Organic thermometry for chondritic parent bodies

Earth and Planetary Science Letters 272 (2008) 446–455

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Earth and Planetary Science Letters

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Organic thermometry for chondritic parent bodies

G.D. Cody a,⁎, C.M.O'D. Alexander b, H. Yabuta a, A.L.D. Kilcoyne c, T. Araki d, H. Ade d, P. Dera a, M. Fogel a,B. Militzer a, B.O. Mysen a

a Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, United Statesb Department of Terrestrial Magnetism, Carnegie Institution of Washington, United Statesc Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, United Statesd Department of Physics, North Carolina State University, Raleigh, NC, United States

⁎ Corresponding author. Tel.: +1 202 478 8980.E-mail address: [email protected] (G.D. Cody).

0012-821X/$ – see front matter © 2008 Elsevier B.V. Adoi:10.1016/j.epsl.2008.05.008

A B S T R A C T

Article history:

Editor: G.D. Cody

Keywords:chondriteparent bodiesinsoluble organic matterthermal metamorphismC-XANESNMRRaman

ure has been identified in a study of twenty-five different samples of meteoriticinsoluble organic matter (IOM) spanning multiple chemical classes, groups, and petrologic types, usingcarbon X-ray Absorption Near Edge Structure (XANES) spectroscopy. The intensity of this feature, a 1s−σ⁎exciton, appears to provide a precise measure of parent body metamorphism. The intensity of this exciton isalso shown to correlate well with a large negative paramagnetic shift observed through solid state 13C NMR.Experiments reveal that upon heating primitive IOM is transformed into material that is indistinguishablefrom that in thermally processed chondrites, including the development of the 1s−σ⁎ exciton. A thermo-kinetic expression is derived from the experimental data that allows the intensity of the 1s−σ⁎ exciton to beused to estimated the effective temperature integrated over time. A good correlation is observed between theintensity of the 1s−σ⁎ exciton and previously published microRaman spectral data. These data provide a self-consistent organic derived temperature scale for the purpose of calibrating Raman based thermometricexpressions.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Very early in Solar System history, during the period directlyfollowing planetesmal accretion, thermal metamorphism gave rise tothe various petrologic subtypes of chondritic meteorites (Brearley andJones, 1998). Mapping these subtypes onto a profile of metamorphictemperature within the parent asteroid body is critical for evaluatinghot versus cold accretion theories (Hutchison, 1996; Rubin andBrearley, 1996), reconciling interior structure models (Taylor et al.,1987; Lipschutz et al., 1989), as well as placing constraints on the sizesand heat sources of primitive planetisimals (Grimm and McSween,1989). Whereas partially equilibrated meteorites can provide well-constrained estimates of temperature (Wlotzka, 2005; Kessel et al.,2007), many chondrites (petrologic types 1, 2, and 3's) are unequi-librated and establishing even a relative metamorphic scale for theseis challenging (Grossman and Brearley, 2005). A classic example is thecase of the CV3 chondrite, Allende. Various analyses of the miner-alogical, textural, and compositional aspects of this well studied fallhave yielded a temperature range that spans from a low of 325 °C

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(Rietmeijer and Mackinnon, 1985) to a high of 600 °C (Huss and Lewis,1994).

Recent microRaman studies have shown that the molecularstructure of insoluble organic matter (IOM) appear to correlate wellwith petrologic type (Quirico et al., 2003; Bonal et al., 2006, 2007;Busemann et al., 2007). In the present study, we present molecularspectroscopic data on meteoritic IOM isolated from a wide range ofchondrite classes, groups, and petrologic types. The analytical toolsapplied here are Carbon X-ray Absorption Near Edge Structure(XANES) spectroscopy and solid-state 13C Nuclear Magnetic Reso-nance (NMR) spectroscopy. We use these data to show that there existmolecular structural characteristics of the bulk of meteoritic IOM thatmay be used to provide a precise estimate of the metamorphictemperature of the parent body.

2. Samples and methods

2.1. Meteorite demineralization

Pure organic isolates from carbonaceous chondrites wereobtained from powdered samples using a concentrated aqueoussolution of CsF with the pH adjusted to near neutral with HF (Codyet al., 2002). The samples included in this study, 25 in number, are

Table 1Sample ID, 1s−σ⁎ intensities, and estimated temperatures

Sample Meteorite namea Abbrev. Meteorite groupb % Exciton intensityc TEFFd °C Te °C literature

1 EET92042 E92 CR2 0.0 ∼02 Bells Bel CM2 0.8±2.2 77±933 Murchison Mur CM2 1.1±1.5 96±654 Orgueil Org CI1 1.2±1.7 101±685 Tagish Lake Tag C2 1.8±1.4 129±466 GRO95577 G95 CR1 3.0±2.5 171±647 ALHA77307 A773 CO3.0 4.2±1.8 203±41 ∼2008 Semarkona Sem LL3.0 4.2±2.6 203±58 200–2609 MET00452 M452 LL3.05 4.2±3.3 203±7010 Y86720 Y86 CM Heated 7.1±2.4 265±42 f400–85011 Kaba Kab CV3.1 13.8±2.5 371±34 300–40012 Vigarano Vig CV3.1/3.4 17.1±1.2 415±25 300–40013 Krymka Kry LL3.1 17.2±1.9 416±3114 Mokoia Mok CV3.2/3.6 17.7±1.7 423±29 300–40015 ALHA77003 A770 CO3.5 17.9±4.3 425±56 475–54016 Leoville Leo CV3.1/3.4 18.6±3.8 434±50 ∼25017 MET00489 M489 L3.6 18.7±3.1 435±4218 Kainsaz Kai CO3.2/3.6 20.2±1.8 453±29 ∼30019 WSG95300 W953 H3.3 21.1±3.9 464±5020 Tieschitz Tie H/L3.6 21.3±2.6 467±3621 Chainpur Cha LL3.4 22.7±5.3 483±6322 Bishunpur Bis LL3.15 28.8±2.6 551±34 300–35023 Allende All CV3.2/N3.6 29.1±1.3 554±25 325–60024 Isna Isn CO3.7–3.8 43.2±3.1 700±37 480–70025 Indarch Ind EH4 68.0±4.5 948±50 ∼640G Graphite Gr 100.0

a Where place name is given the abbreviations are EET = Elephant Morrain, GRO = Grosvenor Mountains, MET = Meteorite Hills, ALHA = Allan Hills, Y = Yamato.b The petrologic Type 3 meteorites are subdivided into tenths; where two values are given, the lower generally reflects designation by thermo-luminescence, the higher

designation is from Raman Spectroscopy.c The percent Exciton intensity is normalized to span from EET92042 to graphite, error range is presented at the 2σ level and is due to white noise.d The TEFF is calculated from the experimental kinetic data assuming a duration of 107 years of parent body heating; the error in temperatures is derived from the combination of

measurement error and the uncertainty in the kinetic data. Mineralogical and geochemical T estimates, Huss et al. (2006) and references therein. Y86720 is a heated CM, TEFFestimates are derived from mineral alteration kinetics (Akai, 1992).

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listed in Table 1, along with their group and petrologic type. Thesespan the CR, CI, CO, CV, CM, ordinary, and enstatite chondrites. Theabundances and elemental and isotopic compositions of the IOMstudied here have been published elsewhere (Alexander et al.,2007).

2.2. X-ray absorption near edge spectroscopy (XANES)

All C-XANES data were acquired using the Scanning TransmissionX-ray Microscope (STXM) located at Beam line 5.3.2, at the AdvancedLight Source, Lawrence Berkeley National Laboratory (Kilcoyne et al.,2003). Sample preparation involved dispersal of grains into droplets ofsuper-cooled liquid sulfur that, once crystallized, were mounted onepoxy stubs and microtomed to 100–140 nm thickness using adiamond knife and a Leica Ultracut microtome. All of the C-XANESspectra presented here are the average over 10's to 100's of 20–50×20–50×140 nm voxels in hyperspectral data sets.

2.3. 13C solid state NMR

Single pulse Magic Angle Spinning MAS NMR spectra wereacquired using a Varian-Chemagnetics Infinity 300 NMR. Theexperimental parameters were 90° pulse widths, 4 µs, 75 kHz 1H RFdecoupling, and MAS frequency (ωr/2π=12 kHz) and a 60 s recycledelay. Additional details may be found in Cody and Alexander (2005).

2.4. X-ray diffraction

X-ray diffraction images of the samples were collected with aRigaku RAXIS/RAPID diffractometer with an Ultrax-18, 18 kW rotatinganode X-ray generator and a hemi-cylindrical image-plate detector.Twenty minute exposures were taken using monochromatic, Mo Kαradiation. Samples were oscillated over a 40° range, to average thegrain orientations.

2.5. Heating experiments

Sub-milligram quantities of Murchison (CM2) IOM were flashheated (500 °C/s) up to 1400 °C in helium using a CDS 1000 pyroprobeinterfaced to an Agilent 6890 Gas Chromatograph and mass spectro-meter. For longer times, 1 to 2 mg quantities of Murchison IOM wereheated platinum foil containers suspended in a DelTech gas mixingFurnace, with a continuous flow of zero-grade argon. The selectedtemperatures were 600, 800, or 1000 °C. Heating times are very fast,typically the thermocouple (within 0.5 cm of the sample) reported atemperature of greater than 90% of that desiredwithin less than 2min.Following reaction, temperature drops rapidly reaching ∼50 °C within5 to 7 min. In all cases, more than 50% of the original organic matterremained.

3. Results

3.1. Carbon XANES of multiple chondrite ion samples

Carbon X-ray Absorption Near Edge Structure (C-XANES) spectro-scopy is useful for detecting and quantifying organic functionalgroups, particularly in the case where only very small quantities ofsample are available. C-XANES has been previously applied to thestudy of organic microfossils (Cody et al., 1995; Boyce et al., 2002),interplanetary dust particles (Flynn et al., 2003) and recently returnedcomet particles (Cody et al., 2008). In general, absorption at the lowestenergies (∼285.0 eV) is well described by photo-excitation of carbon1s electrons to highly localized (bound) states, e.g., low energy π⁎orbitals of alkenyl and aromatic moieties Stöhr (1996). For mostcarbonaceous compounds, absorption of photons at energies exceed-ing the carbon 1s ionization edge (i.e., ∼290.8 eV for benzene) result invery broad spectral features, corresponding to highly delocalized(virtual) excited states, often referred to as 1s−σ⁎ transitions (Stöhr,1996). In the case of graphite (Fig. 1A), however, the C-XANES

Fig. 1. A) C-XANES spectra (offset vertically for clarity) of collection of type three condrites spanning the CO, CV, ordinary, and enstatite chondrite groups. A reference spectrum ofgraphite is included on top. Two peaks are highlighted. The first is at an energy of ∼285 eV and corresponds to the 1s−π⁎ transition of carbon double bonded to carbon, e.g. olefinicand aromatic carbon. In the spectrum of graphite a sharp peak at 291.7 eV is observed that corresponds to a 1s−σ⁎ exciton. The presence of a peak at this energy is observed in manyC-XANES spectra of the type 3.1+ chondrites. B) The first derivative of the C-XANES spectra (from 289 to 300 eV) highlighting the progressive development of the sharp 1s−σ⁎ excitonacross this range of type 3.0 chondrites. Note there is no evidence of the 1s−σ⁎ exciton in the case of Semarkona (LL3.0) and ALHA77307 (CO3.0).

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spectrum also exhibits the presence of a very sharp absorption featureat 291.63 eV. This peak was first observed only relatively recently(Batson, 1993) and is now recognized to correspond to an unusual,highly localized and long lifetime, Frenkel-type 1s−σ⁎ exciton (Maet al., 1993; Brühwiler et al., 1995). The appearance of 1s−σ⁎ exciton ingraphite is due to the presence of extensive planar domains of highlyconjugated graphene sheets. The 1s−σ⁎ exciton is not exclusive tographite, however. It is also observed in carbon nano-tubes (Ponget al., 2001) and amorphous carbon glass (see supplementary data inthe Appendix), what is required are highly conjugated sp2 bondedcarbon domains.

Inspection of Fig. 1A reveals that IOM from many petrologic type 3chondrites exhibit the identical 1s−σ⁎ excitonic feature so prominentin graphite, albeit with diminished intensity but with no change infrequency. The apparent progressive development in exciton intensityspanning the type 3 IOM in this study is more clearly highlighted inthe derivative of the spectra (Fig. 1B), where it is seen that the mostprimitive type 3 chondrites (Semarkona, LL3.0, and ALHA77307,CO3.0) exhibit no evidence of the exciton. Significantly, none of thepetrologic type 1 and 2 IOM samples exhibit evidence of the exciton. Inorder to quantify the relative intensity of the exciton, we use theintensity at 291.1 eV of the derivative spectra (Fig. 1B) and normalizethis such that graphite's exciton intensity is defined to be 100%

(supplementary data in the Appendix). In the group of 25 samplesanalyzed in this study, IOM derived from the CR2 chondrite(EET92042) exhibits the least intensity in its derivative spectrum at291.1 eV; interestingly, this meteorite also appears to be chemically(Cody and Alexander, 2005) and isotopically (Busemann et al., 2006)very primitive. We define, therefore, the scale in exciton developmentas spanning that from EET92042 (0%) to graphite (100%) (supplemen-tary data in the Appendix). These normalized exciton intensities arepresented in Table 1.

3.2. Solid state 13C NMR and intensity of the 1s−σ⁎ exciton

Development of the intensity of the 1s−σ⁎ exciton correlates withanother independent spectroscopic observable. Specifically, in therare case when we are able to obtain sufficient IOM to perform solidstate 13C NMR on type 3 IOM, we have observed a strikingparamagnetic shift of the principal sp2 carbon resonance (aromaticand/or olefinic carbon) to lower frequencies (Fig. 2A). To the best ofour knowledge, such large negative shifts have not been reported forother natural macromolecular material e.g. terrestrial kerogens. Largenegative paramagnetic shifts have been observed, however, in thecase of graphite where the central moment of the C=C resonance isshifted down in frequency to ∼75 ppm (Jiang et al., 2002). Negative

Fig. 3. The correlation between normalized exciton intensity (%) and various estimatesof temperature (gray bars). Included is a single estimate for a temperature of Isna (blackstar) employing Fe/Mg partitioning between olivine-spinel grains (see supplementaldata). The petrologic type 1 and 2 chondrites in this study all fall into the gray region atthe origin of the plot. Curves A, B, and C plot the predicted relationship between excitonintensity and temperature derived from the thermo-kinetic experiments. Curve A usesthe as determined kinetic parameters. Curves B and C are calibrated assuming thatIsna's TEFF=700 °C and Allende's TEFF=325 °C, respectively.

Fig. 2. A) 13C Solid State NMR spectra focusing on the aromatic resonance region for a few chondrites including Tagish Lake (C2), Y86720 (a thermally altered CM), Vigarano (CV3.4),and Allende (CV3.6). A significant paramagnetic shift to lower frequencies is noted moving from Tagish Lake to Allende. B) The correlation between the NMR paramagnetic shift andnormalized exciton intensity.

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paramagnetic shifts in synthetic organic solids are also well known inthe class of organic charge transfer conductors, where the para-magnetic shift is understood to result from the presence of anoccupied conduction band with π⁎-type molecular character (Rybac-zewski et al., 1976). Interestingly, we observe an apparent linearcorrelation between these paramagnetic shifts and exciton intensity(Fig. 2B) that strongly suggests that the evolution in the electronicstructure of IOM leading to the growth in exciton intensity involves aninsulator–conductor transition. Interestingly, X-ray diffraction (XRD)analysis of several IOM samples in this study, that span awide range inexciton intensities reveal broad diffraction peaks with relatively larged002 interlayer spacings (supplementary data in the Appendix),indicating that the exciton intensity is not related to graphitizationof IOM.

In the context of the discussion that follows, it is worthwhileaddressing the issue of graphitization in chondrites. Carbon chemistshave long known that in order to convert organic precursors intographite using temperature alone requires temperatures up to 3000 °Cwith apparent activation energies of ∼1000 kJ/mol (Fishbach, 1971;Pacault, 1971). Indeed, for some organic precursors, e.g. polysacchar-ides and polyvinylidene chloride, heating in excess of 2000 °C neverconverts them to graphite. Rather, they transform into carbon glass,that is, these compounds are “non-graphitizable” (Franklin, 1951). Thefact that enormously high temperatures (2200 °C) were required toconvert high rank coals to graphite was also confirmed (Oberlin,1984); interestingly, lower rank coals were observed in the same studyto be non-graphitizable even at 3000 °C.

On the other hand, graphite is present in terrestrial rocks that werenever subjected to such severe temperatures. For example, theoccurrence of graphite veins and graphitic marble that recordmetamorphic fluid temperature of 600 to 700 °C. Furthermore, widescale graphitization of sedimentary organics is noted to have occurredin Nappe structures in the Alps (Mählmann et al., 2002). The apparentinconsistency between laboratory and natural graphitization wasreconciled through observation and experiments. The transformationof terrestrial carbon into graphite during metamorphism is greatly

accelerated by tectonic shear strain a key point confirmed byexperiment (Ross et al., 1991; Nover et al., 2005). Thus, thetransformation of organic precursors directly to graphite is nowunderstood to only occur in tectonically deformed rocks. Theformation of graphite in marbles and marls is due to the equilibrium

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crystallization from metamorphic fluids with very high partialpressures of CH4 and CO2 (Rumble et al., 1986). The key point is thatthis particular occurrence of graphite does not involve the transfor-mation of organic matter into graphite, rather it involves thehydrothermal crystallization of graphite grains from a solutioncontaining inorganic C1 species.

This discussion of graphitization in important in context with thepresent work because the size of chondritic parent bodies may onlyhave been on the order of ∼100 km in radius, thus pressure is assumedto never have been a significant factor accompanying metamorphism.Certainly, tectonic shear stress was absent. Furthermore, the hightemperature metamorphic fluids likely had low partial pressures ofCH4 and CO2; precluding the precipitation of graphite. It appears likelythat in the case of the chondritic meteorite parent bodies it wouldhave been impossible to thermally transform primitive organic matterinto graphite. Moreover, it is possible that meteoritic IOM falls into thecategory of non-graphitizable carbon, as has been suggested forAllende's IOM (Smith and Buseck, 1981; Harris et al., 2000). In the textthat follows, therefore, it is not correct to conclude that the moleculartransformation leading to exciton growth is due to graphitization.Furthermore, in any study of chondritic IOM it is not appropriate tomake comparison with terrestrial organic matter for the purpose ofthermal calibration.

3.3. Linking 1s−σ⁎ exciton intensity to thermal processing

The relative ranking of type 1, 2, and 3 IOM in Table 1 suggests thatthe growth in exciton intensity may be a primary molecular structuralsignature of the extent of thermal processing that IOM experiencedduring parent body metamorphism. This observation is supported in

Fig. 4. A) C-XANES spectra of Murchison IOM after flash heating to 600, 1000, and 1400 °C. Cvertically for clarity). Note that Murchison IOM flash heated to 1400 °C yields a residue that ethe C-XANES spectra (from 289 to 300 eV) highlighting the development of the sharp 1s−Murchison IOM flash heated to 1400 °C is very similar to that observed in Vigarano.

Fig. 3 where exciton intensity is plotted against various estimates ofmetamorphic temperature derived from a variety of mineralogical andother inorganic indicators (Huss et al., 2006) for the chondritesstudied here. The general consensus appears to be that type 1 and 2IOM did not experience any significant thermal processing (Zolenskyet al., 1997) and all such primitive IOM samples plot near the origin ofFig. 3. Among the type 3 chondrites, there clearly exists a positivecorrelation between the mineralogical estimates of metamorphictemperature and normalized exciton intensity (Fig. 3), albeit withquite a wide ranges of temperature for type 3 chondrites such asAllende.

The positive correlation exhibited in Fig. 3 suggests that if it can beestablished that the development in the exciton intensity isunambiguously linked to thermal metamorphism of IOM, then theexciton intensity might provide a precise and independent measure ofthermal history. To test this possibility, sub-milligram quantities ofIOM isolated from Murchison were flash heated (500 °C/s) in an inert(helium) atmosphere up to 600, 1000, and 1400 °C, respectively. Theresidues (∼70% of the initial mass) were then analyzed via C-XANES(Fig. 4). Inspection of these C-XANES spectra reveals that even a brief,purely thermal excursion can transform primitive IOM from Murch-ison into material that has a C-XANES spectrum remarkably similar tothat of Vigarano (Vig, Fig. 4A). In particular, a shoulder has clearlydeveloped at in the region of the 1s−σ⁎ exciton at 291.63 eV in theMurchison IOM sample heated to 1400 °C and the derivatives of thespectral region spanning 289.0 to 300.0 eV reveals that the 1s−σ⁎exciton (Fig. 4B) in the heated residue is similar in intensity to thatobserved in Vigarano.

These results suggest that one may be able to experimentallyderive kinetic data to provide a quantitative expression linking exciton

-XANES spectra of Vigarano and Allende are included for comparison (spectra are offsetxhibits an electronic structure very similar to that of Vigarano. B) The first derivative ofσ⁎ exciton with heating. Note that the intensity of the 1s−σ⁎ exciton in the residue of

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intensity to time and temperature. Accordingly, Murchison IOM wassubjected to heating for longer times at temperatures of 600, 800, and1000 °C (supplementary data in the Appendix). Subsequent C-XANESanalyses of these residues reveal a well-resolved time–temperaturerelationship for the development of the exciton (Fig. 5). Interestingly,the kinetics exhibit log-linear behavior described by the phenomen-ological rate expression,

dE=dt~n=t: ð1Þ

Here ξ is a temperature-dependent exciton development coeffi-cient. It is important to note that a very fast pyrolytic reaction,volatilizing ∼30–40 wt.% of small molecules (Kitajima et al., 2002),precedes the much slower thermal development of the excitonintensity. The pyrolytic bond cleavage is endothermic and the shiftin the origin (or focus point) of the growth curves from t=0 tot∼200 µs reflects the expected thermal phase lag derived frompyrolysis.

As to the molecular scale details underlying these kinetics, at thispoint we can only discuss the thermal transformation of IOM structureleading to exciton development in phenomenological terms. Fromwhat can be determined from the literature, the development of theexciton requires the formation of highly conjugated π systems withminimal curvature (e.g. large diameter “bucky” tubes exhibit theexciton, narrow diameter “bucky” tubes exhibit a weaker exciton, andfullerenes (e.g. C60) show no exciton (Chen et al., 1991; Pong et al.,2001). The structure of IOM at the initiation of thermal transformationis a highly cross-linked macromolecule composed of highly sub-stituted small ring aromatics (Cody et al., 2002; Cody and Alexander,2005); the structure of IOM differs enormously from terrestrialkerogen. Thermal evolution of primitive IOM structure into a materialthat contains extended domains of conjugated π bonded carboncapable of developing this exciton presumably involves a highlycooperative, multi-step process, involving considerable bond break-ing, bond migration and bond rearrangment. As highly conjugatedcarbon-carbon chains or domains provide a particularly stable

Fig. 5. The experimentally observed Log-linear time–temperature development of the1s−σ⁎ exciton in Murchsion IOM upon heating to 600 (■), 800 ( ), and 1000 (□) °Cfor various times (see supplemental data). The fine dashed lines correspond tolinear fits through these data constrained to pass through 0% exciton intensity at200 µs. These data define an apparent activation energy of ∼ 18 kJ for the excitongrowth. The coarse dashed line is calculated from the lower temperature kineticdata for a temperature of 1400 °C and passes near the single data point for IOMheated at 1400 °C for 10 s.

electronic configuration, these are expected to accumulate with timeincreasing the intensity of the exciton. Evidently, as these conjugateddomains grow, progressively more steps are necessary for continueddomain growth, leading to the observed log-linear kinetic behavior.

It is appealing to apply these kinetic data to derive a quantitativeexpression to determine metamorphic temperatures for these variousIOM samples (Table 1). If we assume an Arrhenius-type temperaturedependence on the magnitude of the transformation constant, ξ, weobtain the following expression,

ln n ¼ Aþ B=T ð2Þ

where T is temperature (K). For the experimental data shown inFig. 5, A=1.504 and B=−2177.1 K (which implies an apparent activationenergy, EA⁎, of ∼18 kJ). Using these kinetic data we can calculate thepredicted transformation curve for 1400 °C, that is shown in Fig. 5 topass close to the single data point acquired at this temperature.

If the time–temperature history of a given chondritic parent bodywere known, then these kinetic data could be used to put chondrites ofdifferent petrologic type in spatial context, i.e., in terms of radial distancefrom parent body center. In the absence of such informationwe set out touse the kinetic data to calculate an effective temperature, TEFF, assumingisothermal heating for 107 years following parent body accretion as beingwithin the range of possible durations (Brearley and Jones, 1998). Usingthe raw kinetic data in Fig. 5. leads to the prediction that the excitonintensity observed in Allende required a TEFF of 893±25 °C (Curve A,Fig. 3., where the temperature uncertainty reflects the error in theexperimentally determined values of ξ and measured exciton intensity).This temperature is much higher than the maximum acceptedtemperature for Allende of 600 °C (Huss and Lewis, 1994). In the caseof Isna, a CO3.8, the kinetics predict an unreasonably high TEFF of∼1200 °C (±37 °C). Indarch (EH4), with a considerably more intenseexciton intensity than Isna (Table 1),would beginmelting at temperaturesbetween 1000 and 1100 °C (McCoy et al., 1999). Clearly this experimen-tally derived kinetic expression predicts unreasonably high values for TEFFfor all of these chondrites (Table 1, Fig. 3).

The excessive temperature estimates derived from the laboratorykinetics expression (Eq. (2)) may result from the obvious difficulty ofexperimentally recreating all of the critical environmental characteristicsthat constitute the natural conditions present in the meteorite parentbodies. This said, it is reasonable to assume that the thermally inducedrate of bond breaking and bond migration, required for conjugated sp2

carbon chain growth and increasing exciton intensity, term B, Eq. (2), isprimarily a property intrinsic to the IOM macromolecule, hence notaffected by the external environment. Differences in external environ-ment may, however, change the value of the temperature independentcoefficient A (Eq. (2)). Coefficient Amay be compared, coarsely, to the so-called frequency factor in the Arrhenius equation (Gladstone et al., 1941).

How the environmental differences may affect coefficient A is amatter of speculation at this point. It is noted, however, that IOM in theasteroidal parent bodies were subjected to significant backgroundionizing radiation derived from the decay of short-lived radiogenicnuclides. It is conceivable that such ionizing radiation sustains a largerconcentration of radicals, increasing the temperature independentcoefficient A and accelerating the molecular transformation abovewhat temperature alone would do. Whether or not this is the cause ofthe kinetic acceleration, it is well established that on Earth ionizingradiation does significantly alter organic structure and accelerate thediagenetic alteration of terrestrial kerogen (Court et al., 2006)principally through ionization enhanced radical reactions.

For the present purposes, wewill assume that some environmentalvariable present during parent body processing lead to a differentvalue for the coefficient A (Eq. (2)). Unfortunately, this leads to thesituation where any temperature scale derived from experimentallydetermined value of coefficient B will have to have coefficient Acalibrated to at least one well-established meteorite temperature in

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the set of chondrites studied here. The ranges in temperatureestimates previously published for some of the chondrites in thepresent study, however, are in many cases very broad (Fig. 3, Table 1).

The choice of any one published temperature value for use as acalibration for the kinetic expression (Eq. (2)) will inevitably lead toagreement with some assessments and not with others. For example,analysis of Fe/Mg diffusion profiles across olivine-spinel grains inAllende were used to propose a TEFF of 327 °C (Weinbruch and Muller,1994). This value is consistent with a previous estimate of 325 °C(Rietmeijer and MacKinnon, 1985) derived from comparison of theelectron diffraction d spacing measured for IOM in Allende with thatof anthracitic coals of known metamorphic grade. If we chose to usethis temperature as a calibration point, then A is increased to 3.263(curve C, Fig. 3). It is observed in Fig. 3 that curve C falls well belowmost published TEFF estimates for meteorites represented in thisstudy. For example, TEFF for Isna using this curve C is estimated to beonly 400 °C, much lower than temperatures derived for inter-granularFe/Mg diffusion in Isna [510–570 °C (Jones and Rubie, 1991)] which isclose to the range of 500 to 600 °C (Keck and Sears, 1987). Similarly,Indarch's (EH4) TEFF would only be 510 °C, while the lowest estimateswould place Indarch at 640 °C (Huss et al., 2006) and otherthermometers have suggested temperatures as high as 925 °C (Fogelet al., 1986).

Recently Wlotzka (2005) applied olivine/spinel thermometry to arange of ordinary chondrites (types 3–6) and obtained closuretemperatures in the range of 650 °C for a number of type 3.8 ordinarychondrites using a calibrated, albeit empirical, relationship betweenthe olivine spinel Fe/Mg KD and Cr/ (Cr+Al) ratio in spinels. The appealof Wlotzka's approach is that a consistent linear relationship isobserved in the partially to thoroughly equilibrated type 3.8, 3.9, and4–6 ordinary chondrites, supporting the proposal for thermodynamicequilibrium. In the case of the type≤3.6 ordinary chondrites, such asTieshitz (H3.6) and Bishunpur (LL3.15), no systematic (linear) relation-ship is observed, consistent with the idea that these meteoritesexperienced temperatures too low for complete equilibration.

In order to establish an independent estimate of temperature for achondrite included in our suite of meteorites (Table 1), to calibrateterm A (Eq. (2)), we have measured coexisting olivine spinel Fe/MgKD's along with the Cr/ (Cr+Al) ratio in the spinel for Isna (CO 3.8) andobtain awell resolved linear isotherm corresponding to a temperatureof 700 °C usingWlotzka's expression (Wlotzka, 2005) (supplementarydata in the Appendix). This temperature is within the range of thatmeasured for type 3.8 ordinary chondrites. It is worth noting thatIsna's temperaturemaximum could actually be higher than this due toa potentially lower closure temperature for this thermometer(Wlotzka, 2005). We will proceed by assuming that 700 °C is areasonable temperature for Isna and use these data to calibrate ourkinetic model by fixing the term A=2.26 (Eq. (2)), yielding Curve B(Fig. 3.).

The estimates for isothermal TEFF are presented in Table 1 and Fig. 3(Curve B). Note that due to the log-linear nature of the transformationkinetics, changing the heating duration to either 106 or 108 years has aminimal effect, shifting the temperature estimates by ∼30 °C eitherway. For the most part, the apparent ranking of chondrite's based onexciton intensity in Table 1 and Fig. 3 is close to what mineralogicalindicators predict, i.e. type 1 and 2 chondrites are slightly less than theleast thermally altered 3.0's. The most primitive type 3 chondrites inthis study, Semarkona, MET00452, and ALHA77307, record TEFFestimates on the order of 200 °C (Fig. 3, Table 1), close to what havebeen proposed based on other estimation methods (Alexander et al.,1989; Huss et al., 2006). Furthermore, Curve B trends through themiddle of various estimates for TEFF's in the mid type 3 chondrites (e.g.Kaba, Vigarano, Tieschitz, and ALH77003). Following curve B, oneobtains an estimate of the TEFF of Allende to be ∼550±25 °C, close to aupper temperature limit proposed byWeinbruch andMuller (1994) of527 °C and slightly less than the estimate of 600 °C favored by Huss

and Lewis (1994). The estimate of TEFF for Indarch following Curve B is948±50 °C. This value is on the high side of previous estimates, but itdoes lie below temperatures (between 1000 and 1100 °C) where thefirst observation of partial melting is known to occur (McCoy et al.,1999).

Not all of the samples in this suite of chondrites, however, are aswellbehaved as the examples chosen above. For example, various miner-alogical studies have concluded that that Bishunpur (variouslyconsidered to be a LL3.1 to 3.15) is only lightly thermally altered(Grossman and Brearley, 2005), possibly having been subjected to a TEFFin the range of 300 to 350 °C (Rambaldi andWasson,1981; Alexander etal., 1989). Based on exciton intensity, however, curve B predicts a muchhigher TEFF for Bishunpur on the order of 550±34 °C, very near that ofAllende (Fig. 3, Table 1) andwell above that of Krymka [TEFF=416±31 °C,and ranked as ≥3.2 relative to Bishunpur's 3.15 (Grossman and Brearley,2005)]. At this point, we can offer no explanation, beyond retrogrademetamorphism, for why the exciton intensity is so strong in Bishunpurrelative to what other mineralogical indicators would have predicted.

Finally, there is the special case of the heated CM Y86720.Following Curve B, the relatively weak exciton intensity exhibited byY86720 predicts a TEFF of only 265±42 °C. Based on mineralogical andtextural arguments, however, temperature estimates for Y86720 mayhave been as high as 850 °C (Akai, 1992). Using the exciton derivedkinetic expression, such a high temperature could only be accom-modated with a considerably shorter isothermal heating time. Forexample, if the temperature is set at 850 °C, only 34 ms is required(using the kinetic expression of curve B) to achieve an excitonintensity of 7.1; if a TEFF of 400 °C is assumed, then a heating time of∼6 hrs is obtained. In either case, an alternative heating mechanismfor Y86720 to internal heating by radiogenic decay is required toreconcile the current observation. It is possible that Y86720 and otheraltered CM's (Kitajima et al., 2002; Yabuta et al., 2005) haveexperienced very short duration heating events that resultedspecifically from impact (Rubin, 1995). If so, then significantdifferences between the exciton intensity andmineralogical indicatorsof TEFF may serve to distinguish impact heating from long terminternal heating. Furthermore, for such short heating times, the log-linear kinetics provide the potential for speedometry, if other robustmeasures of TEFF exist.

3.4. Relating excitonic thermometry to Raman spectroscopic features

The benefit of transmission XANES is that it is an extremely simple,and hence robust, spectroscopy that is not subject to instrumental orsample preparation artifacts that may affect surface and scatteringbased measurements. Thus, the methodology outlined here istransferable and applicable to any soft X-ray XANES instrumentation.The general utility of this thermometer is somewhat restricted,however, due to the requirement for access to specialized equipment,in particular synchrotron based Scanning Transmission X-ray micro-scopes (currently available only in a few countries). In general, onlylimited beam time is made available at these facilities due to oversubscription. Based on the discussions above, the correlation betweenthe paramagnetic shifts observed via solid state 13C NMR and excitonintensity (Fig. 2B) could allow one to get around this limited access tosynchrotron facilities by employing NMR as the primarymeasurementtool. Unfortunately, solid state NMR, while widely available, requireslarge quantities of pure IOM and long data acquisition times (oftenexceeding multiple weeks per sample, depending on the quantity ofIOM available). In the case of some particularly rare chondrites, it isdifficult to obtain sufficient quantities of IOM for µ-XANES, let alonesolid-state NMR. An alternative solution to this dilemma exists in theform of microRaman spectroscopy.

Recently, there has been considerable development in theapplication of micro-Raman analysis for the purpose of rankingchondritic IOM in terms of the extent of parent body processing. The

Fig. 7. The correlation between microRaman width parameter ΓD cm−1 and the excitonderived TEFF estimates. All of the petrologic type 1 and 2 chondrites are grouped withinthe black bordered box. The petrologic type 3–4 chondritic IOM exist in the gray region.The black curve corresponds to the best second order polynomial fit through the data.

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extensive studies of Quirico et al. (2003), Bonal et al. (2006, 2007) (insitu), and Busemann et al. (2007) (with isolated IOM) clearly revealthat the spectral characteristics afforded bymicroRaman spectroscopyfaithfully record the progressive molecular structural evolution ofchondritic organic matter that accompanies parent body metamorph-ism. The studies by Quirico et al. (2003) and Bonal et al. (2006, 2007)have been used to re-evaluate petrologic sub-type ranking, previouslyestablished via induced thermal luminescence studies and mostrecently established a correlation between micro-Raman parametersand other indicators of parent body processing, e.g. nanodiamondabundance and noble gas release profiles (Huss et al., 2003).Busemann et al. (2007) went a step further by correlating a selectgroup of their large sample set (many of them also analyzed here) toindependent estimates of parent body maximum temperature (PMT)in order to derive a Raman based thermometric expression using theline width of the so-called Raman D-band (ΓD cm−1).

It is interesting to see how well exciton intensity (hence excitonictemperature, Curve B) correlates with Busemann et al.'s (2007)measured ΓD and ΓG. The three parameters are compared in Fig. 6,where it is evident that both microRaman line width parametersexhibit negative, non-linear, correlations with exciton intensity, albeitwith considerable scatter. Busemann et al. (2007) used publishedestimates of temperature to constrain a second order polynomialequation in order to derive a thermometric expression,

PMT -Cð Þ ¼ 931−5:10ΓD þ 9:1� 10−3Γ2D ð3Þ

It should be noted that there is no physical basis to assume that thetemperature dependence of the D-band width should be described bya second order polynomial, Busemann et al. (2007) chose thisfunctional form only to capture the non-linear correlation withtemperature. In Eq. (3) the first coefficient has the units, °C, and thesecond and third coefficients have the units, cm °C and cm2 °C,respectively. Given the correlation between ΓD (and to a lesser extent,ΓG) and exciton intensity (Fig. 6), it appears worthwhile to determine asimilar microRaman based thermometric relationship using the currentexciton derived TEFF estimates (Curve B, Fig. 3, Table 1) recognizing thatthe evolution in Raman parameters, like the exciton intensity, reflect a

Fig. 6. The correlation between the micro-Raman band width parameters (ΓD and ΓG,cm−1) with normalized exciton intensity. The gray region corresponds to petrologictype 1 and 2 chondritic IOM. The dashed lines serve to highlight the correlationtrends only.

thermokinetic response of IOM to sustained heating. Figs. 7 and 8compare Busemann et al.'s (2007) tabulated values of ΓD and ΓG,respectively, and the estimated temperatures in Table 1 (Curve B,Fig. 3). It is obvious that use of the exciton derived temperature estimatesdoesnot improve the correlation statistics for theRamandata. Fromthesedata, a similar relationship between TEFF and ΓD is obtained, again fitting

Fig. 8. The correlation between microRaman width parameter ΓG cm−1 and theexciton derived TEFF estimates. All of the petrologic type 1 and 2 chondrites aregrouped within the black bordered box. The petrologic type 3–4 chondritic IOM existin the gray region. The black curve corresponds to the best second order polynomialfit through the data.

454 G.D. Cody et al. / Earth and Planetary Science Letters 272 (2008) 446–455

to a second order polynomial,

TEFF -Cð Þ ¼ 899:9−3:0ΓD þ 1:4� 10−3Γ2D R2 ¼ 0:84� � ð4Þ

Doing the same for the published (Busemann et al., 2007) values ofΓG (Fig. 8) we obtain,

TEFF -Cð Þ ¼ 1594:4−20:4ΓD þ 5:8� 10−2Γ2D R ¼ 0:75ð Þ ð5Þ

It is important to note that in the case of Eq. (3) (Busemann et al.,2007) the minimum in their second order polynomial fit occurs at avalue of ΓD=280 cm−1. This was recognized previously (Busemannet al., 2007) where it was noted that temperature estimates for thetype 1 & 2 IOM, as well as Semarkona (LL3.0), all of which havevalues of ΓD greater than 280 cm−1, cannot be predicted using Eq.(3). The minimum of the second order polynomial fit using theexciton TEFF scale (Curve B, Fig. 3, Table 1) has a minimum at aΓD=1071 cm−1, thus, in principal Eq. (4) is valid across the range ofreported ΓD values.

There is quite a bit of scatter in either fit, leading to some interestinginversions in apparent TEFF ranking from Eq. (5) relative to thosepredicted using Eq. (4) (Table 2). For example, Mokoia appears to have amuch higher TEFF based ΓG than ΓD; Isna, Allende, and ALH77003behave in an opposite sense (Table 2). Why such low covariance existsbetween ΓG than ΓD is not known, but such a high degree of scatterclearly degrades the precision for Ramanbased thermometry. Assumingthat the distribution of points off curves Eqs. (4) and (5) represents theuncertainty in TEFF using Raman, we find the error (2σ) associated withusing ΓD and ΓD is large, ±118 °C and ±120 °C, respectively. Even withsuch uncertainty one can still use IOM Raman data to distinguishbetween low, medium, and high petrologic subtypes; beyond this,however, using Raman data to establish a numerical sub-type rankingmay be dubious. Generally ΓD has been favored for the use ofestablishing metamorphic ranking among chondrites (Quirico et al.,2003; Bonal et al., 2006, 2007; Busemann et al., 2007); however, thegeneral trends predicted by Eqs. (4) and (5) are similar enough thateither equation could be used to make at least coarse, yet still valuable,predictions for TEFF.

Table 2Estimated temperatures based on micro-Raman peak width parameters

Meteorite name Abbrev. Exciton intensity TEFF ΓDa TEFF ΓDb TEFF ΓGb

EET92042 E92 0.0 – 112±118 185±120Bells Bel 0.8 – 94±118 195±120Murchison Mur 1.1 – 113±118 78±120Orguiel Org 1.2 – 113±118 159±120Tagish Lake Tag 1.8 – 101±118 184±120GRO95577 G95 3.0 – 129±118 162±120ALHA77307 A773 4.2 – 222±118 389±120Semarkona Sem 4.2 – 133±118 230±120Y86720 Y86 7.1 228 250±118 172±120Kaba Kab 13.8 314 414±118 568±120Vigarano Vig 17.1 370 481±118 475±120Krymka Kry 17.2 273 352±118 310±120Mokoia Mokoia 17.7 387 499±118 610±120ALHA77003 A770 17.9 497 601±118 401±120Leoville Leo 18.6 260 330±118 373±120MET0489 M048 18.7 NA NA NAKainsaz Kai 20.2 387 499±118 387±120Tieschitz Tie 21.3 368 478±118 340±120Chainpur Cha 22.7 323 426±118 401±120Bishunpur Bis 28.8 321 422±118 422±120Allende All 29.1 589 674±118 515±120Isna Isn 43.2 597 681±118 582±120Indarch Ind 68.0 643 714±118 846±120

aPMT estimates based on thermometric expression in Busemann et al. (2007), Eq. (3).bTEFF estimates based on thermometric expressions linked to exciton temperatures, Eqs.(4) and (5).

4. Conclusions

From this and other studies, it is clear thatmeteoritic IOM carries a selfconsistent record of parent body thermal evolution in its molecularstructure that may be measured via a number of different analyticalspectroscopic techniques (e.g., C-XANES,NMR, andRaman). If the thermo-kinetic models discussed above faithfully reproduce the time–tempera-ture aspects of organicmetamorphism, then the state of IOMmayprovidea independent predictor of parent body thermal alteration. A quantitativemeasure of the range of temperatures associated with various chondriticmeteorites is the most important constraint for models of planetesimalthermal evolution (Grimm and McSween, 1989; Young et al., 1999, 2003)and will resolve any remaining debate regarding hot or cold planetaryaccretionary models (Hutchison, 1996; Rubin and Brearley, 1996).

One of the outstanding aspects of the petrologic type 3 chondrites isthe enormous range inparent body thermalmaxima. For example,withinthe samples studied here, the CO group spans the greatest range inpetrologic type, from 3.0 to 3.8. The estimated TEFF data based on excitonintensity presented in Table 1 and Fig. 3 indicate a range of temperatures,ΔT, for the CO's on the order 500 °C. This range is larger than that derivedfor the CO's by other methods [e.g., ΔT=200 °C (Dodd, 1981), 150 °C(Brearley,1990), and 90–100 °C (Jones and Rubie,1991)], but is consistentwith the range favored in a recent compilation (Huss et al., 2006). Asimilarly large range in temperature is evident for theordinarychondritesmoving from Semarkona up to the 3.8+ range (Wlotzka, 2005).

Spectroscopic studies of extraterrestrial organic matter may providean additional benefit beyond providing independent estimates of parentbody thermal processing. The non-reversible nature of organic structuralevolutionwith thermal stressmeans that themolecular structure of IOMwill not be perturbed by secondary alteration that can complicatemineralogic and compositional estimates of metamorphic grade. Themolecular structure of IOM may provide a less ambiguous measure ofparent body thermal processing in the case of chondrites that may havesuffered secondary alteration post dating primary metamorphism, e.g.,Allende [see, for example, (Brearley, 2006) and references therein).

Finally, a very significant result of the present study is theunambiguous demonstration that the constituent primitive organicmatter in type 1 and 2 chondrites may be thermally transformed into astructural state that is nearly identical to that observed in thermallymetamorphosed type 3 chondrites. As it is generally agreed that type 1and 2 chondrites never experienced temperatures much in excess of100 °C (Zolensky et al., 1997); these results require that, in general,meteoritic organicmatterwas synthesized cold and, at least in the case ofthe type 1 and 2 chondrite parent bodies, accreted cold.

Acknowledgments

The authors gratefully acknowledge financial support through theNASA Astrobiology Institute and Origins of the Solar System Program.All NMR experiments were performed at the W. M. Keck Solid StateNMR facility at the Geophysical Laboratory that received financialsupport through the W. M. Keck Foundation, the National ScienceFoundation, and the Carnegie Institution of Washington. TheAdvanced Light Source is a DOE supported facility. We are gratefulfor thoughtful reviews provided by Adrian Brearley and MichaelZolensky that helped improve the quality of this manuscript.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.epsl.2008.05.008.

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