrices provide a more reliable comparison to meteorites than bulk compositions because components such as chondrules and CAIs are not present in representative proportions within individual AMMs. Divergences from CM-like matrix compositions are generally minor and probably arose during entry heating by the dissociation of volatile-bearing phases at subsolidus temperatures and evaporation and loss of immiscible metallic liquids at higher temperatures. Depletions in Ni and Mg relative to CM matrix are tentatively attributed to terrestrial weathering. No conclusive evidence for contamination during atmospheric residence was observed. Contrary to theory, textural evidence suggests that bow shocks and high thermal gradients existed during deceleration of micrometeoroids in the atmosphere.

Tissint Martian Meteorite: A Fresh Look at the Interior, Surface, and Atmosphere of Mars

A New Rock from Mars On 18 July 2011 a meteorite originating from Mars fell on the moroccan desert. Chennaoui Aoudjehane et al. (p. 785, published online 11 October) show that this meteorite was ejected from the surface of Mars 700,000 years ago and contains components derived from the interior, surface, and atmosphere of the red planet. Previous to this fall, only four other martian meteorites have been collected after being witnessed falling to Earth. All the other martian meteorites that are represented in collections around the world, have been found long after their arrival on Earth, and thus have suffered from exposure to the terrestrial environment. A meteorite that fell in Morocco in July 2011 provides a sample to study processes that operated on Mars 700,000 years ago. Tissint (Morocco) is the fifth martian meteorite collected after it was witnessed falling to Earth. Our integrated mineralogical, petrological, and geochemical study shows that it is a depleted picritic shergottite similar to EETA79001A. Highly magnesian olivine and abundant glass containing martian atmosphere are present in Tissint. Refractory trace element, sulfur, and fluorine data for the matrix and glass veins in the meteorite indicate the presence of a martian surface component. Thus, the influence of in situ martian weathering can be unambiguously distinguished from terrestrial contamination in this meteorite. Martian weathering features in Tissint are compatible with the results of spacecraft observations of Mars. Tissint has a cosmic-ray exposure age of 0.7 ± 0.3 million years, consistent with those of many other shergottites, notably EETA79001, suggesting that they were ejected from Mars during the same event.

The carbon and oxygen isotopic composition of meteoritic carbonates

The 13C12C and 18O16O isotopic ratios of carbonates from carbonaceous and ordinary chondrites have been measured on CO2 released by the action of H3PO4 on whole-rock samples. Carbonates from CI, CM and CR carbonaceous chondrites exhibit a range in δ18O of ca. 15%. (+20.5%. to +35.1%. relative to SMOW). Limited data from CO2-water equilibration experiments suggest that meteoritic carbonates do not possess grossly anomalous 17O isotopic compositions; therefore, they are truly enriched in 13C, with δ13C between +23.7%. and +80.7%. relative to PDB. Large internal variations in δ13C and δ18O were found in individual meteorites and suggest that two or more isotopically distinct carbonates of different origin may be present. The abundance, δ13C and δ18O of carbonate in CM2 chondrites may be related to the extent of aqueous alteration of the meteorites. Carbonates in CI and CR chondrites have a median δ13C ca. +50 to +60%., whereas δ13C of CM meteorites lie in the range +40 to +50%., although exceptions exist in both sets of samples. CV3 and CO3 carbonaceous chondrites and unequilibrated ordinary chondrites release small amounts of CO2 on acid treatment, which might be from carbonate dissolution, but which is not enriched in 13C, exhibiting δ13C values ca. 0 ± 10%. The exception to this is Bishunpur, with δ13C ca. −23.5%.. The difference in δ13C of the CI, CM and CR vs. CV, CO and ordinary chondrite carbonates may be a result of the progressive enrichment in 13C of percolating fluids, brought about by increasing solubilization of “exotic” 13C-enriched grains.