Siderophile elements in chondrules of CV chondrites

Chemie der Erde 74 (2014) 507–516

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Siderophile elements in chondrules of CV chondrites Herbert Palme a,∗ , Bernhard Spettel b , Dominik Hezel c,d a

Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany An den 18 Morgen 10, 55127 Mainz, Germany c Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicherstrasse 49b, D-50674 Köln, Germany d Natural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD London, UK b

a r t i c l e

i n f o

Article history: Received 3 February 2014 Accepted 24 June 2014 Editorial handling - F. Langenhorst Keywords: Chondrules Chondrule formation Ca,Al-rich inclusions CV chondrites Siderophile elements Refractory elements Matrix chondrule complementarity

a b s t r a c t New bulk compositional data for 34 Allende chondrules are presented. Whole chondrules were analyzed by instrumental neutron activation analysis (INAA). The new data set is evaluated together with older INAA data on Allende chondrules and recent INAA data on Mokoia chondrules. The Ni/Co ratios of 200 chondrules are close to the CI- or solar ratio. The chondritic Ni/Co ratios require an unfractionated chondritic metal source and set a limit to the fraction of metal lost from molten chondrules. The bulk chondrule Fe/Ni and Fe/Co ratios are more variable but on average chondritic. Iridium and other refractory metals have extremely variable concentrations in chondrules. High Ir chondrules have chondritic Ir/Sc ratios. They are dominated by CAI (Ca,Al-rich inclusion) components. Low Ir chondrules have approximately chondritic Ir/Ni ratios reflecting mixing with chondritic metal. In low Ir chondrules Ir correlates and in high Ir chondrules Ir does not correlate with Ni or Co. A large fraction of Ir may have entered chondrules in variable amounts as tiny grains of refractory metal alloys. Most Allende chondrules have Ir/Sc ratios below bulk meteorite ratios. Matrix must have a complementary high Ir/Sc ratio, as bulk Allende has approximately chondritic Ir/Sc ratio. Similarly, the high average Ir/Ni ratios of Allende chondrules must be balanced by low Ir/Ni ratios in matrix to obtain the bulk Allende Ir/Ni ratio, which is close to the average solar system ratio. More recent data on single chondrules from Allende by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) and ICP-OES (Inductively Coupled Optical Emission Spectrometry) show the same trends as the INAA data discussed here. © 2014 Elsevier GmbH. All rights reserved.

1. Introduction Chondrules are a major structural component of most chondritic meteorites. In ordinary chondrites they make up some 80 vol.% of the bulk and in carbonaceous chondrites the chondrule fraction accounts for some 50 vol.% of the meteorites (Krot et al., 2014). Ordinary chondrites, therefore, have correspondingly smaller fractions of matrix than carbonaceous chondrites, implying that in ordinary chondrites the chemical composition of matrix is much less important for the bulk meteorite composition than for carbonaceous chondrites (e.g., Hezel and Palme, 2010). The process that made chondrules is still uncertain. Suggestions for chondrule formation encompass a wide range of possible mechanisms, such as condensation from a hot solar gas, formation near the Sun and transport with protostellar jets to the asteroid belt,

∗ Corresponding author. Tel.: +49 6131472732. E-mail addresses: [email protected], [email protected] (H. Palme). http://dx.doi.org/10.1016/j.chemer.2014.06.003 0009-2819/© 2014 Elsevier GmbH. All rights reserved.

collisions of molten planetesimals, heating by shock waves possibly generated by gravitational instabilities in the solar nebula or by supersonic planetesimals, or heating by electromagnetic processes (e.g., Ciesla, 2005). The chondrule formation process is one of the major unresolved questions in cosmochemistry. A wealth of chemical and isotopic data on single chondrules in a large variety of chondritic meteorites has been collected during the last 40 years (e.g., Jones et al., 2005). Some progress has been made in identifying the formation conditions of chondrules. For example, we now know that chondrules formed within the first few million years after the formation of the first solids in the solar system during minutes to hours long high temperature events in a dust enriched region (e.g., Kita and Ushikubo, 2012). The principal mechanism of chondrule formation is, however, still as unknown as it was 140 years ago, when chondrules were first described (Sorby, 1877). One of the problems in studying the composition of chondrules is the large variability in the chemistry of individual chondrules. The bulk chemistry of a single chondrule has no significance (e.g., Grossman and Wasson, 1983; Jones et al., 2005; Hezel and Palme, 2007). A large number (>∼20) chondrules has to be analyzed to

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obtain a statistically significant data set. This is true for chondrules of enstatite, ordinary and carbonaceous chondrites. Some of the variability may be produced by evaporation and recondensation during chondrule formation (e.g., Tissandier et al., 2002; Libourel et al., 2006; Hezel et al., 2003, 2010). However, most of the variability reflects large inhomogeneities in the composition of chondrule precursors (e.g., Grossman and Wasson, 1983; Hezel and Palme, 2007). In ordinary chondrites the variability in chemical composition is the result of mixing of different cosmochemical components in pre-chondrule aggregates. The components are produced in the solar nebula, i.e., their composition is controlled by volatility in a solar nebula environment, such as early formed refractory metal bearing grains, FeNi-metal components formed at lower temperatures (Grossmann and Wasson, 1985), variable refractory element abundances and large variations in the concentrations of volatile elements (Grossman and Wasson, 1982; Grossman and Wasson, 1983). These precursors of chondrules cannot be aggregates of finegrained interstellar dust, but must be larger objects, –>100 ␮m in size: the compositional variability of chondrules is too large (Hezel and Palme, 2007), even if some of the variability is accounted for by reaction of molten chondrules with the surrounding gas. The situation is somewhat different with regard to carbonaceous chondrites, as the fraction of chondrules in carbonaceous chondrites is much smaller than in ordinary chondrites. Carbonaceous chondrites have instead a fairly high proportion of matrix (around 50%). The compositional variability of chondrules of carbonaceous chondrites is similar to that of ordinary chondrites, but in carbonaceous chondrites the chemistry of matrix and chondrules must be complementary in the sense that both components are required to produce a chondritic bulk meteorite composition (Hezel and Palme, 2008, 2010). The small fraction of matrix in ordinary chondrites is no real constraint for the bulk meteorite composition. Matrix in carbonaceous chondrites is compositionally uniform with certain chemical characteristics (Grossman and Wasson, 1983; Hezel and Palme 2010): (1) matrix has lower than bulk meteorite Mg/Si ratios, (2) matrix has higher than bulk meteorite Fe/Mg and Ni/Mg- and other siderophile element to Mg ratios, (3) matrix has lower than bulk meteorite concentrations of refractory lithophile elements, and (4) matrix has higher than bulk meteorite volatile element concentrations, such as S or Se, except for Na, K and Mn. For non-volatile elements average chondrule composition must be complementary to the matrix composition to produce the well-defined ‘solar’ bulk composition. Since individual chondrules show a large range of bulk compositions, at least ∼20 arbitrarily selected chondrules are required to obtain a reliable average chondrule composition. This estimate depends on the elements in question. For example, Ir contents are much more variable than Cr or Mn concentrations in any of the chondrule suites discussed here. In addition, it is often better to use element ratios than element concentrations to interpret chondrule compositions. The Ni and Co contents of chondrules are extremely variable, the Ni/Co ratios are, however, much more constant as discussed in more detail below.

2. Bulk compositions of chondrules Here we consider chondrule bulk compositions obtained by instrumental neutron activation analysis (INAA). We use data by Osborn (1971), Rubin and Wasson (1987), and previously unpublished data for Allende chondrules from the former Cosmochemistry Department of the Max-Planck-Institute for Chemistry in Mainz. In addition, we discuss INAA chondrule data from Mokoia, another CV chondrite (Jones and Schilk, 2009). Finally, we compare our results with new data on single chondrules from Allende by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) and

ICP-OES (Inductively Coupled Optical Emission Spectrometry) reported by Gerber (2012). The advantages and disadvantages of using INAA are discussed at some length in Jones and Schilk (2009). Briefly, with INAA complete or nearly complete chondrules are analyzed, e.g. allowing conclusions regarding the dependence of elemental abundances with chondrule sizes. In addition, the data do not require conversion of data obtained in 2 dimensions by analysis of polished thin sections to 3 dimension bulk chondrule abundances (e.g., Hezel and Kießwetter, 2010). Instrumental neutron activation analysis is extremely sensitive for some elements, whereas others cannot be determined at all. The sensitivity of an element determination depends on neutron capture cross sections and the formation of radioactive nuclei with suitable half-lives and ␥-lines. The sensitivity for detection of ␥-radiation has tremendously improved over the time period when INAA was used. The early study of Osborn (1971) has only few elements on which we want to concentrate in this paper. Fortunately, these cover a range of cosmochemically and geochemically relevant elements. This data set reports data on nine elements, the refractory lithophile elements Al, Ca and Sc, the refractory siderophile element Ir, the common lithophile element Cr, the moderately volatile lithophile element Mn, as well as the siderophile elements Fe, Co and Ni. With a short irradiation of a few minutes, excellent data for Al and Mg can be obtained. The newest data set of Jones and Schilk (2009) lists abundances for 24 elements for each single chondrule.

3. New data on Allende chondrules All chondrules analyzed in this study were randomly taken from several pieces of Allende. Chondrules were removed from bulk Allende with dental tools and ultrasonically cleaned. Only complete chondrules were analyzed. Most chondrules were separated in Mainz, some were separated by Gero Kurat and Franz Brandstätter at the Natural History Museum in Vienna. A significant time after irradiation, chondrules were cut into half and polished sections were prepared. Full records of the petrography of these chondrules are, unfortunately, no more available. Preliminary results of this study can be found in Kurat et al. (1985, 1992), Spettel et al. (1989), Kurat and Palme (1989) and Palme et al. (1992). The first two unusually large chondrules in Table 1 are very large radial pyroxene chondrules. For INAA, routine procedures of the former Cosmochemistry Department at the Max-Planck-Institute for Chemistry in Mainz were applied. These procedures involve neutron irradiation for 6 h in a TRIGA reactor at a neutron flux of 7 × 1011 n/(s × cm2 ) and subsequent counting on Ge-detectors, followed by a short irradiation (3 min) for the determination of Al, Mg, V, and Ti concentrations (see Wänke et al., 1977, for details). Table 1 contains the results of analyses of 34 Allende chondrules. Estimated accuracies based on counting statistics and the analyses of numerous standard rocks are given in the second column of Table 1 (Wänke et al., 1977). The average chondrule compositions along with corresponding standard deviations are given in the two last columns of Table 1. In this paper we compare four sets of INAA data on chondrules in CV chondrites. Three data sets are from Allende, and a fourth with 94 bulk chondrule compositions was recently published by Jones and Schilk (2009). An overview is given in Table 2. The Osborn (1971) chondrule data as well as Allende chondrule data given in this work are whole chondrule analyses. Rubin and Wasson (1987) excavated chondrules from one mm thick slices of Allende, which allowed them to use chondrule fractions for petrographic work. Jones and Schilk (2009) mechanically separated Mokoia chondrules from the host meteorite and in a few cases a

Table 1 Compositions of 34 chondrules from Allende. Ach-1 Mass in mg S.d.% % Mg 5 Al 5 Ca 10 Ti 20 5 Fe 3 20 5 3 10 3 3 3 10 25 20 25 20 25 20 10 5 10 20 20 15 15 20 20 20 5 5

2.61

Ach-2 236.3

1.99 5.25

3740

4440

740 18.8

795 14.6

4520 1610 19.5 310 32 0.5

4790 1900 15.1 170 20 1

1 0.75 0.49 0.17 0.11 0.96 0.55 0.076 0.33

0.011 0.011

0.57 0.39 0.14 0.093 0.90 0.40 0.06 0.27

0.0066 0.0062

Ach-10

Ach-11

7.23

4.83

26.02 2.5 2.44 0.15 3.75

25.04 2.12 2.48 0.10 3.71

2180 135 190 22.5 161 2310 397 59.2 1260 19 1.05 0.19

2310 88 198 21.7 168 2300 448 70.2 1600 23 1.04 0.26

0.38 1.6 0.75 0.46 0.16 0.12 1.01 0.49 0.086 0.44 0.139 1.74 1.45 0.012

0.49 5.5 0.63 0.41 0.15 0.17 1.0 0.44 0.087 0.072 0.88 0.64 0.033

Ach-12

Ach-13

Ach-14

Ach-15

46.8

11.68

5.03

27.053

19.62 1.72 1.62 0.10 8.45

21.36 1.71 1.49 0.15 12.40

14.07 6.7 2.8 0.32 11.73

8.83 13.54 9.84 0.74 2.23

8450 5020 14,570 920 490 1160 765 435 1180 13.1 16.1 40.1 102 143 123 5220 3820 3420 2930 763 690 159 581 478 3270 13,000 11,820 40 60 116 1.8 1.7 5.4 0.82 1.5 2.84 7.55 7.2 4.1 2.4 8.7 1.1 1.7 0.55 0.65 1.40 0.36 0.40 0.96 0.14 0.14 0.21 0.083 0.34 0.59 0.75 2.23 0.353 0.51 0.79 0.0523 0.0705 0.16 0.245 0.87 0.029 0.33 0.20 1.1 5.14 0.163 0.775 4.30 0.076 0.04 0.10

6950 965 535 89.8 879 3220 370 116 3700 114 1.5 0.52 4.8 6.4 6.8 3.71 2.57 0.65 0.65 4.32 2.14 0.425 1.86 0.50 6.72 5.78 0.089

Ach-16

Ach-17

7.44

3.16

15.07 5.01 2.62 0.29 7.84

17.83 5.5 5.31 0.28 5

22,500 2970 1610 16.6 211 3625 956 274 5920 78 4.7 0.48 4 16.6 2.8 1.41 1.04 0.31

11,720 750 1050 38.6 175 3570 980 161 4380 64 2.5 0.49

1.16 0.65 0.056 0.33 0.56 0.443 0.057

3.9 6.7 1.86 1.20 0.39 1.97 1.32 0.20 0.93 0.39 4.66 3.84 0.056

Ach-18 3.227 23.17 4.41 4.1 0.22 3.48 12,650

Ach-19

Ach-20

ACS1

ACS2

ACS3

ACS-4

1.67

1.13

17.03

21.38

20.31

22.85

21.31 3.26 2.88 0.14 5.76

23.96 2.06 1.76 0.09 7.66

24.55 2.22 2.48 0.13 3.98

22.4 1.04 1.07

21.71 2.96 3.34 0.16 5.32

22.51 1.92 1.9 0.17 6.93

4.63

4620

4205

1197

2520

4620

980 30.6 118 2410 456 69.3 2290 44 2.5 0.33

510 18.6 193 3590 1350 141 3100 96 2.86 0.46

453 16.9 64.1 1950 408 339 7550 110 1.6 0.34

440 9.32 96.9 4770 2865 63.8 2090 30 1.24 0.27

0.64

0.95

0.6

1.31 0.84 0.31 0.27 1.39 0.90 0.14 0.53 0.13 1.62 1.36 0.030

0.85 0.53 0.23

0.62 0.38 0.14

0.92 0.57 0.085

0.72 0.41 0.066 0.3 0.034

138 14 127 2530 412 65.9 1730 38 1.05 0.25 1.3 0.12 1.4 0.727 0.473 0.15 0.14 0.71 0.424 0.066 0.27 0.035 0.30 0.277 0.035

0.246 0.034 0.15

464 22.7 91.4 2870 636 213 4800 60 0.9 0.52 3.87 0.49 2.3 0.839 0.547 0.24 0.12 0.83 0.666 0.088 0.38

0.043 0.022

0.26 0.231 0.020

0.25 3.04 2.57 0.12

0.25 0.050

0.43 2.3 0.36 0.216 0.079 0.059

5900 340 885 18.7 96.1 3660 1115 202 4660 42 2.24 0.70 2.6 2.03 0.770 0.477 0.19 0.13 0.82 0.514 0.075 0.24

H. Palme et al. / Chemie der Erde 74 (2014) 507–516

ppm Na Cl K Sc V Cr Mn Co Ni Zn Ga As Se Br Mo La Sm Eu Tb Dy Yb Lu Hf Re Os Ir Au

167.8

0.26 0.226 0.061

509

510

Table 1 (Continued) ACS-5 Mass in mg S.d.% % Mg 5 5 Al Ca 10 Ti 20 5 Fe

ACS-7

ACS-8

3.49

3.55

2.38

8.93 10.64 8.4 0.36 2.44

20.93 1.03 1.17 0.16 11.3

20.08 2.15 2.43 0.16 10.4

24.12 2.32 2.32 0.14 6.78

13,650 2050 847 7.36 414 2260 832 101 3030 40 2.5 0.37 13.6 0.8 1.10 1.153 0.549 0.44 1.54 0.042

0.241 0.031

1370

4970

310 10.8 102 5790 1725 272 6070 54 2.71 0.79

740 20.7 193 4850 1300 374 8020 67 3.27 0.58 6.6 0.64

0.31 0.430 0.250 0.227 0.53 0.45 0.038 0.27 0.58 0.48 0.071

4800 260 602 18.7 149 3110 630 256 5700 57 2.3 1.09

0.960 0.576 0.275 0.19 1.11 0.69 0.095

0.820 0.535 0.143

0.17 2.94 2.45 0.261

0.11 1.5 1.22 0.146

0.85 0.74 0.18

ACS-12 10.0 22.10 3.44 2.06 0.17 6.27 3996 370 27.1 133 3460 480 53.3 1520

ACS-13

ACS-14

ACS-15

ACS-16

9.68

1.70

1.70

0.99

18.87 1.85 1.60 0.13 13.67

26.39 0.92 0.84 0.12 8.76

25.1 1.69 1.51 0.14 10.23

20.81 1.43 1.02 0.10 7.27

1640

1584

2550

3910

ACS-17 22.9 28.69 2.49 1.70 0.19 7.66 6360

ACS-18

ACS-23

8.63

1.27

20.57 1.41 1.33 0.14 5.93

21.06 3.17 3.31 0.31 12.45

3885

1320

260 220 240 460 790 580 420 11.3 9.97 12.3 10.3 16.1 10.7 21.8 102 116 126 58 159 106 163 4450 3810 3610 2690 6910 4640 4520 635 750 728 605 4080 3300 963 737 131 270 103 137 90.2 205 15,500 2920 5720 2670 3380 2600 3690 49 46 66 44 58 94 0.6 2.42 2.1 2.8 1.5 2.4 2.23 6.0 0.13 0.81 0.56 0.26 0.71 0.34 1.4 13.5 1.6 10 0.48 1.1 0.12 0.66 0.60 2.1 0.8 0.910 0.580 0.360 0.540 0.390 0.630 0.500 1.060 0.694 0.34 0.235 0.365 0.296 0.422 0.301 0.659 0.220 0.130 0.068 0.125 0.099 0.137 0.169 0.251 0.22 0.099 1.09 0.56 0.61 0.49 0.66 0.46 1.01 0.34 0.23 0.39 0.36 0.44 0.34 0.72 0.803 0.12 0.042 0.058 0.057 0.047 0.068 0.046 0.12 0.54 0.28 0.28 0.28 0.69 0.079 0.11 0.15 0.29 1.3 0.53 1.2 0.25 2.4 0.125 1.13 0.344 0.82 0.30 0.199 0.0783 1.86 0.011 0.095 0.070 0.126 0.056 0.077 0.027 0.120

ACS-25 43.7 19.03 1.39 1.52 0.13 5.55 4700 1080 470 10.9 101 5310 2760 97.7 2920 33 3.2 0.13 2.48 1.2 0.400 0.284 0.112 0.076 0.51 0.335 0.046 0.19 0.17 0.137 0.022

ACK1

ACK2

24.5

ACK3 1.13

ACK4 9.32

22.39 213 20.86 3.24 2.77 0.20 6.94

2.17

6.85

3.16

2.15

6.87

2.66

5.67

6.86

10,290 1720 710 22.1 2820 912 212 4340 78 3.2 0.5 3.5 6.97

12,650 2540 560 49.3 2650 808 68.4 1450 95 3.1 0.31 1.7 15.5

0.820 0.520 0.200 0.14 0.99 0.64 0.095 0.38

1.73 1.19 0.405 0.27 1.82 1.34 0.18 0.81

0.154 0.028

1.6 1.18 0.024

13,610 990 1380 9.49 3050 795 122 2530 67 3.1

8280 300 1010 20.1 2655 848 173 3890 2.3 1.18

2.9 0.780 0.530 0.200 0.15 0.63 0.69 0.033

0.156 0.076

Average S.d. in %

3.86

7.8 0.770 0.502 0.187 0.83 0.56 0.093 0.30 0.1 1.14 1.05 0.100

22 89 73 67 44

6387 77 86 998 628 55 20.9 73 167 92 3681 31 1189 79 189 84 4341 79 59.17 46 2.33 54 0.56 64 5.08 70 3.47 139 2.87 83 0.90 69 0.61 73 0.21 58 0.18 75 0.98 79 0.65 62 0.09 78 0.47 79 0.16 83 1.62 106 1.01 134 0.06 81

H. Palme et al. / Chemie der Erde 74 (2014) 507–516

ppm Na 3 Cl 20 K 5 3 Sc V 10 3 Cr Mn 3 Co 3 Ni 10 Zn 25 Ga 20 As 25 Se 20 Br 25 Mo 20 La 10 Sm 5 Eu 10 Tb 20 Dy 20 Yb 15 Lu 15 Hf 20 Re 20 Os 20 5 Ir Au 5

ACS-6 7.55

H. Palme et al. / Chemie der Erde 74 (2014) 507–516 Table 2 Mass range and number of analyzed chondrules in the Allende meteorite. No. of chondrules analyzed

Mass range (mg)

Allende Osborn (1971) Rubin and Wasson (1987) This work

61 20 34

0.19–91.2 0.99–11.5 0.99–236.3

Mokoia Jones and Schilk (2009)

94

0.10–22.5

small fraction of material was removed from a few larger chondrules for petrographic studies. The Mokoia chondrule data are therefore representative of bulk chondrules. Gerber (2012) separated chondrules using the freeze–thaw technique. In Table 3 we list average Allende chondrule compositions of the three data sets discussed here. For comparison, we also list the average composition of a large number of chondrules with a total of 1.5 g from Clarke et al. (1971). All data are normalized to bulk Allende of Kallemeyn and Wasson (1981). Judging from the standard deviations, chondrules are quite variable in composition. Nevertheless, there are some general trends obvious in all suites of data. The two refractory lithophile elements Al and Sc are on average enriched relative to bulk Allende by a factor of 1.1–1.9. The alkali elements Na and K show a trend parallel to the refractory lithophile elements, probably indicating the preferred uptake of Na in Al-rich phases of chondrules. Kimura and Ikeda (1995) documented the replacement of plagioclase and chondrule glass by nepheline and/or sodalite. The CaO that was lost from anorthite during Na, K replacement reactions was either lost to the gas phase or was retained forming new minerals, such as hedenbergite, andradite, grossular etc. In the Rubin and Wasson (1987) data set, there is no evidence for a negative correlation of Ca/Al vs Na expected from such a replacement. In the present data set (Table 1) there is a hint of a negative correlation. The two chondrules with the highest Na contents Ach-14 (1.5 wt.%) and Ach16 (2.2 wt.%) have the lowest Ca/Al ratios, 0.42 and 0.52, respectively. Table 3 also indicates that Cr and Mn are more or less equally distributed between chondrules and matrix. As chondrules have approximately the bulk Allende concentrations of Cr and Mn, matrix should have similar concentrations. Large differences in average Allende chondrules are found for Fe, Co and Ni. The mean total Fe in the 20 chondrules reported by Rubin and Wasson (1987) is 13.1 ± 6.3 wt.%, compared to 6.9 ± 3.1 wt.% in

511

the Mainz chondrule suite of 34 chondrules. The Osborn (1971) data give a mean Fe content of 9.8 ± 3.3 wt.%, right in the middle between the LA and Mainz data. The 1.5 g chondrule sample of Clarke et al. (1971) has an Fe content of 8.5 wt.%. The concentrations of Co and Ni seem to co-vary with Fe. The Mainz chondrules are on average nearly a factor of two lower in Fe than the LA data. The large number of unusually FeO-rich chondrules in the Rubin and Wasson (1987) data set may indicate preferred chondrule compositions in certain parts of the Allende meteorite. Given constant bulk compositions of Allende at the gram scale (Stracke et al., 2012), either the fraction of matrix in a given piece of Allende is variable or the average FeO contents of matrix in areas with high FeOchondrules is lower than average matrix. To prove these hypotheses requires analyses of chondrules and bulk samples from single welldefined pieces of Allende. Such detailed studies are presently not available. The average chondrule Ir content of 1.28 (normalized to bulk Allende) in the present data set is significantly above the corresponding values for Fe (0.29), Co (0.29) and Ni (0.33), see Table 3. The high Ir indicates enrichment of the refractory metal Ir over the non-refractory metals Fe, Co and Ni. But the enrichment (relative to bulk Allende) is much less than the enrichment of refractory lithophile elements with enrichment factors of 1.85 for Sc and 1.84 for Al in the same data set (Table 3). Thus, the behaviour of Ir is governed by two contradicting trends: (a) general enrichment of lithophile and siderophile refractory elements in chondrules (perhaps as CAI-components) and (b) general depletion of metallic elements in chondrules. In the other data sets the normalized abundance of Ir is also between the abundance of Fe, Ni, Co and the refractory lithophile elements Al and Sc. A more detailed discussion of siderophile elements is given below. 4. Siderophile elements in Allende and Mokoia chondrules Fig. 1 is a Co–Ni plot, demonstrating approximately chondritic Ni/Co ratios in Allende and Mokoia chondrules. The data for bulk carbonaceous chondrites are from Kallemeyn and Wasson (1981). The range of absolute Ni and Co concentrations is enormous, from 200 ppm to 2.2 wt.% Ni, with essentially chondritic Ni/Co ratios. Nearly all data points are below the bulk CV chondrites with about 600 ppm Co and 1.42 wt.% Ni, reflecting the general depletion of metallic elements in chondrules. To achieve mass balance, enhanced contents of Ni and Co in matrix are required. Data from Osborn (1971) deviate slightly from the CI Ni/Co ratio (Fig. 1). It is not clear whether this is real or reflects analytical prob-

Table 3 Average chondrule compositions and associated 1 standard deviations (in italics) of the various chondrule populations. All data are normalized to bulk Allende (Kallemeyn and Wasson, 1981). Rubin and Wasson (1987) Average of 20 chondrules

This work Average of 34 chondrules

Osborn (1971) Average of 61 chondrules

Clarke et al. (1971) Average of 1.5 g of chondrules

Allende bulk Kallemeyn and Wasson (1981)

Refractory lithophile elements 1.23 ± 0.88 Al 1.12 ± 0.37 Sc

1.84 ± 1.64 1.85 ± 1.36

1.52 ± 1.40 1.76 ± 1.34

1.68

1.76% 11.3 ppm

Alkali elements 1.42 ± 0.85 Na 1.46 ± 0.80 K

1.94 ± 1.50 2.13 ± 1.18

1.87 ± 1.08

1.85 1.70

3290 ppm 294 ppm

Moderately volatile, lithophile elements 1.14 ± 0.38 Cr 1.01 ± 0.73 Mn

1.01 ± 0.32 0.82 ± 0.65

1.21 ± 0.55 0.80 ± 0.56

0.89 0.75

3630 ppm 1450 ppm

Siderophile elements 0.55 ± 0.27 Fe 0.60 ± 0.35 Co 0.70 ± 0.55 Ni

0.29 ± 0.13 0.29 ± 0.24 0.33 ± 0.26

0.42 ± 0.14 0.39 ± 0.30 0.47 ± 0.38

0.36

23.7% 662 ppm 1.33%

Highly siderophile elements 0.99 ± 1.53 Ir

1.28 ± 1.72

0.73 ± 1.08

0.42

0.785 ppm

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Fig. 1. Co–Ni plot for Allende and Mokoia chondrules and bulk CV-chondrites. Data from Rubin and Wasson (1987) and from the present work (Table 1) indicate a chondritic Ni/Co ratio. The Osborn (1971) data deviate slightly with increasing Ni/Co at higher Ni contents. The Mokoia data (Jones and Schilk, 2009) show the opposite trend. Bulk carbonaceous chondrite data are from Kallemeyn and Wasson (1981).

lems. The newer Mokoia data from Jones and Schilk (2009) indicate the opposite trend, a tendency for lower Ni/Co ratios at higher Ni contents (Fig. 1). It could indicate metal loss from chondrules, which would affect Ni much more than Co. The potential loss of metal from chondrules will be discussed below. Fig. 2 displays approximately chondritic Fe/Co ratios in Allende chondrules, although the variations are significantly larger than in the Ni/Co ratios and extend from sub- to super-chondritic. Almost all data points are, however, below the bulk CV meteorite abundances. This requires that matrix, the other volumetrically important component in carbonaceous chondrites, has high Co and Fe balancing the low chondrule abundances (see Hezel and Palme, 2007, 2010). The spread of Fe concentrations from below one weight percent to the bulk Allende level is more or less continuous. The dichotomy

Fig. 2. Co vs Fe plot of Allende and Mokoia chondrules and bulk CV chondrites. There is a rough correlation between Fe and Co. A similar correlation would be obtained for Fe and Ni. At low Fe contents, there is a deficit of Co and at higher Fe contents a Co excess. Bulk carbonaceous chondrite data are from Kallemeyn and Wasson (1981).

Fig. 3. Ir vs Ni in chondrules and bulk CV-chondrites from Allende and Mokoia. Most chondrules have Ir/Ni ratios significantly above the CI or bulk CV ratio. The variability of the Ir/Ni ratios excludes a single metal carrier for Ni, Co and Ir. The excess Ir was most probably delivered by refractory metal grains, as discussed in the text. Bulk carbonaceous chondrite data are from Kallemeyn and Wasson (1981).

of type 1, Fe-poor and type 2, Fe-rich chondrules is not visible in bulk Fe chondrule contents. The general trend in chondrules indicates close to chondritic ratios between Fe–Ni–Co (Table 3) and requires that the three elements were added to chondrules as a single phase, e.g. as metallic FeNiCo alloy. If the alloy formed by condensation, all three elements should be completely condensed at the time of chondrule formation. Early FeNiCo alloys have high Ni/Fe and Co/Fe ratios, as well as fractionated Ni/Co ratios (e.g., Palme and Wlotzka, 1976). They are not suitable candidates. The addition of chondrule precursor components containing FeO without accompanying Ni and Co could explain the higher Fe at low bulk chondrule Fe contents (Fig. 2). At higher bulk Fe contents, Fe is too low for a chondritic Fe/Co ratio. This could indicate loss of iron by oxidation and/or sulfurization of FeNiCo alloys before incorporation into chondrules. The effect can only be minor as it would affect Co much more than Ni and result in low Co/Ni ratios, at high Fe contents, which is generally not observed (see Fig. 1). A metallic chondrule precursor component should also carry the appropriate amounts of Ir, i.e., one would expect chondritic Ir/Ni ratios in bulk chondrules. However, Fig. 3 clearly demonstrates the absence of any Ir–Ni correlation. Almost all data points for Allende or Mokoia chondrules lie above the CI reference line. Most chondrules have significantly higher than chondritic Ir/Ni ratios. They are enriched in the refractory metal Ir compared to the nonrefractory metal Ni. The enrichment can be quite large as is clear from Fig. 3. One way to contribute Ir but not Ni to chondrules is to add Ca,Alrich inclusions (CAI). Coarse-grained CAIs are often enriched in refractory siderophile and simultaneously in refractory lithophile elements; they have high Ir and high Sc. Both elements condense above Mg-silicates and FeNi-metal, respectively (e.g., Wänke et al., 1974; Grossman, 1980). If Ir was introduced into chondrules with CAIs, one would expect a correlation of Sc with Ir. Indeed, chondrules with the highest Ir also have high Sc, but at lower Ir contents there is no co-variation of Ir and Sc. The majority of the data points in Fig. 4 lie below the chondritic correlation line, implying lower CI-normalized Ir than Sc. The comparatively low Ir reflects the general depletion of metallic elements in chondrules, independent of their host-phase.

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Fig. 4. Bulk chondrules display no correlation of Ir to Sc. Bulk CV chondrites have CI chondritic Ir/Sc ratios. The host phases of Ir are not CAIs with uniform enrichment factors for Sc and Ir. Perhaps nuggets of refractory metal alloys were added in variable amounts to chondrule precursors. Almost all Ir/Sc ratios of chondrules are below the bulk meteorite Ir/Sc ratio. Matrix must have higher than bulk meteorite Ir/Sc ratios to compensate chondrules. Bulk carbonaceous chondrite data are from Kallemeyn and Wasson (1981).

This leads to the possibility of a separate host phase for Ir. Refractory metal grains are common in carbonaceous chondrites (e.g., Berg et al., 2009). Such grains are composed of alloys of refractory metals such as W, Os, Re, Ru, Ir, Mo, Pt and Rh. Random addition of such grains to chondrules would result in the observed abundance patterns. The fractionation of refractory lithophile (e.g., Sc) and refractory siderophile (e.g., Ir) elements between matrix and chondrules can be used for understanding Hf/W fractonation. The bahaviour of W is similar to that of Ir, both elements are concentrated in refractory metal nuggets. Chondrules have high but variable Hf/W ratios, matrix has low Hf/W (Becker et al., 2013).

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In Fig. 5 we have plotted the CI-normalized abundances of three refractory lithophile elements (Ca, Sc, Sm), three refractory siderophile elements (Re, Os, Ir) and four non-refractory siderophile elements (Ni, Co, Fe, Au) in the seven chondrules most enriched in Ir and the six chondrules with the lowest Ir contents of the data set introduced here (Table 1). The Ir enriched chondrules have a pattern with rather uniform enrichment factors for all refractory elements (i.e., lithophile and siderophile refractories) and much lower contents of non-refractory elements. The high absolute contents of refractory elements and the relatively flat enrichment pattern of refractory lithophile and refractory siderophile elements are typical of coarse grained Ca,Al-rich inclusions (e.g., Wänke et al., 1974; Grossman, 1980). It appears that these chondrules are mixtures of “normal” chondrules and CAIs. Chondrules with low Ir contents have approximately chondritic ratios of Fe, Ni, Co and Ir (Fig. 5), indicating the presence of a chondritic metal component. This is also apparent from Fig. 3 with a correlation of Ni vs Ir at low contents of Ni and Ir. There are few chondrules with Ir/Ni ratios above the chondritic ratio. Most of these unusual chondrules in Fig. 4 are from the Osborn (1971) study with large uncertainties in Ir contents. Between the high Ir chondrules with approximately chondritic Ir/Sc ratios (CAI source) and low Ir samples with chondritic Ir/Ni ratios (chondritic metal source) there are numerous chondrules with Ir contents that do neither correlate with Ni nor with Sc. Furthermore, Ir contents cover a wider concentration range than the non-refractory lithophile elements. The variation in Ir concentrations among chondrules of the present data set is above a factor of 800 compared to about 100 for Ni, 6 for Fe and 10–15 for refractory lithophile elements such as Sc (Table 1). The other data sets show similar patterns, increasing variability from refractory lithophiles, non-refractory common siderophiles to the highly siderophile and refractory element Ir. The large abundance range of Ir in chondrules of CV meteorites is a combined effect of the enrichment of some chondrules in refractory elements due to the presence of CAI components and the low contents of siderophile elements in some chondrules lacking a CAI component. One could assume that the actual Ir contents in most chondrules, intermediate between the two end members in Fig. 5, are mixtures between these two components. In this case, one would expect

Fig. 5. Patterns (from left to right) of three lithophile refractory, three siderophile refractory elements three common metals and one moderately volatile metal in the seven chondrules with the highest Ir contents and the six chondrules with the lowest Ir contents of the data in Table 1. The carriers of the high Ir chondrules could be CAIs, whereas in low Ir chondrules Ir is provided by chondritic FeNi-metal. In the majority of chondrules Ir neither correlated with Sc nor with Ni. Source of CI-data (Palme et al., 2014).

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Excess Ir relative to Ni (Fig. 7c) and the lack of an Ir–Sc correlation (Fig. 7d), as discussed above, are also visible in the Gerber (2012) data set (cf. Figs. 3 and 4). These trends are probably characteristic of all CV chondrites. Similar trends are also seen in chondrules of ordinary chondrites, except that Co/Ni ratios of low Ni chondrules are enhanced in Semarkona chondrules (Grossmann and Wasson, 1985). The carbonaceous chondrite data show that non-chondritic ratios among siderophile elements and nonchondritic lithophile siderophile element ratios must be compensated by corresponding complementary ratios in matrix. 5. Removal of siderophile elements by expulsion of metal from molten chondrules

Fig. 6. Ir vs Os concentrations in Allende and Mokoia chondrules and bulk CV chondrites. Ir/Os ratios in Allende chondrules are chondritic. Mokoia chondrules have higher Ir/Os ratios. The reason is unclear. Bulk carbonaceous chondrite data are from Kallemeyn and Wasson (1981).

a correlation of Ni/Ir vs Sc/Ir. There is, however, no such correlation. This supports the assumption of a separate carrier phase for Ir and other refractory metals as discussed above. Grossman and Wasson (1985) have suggested such a component for chondrules in Semarkona based on similar observations as those presented here. So far, we have identified three different components contributing Ir and other refractory metals to chondrules: (a) chondritic metal (b) CAIs and (c) refractory metal grains. The refractory metal Ir is representative of other refractory metals, such as Os or Ru. Osmium in chondrules can be determined by INAA, but is significantly less sensitive than Ir. Therefore, Os data are much more sparse and have larger uncertainties. The Osborn (1971) data set contains no Os. In Fig. 6 we present an Os vs Ir correlation. The Ir/Os ratios are chondritic in the Rubin and Wasson (1987) and the present data set. The Mokoia samples have a somewhat higher Ir/Os ratio. The significance of this observation is unclear. The chondritic Os/Ir ratio in Allende chondrules indicates unfractionated patterns of refractory metals in chondrules, which is supported by Re data in the present data set (see Fig. 5, Table 1). Recent data on highly siderophile elements in five Allende chondrules confirms this view (Archer et al., 2014). Although these authors find a spread in Ir from 58.4 to 1270 ppb, the ratios of Os to other refractory metals (Re, Ir, Ru, Pt) are essentially chondritic, supporting the findings reported here. In the Ir–Sc plot of Fig. 4 most chondrules are deficient in Ir compared to Sc, i.e., their Ir/Sc ratios are below the bulk meteorite ratios, which are essentially CI chondritic. Twenty one CV chondrites have an average chondritic Ir/Sc ratio of 0.073 ± 0.005 (Kallemeyn and Wasson, 1981), whereas the average Ir/Sc ratio of 200 chondrules is 0.133 ± 1.2, with only 23 out of 200 chondrules with ratios above chondritic. Thus it is clear that matrix, the other volumetrically important component of carbonaceous chondrites, must have on average the complementary high Ir/Sc ratio to balance the low ratios of chondrules (see also Table 3). In Fig. 7 we show the results of recent ICP-MS and ICP-OES analyses of Allende chondrules by Gerber (2012). Fifteen chondrules were analyzed and Fig. 7 demonstrates exactly the same trends as discussed before for about two hundred INA analyses. Fig. 7a shows the perfect chondritic Ni/Co ratio of Allende chondrules (cf. Fig. 1). The enrichment of Fe at low total Fe contents and the depletion at high Fe relative to Co and Ni are seen in Fig. 7b, analogue to Fig. 2.

Large variations in the concentration of siderophile elements in chondrules can be achieved by removal of liquid metal droplets from molten chondrules as for example suggested by Grossmann and Wasson (1985). The chondritic Ni/Co ratios in CV chondrules limit the extent of this process. As Ni is much more siderophile than Co and Fe, liquid metal that forms during chondrule melting will have high Ni/Co ratios. If this metal is expelled from chondrules, the residual chondrule will be deficient in Ni relative to Co, destroying the chondritic Ni/Co ratios in chondrules. As an example, we calculated the distribution of Ni and Co between liquid metal and liquid silicate, assuming a metal/silicate ratio of 1/1000, i.e., 1 ␮g metal in equilibrium with 1 mg silicate liquid. We assume metal silicate partition coefficients of 104 for Ni and 103 Co corresponding to an oxygen fugacity of IW-3 and a temperature of 1400 ◦ C (Holzheid and Palme, 2007). With these parameters we calculate a Ni/Co ratio in the liquid silicate of 4 compared to the chondritic Ni/Co ratio of about 20. Thus if a 1 ␮g metal grain would be ejected from the chondrule liquid the residual chondrule would have a far lower Ni/Co ratio than that observed. If metal droplets were larger the effect would be larger. Smaller metal grains would reduce the effect. At a metal/silicate mass ratio below 10−4 , metal removal would not have a noticeable effect on the Ni/Co ratio in the residual silicate. Thus removal of very small metal droplets cannot be excluded as they would remove so little Ni and Co that the effect on the coexisting silicate would be too small to be observable. However, any loss of metal from molten chondrules would remove Ir from chondrules more or less quantitatively. Iridium is a highly siderophile element with metal/silicate partition coefficients far above those of Ni and Co (Borisov and Palme, 1995). Given metalsilicate equilibration during the chondrule melting stage, any loss of metal would lead to complete loss of Ir from molten chondrules, assuming equilibrium is reached within the short time of melting. Cosmic spherules often contain tiny nuggets with Ir and other refractory metals formed by a short melting episode of heating during atmospheric entry (Rudraswami et al., 2014). Apparently, formation of refractory metal rich nuggets may be achieved in a few seconds. It is therefore unlikely that ejection of molten metal droplets during chondrule melting occurs and modifies the composition of chondrules. 6. Consequences for chondrule formation The previous discussion has shown that there are three sources for Ir in CV chondrules: (1) chondritic FeNi-metal, (2) CAIs and (3) refractory metal nuggets. Sources (2) and (3) may not be independent. Refractory metal nuggets (RMN) are the main carrier of Ir, Os etc. in CAIs (Palme et al., 1994). The RMNs may form by condensation independent from CAIs, so that CAIs from the same reservoir would be depleted in refractory siderophile elements compared to refractory lithophile elements. Thus, variable amounts of RMN in

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Co (ppm)

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Fig. 7. Data by Gerber (2012) for Allende chondrules by ICP-MS and ICP-OES. These data show the same trend as the INAA data discussed before (see text and Figs. 1–4).

CAIs could produce the large observed variations in Ir in chondrule precursors and consequently in chondrules. The FeNi-component in chondrule precursors formed at lower temperatures. The chondritic Ni/Co ratio requires full condensation of Ni and Co (Palme and Wlotzka, 1976). Thus chondrules contain components formed at different nebular temperatures, as has been observed in chondrules of ordinary chondrites (Grossman and Wasson, 1983). Since chondrules in CV chondrites and also in other groups of chondritic meteorites are low in metallic elements, the FeNi phase formed by condensation processes has only partially entered the chondrule precursor grains. Much of the FeNi metal CV chondrites will be oxidized and end up in matrix, carrying a major fraction of Ir and other metallic elements. 7. Summary New chemical analyses of bulk chondrules in Allende are presented. The data are evaluated together with earlier results on Allende and Mokoia chondrules The dichotomy of type 1 chondrules (FeO-poor) and type 2 chondrules (high FeO) is not visible in the bulk Fe contents. Nickel in chondrules is well correlated with Co defining the chondritic Ni/Co ratio. Iron, although less well correlated with Ni and Co, approximately defines chondritic Fe/Ni and Fe/Co ratios. Iridium and other refractory metals entered chondrule precursors as chondritic FeNi metal, as Ca,Al-rich inclusion and as independent refractory metal nuggets. Some consequences for chondrule formation are discussed. Metal loss by expelling metal grains from molten chondrules was not an important process in modifying chondrule chemistry.

Acknowledgements The authors are grateful to Gero Kurat and Franz Brandstätter (Vienna) for providing some of the chondrules. Klaus Keil and Mario Fischer-Gödde are thanked for helpful comments.

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