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A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites
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Geochimica et Cosmochimica Acta 75 (2011) 3088–3102 www.elsevier.com/locate/gca
A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites Eric Quirico a,⇑, Miche`le Bourot-denise b, Christophe Robin a,1, Gilles Montagnac c, Pierre Beck a b
a Institut de Plane´tologie et d’Astrophysique de Grenoble, UJF-Grenoble 1/CNRS-INSU, UMR 5274, Grenoble F-38041, France Muse´um National d’Histoire Naturelle, Laboratoire de Mine´ralogie et Cosmochimie du Muse´um, CP 52, 57 rue Cuvier, 75231 Paris Cedex 05, France c Laboratoire de ge´ologie de Lyon, CNRS, Ecole Normale Supe´rieure de Lyon, 46 alle´e d’Italie – BP 7000, 69342 Lyon Cedex 07, France
Received 15 June 2010; accepted in revised form 1 March 2011; available online 10 March 2011
Abstract The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1–3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6–3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes. Crown copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
1. INTRODUCTION ⇑ Corresponding author. Tel.: +33 4 76 51 41 56; fax: +33 4 76 51
41 46. E-mail address: [email protected] (E. Quirico). 1 Present address: Institut des Sciences de la Terre, MR 5275 UJF/CNRS/UdS/IRD/LCPC, 1381 rue de la Piscine – BP 5, 38041 Grenoble Cedex 9, France.
The mineralogy of EH3 and EL3 chondrites reflects the highly reducing conditions that were present in the environment in which they were formed. These chondrites are mostly composed of enstatite, minor olivines, glasses and feldspars together with abundant opaque minerals in the form of metals, sulfides and phosphides. The iron budget
0016-7037/$ - see front matter Crown copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.03.009
Thermal metamorphism in EH3 and EL3 chondrites
is dominated by opaque mineralogy and the corresponding zero valence testifies to the reducing conditions that were present in the region in which they were formed within the solar nebula (Brearley and Jones, 1998, and references therein). These conditions also prevailed on the parent body, ruling out the presence of extensive fluid circulation as observed in carbonaceous chondrites. EH3 and EL3 chondrites experienced post-accretion processes on their parent body, such as thermal and shock metamorphism. Unraveling the effects of these two processes appears to be a complex problem, as the former possibly erase the petrographical and mineralogical signatures of the latter (Rubin et al., 1996). The nature and extent of thermal metamorphism have not yet been clarified. The standard classifications and tools (e.g., induced thermoluminescence) used for classifying ordinary and carbonaceous chondrites are not suitable for EH3s and EL3s. So far few studies have been devoted to thermal metamorphism in type 3 enstatite chondrites. Zhang et al. (1995) proposed a classification based upon the composition of opaque minerals and the texture of the mineralogic assemblage, to classify objects over a large range of metamorphism (types 3–6). It is however unclear to what extent this classification provides details on the nature and extent of the metamorphic history in the less metamorphosed (and therefore best preserved) EH3s and EL3s. Zhang and Sears (1996) determined closure temperatures from opaque mineral assemblages and proposed a three-stage thermal history on the parent body. The apparent complexity of the post-accretion history has also been suggested by the numerous silica polymorphs which suggest the presence of wide variations of post-accretion conditions (Kimura et al., 2005). Type 3 enstatite chondrites contain polyaromatic Organic Matter (OM), presolar grains and nobles gases (Anders and Zinner, 1993; Huss and Lewis, 1994a,b; Alexander et al., 1998; Lin et al., 2002; Alexander et al., 2007). In this respect, they share common characteristics with ordinary and carbonaceous chondrites with which they can be compared. The extent of thermal metamorphism can be investigated by the study of the structure of OM by Raman micro-spectroscopy (Quirico et al., 2003; Bonal et al., 2006, 2007; Busemann et al., 2007; Quirico et al., 2009). This approach can be used to determine the petrologic types that are interpreted in terms of peak metamorphism temperature and that have the same physical interpretation whatever the chemical class of chondrites. This paper presents a study of the structure of OM in the matrix from a series of EH3 and EL3 chondrites that experienced mild or moderate shock metamorphism: Sahara 97096, Allan Hills (ALH) 84206, ALH 84170, Parsa, ALH 85119, Qingzhen, MacAlpine Hills (MAC) 88136, MAC 88184 (Table 1). The degree of structural order of OM was characterized by Raman micro-spectroscopy and then compared with the available mineralogical tracers of thermal metamorphism (matrix texture, volatile abundance and composition of opaque minerals). We show that the maturation grade of OM is a sensitive tracer of the metamorphism grade and that EH3 and EL3 chondrites have experienced thermal metamorphism processes similar to those reported in UOCs, CVs and COs. New petrologic
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types are provided and we discuss the post-accretion history. 2. EXPERIMENTAL 2.1. Samples Thin or polished sections of EH3, EL3, CO, CV and ordinary chondrites were provided by the Meteorite Working Group (NASA Johnson Space Center – Houston, USA) and the Museum National d’Histoire Naturelle (Paris, France) (Table 1). Preliminary measurements on the enstatite chondrites were tentatively performed on chips of raw materials. However, due to the low matrix modal abundance, no matrix could be isolated, as opposed to the case of Bonal et al. (2006). OM isolation by HF/HCl acid attack was not performed because this method tends to mix OM with graphite inclusions hosted by kamacite. Therefore, all analyses were performed in situ on thin polished sections. Matrix zones were previously located using an optical microscope in reflected light and by a Field Emission Gun Scanning Electron Microscopy (FEG-SEM). UOCs and CV and CO chondrites were used for internal calibration and petrologic type crossed assignment (see below). 2.2. Instrumentation Raman experiments were performed at Laboratoire de Sciences de la Terre (ENS-Lyon, France) with a LABRAM spectrometer (Horiba Jobin-Yvon). An excitation wavelength of 514.5 nm from a Spectra Physics argon ion laser was used. The laser beam was focused by a microscope equipped with a 50 long-distance objective, leading to a spot diameter of 2–3 lm. The spectrometer was equipped with a 600 gr/mm grating. Experiments were performed in an inert atmosphere (argon), the power at the sample surface was 500 ± 20 lW and the acquisition time was 3 30 s. Such conditions avoided thermal damage. A visual check for sample deterioration was carried out after each acquisition and measurement reproducibility was tested at the same location. Spectra were acquired in the spectral region 700–2000 cm1. Polishing artefacts have been reported in Raman measurements of graphitic compounds (Pasteris, 1989) but are not of concern here given the low degree of structural order of the OM studied. Backscattered electron and secondary electrons images of sections were acquired using a JEOL JSM 840 A SEM at University Paris VI and ZEISS Ultra55 FEG-SEM at CMTC INP Grenoble, operating at 10 kV. The composition of metals and sulfides was determined with a SX100 electron microprobe (EPMA) (University Paris VI) using a 15 kV voltage, a 10 nA current and a PAP algorithm. 3. RAMAN SPECTRA ANALYSIS The typical Raman spectrum of polyaromatic organic matter exhibits several bands, the most intense being the first order band peaking at 1600 cm1 (G-band) and 1350 cm1 (D-band). The G-band (G for graphite) is assigned to the E2g2 vibrational mode of the aromatic plane
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Table 1 Chondrites studied and Raman spectral parameters. Class
Shock stage
Weathering
FWHM-D (cm1)
xD (cm1)
FWHM-G (cm1)
xG (cm1)
ID/IG
Numb. meas.
PTc
Sahara 97096 ALH 84206
EH EH
S3-4b S3a
W1 A/B
1358.8 ± 1.2 1353.9 ± 1.8
104 ± 5.2 88.1 ± 2.8
1599.7 ± 1.3 1600.8 ± 0.9
3.1–3.4 3.1–3.4
EH EH EL EH EL EL LL LL LL H CO CO CO CV CV CV CV
S4a S3a S3aS3-4b S3a S3a
B Fall Be Fall A C Fall Fall Fall Fall Ae Fall Fall Find Find Fall Fall
1351.4 ± 0.6 1350.7 ± 1.3 1352.5 ± 3.8 1351.4 ± 3 1352.4 ± 1.7 1351.2 ± 1.8 1358.0 ± 4.4 1353.2 ± 2.2 1350.8 ± 1.2 1352 ± 1.2 1350.5 ± 2.7 1348.5 ± 0.7 1350.1 ± 1.0 1351.5 ± 3.5 1349.7 ± 1.7 1349.5 ± 1.5 1351.3 ± 1.5
85.9 ± 2.8 80.6 ± 4.7 73.4 ± 8.8 85.8 ± 6.9 84.1 ± 12 79.0 ± 4.7 70.2 ± 3.4 73.2 ± 4.7 73.0 ± 2.2 77.2 ± 3.6 66.7 ± 4.4 75.2 ± 5 73.0 ± 4.2 76.5 ± 3.6 79.6 ± 3 71.8 ± 4.3 66.5 ± 8.2
1602.2 ± 0.9 1601.6 ± 1.4 1599.7 ± 2.8 1600.8 ± 1.3 1599.1 ± 3.9 1599.4 ± 2.0 1603.6 ± 1.9 1606.6 ± 2.3 1604.6 ± 0.7 1604.2 ± 2.0 1603.5 ± 1.1 1602.1 ± 1.1 1602.6 ± 0.9 1602.3 ± 0.4 1601.3 ± 0.8 1603.5 ± 1.3 1600.7 ± 1.6
1.04 ± 0.02 1.07 ± 0.04 1.04 ± 0.03 1.09 ± 0.01 1.09 ± 0.03 1.10 ± 0.08 1.07 ± 0.07 1.20 ± 0.06 1.19 ± 0.12 1.28 ± 0.05 0.795 ± 0.06 0.93 ± 0.12 0.90 ± 0.03 1.11 ± 0.05 0.75 ± 0.03 1.12 ± 0.02 1.33 ± 0.05 0.95 ± 0.02 1.00 ± 0.01 1.12 ± 0.23 1.48 ± 0.11
12 28
ALH 84170 Parsa ALH 85119 Qingzhen MAC 88136 MAC 88184 Semarkona Bishunpur Krymka Tieschitz ALH 77307 Kainsaz Felix Leoville Efremovka Vigarano Allende
188.8 ± 13.5 168.9 ± 19.5 Z1: 186.2 ± 8.3 Z2: 148.8 ± 6.1 140 ± 11 138.5 ± 15 119.3 ± 19.8 116.7 ± 25.2 97.4 ± 16.7 94.2 ± 11.1 255.4 ± 19.7 177.3 ± 29.7 172 ± 5.3 123 ± 6.8 218.4 ± 10.3 136 ± 21.7 95.8 ± 9.3 162.4 ± 5.1 154.5 ± 6.4 109 ± 13 72.5 ± 13.2
40 12 7 31 25 28 52 51 33 11 48 6 23 22 43 60 32
3.5 3.5 3.6 3.6–3.7 3.6–3.7 3.6–3.7 3.0 3.1 3.1 3.6 3.03 3.6 3.7 3.1–3.4 3.1–3.4 3.1–3.4 3.7–3.8
a b c
Shock stage assigned by Rubin et al. (1996). Shock stage assigned by this study. Petrologic type: this study for EH3 and EL3, Bonal et al. (2006, 2007) for CO and CV carbonaceous chondrites. The uncertainties on the Raman spectral parameters are 1 standard deviation.
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Chondrites
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and is present in all OM whatever the degree of structural order. On the other hand, the D-band (D for defects) is not present in perfectly stacked graphite and is induced by structural defects in the material (e.g., Tuinstra and Koenig, 1970). The only structural parameter which has been linked to the Dand G-band spectral parameters is the lateral size of the basic structural units of polyaromatic matter (e.g., Tuinstra and Koenig, 1970; Wopenka and Pasteris, 1993). Unfortunately, the available relationship is highly inaccurate for poorly-ordered OM and in particular cannot be used to study chondritic OM. Nevertheless, some spectral parameters are very sensitive to the degree of structural order of poorly ordered materials (referred to below as maturity tracers), although they do not provide quantitative structural information. Studies on series of coals and other natural sedimentary organic matter have provided different sets of parameters that correlate well with the maturation grade as quantified by vitrinite reflectance (Wopenka and Pasteris, 1993; Quirico et al., 2003, 2005). In the case of chondrites, these parameters are not applicable (Quirico et al., 2009). The issue of the most relevant tracers of chondritic OM maturity will be addressed below. The analysis of the Raman spectra was focused on the first-order G- and D-bands. The second order bands, mostly sensitive to the last stages of the graphitization process, were not considered here. The fluorescence background was first subtracted assuming a linear baseline between 700 and 2000 cm1. Two numerical treatments were applied. The first fit the reduced spectra using a Lorentzian profile for the D band and a Breit–Wigner– Fano profile for the G-band. The second was a Principle Component Analysis (PCA) performed on the whole set of spectra. The input variables were the intensities of the spectra at each wavelength in the range of 800– 1990 cm1. The efficiency of the PCA was evaluated as the number of principal components required to account for >95% of the variance. The LBWF fitting and PCA were performed with IGOR software (Wavemetrics, Inc.). 4. RESULTS 4.1. Texture of the matrix area In unequilibrated ordinary chondrites, matrix presents a fine-grained optically opaque component and a coarsegrained optically transparent component (Huss et al., 1981). The second was assumed to be the recrystallization product of the former due to mild thermal metamorphism. The matrix material is also characterized by its petrological location in interchondrule matrix, chondrule rims or clasts (i.e. areas with well defined borders). Matrix is scarce in enstatite chondrites and few studies have focused on its description. Rubin et al. (2009) described patches of matrix dubbed “clastic matrix” because they contains coarse angular fragments, but strictly speaking they are not clasts as described before. They distinguished two components containing coarse silicates and opaque mineral grains, respectively, and a third component described as an “inferred nebular component” accordingly to its finegrained texture. This description may appear ambiguous,
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given that there is no evidence that the coarse-grained component was not present in the solar nebula nor that the finegrained component was a nebular condensation product. In this study we have followed a three-step procedure to locate carbon-rich matrix areas: (1) fine-grained areas were first located by optical microscopy; (2) the presence of a fine-grained material was further confirmed by SEM-FEG (typical grain size <10 lm) and (3) polyaromatic carbonaceous material (OM) was detected by Raman microspectroscopy. This procedure unambiguously identifies a matrix material that presents similarities with matrix material observed in ordinary and carbonaceous chondrites, i.e. a submicrometer-scaled mixture of silicate grains, organic matter, volatile species (Q phase) and presolar grains (Huss and Lewis, 1994a,b; Lin et al., 2002). This criterion, based on carbon detection, is definitely less ambiguous than mineralogy and texture alone. This also ensures overall consistency with earlier studies focused on ordinary and carbonaceous chondrites and involving the selection of carbon-rich matrix fragments (see Quirico et al., 2003; Bonal et al., 2006, 2007). FEG-SEM images reveal that carbon-rich matrix areas should be better described by an interchondrule matrix. No clasts well separated from the rest of the petrologic assemblage were observed. Coarse-grained and fine-grained components appear intimately mixed in matrix areas, but the relative abundance of the former varies among the chondrites studied (Figs. 1 and 2, Table 2). Quantifying the abundance of the fine-grained versus coarse-grained components of the matrix from the SEM images appeared very tricky. It would have required image analysis software and a dedicated procedure that were not available. Therefore, naked eye analyses were performed independently by two of the authors (MBD and EQ) on two series of SEM images measured with two SEMs (one at University Paris VI, France and the other at CMTC – INP Grenoble, France). This led to the following consistent description. Matrix zones in Sahara 97096 and ALH 84206 present a fine-grained component that appears less fine-grained than in the less equilibrated ordinary chondrites (e.g., Bishunpur LL3.1 and Krymka L3.1: Huss et al., 1981; Quirico et al., 2003). A similar matrix texture was observed in ALH 84170, but the fine-grained regions appeared less compacted and more porous. More compacted and more coarse-grained matrix areas were observed in Parsa. The matrix of ALH 85119 was found to be very heterogeneous. The matrix material in some areas was mostly coarsegrained with irregular and angular coarse fragments of silicates and opaque minerals. Other matrix areas exhibit a very compact material with rounded grains of sulfides and metal that testifies to the impact of shock processes. ALH 85119 is the only chondrite of the series that presents a heavily shock processed matrix. Lastly, fine-grained matrix was found to be scarce in Qingzhen and was mostly unobserved in both MAC 88136 and MAC 88184. 4.2. Maturation grade of the organic matter The first-order G and D bands of the OM were detected in most of the matrix areas (Table 1, Fig. 3). These measurements however suffered from an intense fluorescence
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Fig. 1. Backscattered electron images of matrix areas in the EH3 and EL3 chondrites in this study. (a) Sahara 97096. The yellow circles indicate the location of Raman microscopic measurements in the fine-grained component of the matrix. OM was detected for each. (b) Finegrained and coarse-grained components in the matrix of ALH 84170 and (c and d) Matrix in ALH 84206. Cg: coarse grain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Backscattered electron images of matrix areas in the EH3 and EL3 chondrites in this study. (a) ALH 85119. The yellow circles indicate the location of Raman microscopic measurements. Raman spectra of OM appear to be similar in the shock melt vein and the weakly shocked area (top-left), suggesting the lack of effects of shocks on the structure of OM in this chondrite and (b) MAC 88184. The matrix appears mostly as a coarse-grained component. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 Matrix texture of the EH3 and EL3 chondrites.
Sahara 97096 ALH 84206 ALH 84170 Parsa ALH 85119 Qingzhen MAC 88136 MAC 88184 a b
Class
Shock stage
Texture
EH EH EH EH EL EH EL EL
S3-4b S3a S4a S3a S3a S3a S3a
Coarse and fine-grained components Coarse and fine-grained components Coarse and fine-grained components. More porous than Sahara 97096 and Allan Hills 84206 Less abundant fine-grained component Coarse-grained or very compact Coarse-grained component. Much less fine-grained component Rare fine-grained component Rare fine-grained component
Shock stage assigned by Rubin et al. (1996). Shock stage assigned by this study.
background in several spectra. Chondritic OM in type 3 chondrites does not exhibit a significant fluorescence be-
cause of the high degree of condensation of the polyaromatic units that enhance fluoresced photon absorption.
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Fig. 3. Typical Raman spectra of matrix areas from the 8 enstatite chondrites. All display the first-order G and D bands of polyaromatic organic matter, which point to different degrees of structural order through their spectral variations. The peak intensity of the G band has been normalized to 1. Spectra are vertically off-shifted for the sake of clarity.
Therefore, the embedding resin of the sections appears to be the carrier of this fluorescence background. Practically speaking, this led to uncertainties in the derivation of the spectral characteristics of the G and D bands during the fit-
ting process and spectra with an intense background were rejected. The number of spectra retained for each chondrite appears finally to vary from one object to another, ranging between 6 and 52. The first numerical treatment applied to
Fig. 4. The spectral tracer FWHM-D plotted against ID/IG. The curve points out a trend of increasing degree of order of OM (gray arrow). EH3 and EL3 data are compared with those of UOCs and CO and CV chondrites to assign petrologic types. Sahara 97096 and ALH84206 plot slightly outside the trend.
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the spectra was the LBWF fitting procedure described in Section 3. It provided five spectral parameters: the peak position (xG) and width at half maximum (FWHM-G) of the BWF profile fitting the G band, the peak position (xD) and width at half maximum (FWHM-D) of the Lorentzian profile fitting the D band and the ratio of the peak intensities of the profiles fitting the G and D bands (ID/IG). The results are presented in Table 1 and in Figs. 4 and 5. The second numerical treatment that was applied was a principal component analysis (PCA) as described in Section 3. About 94% of the variance of the whole set of data was accounted for by six principal components. We first address the issue of the spectral tracers of the maturity of OM. FWHM-D and ID/IG have been found to be tracers of the degree of structural order of OM (Quirico et al. 2003; Bonal et al. 2006, 2007; Busemann et al. 2007; Quirico et al., 2009). The measurements in this study confirm this result: FWHM-D and ID/IG define a metamorphic pathway (Fig. 4) and both correlate with the reduced spectra projected along the axis of the principal components (Fig. 6). The width at half maximum of the G band (FWHM-G) has been proposed as a tracer of OM maturity (Busemann et al., 2007). However, this result is not supported by our data (Fig. 5) nor by the studies of Quirico et al. (2003, 2009). In contrast, we observe that FWHM-G is on the average larger in EH3 and EL3 chondrites than in UOCs and CO and CV carbonaceous chondrites (Fig. 5). Within the enstatite chondrites group, Sahara 97096 even exhibits a larger value than in any other chondrites (Fig. 5). Different explanations should be examined. Alexander et al. (2007) have reported that weathering modifies the composition and likely the structure of OM. The low weathering stage (W1) of Sahara 97096 is however not consistent with this explanation. The shock stage of Sahara 97096 (S3-4) leads us to consider a possible shock effect on the structure of OM. To test this possibility, we measured the degree of structural order of OM in shocked melt veins and a weakly shocked matrix area in ALH 85119
(Fig. 2). We observed no difference in the Raman data. It is however difficult to draw a firm conclusion given that shocks have certainly competed with long duration thermal metamorphism that tends to erase their impact. Shock wave effects finally cannot be discarded. A third explanation would consider that the variations of FWHM-G among and within the chondrite groups are accounted for by structural variations of the precursors accreted. Such variations have been suggested in UOCs and CO and CV carbonaceous chondrites (Quirico et al., 2009). Hence, a lack of correlation between the H/C ratio and FWHM-D is observed in the case of chondrites, while both these parameters are linearly correlated in coal samples that originate from more similar precursors. Note also that unlike chondrites, FWHM-G correlates with maturity in coals (Kelemen and Fang, 2001; Quirico et al., 2005). Therefore, it is likely that OM precursors accreted by EH3s and EL3s are not identical to those accreted by other groups. However, the organic precursors are similar enough to ensure that OM maturity is controlled by the degree of thermal metamorphism. A second issue concerns measurement reproducibility and accuracy. The number of measurements for each chondrite ranges between 6 and 52. This is insufficient to determine the statistical distribution of the FWHM-D and ID/IG parameters derived from the fitting procedures. Therefore, the experimental average may not tend to the real average of the statistical distribution when increasing the number of measurements. In the case of Vigarano, the experimental average tends to a finite value p and ffiffiffiffi the standard deviation of the mean r decreases (r = x= N where x = FWHM-D or ID/IG and N is the number of measurements) (Fig. 7). In contrast, this is not the case for Semarkona and the data here shows a much larger structural heterogeneity. The situation obviously gets worse as the number of measurements decreases. Therefore, for each chondrite we have provided the average and standard deviation of the experimental data.
Fig. 5. The parameter FWHM-G plotted against ID/IG. The lack of correlation between the parameters reveals that FWHM-G does not trace the thermal metamorphism grade. FWHM-G in EH3s and EL3s is on the average larger than in UOCs and CO and CV carbonaceous chondrites. Sahara 97096 is peculiar, with the highest FWHM-G.
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Fig. 6. The parameters FWHM-G and ID/IG plotted against the coordinate of the reduced spectra along the second principal component.
To ensure that the measurements are unbiased, the data of this study have been compared with earlier published data. We observe that the FWHM-D and ID/IG parameters linearly correlate with those determined by Bonal et al. (2006, 2007) from raw matrix grains under atmospheric conditions with a longer acquisition time and greater energy deposited onto the sample (Fig. 8). The data by Bonal et al. (2006, 2007) also show a very good linear correlation with those of Busemann et al. (2007) (Fig. 8). The data of Busemann et al. (2007) were acquired from Insoluble Organic Matter (IOM) extracts with a different Raman microprobe and different experimental conditions. The number of measurements was very large (from several 100 to several 1000 spectra for each chondrite), ensuring a reliable estimation of the mean of the spectral parameters. The slope of the linear fit is not equal to 1, as a consequence of different experimental conditions resulting in different sample temperatures and also possibly a different fitting technique
(two Lorentzian profiles). We note however that two objects plot far from the regression line: Bishunpur (LL/3.1) and Mokoia (CV3.6). In the case of Bishunpur, our data and the data of Bonal et al. (2006) are consistent with each other and with its petrologic type 3.1. In the case of Mokoia, we note that the petrologic type derived from the Raman data is consistent with the abundance of the P3 noble gas. We suspect here an artifact induced by IOM isolation (see discussion section). Finally, the FWHM-D and ID/IG parameters reveal a maturation trend of OM in the EH3 and EL3 with three distinct groups (Fig. 4): (1) Lowest OM maturity: Sahara 97096 and ALH 84206; (2) Intermediate OM maturity: ALH84170, Parsa, ALH 85119; (3) Highest OM maturity: Qingzhen, MAC 88136, MAC 88184.
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Fig. 7. The average of FWHM-D plotted against the number of measurements. Bars are the standard deviation of the mean. For Vigarano, the average tends towards a constant value. In the case of Semarkona, the average does not stabilize and the standard deviation of the mean does not decrease.
Chondrites from groups 2 to 3 plot near ordinary and carbonaceous CV and CO chondrites, but Sahara 97096 and ALH 84206 (group 1) plot slightly apart from this trend. ALH 84206 presents two groups of maturities, one plotting near SAH97096 and the other within group 2 (see inset, Fig. 4). Both Sahara 97096 and ALH 84206 spectra are not disturbed by the fluorescence from the embedding resin, therefore the observed variability is not due to incorrect background subtraction. Two explanations are possible in the case of ALH 84206: (1) an effect of brecciation (shocks?), with matrix areas corresponding to different maturation grades; (2) an effect of the low number of measurements that may be not large enough to account for the whole structural heterogeneity. Note that Sahara 97096 has a peculiar position in the FWHM-D vs. ID/IG diagram and the largest FWHM-G value with respect to all other chondrites (see discussion before). 4.3. Composition of opaque minerals In enstatite chondrites, texture and mineral chemistry seem to indicate different thermal histories or a combination of shock and thermal processes. Zhang and Sears (1996) have proposed a classification that uses both mineralogy (rated by the letters a, b, c and d) and texture (rated by a number ranging between 3 and 6). To our knowledge, this is the only systematic classification proposed to date for enstatite chondrites. Its suitability for samples of low metamorphic grades (type 3) has however not been clearly shown. For the EH3 and EL3 series, we have thus determined the composition of Ti and Cr in troilite and Si and
Ni in kamacite (Fig. 9, Table 3). The compositions for ALH 85119 and MAC 88184 are in agreement with those obtained by Zhang et al. (1995). These data show that most objects of this study belong to the so-called type 3a. The EH and EL subclasses are readily distinguished and the composition of opaque minerals does not correlate with the maturity of OM. The standard deviation around the mean of the compositions of Sahara 97096 is larger than for other objects, suggesting it could be better described by an ab type. Once again, we observe that Sahara 97096 is peculiar with respect to other chondrites.
4.4. Shock metamorphism Among the EH3s and EL3s considered in this manuscript, Sahara 97096 and MAC 88184 were not classified by Rubin et al. (1996). Polished sections were analyzed to characterize the extent of shock metamorphism in them. The use of polished sections precluded the direct application of the shock classification of Rubin et al. (1996) which is mostly based on shock features that are only observable for thin sections (i.e. mosaicism, undulose extinctions, etc.). Instead, we used the presence or absence of shock melt veins and melt pockets, as well as the presence of local sulfur phase redistribution. In the case of MAC88184, no shock veins were observed, indicating a shock degree
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Fig. 8. Top: the width at half maximum of the D-band measured in the unequilibrated ordinary chondrites and CO and CV chondrites in this study, plotted against the values obtained by Bonal et al. (2006, 2007). Bottom: the width at half maximum of the D-band measured in unequilibrated ordinary chondrites and CO and CV chondrites by Bonal et al. (2006, 2007), plotted against the values obtained by Busemann et al. (2007). The correlation between the data sets is excellent, showing that internal structural heterogeneity is much lower than the variations between the chondrites. The slight misfit for Bishunpur and Mokoia (bottom) is likely an artifact due to IOM preparation.
5. DISCUSSION 5.1. Organic matter and post-accretion processes As mentioned above, the degree of structural order of OM in the EH3 and EL3 series shows an increasing trend that can be represented by three groups: (1) lowest OM maturity: SAH97096 and ALH84206; (2) intermediate OM maturity: ALH84170, Parsa, ALH85119; (3) Highest OM maturity: Qingzhen, MAC 88136, MAC 88184. In UOCs
and CO and CV carbonaceous chondrites, the degree of structural order of OM is related to the degree of thermal metamorphism and controlled by the temperature of peak metamorphism. This interpretation has been validated by the comparison with independent metamorphism tracers like Fe-zoning at the border of type II chondrules (Bonal et al., 2006, 2007), opaque mineral textures (Quirico et al., 2003) and volatile abundances. A comparative study with coals has further demonstrated that the OM precursors accreted by the parent bodies were not strictly identical, but
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Fig. 9. Composition of metal and troilite plotted according to the study of Zhang et al. (1995) (see text for details). The studied enstatite chondrites all belong to the textural subtypea, except for Sahara 97096. EH3s can be distinguished from EL3s.
Table 3 Composition of opaque minerals.
Sahara 97096 Parsa ALH 85119 Qingzhen MAC 88184
Si in kamacite (wt.%)
Ni in kamacite (wt.%)
Ti in troilite (wt.%)
Cr in troilite (wt.%)
2.780.09
3.51 ± 0.49
0.23 ± 0.05
2.60 ± 1.23
2.56 ± 0.50 0.67 ± 0.13
3.81 ± 1.25 5.50 ± 0.40
0.18 ± 0.09 0.40 ± 0.12
0.72 ± 0.15 0.85 ± 0.84
2.85 ± 0.17 0.60 ± 0.15
3.35 ± 0.73 5.86 ± 0.60
0.27 ± 0.04 0.37 ± 0.08
1.03 ± 0.22 0.73 ± 0.56
Bars are one standard deviation.
similar enough to ensure that the maturity trend was interpretable as a thermal metamorphism tracer (Quirico et al., 2009). This interpretation can be extended to EH3 and EL3 chondrites if their parent bodies accreted similar OM precursors. This condition can be tested using the Raman data because the shapes of the first order bands of OM are controlled by both the temperature of peak metamorphism and the nature of the precursor. Both PCA and LBWF fitting show that the Raman data of EH3 and EL3 chondrites point to precursors similar to those in UOCs and CO and CV carbonaceous chondrites (Figs. 4 and 6). In both cases, Sahara 97096 plots slightly apart from the general trends. We think that the most suitable explanation is related to
Thermal metamorphism in EH3 and EL3 chondrites
shocks but, as already mentioned, the effects of shocks on OM are difficult to unravel. It seems also that the different redox conditions in the solar nebula, which controlled the nature of the mineralogical assemblage, had no significant impact on the OM formation processes, or that this OM was formed elsewhere and was later transported to the accretion region of chondrite parent bodies. The physical and chemical conditions on the parent body had likely no significant influence on the structural evolution of OM. This first-order description supports the use of OM maturity as a metamorphic indicator. In this respect, petrologic types can be calculated using the measurements performed on the ordinary, CV and CO chondrites. Sahara 97096 and ALH 84206 are assigned to petrologic types 3.1–3.4. This large estimation reflects the uncertainty in relation to the peculiar nature of the chondrites, possibly linked to the impact of shock processes. ALH 84170 and Parsa are assigned to petrologic type 3.5, ALH 851119 to petrologic type 3.7 and Qingzhen, MAC 88136 and MAC 88184 to petrologic types 3.6–3.7 (Table 1). As reported in the previous section, the assignment of Qingzhen is supported by the abundance of the P3 component of the noble gases, which also indicates petrologic type 3.6 (Huss and Lewis, 1994b). Though more qualitative (rated by naked eye observations), the abundance of the fine-grained matrix can be compared with the derived petrologic types. As reported in Section 4, the texture of the matrix in unequilibrated ordinary chondrites is related to the degree of recrystallization of an initial fine-grained material and provides a rough estimation of the extent of thermal metamorphism. We have tentatively applied the same criterion to the series of EH3 and EL3 chondrites and obtained the sequence: Sahara 97096 ALH 84206 ALH 84170 < Parsa ALH 85119 < Qingzhen < MAC 88136 MAC 88184 (Figs. 1 and 2). This sequence appears fairly consistent with the derived petrologic types. We also note that the Cr-composition of olivine grains in Sahara 97096, Parsa and Qingzhen is consistent with a higher metamorphism grade in Qingzhen (Weisberg et al., 2005; Bendersky et al., 2007). This observation is consistent with the derived petrologic type in the case of Qingzhen, but Parsa and Sahara 97096 cannot be distinguished. The low petrologic type of Sahara 97096 also appears to be consistent with the mineralogic observations of Weisberg and Prinz (1998). 5.2. Can we define a peak temperature? The determination of a peak metamorphism temperature from OM maturity has been debated for years. Carbon thermometers for chondrites have been proposed in the literature (Rietmeijer and MacKinnon, 1985; Bonal et al., 2007; Busemann et al., 2007; Cody et al., 2008). These studies have led to very different estimations. Rietmeijer and MacKinnon (1985) and Bonal et al. (2007) both used terrestrial carbonaceous rocks to calibrate cosmothermometers. Bonal et al. (2007) restricted the application of this method to the most metamorphosed type 3 chondrites, because the OM structure records the structure and likely composition of the precursor. They considered that above petrologic
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type 3.7, the precursor memory vanishes. They obtained a peak metamorphism temperature of 300 °C for Allende, similar to the estimation by Rietmeijer and MacKinnon (1985). This temperature appears too small with respect to those obtained by mineralogical thermometers, even if the latter are not very accurate and span large ranges. We have recently estimated a more realistic temperature ranging between 550 and 600 °C by modeling Fe-zoning at the border of type II chondrules (manuscript in preparation). Therefore, we think that a carbon cosmothermometer dedicated to chondrites cannot be calibrated using carbonaceous terrestrial rocks. The approach by Busemann et al. (2007) is more realistic. They compiled temperatures determined from chondrites (Huss et al., 2006) to calibrate a Raman carbon cosmothermometer. However, there are serious flaws in the temperature determined in the literature by mineralogical thermometers. The derived temperatures span a broad range of values depending on the thermometric method and both equilibration and peak metamorphism temperatures are provided, although it is sometimes hard to be sure which is which. The temperatures employed for this calibration appear to be suspect. For instance Chainpur (LL3.4) and Tieschitz (H/L3.6) are assigned similar peak temperatures even though they have different petrologic types. Moreover, too few data points are used to calculate the polynomial fit and the calculated correlation coefficient is too low to guarantee the reliability of the calibration curve. We fully agree that FWHM-D correlates with the peak metamorphism temperature (see discussion in Bonal et al., 2007), but reliable and accurate peak temperatures from mineralogical thermometers are still required to guarantee a reliable Raman carbon thermometer. Cody et al. (2008) built a carbon thermometer based on both heating experiments on an Orgueil IOM sample and a calibration using the Isna chondrite. The polyaromatic structure of the IOMs was characterized by X-ray Absorption Near-Edge Structure (XANES) spectroscopy at the carbon K-edge. Their results are compared with Raman measurements in Fig. 10. The data correlate, but not well. Two out of the three data points that plot far from the regression line are Bishunpur and Mokoia. These two samples also plotted far from the regression line of the Raman data (Fig. 8), suggesting an artifact due to sample preparation given that these IOMs were produced and studied within the same organization (Carnegie Institution). Excluding these two chondrites, the data remains relatively uncorrelated, suggesting that Raman micro-spectroscopy offers better sensitivity, accuracy and reliability. Regarding the calibration of the cosmothermometer, we think that this approach is hampered by the relevance of experiments performed at the laboratory timescale while the process occurs over millions of years under natural conditions. Indeed, time and temperature are not exchangeable parameters. Polyaromatic chondritic OM is a complex material that experiences an irreversible and progressive carbonization process upon heating. It is therefore unlikely that similar structural and chemical pathways of the chemical process occur at two very different temperatures, in particular because some chemical pathways are thermodynamically,
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Fig. 10. The FWHM-D parameter from Bonal et al. (2006, 2007) plotted against the exciton intensity from Cody et al. (2008). The correlation is much weaker than that observed between Raman data sets (Fig. 7).
and not only kinetically, inhibited. This is confirmed by the structure of OM in metamorphosed CMs subjected to a high temperature and short duration heating process (Yabuta et al., 2010). The structural evolution of the OM is significantly different than in the case of long duration radiogenic thermal metamorphism. Note that the authors themselves found the experimental kinetic law to be inconsistent with the natural samples. For this reason, they performed a recalibration using a closure temperature of 700 °C for Isna (CO chondrite). The derived temperatures are certainly not reliable. The temperature obtained for Indarch (EH4) is for instance that of a type 6 chondrite (Lin et al., 2002; Slater-Reynolds and McSween, 2005). From this, we conclude that no reliable carbon thermometer is yet available for type 3 chondrites. Raman spectroscopy appears to be the most suitable technique, i.e. the most sensitive, accurate and easy to implement. However the calibration of a Raman carbon thermometer would require progress in the determination of peak metamorphism temperature through mineralogical techniques. 5.3. Connection with mineralogy and texture The mineralogical composition and texture of the assemblage of enstatite chondrites have been used to derive their thermal history (Zhang et al., 1995; Zhang and Sears, 1996). These authors define two criteria. The first is the so-called textural type (ranging from 3 to 7), which describes strictly speaking the texture (chondrules delineation, etc.) of the phases, while also providing mineralogical information such as the dominant pyroxene structure, the abun-
dance of scarce metastable olivine grains and the presence of mesostatic glass/feldspar. The second criterion is the mineralogical type (ranging from a to d), which exclusively accounts for the composition of opaque minerals: sulfides, metals and phosphides. From comprehensive studies covering an extended series of EHs and ELs and a large range of metamorphism grades, the authors concluded that the textural type is related to the peak metamorphism temperature and the mineralogical type to the cooling history. According to the criteria defined by Zhang et al. (1995), all the chondrites studied here belong to EH3a or EL3a, with the exception of Sahara 97096 which should be classified as EH3a,b (Fig. 9). No correlations are reported between the degree of thermal metamorphism derived from OM maturity (petrologic types) and the composition of opaque minerals. In contrast, the closure temperatures derived from the schreibersite-metal thermometer (Zhang and Sears, 1996) correlate relatively well with petrologic types, with the exception of MAC 88136 (Fig. 11). The temperatures derived from the Niningerite-Alabandite thermometer also have trends consistent with the petrologic types. The temperatures derived from opaque mineral assemblage have been interpreted as “closure” temperatures, meaning that kinetic locking takes place as the rock cools down. If this is correct, our data suggest that the peak metamorphism temperature obtained from the OM and the cooling rate obtained from opaque assemblages are not independent parameters. This is fully consistent with an onion shell asteroidal model. In this respect, the higher the metamorphism peak, the longer the duration at the metamorphism peak and the shorter the cooling rate
Thermal metamorphism in EH3 and EL3 chondrites
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Fig. 11. Closure temperatures derived from three thermometers based on opaque mineral assemblages for some of the enstatite chondrites of this study (data from Zhang and Sears, 1996). The chondrites are presented from left to right, within a sequence of increasing petrologic type (e.g., metamorphism peak temperature). The closure temperatures correlate relatively well with petrologic types, consistent with an onion shell asteroidal structure.
(Bennett and McSween, 1998). Therefore, the thermal history of EH3s and EL3s does not appear to be very different than that of ordinary and carbonaceous chondrites.
The OM in Sahara 97096 appears to be peculiar with respect to all other studied chondrites.
6. CONCLUSION
ACKNOWLEDGEMENTS
The metamorphic history of EH3 and EL3 chondrites has been reinvestigated through the study of the structure of its OM. The main results are the following: OM in EH3s and EL3s has a polyaromatic structure similar but not identical to that in UOCs and CV and CO carbonaceous chondrites. Its degree of structural order is controlled by the grade of thermal metamorphism. The following petrologic types were assigned: Sahara 970096 and ALH 84206: 3.1–3.4; ALH 84170 and Parsa: 3.5; ALH 851119: 3.7; Qingzhen, MAC 88136 and MAC 88184: 3.6–3.7. The petrologic types derived from OM maturity are controlled by the temperature of peak metamorphism. However, no reliable calibration can be provided to date. Advances in mineralogic thermometry are required to go further. A comparison of petrologic types with closure temperatures recorded in opaque minerals during asteroid cooling shows that a simple asteroidal onion shell model accounts for the thermal history. Shocks were not observed to have any significant effect on the structure of OM. In the case of the less metamorphosed chondrites, such an effect cannot be fully ruled out.
We warmly thank the NASA Working Meteorite Group (Houston, USA) and the Museum National d’Histoire Naturelle (Paris, France) for providing us with precious meteorite samples. This work has been funded by the Centre National d’Etudes Spatiales (CNES-France). Eric Quirico is very grateful to Nicolas Dauphas (University of Chicago) for valuable discussions on statistical treatment of Raman data, and to Henner Busemann (University of Manchester UK) for valuable and constructive discussions about the Raman analysis he performed at Carnegie Institute (Washington). We are very grateful to three anonymous reviewers for very valuable comments that greatly increase the quality of the manuscript. We thank the Laboratoire de Ge´ologie de Lyon (ENS-Lyon) to give us access to the national Raman instrument supported by the Institut national des sciences de l’Univers, CNRS.
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Bendersky C., Weisberg M. K., Connolly H. Jr., Ebel D. S. (2007) Olivine and the onset of metamorphism in EH3 chondrites. Lunar Planet. Sci. XXXVIII, #2077. Bennett M. E. and McSween H. Y. (1998) Revised model calculations for the thermal histories of ordinary chondrite parent bodies. Meteorit. Planet. Sci 31, 783–792. Bonal L., Quirico E., Bourot-Denise M. and Montagnac G. (2006) Determination of the petrologic type of CV3 chondrites by Raman spectroscopy of included organic matter. Geochim. Cosmochim. Acta 70, 1849–1863. Bonal L., Bourot-Denise M., Quirico E., Montagnac G. and Lewin E. (2007) Organic matter and metamorphic history of CO chondrites. Geochim. Cosmochim. Acta 71, 1605–1623. Brearley A.J. and Jones R.H. (1998) Chondritic Meteorites. In Planetary Materials (J.J. Papike ed.). Mineralogical Society of America, pp. 3.220–225, 3.52–63. Busemann H., Alexander C. M. O. and Nittler L. R. (2007) Characterization of insoluble organic matter in primitive meteorites by microRaman spectroscopy. Meteorit. Planet. Sci. 42, 1387–1416. Cody G. D., Alexander C. M. O., Yabuta H., Kilcoyne A. L. D., Araki T., Ade H., Dera P., Fogel M., Militzer B. and Mysen B. O. (2008) Organic thermometry for chondritic parent bodies. Earth Planet. Sci. Lett. 272, 446–455. Huss, G.R., Rubin A.E., Grossman J.N. (2006) Thermal metamorphism in chondrites. In Meteorites and the Early Solar System II (eds. D. S. Lauretta and H. Y. McSween, Jr.). University of Arizona Press, Tucson. 943pp., pp. 567–586. Huss G. R., Keil K. and Taylor G. J. (1981) The matrices of unequilibrated ordinary chondrites: implications for the origin and history of chondrites. Geochim. Cosmochim. Acta 45, 33– 51. Huss G. R. and Lewis R. S. (1994a) Noble gases in presolar diamonds I: Three distinct components and their implications for diamonds origins. Meteoritics 29, 791–811. Huss G. R. and Lewis R. S. (1994b) Noble gases in presolar diamonds II: Component abundances reflect thermal processing. Meteoritics 29, 811–829. Kelemen S. R. and Fang H. L. (2001) Maturity trends in Raman spectra from kerogen and coal. Energy Fuel. 15, 653– 658. Kimura M., Weisberg M. K., Lin Y., Suzuki A., Ohtani E. and Okazaki R. (2005) Thermal history of enstatite chondrites from silica polymorphs. Meteorit. Planet. Sci 40, 855–868. Lin Y., Amari S. and Pravdivtseva O. (2002) Presolar grains from the Qinqhzen (EH3) meteorite. Astrophys. J. 575, 257– 263. Pasteris J. (1989) In situ analysis in geological thin-sections by laser Raman microprobe spectroscopy – a cautionary note. Appl. Spectrosc. 43, 567–570.
Quirico E., Raynal P. I. and Bourot-Denise M. (2003) Metamorphic grade of organic matter in six unequilibrated ordinary chondrites. Meteorit. Planet. Sci 38, 795–811. Quirico E., Rouzaud J. N., Bonal L. and Montagnac G. (2005) Maturation grade of coals as revealed by Raman spectroscopy: progress and problems. Spectrochim. Acta Part A 61, 2368– 2377. Quirico E., Montagnac G., Rouzaud J. N., Bonal L., BourotDenise M., Duber S. and Reynard B. (2009) Precursor and metamorphic condition effects on Raman spectra of poorly ordered carbonaceous matter in chondrites and coals. Earth Planet. Sci. Lett. 287, 185–193. Rietmeijer F. J. M. and MacKinnon I. D. R. (1985) Poorly graphitized carbon as a new cosmothermometer for primitive extraterrestrial materials. Nature 316, 733–736. Rubin A. E., Scott E. R. D. and Keil K. (1996) Shock metamorphism of enstatite chondrites. Geochim. Cosmochim. Acta 61, 847–858. Rubin A. E., Griset C. D., Choi B.-G. and Wasson J. T. (2009) Clastic matrix in EH3 chondrites. Meteorit. Planet. Sci. 44, 589–601. Slater-Reynolds V. and McSween, Jr., H. Y. (2005) Peak metamorphic temperatures in type 6 ordinary chondrites: an evaluation of pyroxene and plagioclase geothermometry. Meteorit. Planet. Sci. 40, 745–754. Tuinstra F. and Koenig J. L. (1970) Raman spectrum of graphite. J. Chem. Phys. 53, 1126–1130. Weisberg M.K., Kimura M., McCoy T.J., Lin Y. (2005) Olivine and the thermal history of the E chondrite parent body. Lunar Planet. Sci. XXXVI, #1420. Weisberg, and Prinz (1998) Sahara 97096: A highly primitive EH3 chondrite with layered sulfide–metal-rich chondrules. Lunar Planet. Sci. XXIX, #1741. Wopenka B. and Pasteris J. D. (1993) Structural characterization of kerogens to granulite-facies graphite: applicability of Raman microprobe spectroscopy. Am. Mineral. 78, 533–557. Yabuta H., Alexander C. M. O’D., Fogel M. L., Kilcoyne A. L. D. and Cody G. D. (2010) A molecular and isotopic study of the macromolecular organic matter of the ungrouped C2 WIS 91600 and its relationship to Tagish Lake and PCA 91008. Meteorit. Planet. Sci. 45, 1446–1460. Zhang Y., Benoit P. H. and Sears D. W. G. (1995) The classification and complex thermal history of the enstatite chondrites. J. Geophys. Res. 100, 9417–9438. Zhang Y. and Sears D. W. G. (1996) The thermometry of enstatite chondrites: a brief review and update. Meteorit. Planet. Sci. 31, 647–655. Associate editor: Alexander N. Krot
Geochimica et Cosmochimica Acta 75 (2011) 3088–3102 www.elsevier.com/locate/gca
A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites Eric Quirico a,⇑, Miche`le Bourot-denise b, Christophe Robin a,1, Gilles Montagnac c, Pierre Beck a b
a Institut de Plane´tologie et d’Astrophysique de Grenoble, UJF-Grenoble 1/CNRS-INSU, UMR 5274, Grenoble F-38041, France Muse´um National d’Histoire Naturelle, Laboratoire de Mine´ralogie et Cosmochimie du Muse´um, CP 52, 57 rue Cuvier, 75231 Paris Cedex 05, France c Laboratoire de ge´ologie de Lyon, CNRS, Ecole Normale Supe´rieure de Lyon, 46 alle´e d’Italie – BP 7000, 69342 Lyon Cedex 07, France
Received 15 June 2010; accepted in revised form 1 March 2011; available online 10 March 2011
Abstract The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1–3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6–3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes. Crown copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
1. INTRODUCTION ⇑ Corresponding author. Tel.: +33 4 76 51 41 56; fax: +33 4 76 51
41 46. E-mail address: [email protected] (E. Quirico). 1 Present address: Institut des Sciences de la Terre, MR 5275 UJF/CNRS/UdS/IRD/LCPC, 1381 rue de la Piscine – BP 5, 38041 Grenoble Cedex 9, France.
The mineralogy of EH3 and EL3 chondrites reflects the highly reducing conditions that were present in the environment in which they were formed. These chondrites are mostly composed of enstatite, minor olivines, glasses and feldspars together with abundant opaque minerals in the form of metals, sulfides and phosphides. The iron budget
0016-7037/$ - see front matter Crown copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.03.009
Thermal metamorphism in EH3 and EL3 chondrites
is dominated by opaque mineralogy and the corresponding zero valence testifies to the reducing conditions that were present in the region in which they were formed within the solar nebula (Brearley and Jones, 1998, and references therein). These conditions also prevailed on the parent body, ruling out the presence of extensive fluid circulation as observed in carbonaceous chondrites. EH3 and EL3 chondrites experienced post-accretion processes on their parent body, such as thermal and shock metamorphism. Unraveling the effects of these two processes appears to be a complex problem, as the former possibly erase the petrographical and mineralogical signatures of the latter (Rubin et al., 1996). The nature and extent of thermal metamorphism have not yet been clarified. The standard classifications and tools (e.g., induced thermoluminescence) used for classifying ordinary and carbonaceous chondrites are not suitable for EH3s and EL3s. So far few studies have been devoted to thermal metamorphism in type 3 enstatite chondrites. Zhang et al. (1995) proposed a classification based upon the composition of opaque minerals and the texture of the mineralogic assemblage, to classify objects over a large range of metamorphism (types 3–6). It is however unclear to what extent this classification provides details on the nature and extent of the metamorphic history in the less metamorphosed (and therefore best preserved) EH3s and EL3s. Zhang and Sears (1996) determined closure temperatures from opaque mineral assemblages and proposed a three-stage thermal history on the parent body. The apparent complexity of the post-accretion history has also been suggested by the numerous silica polymorphs which suggest the presence of wide variations of post-accretion conditions (Kimura et al., 2005). Type 3 enstatite chondrites contain polyaromatic Organic Matter (OM), presolar grains and nobles gases (Anders and Zinner, 1993; Huss and Lewis, 1994a,b; Alexander et al., 1998; Lin et al., 2002; Alexander et al., 2007). In this respect, they share common characteristics with ordinary and carbonaceous chondrites with which they can be compared. The extent of thermal metamorphism can be investigated by the study of the structure of OM by Raman micro-spectroscopy (Quirico et al., 2003; Bonal et al., 2006, 2007; Busemann et al., 2007; Quirico et al., 2009). This approach can be used to determine the petrologic types that are interpreted in terms of peak metamorphism temperature and that have the same physical interpretation whatever the chemical class of chondrites. This paper presents a study of the structure of OM in the matrix from a series of EH3 and EL3 chondrites that experienced mild or moderate shock metamorphism: Sahara 97096, Allan Hills (ALH) 84206, ALH 84170, Parsa, ALH 85119, Qingzhen, MacAlpine Hills (MAC) 88136, MAC 88184 (Table 1). The degree of structural order of OM was characterized by Raman micro-spectroscopy and then compared with the available mineralogical tracers of thermal metamorphism (matrix texture, volatile abundance and composition of opaque minerals). We show that the maturation grade of OM is a sensitive tracer of the metamorphism grade and that EH3 and EL3 chondrites have experienced thermal metamorphism processes similar to those reported in UOCs, CVs and COs. New petrologic
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types are provided and we discuss the post-accretion history. 2. EXPERIMENTAL 2.1. Samples Thin or polished sections of EH3, EL3, CO, CV and ordinary chondrites were provided by the Meteorite Working Group (NASA Johnson Space Center – Houston, USA) and the Museum National d’Histoire Naturelle (Paris, France) (Table 1). Preliminary measurements on the enstatite chondrites were tentatively performed on chips of raw materials. However, due to the low matrix modal abundance, no matrix could be isolated, as opposed to the case of Bonal et al. (2006). OM isolation by HF/HCl acid attack was not performed because this method tends to mix OM with graphite inclusions hosted by kamacite. Therefore, all analyses were performed in situ on thin polished sections. Matrix zones were previously located using an optical microscope in reflected light and by a Field Emission Gun Scanning Electron Microscopy (FEG-SEM). UOCs and CV and CO chondrites were used for internal calibration and petrologic type crossed assignment (see below). 2.2. Instrumentation Raman experiments were performed at Laboratoire de Sciences de la Terre (ENS-Lyon, France) with a LABRAM spectrometer (Horiba Jobin-Yvon). An excitation wavelength of 514.5 nm from a Spectra Physics argon ion laser was used. The laser beam was focused by a microscope equipped with a 50 long-distance objective, leading to a spot diameter of 2–3 lm. The spectrometer was equipped with a 600 gr/mm grating. Experiments were performed in an inert atmosphere (argon), the power at the sample surface was 500 ± 20 lW and the acquisition time was 3 30 s. Such conditions avoided thermal damage. A visual check for sample deterioration was carried out after each acquisition and measurement reproducibility was tested at the same location. Spectra were acquired in the spectral region 700–2000 cm1. Polishing artefacts have been reported in Raman measurements of graphitic compounds (Pasteris, 1989) but are not of concern here given the low degree of structural order of the OM studied. Backscattered electron and secondary electrons images of sections were acquired using a JEOL JSM 840 A SEM at University Paris VI and ZEISS Ultra55 FEG-SEM at CMTC INP Grenoble, operating at 10 kV. The composition of metals and sulfides was determined with a SX100 electron microprobe (EPMA) (University Paris VI) using a 15 kV voltage, a 10 nA current and a PAP algorithm. 3. RAMAN SPECTRA ANALYSIS The typical Raman spectrum of polyaromatic organic matter exhibits several bands, the most intense being the first order band peaking at 1600 cm1 (G-band) and 1350 cm1 (D-band). The G-band (G for graphite) is assigned to the E2g2 vibrational mode of the aromatic plane
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Table 1 Chondrites studied and Raman spectral parameters. Class
Shock stage
Weathering
FWHM-D (cm1)
xD (cm1)
FWHM-G (cm1)
xG (cm1)
ID/IG
Numb. meas.
PTc
Sahara 97096 ALH 84206
EH EH
S3-4b S3a
W1 A/B
1358.8 ± 1.2 1353.9 ± 1.8
104 ± 5.2 88.1 ± 2.8
1599.7 ± 1.3 1600.8 ± 0.9
3.1–3.4 3.1–3.4
EH EH EL EH EL EL LL LL LL H CO CO CO CV CV CV CV
S4a S3a S3aS3-4b S3a S3a
B Fall Be Fall A C Fall Fall Fall Fall Ae Fall Fall Find Find Fall Fall
1351.4 ± 0.6 1350.7 ± 1.3 1352.5 ± 3.8 1351.4 ± 3 1352.4 ± 1.7 1351.2 ± 1.8 1358.0 ± 4.4 1353.2 ± 2.2 1350.8 ± 1.2 1352 ± 1.2 1350.5 ± 2.7 1348.5 ± 0.7 1350.1 ± 1.0 1351.5 ± 3.5 1349.7 ± 1.7 1349.5 ± 1.5 1351.3 ± 1.5
85.9 ± 2.8 80.6 ± 4.7 73.4 ± 8.8 85.8 ± 6.9 84.1 ± 12 79.0 ± 4.7 70.2 ± 3.4 73.2 ± 4.7 73.0 ± 2.2 77.2 ± 3.6 66.7 ± 4.4 75.2 ± 5 73.0 ± 4.2 76.5 ± 3.6 79.6 ± 3 71.8 ± 4.3 66.5 ± 8.2
1602.2 ± 0.9 1601.6 ± 1.4 1599.7 ± 2.8 1600.8 ± 1.3 1599.1 ± 3.9 1599.4 ± 2.0 1603.6 ± 1.9 1606.6 ± 2.3 1604.6 ± 0.7 1604.2 ± 2.0 1603.5 ± 1.1 1602.1 ± 1.1 1602.6 ± 0.9 1602.3 ± 0.4 1601.3 ± 0.8 1603.5 ± 1.3 1600.7 ± 1.6
1.04 ± 0.02 1.07 ± 0.04 1.04 ± 0.03 1.09 ± 0.01 1.09 ± 0.03 1.10 ± 0.08 1.07 ± 0.07 1.20 ± 0.06 1.19 ± 0.12 1.28 ± 0.05 0.795 ± 0.06 0.93 ± 0.12 0.90 ± 0.03 1.11 ± 0.05 0.75 ± 0.03 1.12 ± 0.02 1.33 ± 0.05 0.95 ± 0.02 1.00 ± 0.01 1.12 ± 0.23 1.48 ± 0.11
12 28
ALH 84170 Parsa ALH 85119 Qingzhen MAC 88136 MAC 88184 Semarkona Bishunpur Krymka Tieschitz ALH 77307 Kainsaz Felix Leoville Efremovka Vigarano Allende
188.8 ± 13.5 168.9 ± 19.5 Z1: 186.2 ± 8.3 Z2: 148.8 ± 6.1 140 ± 11 138.5 ± 15 119.3 ± 19.8 116.7 ± 25.2 97.4 ± 16.7 94.2 ± 11.1 255.4 ± 19.7 177.3 ± 29.7 172 ± 5.3 123 ± 6.8 218.4 ± 10.3 136 ± 21.7 95.8 ± 9.3 162.4 ± 5.1 154.5 ± 6.4 109 ± 13 72.5 ± 13.2
40 12 7 31 25 28 52 51 33 11 48 6 23 22 43 60 32
3.5 3.5 3.6 3.6–3.7 3.6–3.7 3.6–3.7 3.0 3.1 3.1 3.6 3.03 3.6 3.7 3.1–3.4 3.1–3.4 3.1–3.4 3.7–3.8
a b c
Shock stage assigned by Rubin et al. (1996). Shock stage assigned by this study. Petrologic type: this study for EH3 and EL3, Bonal et al. (2006, 2007) for CO and CV carbonaceous chondrites. The uncertainties on the Raman spectral parameters are 1 standard deviation.
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Chondrites
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and is present in all OM whatever the degree of structural order. On the other hand, the D-band (D for defects) is not present in perfectly stacked graphite and is induced by structural defects in the material (e.g., Tuinstra and Koenig, 1970). The only structural parameter which has been linked to the Dand G-band spectral parameters is the lateral size of the basic structural units of polyaromatic matter (e.g., Tuinstra and Koenig, 1970; Wopenka and Pasteris, 1993). Unfortunately, the available relationship is highly inaccurate for poorly-ordered OM and in particular cannot be used to study chondritic OM. Nevertheless, some spectral parameters are very sensitive to the degree of structural order of poorly ordered materials (referred to below as maturity tracers), although they do not provide quantitative structural information. Studies on series of coals and other natural sedimentary organic matter have provided different sets of parameters that correlate well with the maturation grade as quantified by vitrinite reflectance (Wopenka and Pasteris, 1993; Quirico et al., 2003, 2005). In the case of chondrites, these parameters are not applicable (Quirico et al., 2009). The issue of the most relevant tracers of chondritic OM maturity will be addressed below. The analysis of the Raman spectra was focused on the first-order G- and D-bands. The second order bands, mostly sensitive to the last stages of the graphitization process, were not considered here. The fluorescence background was first subtracted assuming a linear baseline between 700 and 2000 cm1. Two numerical treatments were applied. The first fit the reduced spectra using a Lorentzian profile for the D band and a Breit–Wigner– Fano profile for the G-band. The second was a Principle Component Analysis (PCA) performed on the whole set of spectra. The input variables were the intensities of the spectra at each wavelength in the range of 800– 1990 cm1. The efficiency of the PCA was evaluated as the number of principal components required to account for >95% of the variance. The LBWF fitting and PCA were performed with IGOR software (Wavemetrics, Inc.). 4. RESULTS 4.1. Texture of the matrix area In unequilibrated ordinary chondrites, matrix presents a fine-grained optically opaque component and a coarsegrained optically transparent component (Huss et al., 1981). The second was assumed to be the recrystallization product of the former due to mild thermal metamorphism. The matrix material is also characterized by its petrological location in interchondrule matrix, chondrule rims or clasts (i.e. areas with well defined borders). Matrix is scarce in enstatite chondrites and few studies have focused on its description. Rubin et al. (2009) described patches of matrix dubbed “clastic matrix” because they contains coarse angular fragments, but strictly speaking they are not clasts as described before. They distinguished two components containing coarse silicates and opaque mineral grains, respectively, and a third component described as an “inferred nebular component” accordingly to its finegrained texture. This description may appear ambiguous,
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given that there is no evidence that the coarse-grained component was not present in the solar nebula nor that the finegrained component was a nebular condensation product. In this study we have followed a three-step procedure to locate carbon-rich matrix areas: (1) fine-grained areas were first located by optical microscopy; (2) the presence of a fine-grained material was further confirmed by SEM-FEG (typical grain size <10 lm) and (3) polyaromatic carbonaceous material (OM) was detected by Raman microspectroscopy. This procedure unambiguously identifies a matrix material that presents similarities with matrix material observed in ordinary and carbonaceous chondrites, i.e. a submicrometer-scaled mixture of silicate grains, organic matter, volatile species (Q phase) and presolar grains (Huss and Lewis, 1994a,b; Lin et al., 2002). This criterion, based on carbon detection, is definitely less ambiguous than mineralogy and texture alone. This also ensures overall consistency with earlier studies focused on ordinary and carbonaceous chondrites and involving the selection of carbon-rich matrix fragments (see Quirico et al., 2003; Bonal et al., 2006, 2007). FEG-SEM images reveal that carbon-rich matrix areas should be better described by an interchondrule matrix. No clasts well separated from the rest of the petrologic assemblage were observed. Coarse-grained and fine-grained components appear intimately mixed in matrix areas, but the relative abundance of the former varies among the chondrites studied (Figs. 1 and 2, Table 2). Quantifying the abundance of the fine-grained versus coarse-grained components of the matrix from the SEM images appeared very tricky. It would have required image analysis software and a dedicated procedure that were not available. Therefore, naked eye analyses were performed independently by two of the authors (MBD and EQ) on two series of SEM images measured with two SEMs (one at University Paris VI, France and the other at CMTC – INP Grenoble, France). This led to the following consistent description. Matrix zones in Sahara 97096 and ALH 84206 present a fine-grained component that appears less fine-grained than in the less equilibrated ordinary chondrites (e.g., Bishunpur LL3.1 and Krymka L3.1: Huss et al., 1981; Quirico et al., 2003). A similar matrix texture was observed in ALH 84170, but the fine-grained regions appeared less compacted and more porous. More compacted and more coarse-grained matrix areas were observed in Parsa. The matrix of ALH 85119 was found to be very heterogeneous. The matrix material in some areas was mostly coarsegrained with irregular and angular coarse fragments of silicates and opaque minerals. Other matrix areas exhibit a very compact material with rounded grains of sulfides and metal that testifies to the impact of shock processes. ALH 85119 is the only chondrite of the series that presents a heavily shock processed matrix. Lastly, fine-grained matrix was found to be scarce in Qingzhen and was mostly unobserved in both MAC 88136 and MAC 88184. 4.2. Maturation grade of the organic matter The first-order G and D bands of the OM were detected in most of the matrix areas (Table 1, Fig. 3). These measurements however suffered from an intense fluorescence
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Fig. 1. Backscattered electron images of matrix areas in the EH3 and EL3 chondrites in this study. (a) Sahara 97096. The yellow circles indicate the location of Raman microscopic measurements in the fine-grained component of the matrix. OM was detected for each. (b) Finegrained and coarse-grained components in the matrix of ALH 84170 and (c and d) Matrix in ALH 84206. Cg: coarse grain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Backscattered electron images of matrix areas in the EH3 and EL3 chondrites in this study. (a) ALH 85119. The yellow circles indicate the location of Raman microscopic measurements. Raman spectra of OM appear to be similar in the shock melt vein and the weakly shocked area (top-left), suggesting the lack of effects of shocks on the structure of OM in this chondrite and (b) MAC 88184. The matrix appears mostly as a coarse-grained component. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 Matrix texture of the EH3 and EL3 chondrites.
Sahara 97096 ALH 84206 ALH 84170 Parsa ALH 85119 Qingzhen MAC 88136 MAC 88184 a b
Class
Shock stage
Texture
EH EH EH EH EL EH EL EL
S3-4b S3a S4a S3a S3a S3a S3a
Coarse and fine-grained components Coarse and fine-grained components Coarse and fine-grained components. More porous than Sahara 97096 and Allan Hills 84206 Less abundant fine-grained component Coarse-grained or very compact Coarse-grained component. Much less fine-grained component Rare fine-grained component Rare fine-grained component
Shock stage assigned by Rubin et al. (1996). Shock stage assigned by this study.
background in several spectra. Chondritic OM in type 3 chondrites does not exhibit a significant fluorescence be-
cause of the high degree of condensation of the polyaromatic units that enhance fluoresced photon absorption.
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Fig. 3. Typical Raman spectra of matrix areas from the 8 enstatite chondrites. All display the first-order G and D bands of polyaromatic organic matter, which point to different degrees of structural order through their spectral variations. The peak intensity of the G band has been normalized to 1. Spectra are vertically off-shifted for the sake of clarity.
Therefore, the embedding resin of the sections appears to be the carrier of this fluorescence background. Practically speaking, this led to uncertainties in the derivation of the spectral characteristics of the G and D bands during the fit-
ting process and spectra with an intense background were rejected. The number of spectra retained for each chondrite appears finally to vary from one object to another, ranging between 6 and 52. The first numerical treatment applied to
Fig. 4. The spectral tracer FWHM-D plotted against ID/IG. The curve points out a trend of increasing degree of order of OM (gray arrow). EH3 and EL3 data are compared with those of UOCs and CO and CV chondrites to assign petrologic types. Sahara 97096 and ALH84206 plot slightly outside the trend.
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the spectra was the LBWF fitting procedure described in Section 3. It provided five spectral parameters: the peak position (xG) and width at half maximum (FWHM-G) of the BWF profile fitting the G band, the peak position (xD) and width at half maximum (FWHM-D) of the Lorentzian profile fitting the D band and the ratio of the peak intensities of the profiles fitting the G and D bands (ID/IG). The results are presented in Table 1 and in Figs. 4 and 5. The second numerical treatment that was applied was a principal component analysis (PCA) as described in Section 3. About 94% of the variance of the whole set of data was accounted for by six principal components. We first address the issue of the spectral tracers of the maturity of OM. FWHM-D and ID/IG have been found to be tracers of the degree of structural order of OM (Quirico et al. 2003; Bonal et al. 2006, 2007; Busemann et al. 2007; Quirico et al., 2009). The measurements in this study confirm this result: FWHM-D and ID/IG define a metamorphic pathway (Fig. 4) and both correlate with the reduced spectra projected along the axis of the principal components (Fig. 6). The width at half maximum of the G band (FWHM-G) has been proposed as a tracer of OM maturity (Busemann et al., 2007). However, this result is not supported by our data (Fig. 5) nor by the studies of Quirico et al. (2003, 2009). In contrast, we observe that FWHM-G is on the average larger in EH3 and EL3 chondrites than in UOCs and CO and CV carbonaceous chondrites (Fig. 5). Within the enstatite chondrites group, Sahara 97096 even exhibits a larger value than in any other chondrites (Fig. 5). Different explanations should be examined. Alexander et al. (2007) have reported that weathering modifies the composition and likely the structure of OM. The low weathering stage (W1) of Sahara 97096 is however not consistent with this explanation. The shock stage of Sahara 97096 (S3-4) leads us to consider a possible shock effect on the structure of OM. To test this possibility, we measured the degree of structural order of OM in shocked melt veins and a weakly shocked matrix area in ALH 85119
(Fig. 2). We observed no difference in the Raman data. It is however difficult to draw a firm conclusion given that shocks have certainly competed with long duration thermal metamorphism that tends to erase their impact. Shock wave effects finally cannot be discarded. A third explanation would consider that the variations of FWHM-G among and within the chondrite groups are accounted for by structural variations of the precursors accreted. Such variations have been suggested in UOCs and CO and CV carbonaceous chondrites (Quirico et al., 2009). Hence, a lack of correlation between the H/C ratio and FWHM-D is observed in the case of chondrites, while both these parameters are linearly correlated in coal samples that originate from more similar precursors. Note also that unlike chondrites, FWHM-G correlates with maturity in coals (Kelemen and Fang, 2001; Quirico et al., 2005). Therefore, it is likely that OM precursors accreted by EH3s and EL3s are not identical to those accreted by other groups. However, the organic precursors are similar enough to ensure that OM maturity is controlled by the degree of thermal metamorphism. A second issue concerns measurement reproducibility and accuracy. The number of measurements for each chondrite ranges between 6 and 52. This is insufficient to determine the statistical distribution of the FWHM-D and ID/IG parameters derived from the fitting procedures. Therefore, the experimental average may not tend to the real average of the statistical distribution when increasing the number of measurements. In the case of Vigarano, the experimental average tends to a finite value p and ffiffiffiffi the standard deviation of the mean r decreases (r = x= N where x = FWHM-D or ID/IG and N is the number of measurements) (Fig. 7). In contrast, this is not the case for Semarkona and the data here shows a much larger structural heterogeneity. The situation obviously gets worse as the number of measurements decreases. Therefore, for each chondrite we have provided the average and standard deviation of the experimental data.
Fig. 5. The parameter FWHM-G plotted against ID/IG. The lack of correlation between the parameters reveals that FWHM-G does not trace the thermal metamorphism grade. FWHM-G in EH3s and EL3s is on the average larger than in UOCs and CO and CV carbonaceous chondrites. Sahara 97096 is peculiar, with the highest FWHM-G.
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Fig. 6. The parameters FWHM-G and ID/IG plotted against the coordinate of the reduced spectra along the second principal component.
To ensure that the measurements are unbiased, the data of this study have been compared with earlier published data. We observe that the FWHM-D and ID/IG parameters linearly correlate with those determined by Bonal et al. (2006, 2007) from raw matrix grains under atmospheric conditions with a longer acquisition time and greater energy deposited onto the sample (Fig. 8). The data by Bonal et al. (2006, 2007) also show a very good linear correlation with those of Busemann et al. (2007) (Fig. 8). The data of Busemann et al. (2007) were acquired from Insoluble Organic Matter (IOM) extracts with a different Raman microprobe and different experimental conditions. The number of measurements was very large (from several 100 to several 1000 spectra for each chondrite), ensuring a reliable estimation of the mean of the spectral parameters. The slope of the linear fit is not equal to 1, as a consequence of different experimental conditions resulting in different sample temperatures and also possibly a different fitting technique
(two Lorentzian profiles). We note however that two objects plot far from the regression line: Bishunpur (LL/3.1) and Mokoia (CV3.6). In the case of Bishunpur, our data and the data of Bonal et al. (2006) are consistent with each other and with its petrologic type 3.1. In the case of Mokoia, we note that the petrologic type derived from the Raman data is consistent with the abundance of the P3 noble gas. We suspect here an artifact induced by IOM isolation (see discussion section). Finally, the FWHM-D and ID/IG parameters reveal a maturation trend of OM in the EH3 and EL3 with three distinct groups (Fig. 4): (1) Lowest OM maturity: Sahara 97096 and ALH 84206; (2) Intermediate OM maturity: ALH84170, Parsa, ALH 85119; (3) Highest OM maturity: Qingzhen, MAC 88136, MAC 88184.
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Fig. 7. The average of FWHM-D plotted against the number of measurements. Bars are the standard deviation of the mean. For Vigarano, the average tends towards a constant value. In the case of Semarkona, the average does not stabilize and the standard deviation of the mean does not decrease.
Chondrites from groups 2 to 3 plot near ordinary and carbonaceous CV and CO chondrites, but Sahara 97096 and ALH 84206 (group 1) plot slightly apart from this trend. ALH 84206 presents two groups of maturities, one plotting near SAH97096 and the other within group 2 (see inset, Fig. 4). Both Sahara 97096 and ALH 84206 spectra are not disturbed by the fluorescence from the embedding resin, therefore the observed variability is not due to incorrect background subtraction. Two explanations are possible in the case of ALH 84206: (1) an effect of brecciation (shocks?), with matrix areas corresponding to different maturation grades; (2) an effect of the low number of measurements that may be not large enough to account for the whole structural heterogeneity. Note that Sahara 97096 has a peculiar position in the FWHM-D vs. ID/IG diagram and the largest FWHM-G value with respect to all other chondrites (see discussion before). 4.3. Composition of opaque minerals In enstatite chondrites, texture and mineral chemistry seem to indicate different thermal histories or a combination of shock and thermal processes. Zhang and Sears (1996) have proposed a classification that uses both mineralogy (rated by the letters a, b, c and d) and texture (rated by a number ranging between 3 and 6). To our knowledge, this is the only systematic classification proposed to date for enstatite chondrites. Its suitability for samples of low metamorphic grades (type 3) has however not been clearly shown. For the EH3 and EL3 series, we have thus determined the composition of Ti and Cr in troilite and Si and
Ni in kamacite (Fig. 9, Table 3). The compositions for ALH 85119 and MAC 88184 are in agreement with those obtained by Zhang et al. (1995). These data show that most objects of this study belong to the so-called type 3a. The EH and EL subclasses are readily distinguished and the composition of opaque minerals does not correlate with the maturity of OM. The standard deviation around the mean of the compositions of Sahara 97096 is larger than for other objects, suggesting it could be better described by an ab type. Once again, we observe that Sahara 97096 is peculiar with respect to other chondrites.
4.4. Shock metamorphism Among the EH3s and EL3s considered in this manuscript, Sahara 97096 and MAC 88184 were not classified by Rubin et al. (1996). Polished sections were analyzed to characterize the extent of shock metamorphism in them. The use of polished sections precluded the direct application of the shock classification of Rubin et al. (1996) which is mostly based on shock features that are only observable for thin sections (i.e. mosaicism, undulose extinctions, etc.). Instead, we used the presence or absence of shock melt veins and melt pockets, as well as the presence of local sulfur phase redistribution. In the case of MAC88184, no shock veins were observed, indicating a shock degree
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Fig. 8. Top: the width at half maximum of the D-band measured in the unequilibrated ordinary chondrites and CO and CV chondrites in this study, plotted against the values obtained by Bonal et al. (2006, 2007). Bottom: the width at half maximum of the D-band measured in unequilibrated ordinary chondrites and CO and CV chondrites by Bonal et al. (2006, 2007), plotted against the values obtained by Busemann et al. (2007). The correlation between the data sets is excellent, showing that internal structural heterogeneity is much lower than the variations between the chondrites. The slight misfit for Bishunpur and Mokoia (bottom) is likely an artifact due to IOM preparation.
5. DISCUSSION 5.1. Organic matter and post-accretion processes As mentioned above, the degree of structural order of OM in the EH3 and EL3 series shows an increasing trend that can be represented by three groups: (1) lowest OM maturity: SAH97096 and ALH84206; (2) intermediate OM maturity: ALH84170, Parsa, ALH85119; (3) Highest OM maturity: Qingzhen, MAC 88136, MAC 88184. In UOCs
and CO and CV carbonaceous chondrites, the degree of structural order of OM is related to the degree of thermal metamorphism and controlled by the temperature of peak metamorphism. This interpretation has been validated by the comparison with independent metamorphism tracers like Fe-zoning at the border of type II chondrules (Bonal et al., 2006, 2007), opaque mineral textures (Quirico et al., 2003) and volatile abundances. A comparative study with coals has further demonstrated that the OM precursors accreted by the parent bodies were not strictly identical, but
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Fig. 9. Composition of metal and troilite plotted according to the study of Zhang et al. (1995) (see text for details). The studied enstatite chondrites all belong to the textural subtypea, except for Sahara 97096. EH3s can be distinguished from EL3s.
Table 3 Composition of opaque minerals.
Sahara 97096 Parsa ALH 85119 Qingzhen MAC 88184
Si in kamacite (wt.%)
Ni in kamacite (wt.%)
Ti in troilite (wt.%)
Cr in troilite (wt.%)
2.780.09
3.51 ± 0.49
0.23 ± 0.05
2.60 ± 1.23
2.56 ± 0.50 0.67 ± 0.13
3.81 ± 1.25 5.50 ± 0.40
0.18 ± 0.09 0.40 ± 0.12
0.72 ± 0.15 0.85 ± 0.84
2.85 ± 0.17 0.60 ± 0.15
3.35 ± 0.73 5.86 ± 0.60
0.27 ± 0.04 0.37 ± 0.08
1.03 ± 0.22 0.73 ± 0.56
Bars are one standard deviation.
similar enough to ensure that the maturity trend was interpretable as a thermal metamorphism tracer (Quirico et al., 2009). This interpretation can be extended to EH3 and EL3 chondrites if their parent bodies accreted similar OM precursors. This condition can be tested using the Raman data because the shapes of the first order bands of OM are controlled by both the temperature of peak metamorphism and the nature of the precursor. Both PCA and LBWF fitting show that the Raman data of EH3 and EL3 chondrites point to precursors similar to those in UOCs and CO and CV carbonaceous chondrites (Figs. 4 and 6). In both cases, Sahara 97096 plots slightly apart from the general trends. We think that the most suitable explanation is related to
Thermal metamorphism in EH3 and EL3 chondrites
shocks but, as already mentioned, the effects of shocks on OM are difficult to unravel. It seems also that the different redox conditions in the solar nebula, which controlled the nature of the mineralogical assemblage, had no significant impact on the OM formation processes, or that this OM was formed elsewhere and was later transported to the accretion region of chondrite parent bodies. The physical and chemical conditions on the parent body had likely no significant influence on the structural evolution of OM. This first-order description supports the use of OM maturity as a metamorphic indicator. In this respect, petrologic types can be calculated using the measurements performed on the ordinary, CV and CO chondrites. Sahara 97096 and ALH 84206 are assigned to petrologic types 3.1–3.4. This large estimation reflects the uncertainty in relation to the peculiar nature of the chondrites, possibly linked to the impact of shock processes. ALH 84170 and Parsa are assigned to petrologic type 3.5, ALH 851119 to petrologic type 3.7 and Qingzhen, MAC 88136 and MAC 88184 to petrologic types 3.6–3.7 (Table 1). As reported in the previous section, the assignment of Qingzhen is supported by the abundance of the P3 component of the noble gases, which also indicates petrologic type 3.6 (Huss and Lewis, 1994b). Though more qualitative (rated by naked eye observations), the abundance of the fine-grained matrix can be compared with the derived petrologic types. As reported in Section 4, the texture of the matrix in unequilibrated ordinary chondrites is related to the degree of recrystallization of an initial fine-grained material and provides a rough estimation of the extent of thermal metamorphism. We have tentatively applied the same criterion to the series of EH3 and EL3 chondrites and obtained the sequence: Sahara 97096 ALH 84206 ALH 84170 < Parsa ALH 85119 < Qingzhen < MAC 88136 MAC 88184 (Figs. 1 and 2). This sequence appears fairly consistent with the derived petrologic types. We also note that the Cr-composition of olivine grains in Sahara 97096, Parsa and Qingzhen is consistent with a higher metamorphism grade in Qingzhen (Weisberg et al., 2005; Bendersky et al., 2007). This observation is consistent with the derived petrologic type in the case of Qingzhen, but Parsa and Sahara 97096 cannot be distinguished. The low petrologic type of Sahara 97096 also appears to be consistent with the mineralogic observations of Weisberg and Prinz (1998). 5.2. Can we define a peak temperature? The determination of a peak metamorphism temperature from OM maturity has been debated for years. Carbon thermometers for chondrites have been proposed in the literature (Rietmeijer and MacKinnon, 1985; Bonal et al., 2007; Busemann et al., 2007; Cody et al., 2008). These studies have led to very different estimations. Rietmeijer and MacKinnon (1985) and Bonal et al. (2007) both used terrestrial carbonaceous rocks to calibrate cosmothermometers. Bonal et al. (2007) restricted the application of this method to the most metamorphosed type 3 chondrites, because the OM structure records the structure and likely composition of the precursor. They considered that above petrologic
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type 3.7, the precursor memory vanishes. They obtained a peak metamorphism temperature of 300 °C for Allende, similar to the estimation by Rietmeijer and MacKinnon (1985). This temperature appears too small with respect to those obtained by mineralogical thermometers, even if the latter are not very accurate and span large ranges. We have recently estimated a more realistic temperature ranging between 550 and 600 °C by modeling Fe-zoning at the border of type II chondrules (manuscript in preparation). Therefore, we think that a carbon cosmothermometer dedicated to chondrites cannot be calibrated using carbonaceous terrestrial rocks. The approach by Busemann et al. (2007) is more realistic. They compiled temperatures determined from chondrites (Huss et al., 2006) to calibrate a Raman carbon cosmothermometer. However, there are serious flaws in the temperature determined in the literature by mineralogical thermometers. The derived temperatures span a broad range of values depending on the thermometric method and both equilibration and peak metamorphism temperatures are provided, although it is sometimes hard to be sure which is which. The temperatures employed for this calibration appear to be suspect. For instance Chainpur (LL3.4) and Tieschitz (H/L3.6) are assigned similar peak temperatures even though they have different petrologic types. Moreover, too few data points are used to calculate the polynomial fit and the calculated correlation coefficient is too low to guarantee the reliability of the calibration curve. We fully agree that FWHM-D correlates with the peak metamorphism temperature (see discussion in Bonal et al., 2007), but reliable and accurate peak temperatures from mineralogical thermometers are still required to guarantee a reliable Raman carbon thermometer. Cody et al. (2008) built a carbon thermometer based on both heating experiments on an Orgueil IOM sample and a calibration using the Isna chondrite. The polyaromatic structure of the IOMs was characterized by X-ray Absorption Near-Edge Structure (XANES) spectroscopy at the carbon K-edge. Their results are compared with Raman measurements in Fig. 10. The data correlate, but not well. Two out of the three data points that plot far from the regression line are Bishunpur and Mokoia. These two samples also plotted far from the regression line of the Raman data (Fig. 8), suggesting an artifact due to sample preparation given that these IOMs were produced and studied within the same organization (Carnegie Institution). Excluding these two chondrites, the data remains relatively uncorrelated, suggesting that Raman micro-spectroscopy offers better sensitivity, accuracy and reliability. Regarding the calibration of the cosmothermometer, we think that this approach is hampered by the relevance of experiments performed at the laboratory timescale while the process occurs over millions of years under natural conditions. Indeed, time and temperature are not exchangeable parameters. Polyaromatic chondritic OM is a complex material that experiences an irreversible and progressive carbonization process upon heating. It is therefore unlikely that similar structural and chemical pathways of the chemical process occur at two very different temperatures, in particular because some chemical pathways are thermodynamically,
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Fig. 10. The FWHM-D parameter from Bonal et al. (2006, 2007) plotted against the exciton intensity from Cody et al. (2008). The correlation is much weaker than that observed between Raman data sets (Fig. 7).
and not only kinetically, inhibited. This is confirmed by the structure of OM in metamorphosed CMs subjected to a high temperature and short duration heating process (Yabuta et al., 2010). The structural evolution of the OM is significantly different than in the case of long duration radiogenic thermal metamorphism. Note that the authors themselves found the experimental kinetic law to be inconsistent with the natural samples. For this reason, they performed a recalibration using a closure temperature of 700 °C for Isna (CO chondrite). The derived temperatures are certainly not reliable. The temperature obtained for Indarch (EH4) is for instance that of a type 6 chondrite (Lin et al., 2002; Slater-Reynolds and McSween, 2005). From this, we conclude that no reliable carbon thermometer is yet available for type 3 chondrites. Raman spectroscopy appears to be the most suitable technique, i.e. the most sensitive, accurate and easy to implement. However the calibration of a Raman carbon thermometer would require progress in the determination of peak metamorphism temperature through mineralogical techniques. 5.3. Connection with mineralogy and texture The mineralogical composition and texture of the assemblage of enstatite chondrites have been used to derive their thermal history (Zhang et al., 1995; Zhang and Sears, 1996). These authors define two criteria. The first is the so-called textural type (ranging from 3 to 7), which describes strictly speaking the texture (chondrules delineation, etc.) of the phases, while also providing mineralogical information such as the dominant pyroxene structure, the abun-
dance of scarce metastable olivine grains and the presence of mesostatic glass/feldspar. The second criterion is the mineralogical type (ranging from a to d), which exclusively accounts for the composition of opaque minerals: sulfides, metals and phosphides. From comprehensive studies covering an extended series of EHs and ELs and a large range of metamorphism grades, the authors concluded that the textural type is related to the peak metamorphism temperature and the mineralogical type to the cooling history. According to the criteria defined by Zhang et al. (1995), all the chondrites studied here belong to EH3a or EL3a, with the exception of Sahara 97096 which should be classified as EH3a,b (Fig. 9). No correlations are reported between the degree of thermal metamorphism derived from OM maturity (petrologic types) and the composition of opaque minerals. In contrast, the closure temperatures derived from the schreibersite-metal thermometer (Zhang and Sears, 1996) correlate relatively well with petrologic types, with the exception of MAC 88136 (Fig. 11). The temperatures derived from the Niningerite-Alabandite thermometer also have trends consistent with the petrologic types. The temperatures derived from opaque mineral assemblage have been interpreted as “closure” temperatures, meaning that kinetic locking takes place as the rock cools down. If this is correct, our data suggest that the peak metamorphism temperature obtained from the OM and the cooling rate obtained from opaque assemblages are not independent parameters. This is fully consistent with an onion shell asteroidal model. In this respect, the higher the metamorphism peak, the longer the duration at the metamorphism peak and the shorter the cooling rate
Thermal metamorphism in EH3 and EL3 chondrites
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Fig. 11. Closure temperatures derived from three thermometers based on opaque mineral assemblages for some of the enstatite chondrites of this study (data from Zhang and Sears, 1996). The chondrites are presented from left to right, within a sequence of increasing petrologic type (e.g., metamorphism peak temperature). The closure temperatures correlate relatively well with petrologic types, consistent with an onion shell asteroidal structure.
(Bennett and McSween, 1998). Therefore, the thermal history of EH3s and EL3s does not appear to be very different than that of ordinary and carbonaceous chondrites.
The OM in Sahara 97096 appears to be peculiar with respect to all other studied chondrites.
6. CONCLUSION
ACKNOWLEDGEMENTS
The metamorphic history of EH3 and EL3 chondrites has been reinvestigated through the study of the structure of its OM. The main results are the following: OM in EH3s and EL3s has a polyaromatic structure similar but not identical to that in UOCs and CV and CO carbonaceous chondrites. Its degree of structural order is controlled by the grade of thermal metamorphism. The following petrologic types were assigned: Sahara 970096 and ALH 84206: 3.1–3.4; ALH 84170 and Parsa: 3.5; ALH 851119: 3.7; Qingzhen, MAC 88136 and MAC 88184: 3.6–3.7. The petrologic types derived from OM maturity are controlled by the temperature of peak metamorphism. However, no reliable calibration can be provided to date. Advances in mineralogic thermometry are required to go further. A comparison of petrologic types with closure temperatures recorded in opaque minerals during asteroid cooling shows that a simple asteroidal onion shell model accounts for the thermal history. Shocks were not observed to have any significant effect on the structure of OM. In the case of the less metamorphosed chondrites, such an effect cannot be fully ruled out.
We warmly thank the NASA Working Meteorite Group (Houston, USA) and the Museum National d’Histoire Naturelle (Paris, France) for providing us with precious meteorite samples. This work has been funded by the Centre National d’Etudes Spatiales (CNES-France). Eric Quirico is very grateful to Nicolas Dauphas (University of Chicago) for valuable discussions on statistical treatment of Raman data, and to Henner Busemann (University of Manchester UK) for valuable and constructive discussions about the Raman analysis he performed at Carnegie Institute (Washington). We are very grateful to three anonymous reviewers for very valuable comments that greatly increase the quality of the manuscript. We thank the Laboratoire de Ge´ologie de Lyon (ENS-Lyon) to give us access to the national Raman instrument supported by the Institut national des sciences de l’Univers, CNRS.
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