- Email: [email protected]
Formation of solar nebula reservoirs by mixing chondritic components
Earth and Planetary Science Letters 248 (2006) 650 – 660 www.elsevier.com/locate/epsl
Formation of solar nebula reservoirs by mixing chondritic components Brigitte Zanda a,b,⁎, Roger H. Hewins a,b , Michèle Bourot-Denise a , Philip A. Bland c,d , Francis Albarède e a
c
Laboratoire d'Etudes de la Matière Extraterrestre, MNHN and CNRS-UMS2679, 61 rue Buffon, 75005 Paris, France b Department of Geological Sciences, Rutgers University, Piscataway NJ 08855, USA Impacts and Astromaterials Research Centre (IARC), Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK d IARC, Department of Mineralogy, Natural History Museum, London SW7 5BD, UK e Ecole Normale Supérieure de Lyon, 46, Allée d'Italie, 69364 Lyon, France Received 3 December 2005; received in revised form 9 May 2006; accepted 12 May 2006 Available online 28 July 2006 Editor: R.W. Carlson
Abstract We determined proportions of Type I (reduced) and Type II (oxidized) chondrules in ordinary chondrites (OC) and found linear relationships between chondrule abundances and chondrite bulk chemical and oxygen isotopic compositions. Similar relationships exist between bulk oxygen isotopic compositions of carbonaceous chondrites and modal abundances of their chondritic components (matrix, Type I and Type II chondrules, refractory calcium–aluminium-rich inclusions and amoeboid olivine aggregates). These correlations can be used to predict the bulk oxygen isotopic composition of chondrites based on their petrology. We can also define model isotopic compositions associated with each petrologic component, which are not their current actual isotopic compositions due to alteration or mixing. These compositions for refractory inclusions and chondrules plot close to a slope 1 line, consistent with refractory inclusions (RI) forming from an early 16O-rich gas, the evolution of the gas to more 16O-poor compositions, possibly involving photodissociation and subsequent ice transport, followed by chondrule formation. Our results open a new understanding of the oxygen 3-isotope space and explain the unique position of OC as well as the differences between H, L and LL chondrites. They indicate that major chemical and isotopic variations between chondritic reservoirs were established after chondrule and CAI formation. They may have some bearing on the formation of planetary reservoirs: the Δ17O calculated for type I chondrules is appropriate for terrestrial planet progenitors, consistent with their chemical similarity to Earth mantle. © 2006 Elsevier B.V. All rights reserved. Keywords: nebular fractionation; oxygen isotopes; chondritic reservoirs; chondrules; chondritic metal; refractory inclusions
1. Introduction ⁎ Corresponding author. Laboratoire d'Etudes de la Matière Extraterrestre, MNHN and CNRS-UMS2679, 61 rue Buffon, 75005 Paris, France. Tel.: +33 1 4079 3542; fax: +33 1 4079 5772. E-mail address: [email protected] (B. Zanda). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.05.016
Chondrites are considered the most primitive rocks of the solar system because their chemical composition is similar to that of the Sun and they contain calcium– aluminium-rich inclusions (CAI) and related amoeboid
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
olivine aggregates (AOA). CAI, interpreted both as refractory condensates and evaporation residues, are the oldest objects known so far to have formed in the solar system [1]. Chondrites are also made of up to 80% chondrules, sub-millimeter spherical particles formed from a melt, some of which appear as old as CAI [2]. These high temperature petrologic components are embedded in a volatile-rich, low-temperature, finegrained matrix. Except for the CI group, all chondrites experienced chemical fractionation involving refractory/volatile and metal/silicate separation. Variations in the bulk concentration and redox state of Fe were used to class chondrites into several major groups [3] within which abundances of refractory, siderophile and volatile elements define well-known patterns (e.g. [4–6]). Chondrite groups are now also known to each have specific isotopic signatures in oxygen [7–9], copper [10], zinc [11] and chromium [12] as well as distinct populations of petrologic components [13] in terms of their relative abundances and sizes (Fig. 1). The chemical variations between chondrite groups have been interpreted in terms of nebular fractionations of refractory, metallic [4,5] and volatile components [6]. Such chemical and isotopic variations might reflect spatial or temporal heterogeneities in the nebular dust and gas, which would have generated reservoirs from which the different chondrites and their specific population of petrologic components would have formed. Alternatively, only oxygen isotopic compositions of chondritic components reflect spatial or temporal heterogeneities of the nebular dust and gas, whereas bulk oxygen isotopic compositions of chondrites simply reflect different proportions of isotopically distinct petrologic components (chondrules + matrix and CAI + AOA). The components of chondritic meteorites display a hierarchy of oxygen isotopic compositions [14]. The most 16O-rich are unmelted CAI and AOA. Melted CAI range from similar values (δ17O and δ18O ~− 50‰) to less 16O-rich compositions. Then come Al-rich chondrules with relict CAI, followed by Al-rich chondrules without CAI. Type I (reduced) chondrules are generally poorer in 16O, though some have extremely 16O-rich olivine grains, which may be relicts of AOAs [15]. There is some overlap of isotopic composition, between type I and type II (oxidized) chondrules, but type I chondrules are on average more 16O-rich than type II chondrules in CRs [14,16]. There are not a lot of isotopic data for classified chondrules in ordinary chondrites (OC), but a similar trend is likely: Clayton et al. [9] found that small chondrules in OC are more 16 O-rich than large ones and type I chondrules are
651
Fig. 1. Reflected views of polished sections from Ste Marguerite (H4 —a) and Renazzo (CR2—b) (field of view: 4.7 × 7 mm). In the H chondrite, chondrules are small, often fragmented and metal and sulfide grains (resp. white and grey) are located mostly in between chondrules. In the CR chondrite, the chondrules tend to be much larger and metal grains are almost systematically associated with chondrules, either inside or on their surfaces. Sulfide grains are not visible as they are only present in the fine-grained matrix which is much more abundant (∼ 35 vol.%) than it is in OC (∼ 10–15 vol.%).
known to be smaller than type II. High 16O contents in OC chondrules seem to be due to relict grains [17,18]. Type I chondrules in OC are poorer in 16O than those in carbonaceous chondrites (CC). Clayton et al. [8] suggested that the individual chondrules and CAI we now find in chondrites are the result of protracted mixing involving several generations of chondrule (and possibly also CAI) recycling. 16 O-poor compositions reflect the 16O-poor nebular gas within which CAI and chondrules were remelted and with which they exchanged [8]. CAI are known to have undergone isotopic and chemical exchange [19–21] and become less 16O-rich. Type I chondrules in CC are somewhat 16O-rich because of inherited RI material [22–25] or 16O-rich forsteritic xenocrysts [26,27]. Type II chondrules may be more 16O-depleted than type Is [9], possibly due to more extensive interaction with the nebular gas, and hence might be responsible for the 16Odepletion of OC compared to CC. The oxygen isotopic compositions of RIs and chondrules are consistent with
652
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
RI forming from an early 16O-rich gas [8,19–21] and the evolution of the gas to more 16O-poor compositions before chondrules are formed. The gas evolution may involve photodissociation, either in the precursor
molecular cloud or in the solar nebula, and subsequent ice transport [20,28–30]. Matrix may represent mixing between chondrule precursor silicate dust and extremely 17 O and 18O-enriched ice with compositions matching
Table 1 Volume % abundances of petrologic constituents (adjusted to 100%) in primitive chondrites (from this work and from literature data as quoted in text) and oxygen isotopic compositions from [7,8] δ17O
Δ17O
Δ18O
Δ17O calc
Δ18O calc
Type II ch ^
Ordinary chondrites Sharps (H3.4) Tieschitz (H/L3.6) Hallingeberg (L3.5) Bishunpur (L/LL3.1) Krymka (LL3.1) Semarkona (LL3.0) Chainpur (LL3.4)
44.9 36.3 31.3 33.6 19.4 24.7 21.0
36.2 48.4 55.1 48.9 65.2 58.1 61.0
6.4 4.9 3.8 3.6 1.7 1.5 1.9
0.1 0.1 0.1 0.1 0.1 0.1 0.1
12.4 10.3 9.7 13.9 13.6 15.6 16.1
3.95 5.55 5.02 6.00 5.37 6.09 5.72
2.70 3.83 3.59 4.11 3.95 4.24 4.01
0.65 0.94 0.98 0.99 1.16 1.07 1.04
1.25 1.72 1.43 1.89 1.42 1.85 1.71
0.70 0.89 1.00 0.90 1.16 1.04 1.09
1.92 1.77 1.73 2.03 2.01 2.15 2.19
Enstatite chondrite Sahara 97096 (EH3)
60.0
0.0
12.9
0.0
27.0
5.52
2.83
− 0.04
2.69
0.16
2.97
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100.0 100.0 100.0
16.16 15.79 16.84
8.79 8.60 9.23
0.39 0.39 0.47
7.37 7.19 7.61
0.16 0.16 0.16
8.16 8.16 8.16
CM chondrites Murray Mighei Murchison Cold Bokkeweld Essebi Bells Nogoya
34.9 29.4 27.1 18.1 15.1 11.8 8.3
0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.9 0.8 0.6 1.0 0.9 3.1 2.5
4.8 8.6 8.4 6.0 4.7 0.9 1.7
59.3 61.2 64.0 74.9 79.2 84.2 87.5
7.53 7.57 7.30 10.01 11.65 9.59 8.36
0.85 1.44 1.20 2.76 5.02 3.13 2.35
− 3.07 − 2.50 − 2.60 − 2.45 − 1.04 − 1.86 − 2.00
6.68 6.13 6.10 7.25 6.63 6.46 6.01
− 1.77 − 3.25 − 3.16 − 2.21 − 1.73 − 0.21 − 0.53
5.26 5.40 5.60 6.38 6.68 7.04 7.27
CR chondrites Renazzo Al Rais
55.1 37.8
0.0 0.0
7.2 3.6
2.5 2.4
35.1 56.1
6.25 10.94
2.29 4.68
− 0.96 − 1.01
3.96 6.26
− 0.85 − 0.81
3.54 5.04
CH chondrite ALH85085
74.8
0.0
20.0
0.2
5.0
2.08
− 0.54
− 1.62
2.62
0.16
1.40
CO chondrites Kainsaz Lance Felix Ornans Warrenton
47.7 48.4 46.8 38.9 41.1
2.1 1.1 2.9 3.6 3.3
6.5 1.4 2.3 1.6 1.5
10.8 13.9 12.8 13.2 14.7
33.0 35.1 35.3 42.8 39.4
− 1.85 − 0.31 − 2.38 − 1.71 − 0.73
− 5.68 − 4.51 − 5.83 − 5.34 − 4.45
− 4.72 − 4.35 − 4.59 − 4.45 − 4.07
3.83 4.20 3.45 3.63 3.72
− 4.09 − 5.36 − 4.87 − 5.04 − 5.63
3.39 3.54 3.55 4.08 3.84
CV chondrites Efremovka Vigarano Arch Allende Bali Grosnaja Mokoia Kaba
43.5 51.6 48.9 45.9 33.3 38.6 50.4 34.3
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
5.0 1.5 2.9 1.4 1.6 1.5 2.0 3.4
13.2 11.1 10.8 13.0 12.2 6.4 6.4 9.0
38.3 35.9 37.5 39.6 52.9 53.6 41.2 53.3
− 3.42 1.19 − 0.94 1.51 − 1.21 3.21 3.52 2.18
− 6.86 − 3.61 − 5.44 − 2.73 − 5.53 − 1.46 − 0.91 − 2.37
− 5.08 − 4.23 − 4.95 − 3.52 − 4.90 − 3.13 − 2.74 − 3.50
3.44 4.80 4.50 4.24 4.32 4.67 4.43 4.55
− 5.09 − 4.26 − 4.12 − 5.01 − 4.68 − 2.37 − 2.40 − 3.43
3.77 3.59 3.71 3.86 4.81 4.86 3.98 4.83
CI chondrites Orgueil Alais Ivuna
Metal #
δ18O
Type I ch silicates ⁎, ^
CAI + AOA
Matrix
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
those of cosmic spherules [31] and of unmelted micrometeorites from very friable asteroids [32]. The data base for chondrite components is now almost sufficient to develop a comprehensive understanding of the bulk compositions and oxygen isotope systematics of chondrites. We know, for example, that refractory inclusions can represent up to 15 vol.% of carbonaceous chondrites (CC), but are rare (< 1%) in OC [13]. Chondrules have a wide range of bulk compositions, inherited from precursors or explicable by evaporation and condensation [33–41]. Type I are volatile-depleted, Fe-metal-bearing but FeO-poor (unless metamorphosed), while type II are undepleted or even enriched in volatile elements, and FeO-rich. Type II chondrules are rare in CC but abundant in OC [36– 38,41] in which they are larger in diameter than type Is by 20–25%, which translates to a difference in mass close to a factor of 2 [40]. In order to develop a model for chondritic chemical and isotopic reservoirs derived from varying proportions of the petrologic components that make up chondrites, we have assembled data on the abundance of inclusions, type I chondrules, type II chondrules, matrix and metal grains. We find we can explain chondrite chemical and oxygen isotopic compositions as functions of these abundances. 2. Method Our analysis of ordinary chondrites was restricted to observed falls (finds may have lost part of their metal) and unequilibrated meteorites (the chemical difference between type I and type II chondrules gradually disappears with parent-body metamorphism). The seven most studied OC for which oxygen isotopic data are available [8] were selected: Sharps (H3.4), Tieschitz (H/L3.6), Hallingeberg (L3.5), Bishunpur (L/LL3.1), Semarkona (LL3.0), Chainpur (LL3.4), Krymka (LL3.1) (the numbers in parenthesis reflect the degree of equilibration in the parent body, with 3.0 being most
653
unequilibrated, and equilibration of olivine being complete by 4). We used optical microscopy, scanning electron microscopy (SEM) mapping and electron microprobe (EMP) control analyses to determine the mean radius of all the chondrules present in polished sections of these meteorites with areas ranging between 1.5 and 2.5 cm2. We classified chondrules as type I (Fa ≤ 8) or type II (Fa > 8). Matrix abundances of [42] were used and metal abundance in these meteorites was derived (after a density correction) by summing metallic Fe, Ni and Co in averages of all the wet chemistry data available [43]. The total chondrule fraction was taken as 100%—vol.% metal–vol.% matrix and split into type I and type II chondrules based on our measurements. To allow a comparison, the CC data of McSween [36–38] were similarly normalized to 100%. The standard deviations on petrologic component abundances (estimated by comparing values for half surfaces) range between 0.2% and 4%. The samples considered in this study are listed in Table 1 and displayed in a 3-isotope plot in Fig. 2. For convenience, small variations of oxygen isotopic compositions are described in terms of their deviation in parts per thousand from a terrestrial standard, SMOW (Standard Mean Ocean Water): δ 17 O = {[( 17 O / 16 O)sample / ( 17 O / 16 O)SMOW] − 1} × 103 ; δ 18 O = {[( 18 O / 16 O) sample / ( 18 O / 16 O) SMOW ] − 1} × 103. Δ17O = δ17O − 0.52δ18O was defined by Clayton et al. [8]. It corresponds to the excess of δ17O relative to the terrestrial fractionation line (TFL) and reflects the group (or the nebular reservoir) to which a sample belongs. Similarly, we define Δ18O = δ18O − δ17O to describe mass fractionation effects exclusively, and independently of the group. Δ18O corresponds to the excess of δ18O with respect to the slope 1 line going through the origin (δ17O = 0; δ18O = 0) and is shown in a 3-isotope plot in Fig. 2. The calculated (Δ17O, Δ18O) are equivalent to the measured (δ17 O, δ18 O) for characterizing a sample. Their interest lies in
Notes to Table 1: Also shown are Δ17O and Δ18O. After [8], the first one is defined as δ17O–0.52δ18O and measures in the oxygen 3-isotope plot the distance along the δ17O axis between the terrestrial fractionation line (TFL) and the fractionation line on which the sample is located. In a similar fashion, we define Δ18O as δ18O–δ17O, which measures the distance along the δ18O axis between the slope 1 line going through (0,0) and the parallel line on which the sample is located. The last two columns display Δ17O and Δ18O estimated from the petrologic constituents listed in columns 2–6 using the correlation coefficients derived in this work. The proportion of RIs in all the OC are unknown and were set at 0.1 vol.% according to [13]. * Metal volume was substracted from Type I chondrules in carbonaceous chondrites because metal is dominantly present associated with chondrules or as “metallic chondrules” commonly interpreted as having fallen from chondrules (e.g. Fig. 1 and [46,50,63]), so that comparisons could be made with CH, EH and ordinary chondrites in which metal is found predominantly outside chondrules. ^ “Lithic fragments”, “lithic and mineral fragments” or “monomineralic grains and fragments” from [36–38] that may amount to up to 20% of the counted surfaces were treated as chondrules (in relative proportions of the two types when present). The abundances of Type II chondrules in CH, CR and CV chondrites are unknown. They are low enough (at most a few percent) not to affect oxygen isotopes. # Magnetite in CV chondrites has been added to metal because it is believed to have originated from metal oxidation in the course of parent-body alteration (e.g. [64]). For ordinary chondrites, metal based on wet chemistry analyses of [45] and [65] is listed.
654
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
Fe/Mg and Ni/Mg in the bulk OC and the single CH also correlate with type I chondrule abundances in the same chondrites. Conversely, the correlation is poor for the CC in which metal remained associated with chondrules: Fe and Ni are approximately in CI proportions relative to Mg. 4. Oxygen isotopes 4.1. Nebular effects
Fig. 2. Oxygen 3-isotope plot showing the samples considered in this work and the meaning of Δ17O and Δ18O in the case of the CH chondrite. SL1: slope 1 line going through the origin (0,0); TFL— terrestrial fractionation line; Y-R: Young and Russell line (after [48]); dashed lines—examples of constant Δ17O and Δ18O.
decoupling (nebular) group effects from mass fractionation effects as Δ17O only depends on the former and Δ18O on the latter.
In Fig. 4, Δ17O from Clayton et al. [8] is plotted as a function of the volume proportion of type II chondrules we measured in OC. This figure shows that the excess of 17 O relative to the terrestrial fractionation line (TFL) of an OC is directly related to its abundance of type II chondrules, and that there is a continuum between LL and L chondrites. There is only one sample of H chondrite, which appears isolated, in part because unaltered H chondrites which escaped metamorphism sufficiently to permit accurate measurements of
3. Chondrules and metal in ordinary chondrites OC are subdivided into 3 groups based on their total Fe and metal abundance: H (High Fe), L (Low Fe) and LL (Low Fe, Low metal) [3]. In an attempt to explain their bulk chemical and isotopic compositions, we determined type I and type II chondrule relative abundances in seven unequilibrated falls spanning the three OC groups, including intermediate H/L Tieschitz. Such data, including also refractory inclusion abundances, have long been available for CC [36–38] and for a primitive enstatite chondrite (EC) [44] (Table 1). Although the proportion of chondrules remains ≈80 vol.% in OC, their composition varies significantly. The averages for the H, L and LL groups are respectively 45%, 28% and 22% type I chondrule material versus 36%, 58% and 61% type II chondrule material. The abundance of metal varies between 1.5 vol.% (in LL chondrite Semarkona) and 6.5 vol.% (in H chondrite Sharps) [45] and correlates with the abundance of type I chondrule material (Fig. 3a) and the ratio of type I to type II. The data can be fitted by a linear regression, though with considerable scatter when CMs, COs, CVs and CRs, in which metal is usually retained in or at the surface of chondrules (Fig. 1b), are included. A power law (Fig. 3a) allows us to fit EH and CH data as well as OC, indicating a possible relationship between the chondrites in which metal is interstitial to chondrules [46] (Fig. 1). Fig. 3b shows that chondrite-normalized
Fig. 3. (a) Relationship between the amount of metal and the amount of Type I chondrule material in chondrites. Filled symbols for chondrites in which metal is well separated from chondrules (OC, EH and CH) + CR, open symbols for all other CC. Regression curves for OC only. (b) Chondrite normalized Ni/Mg and Fe/Mg in the same objects (wet chemistry bulk analysis not available for EH Sahara 97096, nor for LL3.4 Chainpur). Linear regression coefficients: R2 = 0.944 for Fe/Mg and R2 = 0823 for Ni/Mg. Dotted line in (b) repeats linear regression on OC metal from (a).
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
655
constant for refractory inclusions were determined based on CV and CI chondrites only as the regression is better (R2 = 0.83 if CM, CO, CR and CH chondrites are also considered, which we believe to result from the uncertainties in RI abundance measurements when RI are either very small as in CO chondrites or scarce as in all the other cases). Based on these correlations, we construct a model for bulk rock chondritic Δ17O by making a regression on type IIs and refractory inclusions (RI) in OC, CI and CV chondrites: Fig. 4. Relationship between Δ O and the amount of Type II material present in ordinary chondrites (Sh = Sharps, H3.4; T = Tieschitz, H/ L3.6; B = Bishunpur, L/LL3.1; H = Hallingeberg, L3.5; S = Semarkona, LL3.0; C = Chainpur, LL3.4; K = Krymka, LL3.1). L/LL chondrite Bishunpur may be misclassified as it falls in the field of L or H/L chondrites, but this more likely reflects heterogeneity in the distribution of Type I and Type II chondrules and the need to study a larger surface. Standard deviations on vol.% type II material (estimated by comparing values for half surfaces): Sh, 0.5%; T, 4%; B, 3.7%; H, 1.6%; S, 3.4%; C, 0.2%; K, 1.3%. 17
petrologic component are scarce, and their oxygen isotopic composition is not known except for Sharps (H3.4). The difference in the proportion of type II chondrules may explain the relative positions of H, L and LL chondrites on the 3-isotope plot. We searched for similar relationships between petrologic components and oxygen isotope compositions using literature data [7–9,36–38] and found that refractory inclusion abundances in CC correlate negatively with Δ17O (R2 = 0.92—Fig. 5). Note that although all the carbonaceous chondrites used in this study are plotted in Fig. 5, the correlation coefficient and origin
Fig. 5. Correlation of Δ17O with volume of RIs in CCs. Because of the scatter of the data, in particular for CM and CO, the slope and origin constant of the linear correlation were estimated using only CI and CV chondrites, which yields a better regression (R2 = 0.92 vs. R2 = 0.83). The correlation between Δ17O and volume of Type IIs is also shown: the two lines intercept the origin at ∼ 0.17, close to the CI point.
D17 OðxÞ ¼ 0:398 vol:% RI þ 0:0157 vol:% Type II þ 0:2
ð1Þ
Such equations can be solved by substituting values of 0% and 100% to identify virtual isotopic components associated with RI, etc. The virtual isotopic components carried by Type I chondrules and matrix both have Δ17O = 0.2‰ and consequently do not appear in Eq. (1). 4.2. Mass fractionation effects As explained above in the Method section (Fig. 2), Δ18O describes the excess of δ18O with respect to the slope 1 line going through the origin (δ17O = 0; δ18O = 0) and is characteristic of the amount of mass fractionation experienced by a sample. In agreement with previous observations of Weisberg et al. [47], matrix abundance correlates positively with Δ18O (R2 = 0.875—Fig. 6): D18 O ðxÞ ¼ 0:0712 vol:% matrix þ 1:0
ð2Þ
In (2) the regression line has been set to intercept the X-axis at 1.0‰ (instead of 1.3‰) so that 0% matrix corresponds to the Young and Russell (Y-R) [48] line (i.e. to aqueously unaltered CAI material). Moreover, the isotopic components carried by chondrules and RI
Fig. 6. Correlation of Δ18O with volume of matrix.
656
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
Fig. 7. Δ17O derived from the proportions of chondritic components (mostly RIs in CC and Type II chondrules in OC) using Eq. (1) is plotted as function of measured values in primitive chondrites.
lie close to the Y-R line and consequently do not affect Δ18O. Figs. 7 and 8, where calculated Δ17O and Δ18O are plotted as functions of the measured values [7–9], show that Eqs. (1) and (2) adequately reproduce the observations and therefore that the oxygen isotopic signatures of OC, CC and EC chondrites can be successfully estimated from the proportions of their petrologic components.
5. Discussion 5.1. Formation of OC reservoirs by dynamic sorting Separation of FeNi metal from silicates is known to be involved in the chemical fractionation of chondritic reservoirs [3,4,49]. It has been suggested that metal formed within Type I chondrules [46,50]. The correlation between the abundances of metal and Type I chondrules in OC displayed in Fig. 3 appears to support this hypothesis. The correlation, however, is stronger in those meteorites (OC, EH and CH) in which metal has physically separated from the chondrules where it formed (Fig. 1a) than in CC where metal is still sequestered in chondrules (Fig. 1b), suggesting another mechanism may have played a role. In addition, the formation of metal in Type I chondrules by reduction should not have affected the Fe and Ni content of the bulk rocks and hence does not explain the correlation of bulk Fe and Ni with Type I chondrules and metal abundances (Fig. 3b). This correlation is consistent with the idea that metal segregation from the silicates is responsible for Fe and Ni fractionation between nebula
(CI) and parent body. These two arguments suggest that the correlation between the abundances of metal and Type I chondrules (Fig. 3a) reflects dynamic sorting during accretion rather than metal formation by reduction within chondrules. Metal grains, smaller and denser than silicate chondrules, have stopping times [51] due to gas drag closer to those of the smaller chondrules [51–53] and, in a weakly turbulent nebula, would have accreted preferentially with the smaller (Type I) chondrules in H chondrites. However, such a process would not have worked as efficiently in CC, where metal remains within chondrules, or attached to their surfaces (Fig. 1b). Rather than being derived from parental (more or less reduced) reservoirs, OC groups have been suggested to result from differential accretion of larger objects in LLs, and smaller or denser objects in Hs [8,54]: our data show that the larger oxidized Type IIs are much more abundant than Type Is in the former, and that reduced Type Is and metal grains exceed Type IIs in the latter. The varying proportions of the two types of chondrules in H, L and LL chondrites, and of metal (also a product of chondrule formation), show that the formation of the OC reservoirs post-dates chondrule formation and is related to accretion, which is consistent with the conclusion reached by Alexander [55]. 5.2. Isotopic components carried by chondrules, refractory inclusions and matrix: model and current compositions The conclusion that chondritic reservoirs formed during accretion is also demanded by the correlations, in
Fig. 8. Δ18O derived from the proportion of matrix using Eq. (2) is plotted as function of measured values in primitive chondrites.
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
primitive chondrite falls, between Δ 17 O and the abundances of refractory inclusions and chondrules, and between Δ18O and the abundance of matrix. Oxygen isotopic signatures are considered to characterize each of the chondritic reservoirs [7–9]. Our results indicate that the various petrologic components within a chondrite did not form in a pre-existing reservoir with a fixed chemical and isotopic composition, but that the chondrite groups originated by mixing petrologic components that carried isotopically and chemically distinct compositions, hereafter referred to as “model isotopic compositions”. Although these model isotopic compositions are no longer present in chondrites, they can still be inferred from the correlations that associate the petrologic component abundances and the oxygen isotopic signatures of the bulk chondrites: RI contain a model isotopic component rich in 16O, Type II chondrules one poor in 16 O, and matrix one enriched in 17O and 18O. By setting petrologic component abundances at 0% or 100% in our equations, we can extract the model compositions of the isotopic components associated with each of them (Table 2). For the refractory inclusions, the end-member has a Δ17O value of − 40 ± 18‰, more 16O-rich than, but within error of, the observed value for minerals measured in CAI, and similar to the lower limit to the solar value of Hashizume and Chaussidon [56]. The isotopic component carried by Type II chondrules, has a Δ17O value of 1.7 ± 0.1‰, corresponding to the most 16 O-poor chondrules analysed, excluding R chondrites. The virtual isotopic component carried by matrix corresponds to Δ18O = 8.2 ± 0.5‰. Type I chondrules contain a fourth isotopic component, defined when all the petrologic components in Eqs. (1) and (2) are zero, at the point Δ17O = 0.2 ± 0.4‰ on the Y-R line in the oxygen 3-isotope plot. Although these model isotopic compositions are no longer found in chondrites, trends are retained in the present isotopic composition of the petrologic components that once carried them: CAI remain 16O-rich and the isotopic and chemical exchange they are all known to have undergone [19–21] explains why even the most pristine ones have become less 16O-rich than our calculated component. Al-rich chondrules (and espeTable 2 Summary of isotopic model component compositions
1 2 3 4
657
cially the ones containing CAI-remnants) remain more 16 O-rich than Type I chondrules, which are in turn more 16 O-rich than Type II chondrules [23]. Type I chondrules in CC are more 16O-rich than our calculated value because of inherited CAI and AOA material or 16O-rich forsteritic xenocrysts [22–27]. This difference between the model isotopic compositions of chondrules, RI and matrix and their current ones may be ascribed to nebular or parent-body transformations and/or mixing. In the case of the matrix, for example, it could be argued that at the time of accretion it consisted of a mixture of dry silicates carrying the isotopic composition of Type I material, together with ice with an isotopic composition close to that of cosmic spherules [31]. The water, produced from the melting of the ice after accretion and reheating on the parent-body, would have circulated and reacted with minerals throughout the chondrite, thus dispersing the isotopic component originally contained within the matrix. As this process would work in a closed system on the parent-body, the correlation between the amount of matrix and Δ18O would be preserved even though the matrix itself would no longer hold the model isotopic composition. It is more difficult to find a straightforward explanation in the case of the Δ 17 O variations, carried by chondrules and refractory inclusions. It is possible, however, that the RIs reacted with chondrule material at the time of accretion or, more likely, as a result of successive episodes of CAI and chondrule formation involving recycling of pre-existing material. This would imply, as suggested by Clayton et al. [8], that the individual chondrules and RI we now find in chondrites are the result of protracted mixing involving several generations of chondrule (and possibly also CAI) recycling. Yet another possibility is that the mixing between RIs and chondrules occurred on an early generation of parent-body that preceded the formation of the chondrules we now find in chondrites as advocated by Libourel et al. (LPSC, 2006). Such a body might correspond to the mixes 4 and 5 (comprising Type I chondrules, CAI and metal) from which Zanda and Hewins [57] generate CM, CO, CR, CH and EH chondrites (Fig. 9). 5.3. Origin of the isotopic components
Carrier
Δ17O (‰)
Δ18O (‰)
Chemical dominance
CAI + AOA Type II chondrules Matrix Type I chondrules
− 39.6 1.7 0.2 0.2
1.0 1.0 8.2 1.0
CaO, Al2O3 FeO FeS and H2O MgO
We have identified four isotopic components (Table 2), which explain chondrite compositions, carried by matrix, refractory inclusions, Type II chondrules and Type I chondrules. The 16O-rich composition of the model component carried by RI is likely to reflect that of
658
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
Fig. 9. Oxygen 3-isotope plot showing the model components and the effect of adding them to the Type I one, located at the intercept between the Y-R line and the TFL. The first component (contained in RIs) is too far along the Y-R line to be plotted. The primitive chondrites used to establish the correlations are also indicated.
the early solar nebula in the vicinity of the Sun: it is similar to the lower limit to the solar value of Hashizume and Chaussidon [56] and in the currently favored model of self-shielding [28–30], the original Δ17O of the solar nebula is ~− 25‰.). AOAs appear as aggregates of solar nebula condensates that escaped melting, so they probably reflect the oxygen isotopic composition of the early nebular gas (Δ17O c − 25‰). 16 O-poor compositions, on the other hand, may reflect the later 16O-poor nebular gas within which CAI and chondrules were remelted and with which they exchanged [8]. The model isotopic compositions of Type II chondrules are more 16O-depleted than those of Type Is, probably due to more extensive interaction with the nebular gas, and are responsible for the 16Odepletion of OC compared to CC. The Δ17O of the calculated Type I chondrule component is close to that of CI (Δ17O ≈ 0.4‰), which suggests that, except for low temperature hydration [58,59], both materials may share the same source. The co-linearity of the isotopic components carried by refractory inclusions, Type Is and Type IIs is striking. It is consistent with RI forming from an early 16O-rich gas [8,19–21], the evolution of the gas to more 16O-poor compositions (possibly involving photodissociation [28,29] and subsequent ice transport [20,30]), followed by the formation of the isotopic component carried by Type I chondrules. This latter component is very similar to the terrestrial planet progenitors of Young et al. [60] (who derive the primitive Earth mantle [61] from these by hydration), in agreement with the bulk chemical resemblance between the Earth's mantle and Type I chondrules [62]. It probably represents the precursor
material from which matrix and Type II chondrules were derived, by hydration for matrix and repeated processing in a gas which continued to evolve to more 16Opoor compositions for Type II chondrules. Matrix may represent mixing between such Type I precursor silicate dust and extremely 17O and 18O-enriched ice with compositions matching those of cosmic spherules [31] and of unmelted micrometeorites from very friable asteroids [32]. Formation of chondrules or CAIs from such a material would have removed the 17O and 18Orich water it originally contained bringing its oxygen signature back towards the present chondritic compositions. Alternatively, matrix can be accounted for by lowtemperature aqueous alteration of a RI + Type II chondrule mixture: it may represent the scattered hydrous regolith of small early objects formed in a distant position sputtered by impacts and subsequently re-accreted with dry material from the more proximal parts of the Solar System. The apparent homogeneity of this matrix component suggests that it remained long enough in space to be efficiently mixed by turbulent movement before its final accretion to the meteorite parent body. 6. Conclusion The petrologic constituents of chondrites, Type I and Type II chondrules, matrix and refractory inclusions each carry a distinctive oxygen isotopic component. The origin of these four isotopic components still needs to be clarified. Our results, however, demonstrate that chemical and isotopic fractionations between solar system bodies were established after CAI and chondrule formation, and involved differential accretion of these high temperature objects in a turbulent regime. Similar mechanisms may have played a role in establishing the chemical and isotopic compositions of the terrestrial planets. Acknowledgements R.N. Clayton is gratefully acknowledged for kind suggestions made on an early version of this manuscript. B.Z. thanks Gene Jarosewich for sending her an electronic version of his meteorite analysis database and Claude Perron for sharing with her his oxygen data spreadsheet. We thank Sasha Krot and an anonymous reviewer for detailed comments, which led to considerable improvements to the manuscript. This work was supported by the Program National de Planétologie and NASA Cosmochemistry Program grants NNG05GK11G and NAG5-10494.
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
References [1] Y. Amelin, A.N. Krot, I.D. Hutcheon, A.A. Ulyanov, Lead isotopic ages of chondrules and calcium–aluminum-rich inclusions, Science 297 (2002) 1678–1683. [2] M. Bizzarro, J.A. Baker, H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431 (2004) 275–278. [3] H.C. Urey, H. Craig, The composition of the stone meteorites and the origin of the meteorites, Geochim. Cosmochim. Acta 4 (1953) 36–82. [4] J.W. Larimer, J.T. Wasson, Siderophile element fractionation, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 416–435. [5] J.W. Larimer, J.T. Wasson, Refractory lithophile elements, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 394–415. [6] H. Palme, Chemical and isotopic heterogeneity in protosolar matter, Philos. Trans. R. Soc. Lond., A 359 (2001) 2061. [7] R.N. Clayton, Oxygen isotope studies of carbonaceous chondrites, Geochim. Cosmochim. Acta 63 (1999) 2089–2104. [8] R.N. Clayton, T.K. Mayeda, J.N. Goswami, E.J. Olsen, Oxygen isotope studies of ordinary chondrites, Geochim. Cosmochim. Acta 55 (1991) 2317–2337. [9] R.N. Clayton, T.K. Mayeda, E.J. Olsen, J.N. Goswami, Oxygen isotopic compositions of ordinary chondrites and their chondrules, Lunar Planet. Sci. XXI (1990) 201–202. [10] J.M. Luck, D. Ben Othman, J.A. Barrat, F. Albarède, Coupled 63 Cu and 16O excesses in chondrites, Geochim. Cosmochim. Acta 67 (2003) 143–151. [11] J.M. Luck, D. Ben Othman, F. Albarède, Zn and Cu isotopic variations in chondrites and iron meteorites: early solar nebula reservoirs and parent-body processes, Geochim. Cosmochim. Acta 69 (2005) 5351–5363. [12] A. Trinquier, J.-L. Birck, C.J. Allègre, 54Cr anomalies in the solar system: their extent and origin, Lunar Planet. Sci. XXXVI (2005) (1259.pdf). [13] A.J. Brearley, R.H. Jones, Chondritic meteorites, in: J.J. Papike (Ed.), Planetary Materials, Reviews in Mineralogy, vol. 36, Mineralogical Society of America, Washington, DC, 1998, pp. 1–398. [14] A.N. Krot, G. Libourel, M. Chaussidon, Oxygen isotope compositions of chondrules in CR chondrites, Geochim. Cosmochim. Acta 70 (2006) 767–779. [15] R.H. Jones, L.A. Leshin, Y. Guan, Z.D. Sharp, T. Durakiewicz, A.J. Schilk, Oxygen isotope heterogeneity in chondrules from the Mokoia CV3 carbonaceous chondrite, Geochim. Cosmochim. Acta 68 (2004) 3423–3438. [16] L.A. Leshin, K.D. McKeegan, C. Engrand, B. Zanda, M. BourotDenise, R.H. Hewins, Oxygen isotopic studies of isolated and chondrule olivine from Renazzo and Allende, Meteorit. Planet. Sci. 33, p. A93 33 (1998) 93. [17] N.T. Kita, H. Nagahara, S. Tomomura, S. Tachibana, J.W. Valley, Systematic oxygen isotopic variations among chondrules from the least equilibrated ordinary chondrites: improved ion microprobe precision, Lunar Planet. Sci. XXXVII (2006) (1496.pdf). [18] D.W.G. Sears, I. Lyon, J. Saxton, G. Turner, The oxygen isotope properties of olivines in the Semarkona ordinary chondrite, Meteorit. Planet. Sci. 33 (1998) 1029–1032. [19] P. Cassen, Nebular thermal evolution and the properties of primitive planetary materials, Meteorit. Planet. Sci. 36 (2001) 671–700.
659
[20] A.N. Krot, I.D. Hutcheon, H. Yurimoto, J.N. Cuzzi, K.D. McKeegan, E.R.D. Scott, G. Libourel, M. Chaussidon, J. Aléon, M.I. Petaev, Evolution of oxygen isotopic composition in the inner solar nebula, Astrophys. J. 622 (2005) 1333–1342. [21] E.R.D. Scott, A.N. Krot, Oxygen isotopic compositions and origins of calcium–aluminum-rich inclusions and chondrules, Meteorit. Planet. Sci. 36 (2001) 1307–1319. [22] A.N. Krot, I.D. Hutcheon, K. Keil, Plagioclase-rich chondrules in the reduced CV chondrites: evidence for complex formation history and genetic links between calcium–aluminum-rich inclusions and ferromagnesian chondrules, Meteorit. Planet. Sci. 37 (2002) 155–182. [23] A.N. Krot, R.H. Jones, M. Yoshitake, H. Yurimoto, Oxygen isotopic compositions of chondrules, Workshop on Oxygen in the Earliest Solar System (LPI Contribution no. 1278, Gatlinburg, Tennessee, 2005). [24] A.N. Krot, K. Keil, Anorthite-rich chondrules in CR and CH carbonaceous chondrites: genetic link between Ca,Al-rich inclusions and ferromagnesian chondrules, Meteorit. Planet. Sci. 37 (2002) 91–111. [25] K. Misawa, N. Nakamura, Origin of Refractory Precursor Component of Chondrules from Carbonaceous Chondrites, in: R. Hewins, R. Jones, E. Scott (Eds.), Chondrules and the Protoplanetary Disk, Cambridge University Press, Cambridge, 1996, pp. 99–105. [26] A. Pack, H. Yurimoto, H. Palme, Petrographic and oxygenisotopic study of refractory forsterites from R.-chondrite Dar al Gani 013 (R3. 5–6), unequilibrated ordinary and carbonaceous chondrites, Geochim. Cosmochim. Acta 68 (2004) 1135–1157. [27] H. Yurimoto, J.T. Wasson, Extremely rapid cooling of a carbonaceous-chondrite chondrule containing very 16O-rich olivine and a 26Mg-excess, Geochim. Cosmochim. Acta 66 (2002) 4355–4363. [28] R.N. Clayton, Solar System: self-shielding in the solar nebula, Nature 415 (2002) 860–861. [29] J.R. Lyons, E.D. Young, CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula, Nature 435 (2005) 317–320. [30] H. Yurimoto, K. Kuramoto, Molecular cloud origin for the oxygen isotope heterogeneity in the solar system, Science 305 (2004) 1763–1766. [31] T. Yada, T. Nakamura, T. Noguchi, T. Ushikubo, N. Matsumoto, H. Kojima, N. Takaoka, Variations of oxygen isotopic compositions of silicate spherules collected from Antarctic Blue Ice, Lunar Planet. Sci. XXXIV (2003) (1587.pdf). [32] G. Matrajt, Y. Guan, L. Leshin, S. Taylor, M. Genge, D. Joswiak, D. Brownlee, Oxygen isotope measurements of bulk unmelted antarctic micrometeorites, Workshop on Dust in Planetary Systems, Kauaiʻi, Hawaiʻi, 2005, p. 4060. [33] J.N. Grossman, A.E. Rubin, H. Nagahara, E.A. King, Properties of chondrules, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 619–659. [34] R.H. Hewins, Y. Yu, B. Zanda, M. Bourot-Denise, Do nebular fractionations, evaporative losses, or both, influence chondrule compositions? Antarct. Meteor. Res. 10 (1997) 275. [35] R.H. Jones, FeO-rich, porphyritic pyroxene chondrules in unequilibrated ordinary chondrites, Geochim. Cosmochim. Acta 60 (1996) 3115–3117. [36] H.Y. McSween Jr., Petrographic variations among carbonaceous chondrites of the Vigarano type, Geochim. Cosmochim. Acta 41 (1977) 1777–1790.
660
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
[37] H.Y. McSween Jr., Carbonaceous chondrites of the Ornans type —a metamorphic sequence, Geochim. Cosmochim. Acta 41 (1977) 477–481. [38] H.Y. McSween, Matrix variations in CM chondrites, Lunar Planet. Sci. X (1979) 810–812. [39] E.R.D. Scott, G.J. Taylor, Chondrules and other components in C, O, and E chondrites Similarities in their properties and origins, J. Geophys. Res. Supp. 88 (1983) 275. [40] D.W.G. Sears, S. Huang, P.H. Benoit, Open-system behaviour during chondrule formation, in: R.H. Hewins, R.H. Jones, E.R.D. Scott (Eds.), Chondrules and the Protoplanetary Disk, Cambridge University Press, Cambridge, 1996, pp. 221–231. [41] B. Zanda, Chondrules, Earth Planet. Sci. Lett. 224 (2004) 1–2. [42] G.R. Huss, K. Keil, G.J. Taylor, The matrices of unequilibrated ordinary chondrites—implications for the origin and history of chondrites, Geochim. Cosmochim. Acta 45 (1981) 33–51. [43] J. Köblitz, Metbase 6.0, Meteorite Data Retrieval Software, CDROM 2003. [44] M.K. Weisberg, M. Prinz, Sahara 97096: a highly primitive EH3 chondrite with layered sulfide-metal-rich chondrules, Lunar Planet. Sci. XXIX (1998) 1741. [45] E. Jarosewich, Chemical analyses of meteorites—a compilation of stony and iron meteorite analyses, Meteoritics 25 (1990) 323–337. [46] A.J. Campbell, B. Zanda, C. Perron, A. Meibom, M.I. Petaev, Origin and thermal history of FeNi-Metal in primitive chondrites, in: A.N. Krot, E.R.D. Scott, B. Reipurth (Eds.), Chondrites and the Protoplanetary Disk, Proceedings of the Astronomical Society of the Pacific Conference Series, 2005, pp. 407–431. [47] M.K. Weisberg, M. Prinz, R.N. Clayton, T.K. Mayeda, The CR (Renazzo-type) carbonaceous chondrite group and its implications, Geochim. Cosmochim. Acta 57 (1993) 1567–1586. [48] E.D. Young, S.S. Russell, Oxygen reservoirs in the early solar nebula inferred from an Allende CAI, Science 282 (1998) 452. [49] D.W.G. Sears, R.T. Dodd, Overview and classification of meteorites, meteorites and the early solar system, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 3–31. [50] B. Zanda, M. Bourot-Denise, C. Perron, R.H. Hewins, Origin and metamorphic redistribution of silicon, chromium, and phosphorus in the metal of chondrites, Science 265 (1994) 1846–1849. [51] J.N. Cuzzi, R.C. Hogan, J.M. Paque, A.R. Dobrovolskis, Sizeselective concentration of chondrules and other small particles in
[52]
[53]
[54]
[55]
[56]
[57] [58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
protoplanetary nebula turbulence, Astrophys. J. 546 (2001) 496–508. K.E. Kuebler, H.Y. McSween, W.D. Carlson, D. Hirsch, Sizes and masses of chondrules and metal–troilite grains in ordinary chondrites: possible implications for nebular sorting, Icarus 141 (1999) 96–106. J.W. Nettles, H.Y. McSween Jr., X-ray computed tomography data used to measure chondrule and metal grain size/shape distributions, Meteorit. Planet. Sci. 40 (2005) A112. H. Haack, E.R.D. Scott, Nebula formation of the H, L, and LL parent bodies from a single batch of chondritic materials, Meteoritics 28 (1993) 358. C.M.O.D. Alexander, Re-examining the role of chondrules in producing the elemental fractionations in chondrites, Meteorit. Planet. Sci. 40 (2005) 943–965. K. Hashizume, M. Chaussidon, A non-terrestrial 16O-rich isotopic composition for the protosolar nebula, Nature 434 (2005) 619–622. B. Zanda, R. Hewins, A mixing origin for chondrite groups, Lunar Planet. Sci. XXXVII (2006) (2199.pdf). R.N. Clayton, T.K. Mayeda, The oxygen isotope record in Murchison and other carbonaceous chondrites, Earth Planet. Sci. Lett. 67 (1984) 151–161. M.W. Rowe, R.N. Clayton, T.K. Mayeda, Oxygen isotopes in separated components of CI and CM meteorites, Geochim. Cosmochim. Acta 58 (1994) 5341–5347. E.D. Young, R.D. Ash, P. England, D.I. Rumble, Fluid flow in chondritic parent bodies: deciphering the compositions of planetesimals, Science 286 (1999) 1331–1335. D. Mattey, D. Lowry, C. MacPherson, Oxygen isotope composition of mantle peridotite, Earth Planet. Sci. Lett. 128 (1994) 231–241. R.H. Hewins, C.T. Herzberg, Nebular turbulence, chondrule formation, and the composition of the Earth, Earth Planet. Sci. Lett. 144 (1996) 1–8. E.R.D. Scott, H. Haack, Chemical fractionation in chondrites by aerodynamic sorting of chondritic materials, Meteoritics 28 (1993) 434. A.N. Krot, M.E. Zolensky, J.T. Wasson, E.R.D. Scott, K. Keil, K. Ohsumi, Carbide–magnetite assemblages in type-3 ordinary chondrites, Geochim. Cosmochim. Acta 61 (1997) 219–237. H.B. Wiik, On the regular discontinuities in the composition of meteorites. Comment. Phys.-Math. 34 (1969) 135–145.
Formation of solar nebula reservoirs by mixing chondritic components Brigitte Zanda a,b,⁎, Roger H. Hewins a,b , Michèle Bourot-Denise a , Philip A. Bland c,d , Francis Albarède e a
c
Laboratoire d'Etudes de la Matière Extraterrestre, MNHN and CNRS-UMS2679, 61 rue Buffon, 75005 Paris, France b Department of Geological Sciences, Rutgers University, Piscataway NJ 08855, USA Impacts and Astromaterials Research Centre (IARC), Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK d IARC, Department of Mineralogy, Natural History Museum, London SW7 5BD, UK e Ecole Normale Supérieure de Lyon, 46, Allée d'Italie, 69364 Lyon, France Received 3 December 2005; received in revised form 9 May 2006; accepted 12 May 2006 Available online 28 July 2006 Editor: R.W. Carlson
Abstract We determined proportions of Type I (reduced) and Type II (oxidized) chondrules in ordinary chondrites (OC) and found linear relationships between chondrule abundances and chondrite bulk chemical and oxygen isotopic compositions. Similar relationships exist between bulk oxygen isotopic compositions of carbonaceous chondrites and modal abundances of their chondritic components (matrix, Type I and Type II chondrules, refractory calcium–aluminium-rich inclusions and amoeboid olivine aggregates). These correlations can be used to predict the bulk oxygen isotopic composition of chondrites based on their petrology. We can also define model isotopic compositions associated with each petrologic component, which are not their current actual isotopic compositions due to alteration or mixing. These compositions for refractory inclusions and chondrules plot close to a slope 1 line, consistent with refractory inclusions (RI) forming from an early 16O-rich gas, the evolution of the gas to more 16O-poor compositions, possibly involving photodissociation and subsequent ice transport, followed by chondrule formation. Our results open a new understanding of the oxygen 3-isotope space and explain the unique position of OC as well as the differences between H, L and LL chondrites. They indicate that major chemical and isotopic variations between chondritic reservoirs were established after chondrule and CAI formation. They may have some bearing on the formation of planetary reservoirs: the Δ17O calculated for type I chondrules is appropriate for terrestrial planet progenitors, consistent with their chemical similarity to Earth mantle. © 2006 Elsevier B.V. All rights reserved. Keywords: nebular fractionation; oxygen isotopes; chondritic reservoirs; chondrules; chondritic metal; refractory inclusions
1. Introduction ⁎ Corresponding author. Laboratoire d'Etudes de la Matière Extraterrestre, MNHN and CNRS-UMS2679, 61 rue Buffon, 75005 Paris, France. Tel.: +33 1 4079 3542; fax: +33 1 4079 5772. E-mail address: [email protected] (B. Zanda). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.05.016
Chondrites are considered the most primitive rocks of the solar system because their chemical composition is similar to that of the Sun and they contain calcium– aluminium-rich inclusions (CAI) and related amoeboid
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
olivine aggregates (AOA). CAI, interpreted both as refractory condensates and evaporation residues, are the oldest objects known so far to have formed in the solar system [1]. Chondrites are also made of up to 80% chondrules, sub-millimeter spherical particles formed from a melt, some of which appear as old as CAI [2]. These high temperature petrologic components are embedded in a volatile-rich, low-temperature, finegrained matrix. Except for the CI group, all chondrites experienced chemical fractionation involving refractory/volatile and metal/silicate separation. Variations in the bulk concentration and redox state of Fe were used to class chondrites into several major groups [3] within which abundances of refractory, siderophile and volatile elements define well-known patterns (e.g. [4–6]). Chondrite groups are now also known to each have specific isotopic signatures in oxygen [7–9], copper [10], zinc [11] and chromium [12] as well as distinct populations of petrologic components [13] in terms of their relative abundances and sizes (Fig. 1). The chemical variations between chondrite groups have been interpreted in terms of nebular fractionations of refractory, metallic [4,5] and volatile components [6]. Such chemical and isotopic variations might reflect spatial or temporal heterogeneities in the nebular dust and gas, which would have generated reservoirs from which the different chondrites and their specific population of petrologic components would have formed. Alternatively, only oxygen isotopic compositions of chondritic components reflect spatial or temporal heterogeneities of the nebular dust and gas, whereas bulk oxygen isotopic compositions of chondrites simply reflect different proportions of isotopically distinct petrologic components (chondrules + matrix and CAI + AOA). The components of chondritic meteorites display a hierarchy of oxygen isotopic compositions [14]. The most 16O-rich are unmelted CAI and AOA. Melted CAI range from similar values (δ17O and δ18O ~− 50‰) to less 16O-rich compositions. Then come Al-rich chondrules with relict CAI, followed by Al-rich chondrules without CAI. Type I (reduced) chondrules are generally poorer in 16O, though some have extremely 16O-rich olivine grains, which may be relicts of AOAs [15]. There is some overlap of isotopic composition, between type I and type II (oxidized) chondrules, but type I chondrules are on average more 16O-rich than type II chondrules in CRs [14,16]. There are not a lot of isotopic data for classified chondrules in ordinary chondrites (OC), but a similar trend is likely: Clayton et al. [9] found that small chondrules in OC are more 16 O-rich than large ones and type I chondrules are
651
Fig. 1. Reflected views of polished sections from Ste Marguerite (H4 —a) and Renazzo (CR2—b) (field of view: 4.7 × 7 mm). In the H chondrite, chondrules are small, often fragmented and metal and sulfide grains (resp. white and grey) are located mostly in between chondrules. In the CR chondrite, the chondrules tend to be much larger and metal grains are almost systematically associated with chondrules, either inside or on their surfaces. Sulfide grains are not visible as they are only present in the fine-grained matrix which is much more abundant (∼ 35 vol.%) than it is in OC (∼ 10–15 vol.%).
known to be smaller than type II. High 16O contents in OC chondrules seem to be due to relict grains [17,18]. Type I chondrules in OC are poorer in 16O than those in carbonaceous chondrites (CC). Clayton et al. [8] suggested that the individual chondrules and CAI we now find in chondrites are the result of protracted mixing involving several generations of chondrule (and possibly also CAI) recycling. 16 O-poor compositions reflect the 16O-poor nebular gas within which CAI and chondrules were remelted and with which they exchanged [8]. CAI are known to have undergone isotopic and chemical exchange [19–21] and become less 16O-rich. Type I chondrules in CC are somewhat 16O-rich because of inherited RI material [22–25] or 16O-rich forsteritic xenocrysts [26,27]. Type II chondrules may be more 16O-depleted than type Is [9], possibly due to more extensive interaction with the nebular gas, and hence might be responsible for the 16Odepletion of OC compared to CC. The oxygen isotopic compositions of RIs and chondrules are consistent with
652
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
RI forming from an early 16O-rich gas [8,19–21] and the evolution of the gas to more 16O-poor compositions before chondrules are formed. The gas evolution may involve photodissociation, either in the precursor
molecular cloud or in the solar nebula, and subsequent ice transport [20,28–30]. Matrix may represent mixing between chondrule precursor silicate dust and extremely 17 O and 18O-enriched ice with compositions matching
Table 1 Volume % abundances of petrologic constituents (adjusted to 100%) in primitive chondrites (from this work and from literature data as quoted in text) and oxygen isotopic compositions from [7,8] δ17O
Δ17O
Δ18O
Δ17O calc
Δ18O calc
Type II ch ^
Ordinary chondrites Sharps (H3.4) Tieschitz (H/L3.6) Hallingeberg (L3.5) Bishunpur (L/LL3.1) Krymka (LL3.1) Semarkona (LL3.0) Chainpur (LL3.4)
44.9 36.3 31.3 33.6 19.4 24.7 21.0
36.2 48.4 55.1 48.9 65.2 58.1 61.0
6.4 4.9 3.8 3.6 1.7 1.5 1.9
0.1 0.1 0.1 0.1 0.1 0.1 0.1
12.4 10.3 9.7 13.9 13.6 15.6 16.1
3.95 5.55 5.02 6.00 5.37 6.09 5.72
2.70 3.83 3.59 4.11 3.95 4.24 4.01
0.65 0.94 0.98 0.99 1.16 1.07 1.04
1.25 1.72 1.43 1.89 1.42 1.85 1.71
0.70 0.89 1.00 0.90 1.16 1.04 1.09
1.92 1.77 1.73 2.03 2.01 2.15 2.19
Enstatite chondrite Sahara 97096 (EH3)
60.0
0.0
12.9
0.0
27.0
5.52
2.83
− 0.04
2.69
0.16
2.97
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
100.0 100.0 100.0
16.16 15.79 16.84
8.79 8.60 9.23
0.39 0.39 0.47
7.37 7.19 7.61
0.16 0.16 0.16
8.16 8.16 8.16
CM chondrites Murray Mighei Murchison Cold Bokkeweld Essebi Bells Nogoya
34.9 29.4 27.1 18.1 15.1 11.8 8.3
0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.9 0.8 0.6 1.0 0.9 3.1 2.5
4.8 8.6 8.4 6.0 4.7 0.9 1.7
59.3 61.2 64.0 74.9 79.2 84.2 87.5
7.53 7.57 7.30 10.01 11.65 9.59 8.36
0.85 1.44 1.20 2.76 5.02 3.13 2.35
− 3.07 − 2.50 − 2.60 − 2.45 − 1.04 − 1.86 − 2.00
6.68 6.13 6.10 7.25 6.63 6.46 6.01
− 1.77 − 3.25 − 3.16 − 2.21 − 1.73 − 0.21 − 0.53
5.26 5.40 5.60 6.38 6.68 7.04 7.27
CR chondrites Renazzo Al Rais
55.1 37.8
0.0 0.0
7.2 3.6
2.5 2.4
35.1 56.1
6.25 10.94
2.29 4.68
− 0.96 − 1.01
3.96 6.26
− 0.85 − 0.81
3.54 5.04
CH chondrite ALH85085
74.8
0.0
20.0
0.2
5.0
2.08
− 0.54
− 1.62
2.62
0.16
1.40
CO chondrites Kainsaz Lance Felix Ornans Warrenton
47.7 48.4 46.8 38.9 41.1
2.1 1.1 2.9 3.6 3.3
6.5 1.4 2.3 1.6 1.5
10.8 13.9 12.8 13.2 14.7
33.0 35.1 35.3 42.8 39.4
− 1.85 − 0.31 − 2.38 − 1.71 − 0.73
− 5.68 − 4.51 − 5.83 − 5.34 − 4.45
− 4.72 − 4.35 − 4.59 − 4.45 − 4.07
3.83 4.20 3.45 3.63 3.72
− 4.09 − 5.36 − 4.87 − 5.04 − 5.63
3.39 3.54 3.55 4.08 3.84
CV chondrites Efremovka Vigarano Arch Allende Bali Grosnaja Mokoia Kaba
43.5 51.6 48.9 45.9 33.3 38.6 50.4 34.3
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
5.0 1.5 2.9 1.4 1.6 1.5 2.0 3.4
13.2 11.1 10.8 13.0 12.2 6.4 6.4 9.0
38.3 35.9 37.5 39.6 52.9 53.6 41.2 53.3
− 3.42 1.19 − 0.94 1.51 − 1.21 3.21 3.52 2.18
− 6.86 − 3.61 − 5.44 − 2.73 − 5.53 − 1.46 − 0.91 − 2.37
− 5.08 − 4.23 − 4.95 − 3.52 − 4.90 − 3.13 − 2.74 − 3.50
3.44 4.80 4.50 4.24 4.32 4.67 4.43 4.55
− 5.09 − 4.26 − 4.12 − 5.01 − 4.68 − 2.37 − 2.40 − 3.43
3.77 3.59 3.71 3.86 4.81 4.86 3.98 4.83
CI chondrites Orgueil Alais Ivuna
Metal #
δ18O
Type I ch silicates ⁎, ^
CAI + AOA
Matrix
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
those of cosmic spherules [31] and of unmelted micrometeorites from very friable asteroids [32]. The data base for chondrite components is now almost sufficient to develop a comprehensive understanding of the bulk compositions and oxygen isotope systematics of chondrites. We know, for example, that refractory inclusions can represent up to 15 vol.% of carbonaceous chondrites (CC), but are rare (< 1%) in OC [13]. Chondrules have a wide range of bulk compositions, inherited from precursors or explicable by evaporation and condensation [33–41]. Type I are volatile-depleted, Fe-metal-bearing but FeO-poor (unless metamorphosed), while type II are undepleted or even enriched in volatile elements, and FeO-rich. Type II chondrules are rare in CC but abundant in OC [36– 38,41] in which they are larger in diameter than type Is by 20–25%, which translates to a difference in mass close to a factor of 2 [40]. In order to develop a model for chondritic chemical and isotopic reservoirs derived from varying proportions of the petrologic components that make up chondrites, we have assembled data on the abundance of inclusions, type I chondrules, type II chondrules, matrix and metal grains. We find we can explain chondrite chemical and oxygen isotopic compositions as functions of these abundances. 2. Method Our analysis of ordinary chondrites was restricted to observed falls (finds may have lost part of their metal) and unequilibrated meteorites (the chemical difference between type I and type II chondrules gradually disappears with parent-body metamorphism). The seven most studied OC for which oxygen isotopic data are available [8] were selected: Sharps (H3.4), Tieschitz (H/L3.6), Hallingeberg (L3.5), Bishunpur (L/LL3.1), Semarkona (LL3.0), Chainpur (LL3.4), Krymka (LL3.1) (the numbers in parenthesis reflect the degree of equilibration in the parent body, with 3.0 being most
653
unequilibrated, and equilibration of olivine being complete by 4). We used optical microscopy, scanning electron microscopy (SEM) mapping and electron microprobe (EMP) control analyses to determine the mean radius of all the chondrules present in polished sections of these meteorites with areas ranging between 1.5 and 2.5 cm2. We classified chondrules as type I (Fa ≤ 8) or type II (Fa > 8). Matrix abundances of [42] were used and metal abundance in these meteorites was derived (after a density correction) by summing metallic Fe, Ni and Co in averages of all the wet chemistry data available [43]. The total chondrule fraction was taken as 100%—vol.% metal–vol.% matrix and split into type I and type II chondrules based on our measurements. To allow a comparison, the CC data of McSween [36–38] were similarly normalized to 100%. The standard deviations on petrologic component abundances (estimated by comparing values for half surfaces) range between 0.2% and 4%. The samples considered in this study are listed in Table 1 and displayed in a 3-isotope plot in Fig. 2. For convenience, small variations of oxygen isotopic compositions are described in terms of their deviation in parts per thousand from a terrestrial standard, SMOW (Standard Mean Ocean Water): δ 17 O = {[( 17 O / 16 O)sample / ( 17 O / 16 O)SMOW] − 1} × 103 ; δ 18 O = {[( 18 O / 16 O) sample / ( 18 O / 16 O) SMOW ] − 1} × 103. Δ17O = δ17O − 0.52δ18O was defined by Clayton et al. [8]. It corresponds to the excess of δ17O relative to the terrestrial fractionation line (TFL) and reflects the group (or the nebular reservoir) to which a sample belongs. Similarly, we define Δ18O = δ18O − δ17O to describe mass fractionation effects exclusively, and independently of the group. Δ18O corresponds to the excess of δ18O with respect to the slope 1 line going through the origin (δ17O = 0; δ18O = 0) and is shown in a 3-isotope plot in Fig. 2. The calculated (Δ17O, Δ18O) are equivalent to the measured (δ17 O, δ18 O) for characterizing a sample. Their interest lies in
Notes to Table 1: Also shown are Δ17O and Δ18O. After [8], the first one is defined as δ17O–0.52δ18O and measures in the oxygen 3-isotope plot the distance along the δ17O axis between the terrestrial fractionation line (TFL) and the fractionation line on which the sample is located. In a similar fashion, we define Δ18O as δ18O–δ17O, which measures the distance along the δ18O axis between the slope 1 line going through (0,0) and the parallel line on which the sample is located. The last two columns display Δ17O and Δ18O estimated from the petrologic constituents listed in columns 2–6 using the correlation coefficients derived in this work. The proportion of RIs in all the OC are unknown and were set at 0.1 vol.% according to [13]. * Metal volume was substracted from Type I chondrules in carbonaceous chondrites because metal is dominantly present associated with chondrules or as “metallic chondrules” commonly interpreted as having fallen from chondrules (e.g. Fig. 1 and [46,50,63]), so that comparisons could be made with CH, EH and ordinary chondrites in which metal is found predominantly outside chondrules. ^ “Lithic fragments”, “lithic and mineral fragments” or “monomineralic grains and fragments” from [36–38] that may amount to up to 20% of the counted surfaces were treated as chondrules (in relative proportions of the two types when present). The abundances of Type II chondrules in CH, CR and CV chondrites are unknown. They are low enough (at most a few percent) not to affect oxygen isotopes. # Magnetite in CV chondrites has been added to metal because it is believed to have originated from metal oxidation in the course of parent-body alteration (e.g. [64]). For ordinary chondrites, metal based on wet chemistry analyses of [45] and [65] is listed.
654
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
Fe/Mg and Ni/Mg in the bulk OC and the single CH also correlate with type I chondrule abundances in the same chondrites. Conversely, the correlation is poor for the CC in which metal remained associated with chondrules: Fe and Ni are approximately in CI proportions relative to Mg. 4. Oxygen isotopes 4.1. Nebular effects
Fig. 2. Oxygen 3-isotope plot showing the samples considered in this work and the meaning of Δ17O and Δ18O in the case of the CH chondrite. SL1: slope 1 line going through the origin (0,0); TFL— terrestrial fractionation line; Y-R: Young and Russell line (after [48]); dashed lines—examples of constant Δ17O and Δ18O.
decoupling (nebular) group effects from mass fractionation effects as Δ17O only depends on the former and Δ18O on the latter.
In Fig. 4, Δ17O from Clayton et al. [8] is plotted as a function of the volume proportion of type II chondrules we measured in OC. This figure shows that the excess of 17 O relative to the terrestrial fractionation line (TFL) of an OC is directly related to its abundance of type II chondrules, and that there is a continuum between LL and L chondrites. There is only one sample of H chondrite, which appears isolated, in part because unaltered H chondrites which escaped metamorphism sufficiently to permit accurate measurements of
3. Chondrules and metal in ordinary chondrites OC are subdivided into 3 groups based on their total Fe and metal abundance: H (High Fe), L (Low Fe) and LL (Low Fe, Low metal) [3]. In an attempt to explain their bulk chemical and isotopic compositions, we determined type I and type II chondrule relative abundances in seven unequilibrated falls spanning the three OC groups, including intermediate H/L Tieschitz. Such data, including also refractory inclusion abundances, have long been available for CC [36–38] and for a primitive enstatite chondrite (EC) [44] (Table 1). Although the proportion of chondrules remains ≈80 vol.% in OC, their composition varies significantly. The averages for the H, L and LL groups are respectively 45%, 28% and 22% type I chondrule material versus 36%, 58% and 61% type II chondrule material. The abundance of metal varies between 1.5 vol.% (in LL chondrite Semarkona) and 6.5 vol.% (in H chondrite Sharps) [45] and correlates with the abundance of type I chondrule material (Fig. 3a) and the ratio of type I to type II. The data can be fitted by a linear regression, though with considerable scatter when CMs, COs, CVs and CRs, in which metal is usually retained in or at the surface of chondrules (Fig. 1b), are included. A power law (Fig. 3a) allows us to fit EH and CH data as well as OC, indicating a possible relationship between the chondrites in which metal is interstitial to chondrules [46] (Fig. 1). Fig. 3b shows that chondrite-normalized
Fig. 3. (a) Relationship between the amount of metal and the amount of Type I chondrule material in chondrites. Filled symbols for chondrites in which metal is well separated from chondrules (OC, EH and CH) + CR, open symbols for all other CC. Regression curves for OC only. (b) Chondrite normalized Ni/Mg and Fe/Mg in the same objects (wet chemistry bulk analysis not available for EH Sahara 97096, nor for LL3.4 Chainpur). Linear regression coefficients: R2 = 0.944 for Fe/Mg and R2 = 0823 for Ni/Mg. Dotted line in (b) repeats linear regression on OC metal from (a).
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
655
constant for refractory inclusions were determined based on CV and CI chondrites only as the regression is better (R2 = 0.83 if CM, CO, CR and CH chondrites are also considered, which we believe to result from the uncertainties in RI abundance measurements when RI are either very small as in CO chondrites or scarce as in all the other cases). Based on these correlations, we construct a model for bulk rock chondritic Δ17O by making a regression on type IIs and refractory inclusions (RI) in OC, CI and CV chondrites: Fig. 4. Relationship between Δ O and the amount of Type II material present in ordinary chondrites (Sh = Sharps, H3.4; T = Tieschitz, H/ L3.6; B = Bishunpur, L/LL3.1; H = Hallingeberg, L3.5; S = Semarkona, LL3.0; C = Chainpur, LL3.4; K = Krymka, LL3.1). L/LL chondrite Bishunpur may be misclassified as it falls in the field of L or H/L chondrites, but this more likely reflects heterogeneity in the distribution of Type I and Type II chondrules and the need to study a larger surface. Standard deviations on vol.% type II material (estimated by comparing values for half surfaces): Sh, 0.5%; T, 4%; B, 3.7%; H, 1.6%; S, 3.4%; C, 0.2%; K, 1.3%. 17
petrologic component are scarce, and their oxygen isotopic composition is not known except for Sharps (H3.4). The difference in the proportion of type II chondrules may explain the relative positions of H, L and LL chondrites on the 3-isotope plot. We searched for similar relationships between petrologic components and oxygen isotope compositions using literature data [7–9,36–38] and found that refractory inclusion abundances in CC correlate negatively with Δ17O (R2 = 0.92—Fig. 5). Note that although all the carbonaceous chondrites used in this study are plotted in Fig. 5, the correlation coefficient and origin
Fig. 5. Correlation of Δ17O with volume of RIs in CCs. Because of the scatter of the data, in particular for CM and CO, the slope and origin constant of the linear correlation were estimated using only CI and CV chondrites, which yields a better regression (R2 = 0.92 vs. R2 = 0.83). The correlation between Δ17O and volume of Type IIs is also shown: the two lines intercept the origin at ∼ 0.17, close to the CI point.
D17 OðxÞ ¼ 0:398 vol:% RI þ 0:0157 vol:% Type II þ 0:2
ð1Þ
Such equations can be solved by substituting values of 0% and 100% to identify virtual isotopic components associated with RI, etc. The virtual isotopic components carried by Type I chondrules and matrix both have Δ17O = 0.2‰ and consequently do not appear in Eq. (1). 4.2. Mass fractionation effects As explained above in the Method section (Fig. 2), Δ18O describes the excess of δ18O with respect to the slope 1 line going through the origin (δ17O = 0; δ18O = 0) and is characteristic of the amount of mass fractionation experienced by a sample. In agreement with previous observations of Weisberg et al. [47], matrix abundance correlates positively with Δ18O (R2 = 0.875—Fig. 6): D18 O ðxÞ ¼ 0:0712 vol:% matrix þ 1:0
ð2Þ
In (2) the regression line has been set to intercept the X-axis at 1.0‰ (instead of 1.3‰) so that 0% matrix corresponds to the Young and Russell (Y-R) [48] line (i.e. to aqueously unaltered CAI material). Moreover, the isotopic components carried by chondrules and RI
Fig. 6. Correlation of Δ18O with volume of matrix.
656
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
Fig. 7. Δ17O derived from the proportions of chondritic components (mostly RIs in CC and Type II chondrules in OC) using Eq. (1) is plotted as function of measured values in primitive chondrites.
lie close to the Y-R line and consequently do not affect Δ18O. Figs. 7 and 8, where calculated Δ17O and Δ18O are plotted as functions of the measured values [7–9], show that Eqs. (1) and (2) adequately reproduce the observations and therefore that the oxygen isotopic signatures of OC, CC and EC chondrites can be successfully estimated from the proportions of their petrologic components.
5. Discussion 5.1. Formation of OC reservoirs by dynamic sorting Separation of FeNi metal from silicates is known to be involved in the chemical fractionation of chondritic reservoirs [3,4,49]. It has been suggested that metal formed within Type I chondrules [46,50]. The correlation between the abundances of metal and Type I chondrules in OC displayed in Fig. 3 appears to support this hypothesis. The correlation, however, is stronger in those meteorites (OC, EH and CH) in which metal has physically separated from the chondrules where it formed (Fig. 1a) than in CC where metal is still sequestered in chondrules (Fig. 1b), suggesting another mechanism may have played a role. In addition, the formation of metal in Type I chondrules by reduction should not have affected the Fe and Ni content of the bulk rocks and hence does not explain the correlation of bulk Fe and Ni with Type I chondrules and metal abundances (Fig. 3b). This correlation is consistent with the idea that metal segregation from the silicates is responsible for Fe and Ni fractionation between nebula
(CI) and parent body. These two arguments suggest that the correlation between the abundances of metal and Type I chondrules (Fig. 3a) reflects dynamic sorting during accretion rather than metal formation by reduction within chondrules. Metal grains, smaller and denser than silicate chondrules, have stopping times [51] due to gas drag closer to those of the smaller chondrules [51–53] and, in a weakly turbulent nebula, would have accreted preferentially with the smaller (Type I) chondrules in H chondrites. However, such a process would not have worked as efficiently in CC, where metal remains within chondrules, or attached to their surfaces (Fig. 1b). Rather than being derived from parental (more or less reduced) reservoirs, OC groups have been suggested to result from differential accretion of larger objects in LLs, and smaller or denser objects in Hs [8,54]: our data show that the larger oxidized Type IIs are much more abundant than Type Is in the former, and that reduced Type Is and metal grains exceed Type IIs in the latter. The varying proportions of the two types of chondrules in H, L and LL chondrites, and of metal (also a product of chondrule formation), show that the formation of the OC reservoirs post-dates chondrule formation and is related to accretion, which is consistent with the conclusion reached by Alexander [55]. 5.2. Isotopic components carried by chondrules, refractory inclusions and matrix: model and current compositions The conclusion that chondritic reservoirs formed during accretion is also demanded by the correlations, in
Fig. 8. Δ18O derived from the proportion of matrix using Eq. (2) is plotted as function of measured values in primitive chondrites.
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
primitive chondrite falls, between Δ 17 O and the abundances of refractory inclusions and chondrules, and between Δ18O and the abundance of matrix. Oxygen isotopic signatures are considered to characterize each of the chondritic reservoirs [7–9]. Our results indicate that the various petrologic components within a chondrite did not form in a pre-existing reservoir with a fixed chemical and isotopic composition, but that the chondrite groups originated by mixing petrologic components that carried isotopically and chemically distinct compositions, hereafter referred to as “model isotopic compositions”. Although these model isotopic compositions are no longer present in chondrites, they can still be inferred from the correlations that associate the petrologic component abundances and the oxygen isotopic signatures of the bulk chondrites: RI contain a model isotopic component rich in 16O, Type II chondrules one poor in 16 O, and matrix one enriched in 17O and 18O. By setting petrologic component abundances at 0% or 100% in our equations, we can extract the model compositions of the isotopic components associated with each of them (Table 2). For the refractory inclusions, the end-member has a Δ17O value of − 40 ± 18‰, more 16O-rich than, but within error of, the observed value for minerals measured in CAI, and similar to the lower limit to the solar value of Hashizume and Chaussidon [56]. The isotopic component carried by Type II chondrules, has a Δ17O value of 1.7 ± 0.1‰, corresponding to the most 16 O-poor chondrules analysed, excluding R chondrites. The virtual isotopic component carried by matrix corresponds to Δ18O = 8.2 ± 0.5‰. Type I chondrules contain a fourth isotopic component, defined when all the petrologic components in Eqs. (1) and (2) are zero, at the point Δ17O = 0.2 ± 0.4‰ on the Y-R line in the oxygen 3-isotope plot. Although these model isotopic compositions are no longer found in chondrites, trends are retained in the present isotopic composition of the petrologic components that once carried them: CAI remain 16O-rich and the isotopic and chemical exchange they are all known to have undergone [19–21] explains why even the most pristine ones have become less 16O-rich than our calculated component. Al-rich chondrules (and espeTable 2 Summary of isotopic model component compositions
1 2 3 4
657
cially the ones containing CAI-remnants) remain more 16 O-rich than Type I chondrules, which are in turn more 16 O-rich than Type II chondrules [23]. Type I chondrules in CC are more 16O-rich than our calculated value because of inherited CAI and AOA material or 16O-rich forsteritic xenocrysts [22–27]. This difference between the model isotopic compositions of chondrules, RI and matrix and their current ones may be ascribed to nebular or parent-body transformations and/or mixing. In the case of the matrix, for example, it could be argued that at the time of accretion it consisted of a mixture of dry silicates carrying the isotopic composition of Type I material, together with ice with an isotopic composition close to that of cosmic spherules [31]. The water, produced from the melting of the ice after accretion and reheating on the parent-body, would have circulated and reacted with minerals throughout the chondrite, thus dispersing the isotopic component originally contained within the matrix. As this process would work in a closed system on the parent-body, the correlation between the amount of matrix and Δ18O would be preserved even though the matrix itself would no longer hold the model isotopic composition. It is more difficult to find a straightforward explanation in the case of the Δ 17 O variations, carried by chondrules and refractory inclusions. It is possible, however, that the RIs reacted with chondrule material at the time of accretion or, more likely, as a result of successive episodes of CAI and chondrule formation involving recycling of pre-existing material. This would imply, as suggested by Clayton et al. [8], that the individual chondrules and RI we now find in chondrites are the result of protracted mixing involving several generations of chondrule (and possibly also CAI) recycling. Yet another possibility is that the mixing between RIs and chondrules occurred on an early generation of parent-body that preceded the formation of the chondrules we now find in chondrites as advocated by Libourel et al. (LPSC, 2006). Such a body might correspond to the mixes 4 and 5 (comprising Type I chondrules, CAI and metal) from which Zanda and Hewins [57] generate CM, CO, CR, CH and EH chondrites (Fig. 9). 5.3. Origin of the isotopic components
Carrier
Δ17O (‰)
Δ18O (‰)
Chemical dominance
CAI + AOA Type II chondrules Matrix Type I chondrules
− 39.6 1.7 0.2 0.2
1.0 1.0 8.2 1.0
CaO, Al2O3 FeO FeS and H2O MgO
We have identified four isotopic components (Table 2), which explain chondrite compositions, carried by matrix, refractory inclusions, Type II chondrules and Type I chondrules. The 16O-rich composition of the model component carried by RI is likely to reflect that of
658
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
Fig. 9. Oxygen 3-isotope plot showing the model components and the effect of adding them to the Type I one, located at the intercept between the Y-R line and the TFL. The first component (contained in RIs) is too far along the Y-R line to be plotted. The primitive chondrites used to establish the correlations are also indicated.
the early solar nebula in the vicinity of the Sun: it is similar to the lower limit to the solar value of Hashizume and Chaussidon [56] and in the currently favored model of self-shielding [28–30], the original Δ17O of the solar nebula is ~− 25‰.). AOAs appear as aggregates of solar nebula condensates that escaped melting, so they probably reflect the oxygen isotopic composition of the early nebular gas (Δ17O c − 25‰). 16 O-poor compositions, on the other hand, may reflect the later 16O-poor nebular gas within which CAI and chondrules were remelted and with which they exchanged [8]. The model isotopic compositions of Type II chondrules are more 16O-depleted than those of Type Is, probably due to more extensive interaction with the nebular gas, and are responsible for the 16Odepletion of OC compared to CC. The Δ17O of the calculated Type I chondrule component is close to that of CI (Δ17O ≈ 0.4‰), which suggests that, except for low temperature hydration [58,59], both materials may share the same source. The co-linearity of the isotopic components carried by refractory inclusions, Type Is and Type IIs is striking. It is consistent with RI forming from an early 16O-rich gas [8,19–21], the evolution of the gas to more 16O-poor compositions (possibly involving photodissociation [28,29] and subsequent ice transport [20,30]), followed by the formation of the isotopic component carried by Type I chondrules. This latter component is very similar to the terrestrial planet progenitors of Young et al. [60] (who derive the primitive Earth mantle [61] from these by hydration), in agreement with the bulk chemical resemblance between the Earth's mantle and Type I chondrules [62]. It probably represents the precursor
material from which matrix and Type II chondrules were derived, by hydration for matrix and repeated processing in a gas which continued to evolve to more 16Opoor compositions for Type II chondrules. Matrix may represent mixing between such Type I precursor silicate dust and extremely 17O and 18O-enriched ice with compositions matching those of cosmic spherules [31] and of unmelted micrometeorites from very friable asteroids [32]. Formation of chondrules or CAIs from such a material would have removed the 17O and 18Orich water it originally contained bringing its oxygen signature back towards the present chondritic compositions. Alternatively, matrix can be accounted for by lowtemperature aqueous alteration of a RI + Type II chondrule mixture: it may represent the scattered hydrous regolith of small early objects formed in a distant position sputtered by impacts and subsequently re-accreted with dry material from the more proximal parts of the Solar System. The apparent homogeneity of this matrix component suggests that it remained long enough in space to be efficiently mixed by turbulent movement before its final accretion to the meteorite parent body. 6. Conclusion The petrologic constituents of chondrites, Type I and Type II chondrules, matrix and refractory inclusions each carry a distinctive oxygen isotopic component. The origin of these four isotopic components still needs to be clarified. Our results, however, demonstrate that chemical and isotopic fractionations between solar system bodies were established after CAI and chondrule formation, and involved differential accretion of these high temperature objects in a turbulent regime. Similar mechanisms may have played a role in establishing the chemical and isotopic compositions of the terrestrial planets. Acknowledgements R.N. Clayton is gratefully acknowledged for kind suggestions made on an early version of this manuscript. B.Z. thanks Gene Jarosewich for sending her an electronic version of his meteorite analysis database and Claude Perron for sharing with her his oxygen data spreadsheet. We thank Sasha Krot and an anonymous reviewer for detailed comments, which led to considerable improvements to the manuscript. This work was supported by the Program National de Planétologie and NASA Cosmochemistry Program grants NNG05GK11G and NAG5-10494.
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
References [1] Y. Amelin, A.N. Krot, I.D. Hutcheon, A.A. Ulyanov, Lead isotopic ages of chondrules and calcium–aluminum-rich inclusions, Science 297 (2002) 1678–1683. [2] M. Bizzarro, J.A. Baker, H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431 (2004) 275–278. [3] H.C. Urey, H. Craig, The composition of the stone meteorites and the origin of the meteorites, Geochim. Cosmochim. Acta 4 (1953) 36–82. [4] J.W. Larimer, J.T. Wasson, Siderophile element fractionation, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 416–435. [5] J.W. Larimer, J.T. Wasson, Refractory lithophile elements, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 394–415. [6] H. Palme, Chemical and isotopic heterogeneity in protosolar matter, Philos. Trans. R. Soc. Lond., A 359 (2001) 2061. [7] R.N. Clayton, Oxygen isotope studies of carbonaceous chondrites, Geochim. Cosmochim. Acta 63 (1999) 2089–2104. [8] R.N. Clayton, T.K. Mayeda, J.N. Goswami, E.J. Olsen, Oxygen isotope studies of ordinary chondrites, Geochim. Cosmochim. Acta 55 (1991) 2317–2337. [9] R.N. Clayton, T.K. Mayeda, E.J. Olsen, J.N. Goswami, Oxygen isotopic compositions of ordinary chondrites and their chondrules, Lunar Planet. Sci. XXI (1990) 201–202. [10] J.M. Luck, D. Ben Othman, J.A. Barrat, F. Albarède, Coupled 63 Cu and 16O excesses in chondrites, Geochim. Cosmochim. Acta 67 (2003) 143–151. [11] J.M. Luck, D. Ben Othman, F. Albarède, Zn and Cu isotopic variations in chondrites and iron meteorites: early solar nebula reservoirs and parent-body processes, Geochim. Cosmochim. Acta 69 (2005) 5351–5363. [12] A. Trinquier, J.-L. Birck, C.J. Allègre, 54Cr anomalies in the solar system: their extent and origin, Lunar Planet. Sci. XXXVI (2005) (1259.pdf). [13] A.J. Brearley, R.H. Jones, Chondritic meteorites, in: J.J. Papike (Ed.), Planetary Materials, Reviews in Mineralogy, vol. 36, Mineralogical Society of America, Washington, DC, 1998, pp. 1–398. [14] A.N. Krot, G. Libourel, M. Chaussidon, Oxygen isotope compositions of chondrules in CR chondrites, Geochim. Cosmochim. Acta 70 (2006) 767–779. [15] R.H. Jones, L.A. Leshin, Y. Guan, Z.D. Sharp, T. Durakiewicz, A.J. Schilk, Oxygen isotope heterogeneity in chondrules from the Mokoia CV3 carbonaceous chondrite, Geochim. Cosmochim. Acta 68 (2004) 3423–3438. [16] L.A. Leshin, K.D. McKeegan, C. Engrand, B. Zanda, M. BourotDenise, R.H. Hewins, Oxygen isotopic studies of isolated and chondrule olivine from Renazzo and Allende, Meteorit. Planet. Sci. 33, p. A93 33 (1998) 93. [17] N.T. Kita, H. Nagahara, S. Tomomura, S. Tachibana, J.W. Valley, Systematic oxygen isotopic variations among chondrules from the least equilibrated ordinary chondrites: improved ion microprobe precision, Lunar Planet. Sci. XXXVII (2006) (1496.pdf). [18] D.W.G. Sears, I. Lyon, J. Saxton, G. Turner, The oxygen isotope properties of olivines in the Semarkona ordinary chondrite, Meteorit. Planet. Sci. 33 (1998) 1029–1032. [19] P. Cassen, Nebular thermal evolution and the properties of primitive planetary materials, Meteorit. Planet. Sci. 36 (2001) 671–700.
659
[20] A.N. Krot, I.D. Hutcheon, H. Yurimoto, J.N. Cuzzi, K.D. McKeegan, E.R.D. Scott, G. Libourel, M. Chaussidon, J. Aléon, M.I. Petaev, Evolution of oxygen isotopic composition in the inner solar nebula, Astrophys. J. 622 (2005) 1333–1342. [21] E.R.D. Scott, A.N. Krot, Oxygen isotopic compositions and origins of calcium–aluminum-rich inclusions and chondrules, Meteorit. Planet. Sci. 36 (2001) 1307–1319. [22] A.N. Krot, I.D. Hutcheon, K. Keil, Plagioclase-rich chondrules in the reduced CV chondrites: evidence for complex formation history and genetic links between calcium–aluminum-rich inclusions and ferromagnesian chondrules, Meteorit. Planet. Sci. 37 (2002) 155–182. [23] A.N. Krot, R.H. Jones, M. Yoshitake, H. Yurimoto, Oxygen isotopic compositions of chondrules, Workshop on Oxygen in the Earliest Solar System (LPI Contribution no. 1278, Gatlinburg, Tennessee, 2005). [24] A.N. Krot, K. Keil, Anorthite-rich chondrules in CR and CH carbonaceous chondrites: genetic link between Ca,Al-rich inclusions and ferromagnesian chondrules, Meteorit. Planet. Sci. 37 (2002) 91–111. [25] K. Misawa, N. Nakamura, Origin of Refractory Precursor Component of Chondrules from Carbonaceous Chondrites, in: R. Hewins, R. Jones, E. Scott (Eds.), Chondrules and the Protoplanetary Disk, Cambridge University Press, Cambridge, 1996, pp. 99–105. [26] A. Pack, H. Yurimoto, H. Palme, Petrographic and oxygenisotopic study of refractory forsterites from R.-chondrite Dar al Gani 013 (R3. 5–6), unequilibrated ordinary and carbonaceous chondrites, Geochim. Cosmochim. Acta 68 (2004) 1135–1157. [27] H. Yurimoto, J.T. Wasson, Extremely rapid cooling of a carbonaceous-chondrite chondrule containing very 16O-rich olivine and a 26Mg-excess, Geochim. Cosmochim. Acta 66 (2002) 4355–4363. [28] R.N. Clayton, Solar System: self-shielding in the solar nebula, Nature 415 (2002) 860–861. [29] J.R. Lyons, E.D. Young, CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula, Nature 435 (2005) 317–320. [30] H. Yurimoto, K. Kuramoto, Molecular cloud origin for the oxygen isotope heterogeneity in the solar system, Science 305 (2004) 1763–1766. [31] T. Yada, T. Nakamura, T. Noguchi, T. Ushikubo, N. Matsumoto, H. Kojima, N. Takaoka, Variations of oxygen isotopic compositions of silicate spherules collected from Antarctic Blue Ice, Lunar Planet. Sci. XXXIV (2003) (1587.pdf). [32] G. Matrajt, Y. Guan, L. Leshin, S. Taylor, M. Genge, D. Joswiak, D. Brownlee, Oxygen isotope measurements of bulk unmelted antarctic micrometeorites, Workshop on Dust in Planetary Systems, Kauaiʻi, Hawaiʻi, 2005, p. 4060. [33] J.N. Grossman, A.E. Rubin, H. Nagahara, E.A. King, Properties of chondrules, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 619–659. [34] R.H. Hewins, Y. Yu, B. Zanda, M. Bourot-Denise, Do nebular fractionations, evaporative losses, or both, influence chondrule compositions? Antarct. Meteor. Res. 10 (1997) 275. [35] R.H. Jones, FeO-rich, porphyritic pyroxene chondrules in unequilibrated ordinary chondrites, Geochim. Cosmochim. Acta 60 (1996) 3115–3117. [36] H.Y. McSween Jr., Petrographic variations among carbonaceous chondrites of the Vigarano type, Geochim. Cosmochim. Acta 41 (1977) 1777–1790.
660
B. Zanda et al. / Earth and Planetary Science Letters 248 (2006) 650–660
[37] H.Y. McSween Jr., Carbonaceous chondrites of the Ornans type —a metamorphic sequence, Geochim. Cosmochim. Acta 41 (1977) 477–481. [38] H.Y. McSween, Matrix variations in CM chondrites, Lunar Planet. Sci. X (1979) 810–812. [39] E.R.D. Scott, G.J. Taylor, Chondrules and other components in C, O, and E chondrites Similarities in their properties and origins, J. Geophys. Res. Supp. 88 (1983) 275. [40] D.W.G. Sears, S. Huang, P.H. Benoit, Open-system behaviour during chondrule formation, in: R.H. Hewins, R.H. Jones, E.R.D. Scott (Eds.), Chondrules and the Protoplanetary Disk, Cambridge University Press, Cambridge, 1996, pp. 221–231. [41] B. Zanda, Chondrules, Earth Planet. Sci. Lett. 224 (2004) 1–2. [42] G.R. Huss, K. Keil, G.J. Taylor, The matrices of unequilibrated ordinary chondrites—implications for the origin and history of chondrites, Geochim. Cosmochim. Acta 45 (1981) 33–51. [43] J. Köblitz, Metbase 6.0, Meteorite Data Retrieval Software, CDROM 2003. [44] M.K. Weisberg, M. Prinz, Sahara 97096: a highly primitive EH3 chondrite with layered sulfide-metal-rich chondrules, Lunar Planet. Sci. XXIX (1998) 1741. [45] E. Jarosewich, Chemical analyses of meteorites—a compilation of stony and iron meteorite analyses, Meteoritics 25 (1990) 323–337. [46] A.J. Campbell, B. Zanda, C. Perron, A. Meibom, M.I. Petaev, Origin and thermal history of FeNi-Metal in primitive chondrites, in: A.N. Krot, E.R.D. Scott, B. Reipurth (Eds.), Chondrites and the Protoplanetary Disk, Proceedings of the Astronomical Society of the Pacific Conference Series, 2005, pp. 407–431. [47] M.K. Weisberg, M. Prinz, R.N. Clayton, T.K. Mayeda, The CR (Renazzo-type) carbonaceous chondrite group and its implications, Geochim. Cosmochim. Acta 57 (1993) 1567–1586. [48] E.D. Young, S.S. Russell, Oxygen reservoirs in the early solar nebula inferred from an Allende CAI, Science 282 (1998) 452. [49] D.W.G. Sears, R.T. Dodd, Overview and classification of meteorites, meteorites and the early solar system, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, Univ. Arizona Press, Tucson, 1988, pp. 3–31. [50] B. Zanda, M. Bourot-Denise, C. Perron, R.H. Hewins, Origin and metamorphic redistribution of silicon, chromium, and phosphorus in the metal of chondrites, Science 265 (1994) 1846–1849. [51] J.N. Cuzzi, R.C. Hogan, J.M. Paque, A.R. Dobrovolskis, Sizeselective concentration of chondrules and other small particles in
[52]
[53]
[54]
[55]
[56]
[57] [58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
protoplanetary nebula turbulence, Astrophys. J. 546 (2001) 496–508. K.E. Kuebler, H.Y. McSween, W.D. Carlson, D. Hirsch, Sizes and masses of chondrules and metal–troilite grains in ordinary chondrites: possible implications for nebular sorting, Icarus 141 (1999) 96–106. J.W. Nettles, H.Y. McSween Jr., X-ray computed tomography data used to measure chondrule and metal grain size/shape distributions, Meteorit. Planet. Sci. 40 (2005) A112. H. Haack, E.R.D. Scott, Nebula formation of the H, L, and LL parent bodies from a single batch of chondritic materials, Meteoritics 28 (1993) 358. C.M.O.D. Alexander, Re-examining the role of chondrules in producing the elemental fractionations in chondrites, Meteorit. Planet. Sci. 40 (2005) 943–965. K. Hashizume, M. Chaussidon, A non-terrestrial 16O-rich isotopic composition for the protosolar nebula, Nature 434 (2005) 619–622. B. Zanda, R. Hewins, A mixing origin for chondrite groups, Lunar Planet. Sci. XXXVII (2006) (2199.pdf). R.N. Clayton, T.K. Mayeda, The oxygen isotope record in Murchison and other carbonaceous chondrites, Earth Planet. Sci. Lett. 67 (1984) 151–161. M.W. Rowe, R.N. Clayton, T.K. Mayeda, Oxygen isotopes in separated components of CI and CM meteorites, Geochim. Cosmochim. Acta 58 (1994) 5341–5347. E.D. Young, R.D. Ash, P. England, D.I. Rumble, Fluid flow in chondritic parent bodies: deciphering the compositions of planetesimals, Science 286 (1999) 1331–1335. D. Mattey, D. Lowry, C. MacPherson, Oxygen isotope composition of mantle peridotite, Earth Planet. Sci. Lett. 128 (1994) 231–241. R.H. Hewins, C.T. Herzberg, Nebular turbulence, chondrule formation, and the composition of the Earth, Earth Planet. Sci. Lett. 144 (1996) 1–8. E.R.D. Scott, H. Haack, Chemical fractionation in chondrites by aerodynamic sorting of chondritic materials, Meteoritics 28 (1993) 434. A.N. Krot, M.E. Zolensky, J.T. Wasson, E.R.D. Scott, K. Keil, K. Ohsumi, Carbide–magnetite assemblages in type-3 ordinary chondrites, Geochim. Cosmochim. Acta 61 (1997) 219–237. H.B. Wiik, On the regular discontinuities in the composition of meteorites. Comment. Phys.-Math. 34 (1969) 135–145.