MURCHISON

On a Sunday morning around 10:45, a fireball exploded with loud detonations and hissing noises. Hundreds of stones rained down over Victoria, Australia covering 5 square miles, permeating the town with the odor of alcohol. Over 700 charcoal-colored stones with a total weight of over 100 kg were collected, the largest mass weighing 7 kg.

Murchison is a breccia that contains mm- to cm-sized xenoliths of mostly type-3 carbonaceous chondrite provenance. Murchison has a shock stage of S1–2 and a weathering grade of W1–2. The CM chondrites have undergone extensive aqueous alteration on the parent body, the degree of which having been determined by such factors as water/rock ratio, temperature, and duration of the process. An early alteration sequence for CM chondrites was proposed by Ikeda (1983), by which most Murchison chondrules would be designated stage-II:

A new aqueous alteration sequence for CM chondrites was proposed by Rubin et al. (2005, 2007). Based on eight major diagnostic parameters of progressive alteration, they classified Murchison as a petrologic type 2.5, which, along with the other CM chondrites, was derived from a hypothetical precursor lithology having a petrologic type 3.0, broadly similar to the anhydrous, CM-related, type-3.0 Acfer 094, or perhaps similar to a CO3.0 such as ALHA77307. Following are the major parameters developed by Rubin et al. (2007) for estimating the degree of alteration of the CM chondrites:

early to intermediate stage alteration processes:

1. hydration of fine-grained matrices to form phyllosilicates, which gradually consists of Mg-rich serpentines
2. conversion of primary chondrule glass to phyllosilicate
3. production of large PCP (serpentine–tochilinite) clumps

processes occurring throughout the alteration sequence:

4. oxidation of FeNi-metal
5. alteration of chondrule silicate phenocrysts
6. compositional changes of PCP; e.g., S depletion
7. formation of increasingly complex carbonates; e.g., calcite => dolomite
8. compositional changes in sulfides

In their study, de Leuw et al. (2009) found that CM chondrites which have lower petrographic subtypes experienced longer durations of aqueous alteration. Analyzing carbonates in CM chondrites, they determined that correlations in the Mn/Cr systematics were indicative of in situ decay of 53Mn during carbonate formation. They were able to utilize the correlation between the degree of aqueous alteration and the age of carbonate formation to arrive at a duration for such alteration of at least 4 m.y. It was shown that the carbonates that are the least altered are the oldest and vice versa. Features present in the FeNi-metal and sulfides in CM chondrites have been identified which reflect the heating stage as follows (Kimura et al., 2009):

Murchison contains sparse chondrules with diameters between 0.1 mm and 0.5 mm, composed of individual phenocrysts of olivine and pyroxene. Porphyritic chondrules contain some FeNi-metal grains. The chondrule mesostasis has been altered to phyllosilicates—Fe- and Mg-serpentines. The black matrix constitutes ~48 vol% of the meteorite and is similar to that of the CI matrix, but contains less magnetite. Chondrule fragments and olivine crystals are abundant in the Murchison matrix, while the CAI content is low. Isotopic research indicates that the matrix and chondrules of pristine carbonaceous chondrites probably condensed during the same heating event (Nyquist et al., 2009). Carbonate grains are present in a few vol%, consisting predominantly of calcite/aragonite with minor amounts of dolomite, in addition to low abundances of sulfides.

Interestingly, newly identified refractory inclusions in Murchison composed primarily of hibonite represent some of the earliest condensed solids or residues from the early, hot, solar nebula (Liu et al., 2009). These refractory grains comprise platy crystals (PLACs), spinel–hibonite spherules (SHIBs), and blue aggregates (BAGs). PLACs lack resolvable 26Mg-excesses and were formed within a timespan of ~100,000 years in an 16O-enriched, heterogeneous region (based on anomalous δ48Ca and δ50Ti isotopic signatures) prior to incorporation and mixing of short-lived nuclides like 26Al into the solar nebula through injection of interstellar dust. Both PLAC and BAG formation occurred hundreds of thousands of years prior to the formation of CV CAIs. In contrast, SHIBs formed later by condensation of precursor material in a lower temperature environment than that of PLACs. They record in situ 26Al decay and "canonical" initial levels of 26Al/27Al, and they are considered to have formed 100,000–300,000 years after the formation of CV CAIs. Some hibonite grains in Murchison have experienced Rayleigh fractionation through distillation/evaporation, while some also exhibit nuclear anomalies showing similarities to the FUN group of CAIs studied in CV3 Allende.

X-ray diffraction techniques and Mössbauer spectroscopy have been used by Bland et al. (2004) to determine the modal mineralogy of several carbonaceous chondrites including Murchison. They were also able to quantify the compositional range of the olivine phases. In addition, the grain density can be readily estimated from the mode data, and therefore, in combination with the calculated bulk density, the porosity can be determined. The modal mineralogy (vol%) of Murchison was determined to be as follows:

• Olivine
• Fo100 -------------------- 6.8
• Fo80 ---------------------- 1.9
• Fo50 ---------------------- 1.6
• Clinoenstatite (En98) ------- 1.9
• Pyrrhotite ----------------------- 1.8
• Pentlandite --------------------- 0.3
• Magnetite ----------------------- 0.2
• Serpentine --------------------- 26.2
• Calcite --------------------------- 1.2
• Cronstedtite/Tochilinite --- 58.1
• TOTAL -------------------------- 100.0
• grain density = 2.93 g per cubic cm

A more technologically advanced determination of the Murchison modal (vol%) mineralogy was conducted by Howard et al. (2009) with results as follows:

• Olivine ----------------------------15
• Clinoenstatite (En98) -------- 8.3
• Pyrrhotite ----------------------- 1.2
• Pentlandite --------------------- 0.65
• Magnetite ----------------------- 1.1
• Serpentine ---------------------- 22
• Calcite ---------------------------- 1.2
• Cronstedtite -------------------- 50
• TOTAL --------------------------- 99.45

Dark, fine-grained (<1 µm) material (DFM) which forms opaque mantles surrounding chondrules, refractory inclusions, and matrix silicates, are believed by some to be accretionary features from solar nebula dust reservoirs unrelated to the silicate cores on which they are found. However, results of an investigation by Trigo-Rodriguez et al. (2006) of the variable porosities within these dark rims indicate that they were formed on the parent body through impact-compaction of fine-grained, porous matrix material, followed by aqueous alteration and the deposition within the mantle of what was historically called "poorly characterized phases" (PCP), which has now been determined to be tochilinite–serpentine intergrowths (TSI). The TSI probably formed when metal particles reacted with ionized, S-bearing, alkaline water at temperatures of ~50–100°C under reducing conditions having a stable temperature and pressure (Peng et al., 2007). Unlike the results of previous studies, it was demonstrated that the DFM is not confined to rims around discrete objects, but instead, can form indistinct boundaries blending seamlessly from the objects into the matrix.

Moreover, the dark, fine-grained material can be found extending beyond its associated object, comprising isolated patches within matrix space, forming single rims around multiple objects, and surrounding post-aqueous altertion phases. Many smaller discrete objects have no mantles at all, inconsistent with a common nebular origin. Where DFM composes mantles around discrete objects, it forms a layered structure in which the lowest porosities adjacent to the enclosed objects reflect a high degree of compaction. Because agglomeration modeling shows that such a high degree of compaction is not attainable through nebular processes, the investigators argue that this provides supporting evidence for a parent body origin for DFM mantling through multiple impact-compaction events. Notably, foliations in some chondrules and phyllosilicates are likely the result of impact-induced deformation.

Following the agglomeration and impact-induced compaction of the various components in Murchison, the material experienced high degrees of aqueous alteration at temperatures of 20–35°C within a zone ~100–250 m thick at a depth of 1–1.8 km (Guo and Eiler, 2007). Studies of CM chondrite REE patterns indicate the water had a pH of 6–8 during the alteration process (Inoue et al., 2009). Low-temperature reactions involving thin fluid films promoted precipitation–dissolution processes in which Fe- and S-bearing phyllosilicates replaced host minerals. Nanotubes have been identified in both Murchison and Mighei (Zega et al., 2004) which are believed to have condensed from low-temperature, S-rich, aqueous solutions. They are composed of a new serpentine phase intermediate between cronstedtite and chrysotile.

As shown above, Rubin et al. (2007) consider Murchison to be one of the least altered CM chondrites, and it can be placed within an aqueous alteration sequence of other CM members from most to least aqueously altered as follows: MET 01070 [2.0], QUE 93005 [2.1], Cold Bokkeveld [2.2], QUE 99355 [2.3], Y-791198 [2.4], Murray [2.4/2.5], Murchison [2.5], and QUE 97990 [2.6]. The CM chondrite Paris is a newly discovered representative reflecting a lower degree of aqueous alteration and mild thermal metamorphism. It contains an abundance of well-preserved metal with little magnetite, and has large unaltered zones with less matrix component than other CM members (Zanda et al., 2010). The unaltered precursor material of the CM group is considered to be type CM3.0. Although Paris does contain some phyllosilicates, most of its features, such as a high content of FeS in PCPs and a high chromium oxide content, indicate a tentative petrologic assignment of type 3.0, similar to that of the CM-related Acfer 094 and the CO ALHA77307. Interestingly, Paris exhibits many intermediate characteristics to the CO chondrite group (Bourot-Denise et al, 2010).

Based on a technologically advanced study in which the modal mineralogy of a suite of CM chondrites (Mighei, Nogoya, Murchison, Murray, and Cold Bokkeveld) was quantified to a high degree of accuracy, Howard et al. (2009) more accurately resolved the degree of aqueous alteration experienced by these CM chondrites based on their total phyllosilicate abundances (Mg-serpentine + Fe-cronstedtite). With the exception of Cold Bokkeveld, which shows anomalous modal abundances, the investigators found that a narrow range exists in the modal abundances of these CM chondrites with regard to phyllosilicates, anhydrous silicates, and other phases. They showed that the inverse relationship which exists between the anhydrous silicates and the phyllosilicates is reliable evidence that the latter formed from the former through aqueous alteration processes. In addition, the inverse relationship apparent between the abundance of Mg-serpentine and Fe-cronstedtite supports an aqueous alteration process as well; Fe-cronstedtite loses Fe and recrystallizes to form Mg-serpentine in the presence of water.

At the same time, since this alteration process is controlled by variable abundances of anhydrous silicates within different CM samples, the initial composition of each CM chondrite may have varied, rendering it impossible to determine the actual extent of aqueous alteration by this method. In light of the fact that accurate measurements of phyllosilicate abundances among the CM samples have now been obtained by Howard et al. (2009), and that these abundances have been ascertained to be the same within a few percent (the total ranges from 73% to 79%), it may be inferred that each CM chondrite has experienced an equal degree of aqueous alteration, and the subtypes proposed by Rubin et al. (2007) (shown above) are not accurately defined. As a result, the team argues that it is not necessary to resolve the subtype of the CM group further than CM2.

Murchison, which is among the most primitive meteorites known, contains water-bearing minerals including serpentines. These phyllosilicates provide a high water content of 4–18 wt%, water that initially condensed at a distance of 4 AU from the Sun (Eiler and Kitchen, 2004). Fe-rich aureoles are among the products resulting from in situ aqueous alteration processes. Additionally, Murchison contains carbon (2–2.5 wt%) and nitrogen (0.09–0.16 wt%) as constituents of free organic matter, diamond, and soluble, complex macromolecular organic compounds (10–15 ring polyaromatic hydrocarbons [PAH]), including at least 80 amino acids and the bases that make up the biological coding elements of RNA and DNA. All of these organic compounds have a nonbiogenic origin which were formed by such processes as irradiation of interstellar organic ices by cosmic rays. A listing of amino acids identified in Murchison before 1991 can be found at the Murchison organics page.

In addition, studies at the University of Bremen in Germany have identified seven diamino acids, which are the building blocks of peptide nucleic acids considered to have preceded RNA and DNA in the genesis of life. Also having an abiotic origin, a suite of over fifty monocarboxylic acids have been identified in Murchison (Huang et al., 2004), some of which have also been found in the C2 ungrouped chondrites Tagish Lake (Herd, University of Alberta, 2009) and EET 96029 (the latter containing a high abundance of formic acid). Studies of Murchison at the University of California at Davis (D. Deamer) have revealed the existence of lipid-like organic chemicals able to self-assemble into a membrane-like film enclosing fluid, an analog to a cell membrane. Although CM chondrites with lower petrologic types (experiencing the most extensive aqueous alteration) contain significantly less abundant amino acids compared to those with higher petrologic types, their similar relative abundances suggests that a common parent body link may exist (Botta et al, 2007).

The case has been made, based on O-isotopic compositions, for the pre-terrestrial production of water-soluble sulfate by the oxidation of sulfides in the presence of water (Airieau, et al., 2005). This process occurred as fluid flowed through unaltered rock that still preserved a component of its original oxygen ratio. This sulfate is isotopically stable, and preserves the oxygen isotopic signature of the water that was present at the time of sulfate formation, and may also, to some degree, reflect that of the water which was present during aqueous alteration processes of the meteorite.

The isotopic composition of Murchison and other CM chondrites reflects contributions from two primary reservoirs: 1) an anhydrous silicate component similar to the primitive CO3.0 chondrite, ALHA77307, and 2) an aqueous component manifest as phyllosilicates formed through parent body processes (Clayton and Mayeda, 1999). A parent body origin for the phyllosilicates is revealed by the heavy carbon (13C) content of the carbonates, which is heavier than nebular C gas; rather than a mass fractionation process, the heavy C could have been derived from presolar carbide grains, or, as proposed by Guo and Eiler (2007), through the production and escape of 13C-depleted methane during aqueous alteration. A low water/rock ratio employing isotopically heavy-water at temperatures near 0°C is considered the most likely parent body alteration environment.

Of possible historic significance is the recent discovery of a variety of sugar compounds, collectively known as polyols, within Murchison and the similar CM2 Murray. These compounds are constituents of RNA and DNA, and serve a role in cellular chemistry. Equally remarkable, fatty acids have been isolated from Murchison that independently form boundary membranes in alkaline conditions creating a rudimentary cell structure that could theoretically lead to self-replication. In addition, two of the known nucleobases, which, together with a sugar and a phosphate group constitute the nucleic acids which in turn compose the genetic code, have been identified as indigenous components in Murchison (Martinsa et al, 2009). The one-ring pyrimidine uracil which is present is a natural component of RNA, while the two-ring purine xanthine is instrumental in the synthesis of other purine nucleotides.

Besides these discoveries, isotopic studies suggest that organic sulfur compounds within Murchison may have been created by interaction of carbon disulfide molecules with light in the low-temperature, pre-planetary environment of interstellar space. Similarly, it was found that UV photolysis of interstellar ice could lead to the formation of the amino acids glycine, alanine, and serine, and could explain the existence of an L-enantiomer bias. In light of the fact that these naturally synthesized ingredients necessary for life are present in asteroidal material, the question arises: what influence did meteorite accumulation on Earth have on the genesis of terrestrial life? Interestingly, an ultrahigh-resolution analysis of the extraterrestrial organic matter in Murchison has revealed its indigenous chemical diversity encompasses tens of thousands to millions of different molecular compositions, exhibiting a chemical complexity that is high compared to biological systems on Earth (Schmitt-Kopplina, 2010).

Murchison also contains insoluble organic compounds. One component (20%) consists of presolar diamonds in concentrations of 1000 ppm, while another (10%) consists of grains of presolar graphite and SiC. Croat et al. (2008) discovered that the graphite in Murchison is present as both ordered (onion types) and disordered (platy types and scaly types) morphologies. Some platy graphite grains contain internal refractory grains of oxides (e.g., eskolaite and magnetite), carbides, and RuFe-metal, similar to constituents found in onion type graphites. These platy grains also show enrichments in 12C. The combined isotopic and compositional evidence indicates that both the onion and platy graphite grains formed during s-process nucleosynthesis (in which neutrons are slowly added to nuclei over thousands of years) in an AGB carbon star(s). It was further determined by the research team that the most disordered grain type, the scaly graphite, has O-isotopic ratios and other features more consistent with having originated during r-process nucleosynthesis in a supernova(e).

The SiC grains are comprised of a predominant mainstream type (from low-mass, C-rich AGB stars) plus the rare types A + B (from J-type carbon stars), X (from type II supernovae), and Y + Z (from low-metallicity AGB stars) (Nguyen et al., 2007). The CRE ages of some large SiC grains range up to 1.5 b.y. The largest insoluble organic component (~70%) is a kerogen-like material (J. Mao, 2005). These components were produced during s-process nucleosynthesis in aging, medium-size, carbon stars (TP-AGB phase), or more rarely (~1%), in supernovae. TiC crystals thought to have been produced in supernovae, and rutile grains of variable composition thought to have been produced in AGB star outflows (Croat, 2007), along with rare SiC grains of uncertain origin (Hynes and Croat, 2007), have become internal constituents of later formed graphite grains. While one SiC grain has been determined to be from a nova, the origin of another anomalous grain highly enriched in 30Si has yet to be established. Additionally, the discovery of enhanced isotopic compositions of heavy elements in some X-Type SiC grains has given rise to the new formation theory of neutron burst nucleosynthesis. All of these interstellar grains, including aluminum oxide, spinel, and silicon nitride, are remnants of the dust cloud from which our Solar System was formed.

The Murchison meteoroid experienced a simple one-stage exposure history in transit to Earth with an exposure age of 1.8 (±0.3) m.y. The CM chondrites may be linked by spectral properties to the C-type, G-class asteroids 19 Fortuna and 13 Egeria, both of which are located near resonances that should be supplying fragments to Earth. On the other hand, Rubin and Bottke (2007) argue that the prevalence of CM xenolithic clasts in ordinary chondrites and HED breccias is consistent with a recent collisional fragmentation event. They believe the source to be an ~170-km C-type asteroid located in the inner asteroid belt which had been disrupted ~160 m.y. ago, and which now comprises the Baptistina asteroid family. The close proximity of this asteroid family to the ν6 resonance and the similar orbits of the OC and HED parent bodies are consistent with such a scenario. However, contradictory spectroscopic data has been obtained by Reddy et al. (2009), including analysis of absorption features and albedo values, which suggest 298 Baptistina is more likely an S-type asteroid, perhaps similar to the LL chondrites, and is inconsistent with a carbonaceous chondrite. The above specimen is a 3.1 g cut fragment exhibiting a portion of the fresh black fusion crust.

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