I. Gilmour

Global fire at the Cretaceous– Tertiary boundary

Cretaceous-Tertiary (K-T) boundary clays from five sites in Europe and New Zealand are 102-104-fold enriched in elemental C (mainly soot), which is isotopically uniform and apparently comes from a single global fire. The soot layer coincides with the Ir layer, suggesting that the fire was triggered by meteorite impact and began before the ejecta had settled.

δ13C of free and macromolecular aromatic structures in the murchison meteorite

Abstract Analyses of the organic compounds in the Murchison meteorite have led to a greater understanding of the nature of extraterrestrial organic materials. However, the relationship between low and high molecular weight material remains poorly understood. To investigate this relationship, untreated Murchison was subjected to supercritical fluid extraction (SFE) to obtain the free organic components in the meteorite. Toluene and other volatile aromatic hydrocarbons dominated the extract, and the carbon isotopic composition of these molecules was determined by gas chromatography-isotope ratio-mass spectrometry (GCIRMS). δ 13 C values of the aromatic hydrocarbons ranged from −28.8 to −5.8‰. These compounds displayed a 13 C-enrichment with increasing carbon number suggesting an origin by cracking. The high molecular weight organic material in the meteorite was isolated and subjected to hydrous pyrolysis. This procedure produced a number of aromatic products, the majority of which were volatile aromatic hydrocarbons, particularly toluene. SFE was used to extract and successfully retain them. This enabled the first carbon isotopic analysis of this poorly understood material to be performed at the molecular level by GCIRMS. δ 13 C values for aromatic pyrolysis products occupied a range from −24.6 to −5.6‰. The trend of 13 C-enrichment with increasing carbon number, observed in the free compounds, was also evident in the macromolecular fragments. Furthermore, the organic fragments of the macromolecular material were consistently 13 C-enriched when compared to structurally identical free molecules. This suggested that the free aromatic hydrocarbons in Murchison were produced by the preterrestrial degradation of the organic macromolecular material. This natural degradation event was extended by the hydrous pyrolysis experiment.

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.

Aromatic moieties in meteoritic macromolecular materials: analyses by hydrous pyrolysis and δ13C of individual compounds

Abstract Hydrous pyrolysis, supercritical fluid extraction (SFE), gas chromatography-mass-spectrometry (GC-MS) and isotope ratio monitoring-gas chromatography-mass spectrometry (irm-GC-MS) were used to investigate the constitution of macromolecular materials in meteorites. Results from the carbonaceous chondrites Orgueil (CI1) and Cold Bokkeveld (CM2) were compared with those obtained previously from Murchison (CM2). Fragments of meteoritic macromolecular materials were produced by hydrous pyrolysis, extracted by SFE, and identified by GC-MS. The CI1 and CM2 hydrous pyrolysates all contain volatile aromatic compounds, some of which have aliphatic side chains, hydroxyl groups, and thiophene rings attached. The results indicate that the macromolecular materials in these meteorites are qualitatively similar. However, the pyrolysates show significant quantitative differences, with the products of ether linkages and condensed aromatic networks being less abundant in the more aqueously altered meteorites. In addition, the methylnaphthalene maturity parameter negatively correlates with aqueous alteration. These features are interpreted as the result of chemical reactions favored under hydrous conditions. Hence, the extent of aqueous alteration on the meteorite parent body appears to be the most important evolutionary stage in determining the final structure of macromolecular materials in the CI1 and CM2 meteorites. The carbon isotopic compositions of the fragments of macromolecular materials were determined by irm-GC-MS. δ13C values for the hydrous pyrolysis products range from −25.5 to −10.2‰ for Orgueil and −22.9 to +4.0‰ for Cold Bokkeveld. These values can be compared to the −24.6 to −5.6‰ range obtained previously for Murchison. The low molecular weight components in each hydrous pyrolysate display shifts to increased 13C contents with carbon number. This indicates the production of simple organic entities by the preferential cracking of 12C-12C bonds in more complex starting materials. The shifts extend from C7 to C8 for Orgueil and Cold Bokkeveld but from C7 to C10 for Murchison. Higher molecular weight components for all of the hydrous pyrolysates show a general trend of decreasing 13C content with carbon number. The higher molecular weight features can be explained by the preferential addition of 12C during the primary synthesis of the macromolecular materials. In addition, δ13C values for the methylnaphthalenes are consistent with the addition of 12C to the most reactive site on the naphthalene parent molecule providing supporting evidence for synthesis. Hence, the macromolecular materials are composed of organic units created by both synthesis and cracking. Therefore, secondary processing by liquid water on the meteorite parent body exerts a strong control on the final molecular architecture of meteoritic macromolecular materials. Yet, the carbon isotopic compositions of some individual moieties may retain a record of primary synthesis.

Diamonds from the Popigai impact structure, Russia

Diamonds were found in impact melt rocks and breccias at the Popigai impact structure in Siberia. The diamonds preserve the crystallographic habit and twinning of graphites in the preimpact target rocks, from which they formed by shock transformation. Secondary and transmission electron microscopy indicate that the samples are polycrystalline and contain abundant very thin lamellae, which could represent stacking faults, with local hexagonal symmetry, or microtwins. Microcrystalline units are ≤1 µm. Infrared spectroscopy indicates the presence of solid CO 2 and water in microinclusions in the diamonds, CO 2 being under a pressure greater than 5 GPa (at room temperature). Trace element and isotopic compositions confirm the derivation from graphite precursors.

THE GARDNOS IMPACT STRUCTURE, NORWAY : PETROLOGY AND GEOCHEMISTRY OF TARGET ROCKS AND IMPACTITES

Abstract The Gardnos structure, Norway is an approximately circular area of anomalously fractured and brecciated rock, about 4.5 km in diameter, emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite. The original recognition of Gardnos as a deeply eroded impact structure between 900 and 400 Ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks (impactites). Deep erosion of the structure has erased the original rim, removed much of the crater-fill deposits, and exposed large areas at or near the original crater floor. However, a wide variety of distinctive impactites —fractured and blackened quartzites in the sub-crater basement rocks, lithic breccias, and melt-bearing breccias—are still preserved. These impactites show petrographic and chemical characteristics that confirm an impact origin: distinctive Planar Deformation Features (PDFs) in quartz and feldspar, incipient melting of feldspar clasts in the melt-bearing breccias, close matches between the chemical composition of the breccias and mixtures of the target lithologies, and the detection of an extraterrestrial component. A minor extraterrestrial component (≤0.15%) was detected in the melt-bearing breccias, based on significantly elevated Ir and Os contents and lower 187Os/188Os ratios compared to those in the target rocks. The Gardnos impactites are significantly enriched in C (5–10X) over the exposed target rocks. This may reflect the presence of a C-rich shale overlying the metamorphic basement at the time of impact; this idea is supported by δ13C values of −28.1 to −31.5%. measured in the impactites. Mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60–90 wt% granite gneiss, 0–30 wt% amphibolite, 0–12 wt% quartzite, and 3–19 wt% of a C-rich shale component. The deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult. One possible scenario involves the impact of a 300 m diameter stony meteorite that released 1019 J of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m. The original crater was filled with at least 0.3 km3 of allochthonous melt-matrix breccias containing about 0.06 km3 of impact melt. The structure underwent low-grade (greenschist?) metamorphism in Caledonian time (about 400 Ma ago) and was subsequently eroded to its present appearance. The value of the Gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone, in the preservation of a possibly complete sequence of crater-fill breccias beneath a cap of elastic sediments, and in the unusual carbon enrichment of its impactites.

Small-scale hydrous pyrolysis of macromolecular material in meteorites

Abstract The hydrous pyrolysis method, usually performed on several hundred grams of terrestrial rock sample, has been scaled down to accommodate less than two grams of meteorite sample. This technique makes full use of the high yields associated with hydrous pyrolysis experiments and permits the investigation of the meteorite macromolecular material, the major organic component in carbonaceous meteorites. The hydrous pyrolysis procedure transforms the high molecular weight macromolecular material into low molecular weight fragments. The released entities can then be extracted with supercritical fluid extraction. In contrast to the parent structure, the pyrolysis products are amenable for analysis by gas chromatography-based techniques. When subjected to hydrous pyrolysis, two carbonaceous chondrites (Orgueil and Cold Bokkeveld) released generally similar products, which consisted of abundant volatile aromatic and alkyl-substituted aromatic compounds. These results revealed the ability of small-scale hydrous pyrolysis to dissect extraterrestrial macromolecular material and thereby reveal its organic constitution.

Cretaceous-tertiary boundary event: Evidence for a short time scale

Abstract Three non-meteoritic trace elements (Sb, As and Zn) are strongly enriched at eleven K-T boundary sites, along with mainly or partly meteoritic elements (Ir, Ni, Cr, Fe and Co). The proportions (As, Sb, Zn/Ir) are remarkably constant over a ≈ 100-fold range in concentration. This correlation persists in sub-layers of boundary clay and even extends to soot (from burned land biomass). Apparently, all the components, despite their diverse origins, became associated in a single, global component prior to deposition. No wholly satisfactory source is available for As, Sb and Zn: the trace element pattern in volcanic gases does not match that in KT boundary clay, with ratios to Ir falling short by 1 to 2 orders of magnitude, terrestrial rocks do not reach high enough concentrations and (modern) ocean water contains too little Zn, but on balance, the latter source seems preferable—perhaps augmented by volatiles from the impact crater. Apparently, Ir-bearing ejecta and soot from forest fires coagulated in the stratosphere and then fell out together, sweeping out oceanic biomass and anoxically precipiting As, Sb and Zn. Significantly, the amounts of marine and land biomass at the KT boundary correspond to about the steady-state global inventory (1 generation), and the amounts of As, Sb and Zn are also within a factor of ≈5 of the global inventory. This is expected in a catastrophic but not a gradualist scenario.

Normal alkanes in meteorites: molecular δ13C values indicate an origin by terrestrial contamination

In the time period since 1961, n-alkanes (straight-chain hydrocarbons) have been detected in varying amounts in many meteorites. Proposed origins for these compounds have included extraterrestrial biotic, extraterrestrial abiotic and terrestrial contamination processes. To help establish the source of these compounds, we determined the carbon isotopic compositions of individual n-alkanes in meteorites of several different classes and terrestrial histories: Orgueil (CI1), Cold Bokkeveld (CM2), Murchison (CM2), Vigarano (CV3), Allende (CV3), Ornans (CO3), and Bishunpur (LL3). The efficiency of supercritical fluid extraction (SFE) was exploited to provide extractable non-polar organic matter from the meteorites. The n-alkanes in these extracts were then identified with gas chromatography–mass spectrometry (GC–MS) and the carbon isotopic composition of individual molecules was determined using isotope ratio monitoring–gas chromatography–mass spectrometry (irm–GC–MS). n-Alkanes were found in all but one of the meteorites analysed (Allende), but carbon number distributions varied between samples. Pristane and Phytane were also detected in five of the seven meteorites. δ13C values for the individual n-alkanes occupied a range from −25.3 to −38.7‰. The δ13C values for the meteoritic n-alkanes have a similar range as those for n-alkanes measured from petroleum in the literature. Therefore, the n-alkanes in meteorites appear to be terrestrial contaminants which may have originated from fossil hydrocarbons or petroleum products. This type of contamination is analogous to that which will be threatening future meteorite falls and the samples returned from spaceflight missions over the next two decades and beyond. For falls already contaminated, irm–GC–MS appears useful in discriminating between indigenous compounds and those introduced by terrestrial contamination.

High molecular weight organic matter in martian meteorites

We have performed an investigation to detect high molecular weight organic matter in martian meteorites. Solvent-extracted samples of two Antarctic 5nds (ALH 84001, sub-sample 106 and EET A79001, sub-sample 351) and one non-Antarctic fall (Nakhla) were analysed by :ash pyrolysis–gas chromatography–mass spectrometry. Results suggest that our sub-sample of ALH 84001 contains no pyrolysable organic matter. In contrast, our samples of EET A79001 and Nakhla contain organic matter of high molecular weight, which releases aromatic and alkylaromatic hydrocarbons, phenol and benzonitrile as major compounds upon pyrolysis. The detection of similar pyrolysis products from Nakhla and EET A79001 indicates that these martian meteorites may have a common high molecular weight organic phase. Carbon isotopic measurements of individual molecules in the Nakhla pyrolysate, by :ash pyrolysis– gas chromatography–isotope ratio mass spectrometry, reveal that this high molecular weight organic matter has some similarities to that found in carbonaceous chondrites. At this point, an origin by terrestrial contamination cannot be unequivocally ruled out, but the data seem to support proposals that martian samples contain organic matter originating from meteoritic infall on Mars. The results suggest that a wider, pyrolysis-based study of martian meteorites would be a justi5able use of these precious samples. ? 2002 Elsevier Science Ltd. All rights reserved.