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Relationship between siderophilic-element content and oxidation state of ordinary chondrites
Geochimica et Cosmochimicr Aeta,X971,Vol.35,pp.1111to 1137.Pergamon
Re~tio~~~
Press.
Printed inNorthern Ireland
between ~dero~~c-element content and oxidation state of ordinary chondrites
OTTO &&LLER,* PHILIP A. BAEDECKER and JOHN T. WASSON Institute of Geophysics and Planetary Physics, Department of Chemistry University of California, Los Angeles, California 90024 (Receiaed 22 July 1970; ~cc~~te~ ila revised
form 28 Jurae 1971)
&&r~&--The concentrations of Ni and Ir have been determined by neutron activation in a suite of ordinary chondrites for which accurate ferromagnesian-mineral compositional data were available. Although hiatus between the individual groups exist, the trends within the groups a,re in keeping with the hypothesis that the ordinary chondrites form a continuous fractionation sequence. A significant negativo correlation is observed between the abundance of Ni or Ir and the Fe content of the ferromagnesian minerals in the H and L groups, and for Ir in the LL group, as expected if the metal-silicate fractionation and the variation in oxidation states were produced by the same or related processes. The Ir//h’i ratio decreases by a factor of l-2 between the II and LL groups. This fractionation must have occurred at an early stage in the condensation phase of the solar nebula. INTRODUCTION THE H-, L- and LL-groups of chondrites are the largest three classes of falls, accounting for about 32, 39 and 6% respectively of falls. The three groups are very similar in many respects and have long been grouped together as the ordinary chondrites. They differ ~ompositionally in their contents of total iron, in their relative amounts of oxidized iron and in their abundances of siderophilic elements. AHRENS et al. (1969) have shown that the Mg, Al and Ca abundances in the three classes are remarkably similar, and quite different from those in the two other major chondrite classes (carbonaceous and enstatite). LARIMER and ANDERS (1967) have noted that the trace-element fractionation patterns are similar in the three groups, and different from those in other classes. PRIOR (1920) recognized the differences in degree of oxidation between the bronzite (H) and hypersthene (L and LL) chondrites, and attributed them to an origin (together with the other chondrite classes) from a single magma subjected to differing degrees of oxidation in different locations. A single-magma origin was refuted by UREY and CRAIG (1953), who showed that these classes differed greatly in total iron contents (i.e. in bulk Fe/Si ratios), and proposed that the two classes must have originated in different parent bodies. MASON (1963) showed that there was also a distinct hiatus in olivine composition between the same groups. CRAIG (1964) gave a careful discussion of prior-type relationships, and argued for the existence of a distinct LL-group (Soko-Banjites) on compositional grounds. KEIL and FREDRIKSSON (1964) showed that the H-L hiatus was also observable in pyroxene compositions, and that the “hypersthene” chondrites could be divided into distinct L- and LL-groups on the basis of smaller hiatus in the compositions of their ferromagnesian minerals. * Present address: 3
E/I~~-Pla,nck-Inst.itut fiir Kernphysik, Heidelberg, Germany. 1121
1122
OTTO MILLER, PHILIPA. BAEDECKERand JOHN T. WASSON
There is general agreement among meteorite researchers that the three groups of ordinary chondrites had very similar origins, and that they probably formed from some primordial material by a process involving both metal-silicate fractionation and oxidation-reduction. The situation is illustrated graphically in Fig. 1, a Urey-Craig-Yavnel’ plot* in which the abundance of Fe present as metal (Fe,& and sulfide (Fe,,,) phases is plotted against the amount of Fe present in silicates
‘\ 0
I
0.1
1
0.2 Fe,ttfSi
I
I
0.3 Mom
0.4
\
‘\ \
0.5
’
0.6
ratio)
Fig. 1. Urey-Craig-Yavnel’ plot showing the variations in the chemical state of iron in the ordinary chondrites. The dashed lines represent constant bulk Fe/Si ratios; the solid line represents a possible differentiationpath linking the three groups of ordinary chondrites.
(Fe,,); both axes are normalized by dividing through by total Si. The plotted data include all published analyses (save one) of falls of ordinary chondrites by the following analysts: H. B. Wiik, E. Jarosewich, M. E. Dyakonova, V. Ya. Kharitonova, D. Maynes and H. Haramura. Where two analyses of the same meteorite were available an average value was plotted. A single analysis has been discarded, Wiik’s result for the highly shocked, very inhomogeneous Rose City chondrite. The variation in oxidation state and bulk Fe/Si (Fe&i) ratio among the ordinary chondrites is apparent on the Urey-Craig-Yavnel’ diagram. On such a plot chondrites having the same Fe&&i ratios but different Fesi,/Fetot ratios * UREY and CRAIG (1953) originally plotted these values in weight per cent, a procedure followed by numerous other authors until YAVNEL’ (1968) recognizedthat plotting Fe/Si ratios gives diagrams which have a simpler graphical interpretation.
Siderophilic-element content and oxidation state of ordinary chondrites
1123
Dashed lines representing the mean Fe,,,/Si ratios fall on lines of -1 slope. for each of the three groups of ordinary chondrites are shown on Fig. 1. That the decrease in mean Fe,,JSi ratio as one goes from H- to L- to LL-groups is accompanied by an increase in the mean Feail/Fetot ratio is evident. The properties of the ordinary chondrites, which are reflected by the UreyCraig-Yavnel’ diagram, can be discussed with reference to a number of models. The existence of hiatus between the H- and L-groups and (less well defined) between the L- and LL-groups suggests that the different chondrite groups formed TANDON and WASSON (1968) have summarized four possible independently. models for the formation of the individual groups. These are: (1) the OR modelthe members of a group exhibit a range in oxidation states, but the abundances (relative to Si) of Fe and the siderophilic elements remain constant; (2) the MSF model-metal-silicate fractionation occurred but the oxidation states of the two fractions remain constant; (3) the OR-inh-oxidation or reduction occurred, the abundances of Fe and the siderophilic elements remain constant over a scale of meters, but local inhomogeneities in the distribution of metal and silicate are found on the cm scale which is involved in typical sampling of specimens (such a model would predict no correlation between the Fe&,JSi and Fe,u/Fet,t ratios within the chondrite groups) ; (4) the MSF-OR model-metal-silicate fractionation was accompanied by the fractionation of substances affecting the final redox state of the chondritic mixture (and specifically, a decrease in the amount of metal relative to silicate was accompanied by an increase in the final oxidation state). TANDON and WASSON (1968) showed that the final model was the only one which could account for the combination of (a) strong, positive correlations between siderophilic elements, and (b) negative correlations between the contents of siderophilic elements and the Fe content of the olivine, as observed in their data on L-group chondrites. The present study was undertaken in order to determine whether the MSF-OR model appeared to hold in the other groups of ordinary chondrites, and, if so, whether the fractionation trends observed within the chondrite groups were part of a continuum extending throughout the ordinary chondrites as a whole. The formation of the chondrites might then be discussed in terms of a continuous-fractionation-sequence (CFS) model, whereby the chondrites formed following condensation of an initially homogeneous gas cloud, or one varying continuously in composition. The different chondrite groups would then represent samplings of different portions of such a sequence, represented by the solid curve (visual fit) in Fig. 1. The tendency of some analyses to scatter along lines of -1 slope on Urey-CraigYavnel’ plots might be explained by the uncertainties in the analytical data used to derive such plots. Errors in the determination of Fe,,,/Si ratios are relatively small, whereas larger errors in the determination of Fe,iJFe,,t (which are not well determined: see, e.g. WIIK, 1956) would tend to simulate such trends. Support for the CFS model is provided by the apparent relationship between bulk FeJSi ratios and the mean Fe,il/Fet,, ratios between chondrite groups. However, there is too much scatter in the Fig. 1 data to allow definition of an accurate model. A better set of data for testing various models is obtained by investigating the relationship between siderophilic-element concentration and the
1124
OTTO MUELLER,PHILIP A. BAEDEC~ER and JOHN T. WASSON
Fe content (1968)
via X-ray nesian study this
of the ferromagnesian
used
the olivine
diffraction.
minerals
we have chosen technique
studied
by
data
Microprobe
appear
17 H-group and
a factor
chondrites
belonging
accurate.
chondrifes
(1964),
and
were
types
obtained
of ferromag-
For the present
which
were studied
10 LL-group
In all cases we have fo petrologic
which
of the composition
of 2 more
and 16 L-group
FREDRIKSSON
by FREDRIKSSON et al. (1968).
“recrystallized”
of MASON (1963),
determinations
to be about
KEIL
In their work, TANDON and WASSON
minerals.
composition
attempted
4-6,
by
chondrites
as defined
to choose by VAN
SCHMUSand WOOD (1967). EXPERIMENTAL Samples
and sample
preparation
The source and catalog numbers for the 45 chondrites which were analyzed in this work are listed in Table 3. We have analyzed fresh interior fragments whenever possible. In many cases only small samples were available, and we were forced to clean existing surfaces with an Al,O, wheel, following this by rinsing in reagent grade acetone. Separate chips weighing about 500 mg were taken for each analysis. Each sample was powdered in a Diamond percussion mortar, and packaged in 2-ml polyethylene vials for irradiation. Duplicate flux monitors were prepared by evaporating Mg(NO,), solutions containing approximately 230 yg Ni and 0.4 pg Ir onto separate high purity aluminum foils. The samples and flux monitors were irradiated for three hours at a flux of about 2 x lOla neutrons crnm2see-1 in either the UCLA or Northrop reactors.
Analytical
and radiometric
procedures
Nickel and Ir were determined simultaneously in each sample. About 100 mg of Ni carrier and 10 mg of Ir carrier were pipetted into Zr crucibles, and evaporated to dryness. The irradiated samples were added and fused with about 5 g of Na,O,. After cooling the fusion cake was dissolved in H,O, the resulting solution made acid with HCl, HNO, added and the mixture boiled to insure complete dissolution of the fusion cake. An aliquot containing about 10 per cent of the total solution was transferred to a centrifuge tube for Ni analysis, and the remainder of the solution was retained for the Ir determination. One drop of 3 per cent H202 was added to the solution in the centrifuge tube, and the mixture made basic with 5 M NH,OH. The resulting precipitate [mainly Fe(OH)s] was centrifuged and discarded. The Ni containing supernatant was acidified, Fe and Mn carriers were added, and the precipitation with NH,OH repeated. The supernatant was treated with dimethylglyoxime, and the resulting precipitate collected by centrifugation. The Ni(dmg), precipitate was dissolved in 2 N HCl, and any undissolved residue removed by centrifugation. The supernatant was transferred to a beaker, 100 ml of water added, and the solution heated to 60°C. Dimethylgloxime was again added, and the Ni precipitated by the dropwise addition of 3 N NH,OH. The precipitate was collected on glass-fiber filter paper, washed with water, dried to 105”C, weighed and counted. The chemical yield ranged from 60 to 80 per cent. The Ir procedure is similar to that employed by TANDON and WASSON (1968). The Ir
containing solution was evaporated to approximately 50 ml, and the silicic acid removed by ccntrifugation. The solution was diluted to 200 ml with H,O and the IrCle2- anion absorbed onto a Dowex-1 x 8 anion-exchange column. The column was washed with O-1 N HCl (200250 ml) until the effluent was free of iron (i.e. showed no color with NH,SCN solution). The upper half of the column was transferred to a beaker, 10 ml of a 10 per cent solution of NH,OH* HCl added, and the mixture warmed on a hot plate to reduce the Ir(IV) to Ir(II1). The resin was filtered and Ir(II1) eluted with 150 ml of 8 N HCl. The eluate was evaporated to 10 ml and the Ir(II1) oxidized to the +4 state with HNO,. About 100 mg of NaBrOs was added, the pH made about 8 with NaOH and NaHCO,, and the mixture boiled to precipitate hydrous IrO,. The precipitate was centrifuged, washed with H,O, and dissolved in 2 ml cont. HCl.
Siderophilic-element The resulting
solution
content
was transferred
and oxidation
to a plastic
chondrites
1125
The yield determination
was
state of ordinary
counting
tube.
by reactivation, and was generally from 40 to 60 per cent. Gamma counting was carried out using a 3 x 3 in. NaI well detector coupled to a 400-channel analyzer. Nickel was determined using the 1.43MeV photopeak of 65Ni, while Ir was determined using the 0.31-MeV photopeak of ls21r.
Precision
ad
accuracy
In order to measure the precision of our method, replicate determinations have been carried out on six chips from the Peace River chondrite and four samples of the Holbrook chondrite. The results are shown in Table 1. The relative standard deviation was about 9 per cent for Ir, and about 13 per cent for Ni. Although this precision was obtained in most of our analyses, the data for some of the meteorites listed in Table 3 show greater scatter. This is apparently the result of occasional fluctuations in the amount of metallic fraction present on the 0.5-g sampling level, since Ni and Ir generally vary coherently. Table
1. Replicate Peace
Mass (g)
Mean
(%)
on two chondrites Holbrook Ir
Nl
Mass
%
(g)
(ppm)
(%)
0.41 0.50 0.42 0.47 0.46 0.45
1.04 1.28 1.29 1.36 1.31 1.10
0.511 0.509 0.338 0.421
0.45 0.36 0.40 0.45
1.51 1.09 1.16 1.18
0.45 0.033 7.3
1.23 0.128 10.4
0.42 0.044 10.5
1.24 0.187 15.1%
(ppm)
0.511 0.497 0504 0.512 0.544 0.491
o/Me:
River Ir
Ir and Ni determinations
Ni
Errors involved in the measurement of flux-monitor specific activities can cause systematic errors in the values obtained from a given irradiation. We have taken steps to prevent such biases from affecting the statistical arguments to be presented later in the paper. The samples were first divided into hH, EH, hL, IL, hLL and ILL groups, where the small letters indicate whether the meteorite was reported by KEIL and FREDRIESSON (1964) to have a high or low Fe content in the ferromagnesian minerals as compared to the mean of that group. Six samples were included in each neutron irradiation, and consisted of three pairs, each made up of an h and 1 member from a single group. At least two groups were represented in each irradiation, and two samples of the same meteorite were never irradiated together. We believe that these techniques have completely avoided biases in the total data, and in the data for all members of each group. The accuracy of our results can be judged by comparison of our analyses with those of other research groups, where comparisons are possible for specific meteorites. Table 2 provides a comparison of our data with those of other researchers for eight chondrites. NICHIPORUK et al. (1967) have determined Ni in 18 of the same chondrites by X-ray fluorescence, while GREENLAND and LOVERING (1965) employed a spectrophotometric method in analyzing Ni in eight of the same meteorites. Our Ni results are found to be very similar to or slightly higher than those of NICHIPORUK et al. (1967), GREENLAND and LOVERING (1965), and various analysts using standard wet chemical methods. It appears that our results are systematically 5-8 per cent higher than those of other investigators. However, this will have no effect on the conclusions which we draw on the basis of our data. We have determined Ir in 14 meteorites studied by EHMANN et ~2. (1970), who utilized a non-destructive neutron-activation analysis procedure. Again, no systematic differences can be observed between the results obtained by EHICANN and coworkers and ourselves. The scatter of the Ni and Ir data obtained by other research groups appears to be slightly greater than ours.
1126
OTTO MILLER, PHILIP A. BAEDECKERand JOHN T. WASSON
Table 2. Ni and Ir concentrations in ordinary chondrites as determined by different research groups Meteorite Allegan Dhurmsala B jurbiile Farmington Forest City Holbrook Mount Browne Ochansk
* 1.62 0.94 l-18 1.28 1.72 1.24 1.91 I*89
Ni (%) t
$
1.76 0.93 I.01 1.10 1.67 1.28
2.03
1.768
1.02 1.28
1*05** l-06** l-69** 1.09**
I.79
1.22 1.93
§
1&3** 1.687?
*
Ir @pm) II
0.64 0.32 0.41 0.49 0.67 0.42 0.77
0.75 0.35 0.38 0.49 0.78
0.78
0.79
0.89
* This work, t NICHIPORUKet al. (1967). $ GREENLANDand LOVERING (1965). 5;Wet chemical analyses. 1)EHMANN et al. (1970). T[JAROSEWICHE. and MASON B. (1969) Geochim. Cosmochim. Acta 33, 411-416. ** WIIK H. B. (1969) Conzmeratationes Physico-Mathenzaticae34, 135-145. tt DYAKONOVAM. E. and KHARITONOVAV. YA. (1959) Meteoritikal&48-67. RESULTS AND INTERPRETATION
Individual determinations of Ni and Ir in the investigated chondrites are listed in Table 3. The chondrites are listed alphabetically within groups. Most of the replicated analyses agree to within about 10 per cent. A few analyses show relatively large, correlated variations in Ni and Ir, which can be attributed to sampling variations on a scale of O-5 g. The most striking example of this behavior is provided by Allegan. In one case, Fukutomi, there were large fluctuations in the observed Ni concentration, while the Ir content measured remained relatively constant. This apparently reflects experimental errors, although differences in the distribution of Ni and Ir between different metallic phases is not inconceivable. In Table 4 are listed mean Ni and Ir concentrations, and Si, Mg, metallic Fe (Fe,,& total Fe (Fe& and the Fe contents of the olivine of the investigated chondrites, in order of decreasing Fe content of the olivine. The latter values were taken from the work of KEIL and FREDRIKSSON (1964) for the H- and L-group stones, and from the work of FREDRIKSSON et al. (1968) for the LL-group chondrites. As will be described later, the values have been corrected slightly on the basis of pyroxene determinations reported by these groups. No attempt was made to correct for the fact that in the 1968 work the L- and LL-group values are systematically about O-4 wt.% higher than those reported in the earlier study, as this will have no appreciable effect on the correlations investigated in the following section. The Mount Browne and Peace River values were calculated by averaging the composition of the ferromagnesian minerals as reported by KEIL and FREDRIKSSON (1964) for those meteorites which MASON (1963, 1967) reported to have the same olivine compositions as Mount Browne and Peace River. The Si, Femet and Fetot values are taken exclusively from investigations carried
Siderophilic-element Table
Meteorite H-group Allegan
content
3. Individual Source* and cat. no.
and oxidation
state of ordinary
Ir and Ni determinations
Type
Ir (g/lo6 Replicates
0.85, 0.45, 0.76, 0.79, 0.73, 0.69, 0.72, 0.64, 0.76, 0.72, 0.78, 0.72, 0.62, 0.76, 0.78, 0.71, 0.74, 0.81, 0.68, 0.74
0.51, 0.74 0.68 0.85 0.73 0.67 0.80 0.69 0.74 0.69 0.71 0.84 0.77 0.78 0.77 0.77 0.79 0.73 0.67,
chondrites
on 45 ordinary g) Mean
0.64
chondrites
Ni (%I Replicates
2.01, 1.04, 1.86, 1.75, 1.80, 1.65, 1.84, 1.73, 1.80, 1.82, 1.59, 1.57, 1.64, 2.07, 1.98, 1.82, 1.91, 1.68, 1.68, 1.49
1127
1.24, 2.18 1.72 1.98 1.80 1.63 1.97 1.70 1.83 1.92 1.73 1.81 1.99 1.75 1.80 1.70 2.02 1.77 1.39,
Mean
1.62
SI 15
H5
UChiFMNH Me352 BM 34795 MHNP 1494 GIULAMNH 2421 NMS 69:0354 AMNH 3940 FMNH 1389 BM 1924,134 UCLA 266 AM DR2494 FMNH Me1441 UCLA 322 AMNH 662 NMS 061893 ASU 743
H5 H4 H6 H6 H H5 H5 H4 H4 H H4 H6 H4 H5 H5 H5 H
MHNP FMNH FMNH
106 Me2298 Me2392
L5 L L4
Bath Furnace Bjurbole Bruderheim Farmington Fukutomi
ASU 243A FMNH Mel426 UAlb B3 FMNH Me345 FMNH Me1491
L6 L4 L6 L5
Harleton
SI 2576
L6
Holbrook Homestead Knyahinya Kuttipuram Kyushu L’Aigle Mocs Peace River
AMNH 586 YalUAMNH 1068 BM 191588 AMNH 1072 UCLA 248 AMNH 1079 UAlb PR4
L6 L5 L5 L L6 L6 L6 L6
BM 1920,40 GSC 218101
LL6 LL6
0.32, 0.40 0.18, 0.27, 0.31
0.36 0.25
0.94, 1.01 0.98, 1.35, 0.94
0.98 1.09
ASU
LL6
0.39, 0.42, 0.32
0.38
1.10, 1.35, 0.87
1.11
Beardsley Beaver Creek Butsura Djati-Pengilon Ekeby Forest City Hessle Kesen Menow Merua Monroe Mount Browne Ochansk Pultusk Richardton Stalldalen Zhovtnevyi L-group Ausson Aztec Bald Mountain
LL-group Appley Benton
Bridge
Cherokee Springs
319.1 x
L4t
0.50, 0.49 0.52, 0.48 0.33, 0.46, 0.48 0.57, 0.55 0.42, 0.40 0.45, 0.52 0.47, 0.51 0.50, 0.50, 0.54, 0.56 0.52, 0.59, 0.51 see Table 1 0.48, 0.43 0.31, 0.32 0.44, 0.43 0.45, 0.48 0.46, 0.49 0.44, 0,42 see Table 1
0.72 0.82 0.73 0.68 0.76 0.67 0.75 0.71 0.75 0.78 0.70 0.77 0.78 0.74 0.77 0.77 0.70
0.50 0.50 0.42 0.56 0.41 0.49 0.49 053 0.54 0.42 0.46 0.32 0.44 0.47 0.48 0.43 0.45
1.36, 1.33 1.31, 1.28 155, 1.21, 1.17 1.52, 1.57 1.16, 1.20 1.26, 1.28 1.20, 1.35 1.71, 2.22, 1.35, 1.32 1.66, 1.29, 1.36 see Table 1 1.46, 1.30 0.98, 1.04 1.41, 1.30 1.16, 1.24 1.29, 1.22 1.28, 1.19 see Table I
1.79 1.87 1.80 1.64 1.91 1.72 1.82 1.87 1.66 1.69 1.82 1.91 1.89 1.76 1.97 1.73 1.52
1.35 1.30 1.38 1.55 1.18 l-27 1.28 1.65 1.44 1.24 1.38 I.01 1.36 1.20 1.26 1.24 1.23
1128
OTTO MILLER, PHILIP A. BAEDECKER and JOHW T. WASSON Table 3. (continued) source*
Ir k/lo6 g)
and cat. no.
Type
Replicates
Mean
Dhurmsala Ensisheim Guidder Jelica Karatu Manbhoom
FMNH Mel348 MHNP 1 MHNP 2262 AMNH 464 SI 2492 AMNH-
LL6 LL6 LL LL6 LL LL6
0.31 0.35 0.32 0.35 0.31 0.44,
0.32 0.36 0.36 0.35 0.35 0.27
Ottawa
AMNH 366
LL6
0.32, 0.36, 0.39, 0.34, 0.38, 0.19, 0.19 0.36,
0.43
0.40
Meteorite
Ni (%I
Replicates
0.92, 0.95 1.09, 0.97
0.91, 0.95, 1.01, 0.86, 0.96 1.13,
Mean 0.94
0.95 1.12 0.85 1.19,
1.03 0.93 1.04 0.93 1.00
1.22
1.18
* The sources are abbreviated as follows: AMNH-American Museum of Natural History; ASU-Arizona State University; AM-Australian Museum; BM-British Museum; GSCGeological Survey of Canada; FMNH-Field Museum of Natural History; GIUL-Geologic Institute, University of Lund; UCLA-University of California, Los Angeles; MHNP-Museum d’Histoire Nature110 Paris; NMS-Naturhistoriska Museet, Stockholm; UAlb--University YalU-Yale Uniof Alberta, UChi-University of Chicago; SI-Smithsonian Institution; versity. t W. R. VAN SCHMUShas informed us that he has seen a FMNH sample of Fukutomi which is L3,4. We have studied our sample by electron microprobe, and fmd it to be an “equilibrated” L-group chondrite. Thus the correct classification seems to be L4, rather than L5, as given by Vm SCHD~S and WOOD (1967).
out in the last 11 years, with the exception of two meteorites analyzed by Prior ha.lf a century ago. Where more than one analysis wa,s available an arithmetic mean of the values of the referenced authors is listed. In the case of Si, if no recent value by the chosen researchers was available, the mean value for the group is listed and italicized. Covariation
of siderophilic
elements
An important test for possible models for the formation of the ordinary chondrites is their ability to explain the variation in the relative concentrations of siderophilic metals. Figure 2 shows a plot of Ir vs. Ni. Both elements are normalized to the concentration of Si observed in the individual chondrites, as tabulated in Table 4. It makes little difference whether the normalization is to Si or Mg, since the data of VON MICHAELIS (1969-AHRENS et al., 1969) show that the mean Si/Mg ratio increases by only 2 per cent in going from the H- to L-groups, an amount which is unlikely to have any observable effect on our results. Figure 2 illustrates very nicely the large fractionation of the NilSi and Ir/Si ratios among the various groups. Either of these parameters would be sufficient to classify any of the meteorites studied, with the exception of Ottawa and Knyahinya. We also observe that the mean Ir/Ni ratio drops from 1.25 x 10e5 in the H-group to about 1.00x 1O-5 in the LL-group chondrites. The difference is so great that in most cases, our Ir/Ni ratios can be used to distinguish between H-group chondrites and the more oxidized ordinary chondrites. The index ratio of 1.18 x 10es is illustrated by the dashed line in Fig. 2. The solid line approximates a possible differentiation path if the ordinary chondrites are part of a continuous fractionation sequence. If this interpretation is correct, the Ir/Ni ratio changed by a
Siderophilic-element
content and oxidation state of ordinary ohondrites
1129
AMa
2
, 2
,
.Sn, 3
I
4
I
! 5
1
I 6
Ni/Si bxns/1020toms)
Fig. 2. A plot of Ir abundance vs. Ni abundance in a suite of ordinary chondrites. The dashed line corresponds to a constant Ir/Ni ratio of 1.18 x 10e5, and tends to separate the H-group points, which have Ir/Ni ratios higher than this value from the L- and LL-group points, which have lower values. The solid line is a possible differentiation path linking the three groups.
process. It is important to note that, because of the precautions which were taken (and described in the experimental section), these changes cannot have resulted from systematic errors. EHMANN et al. (1970) report Ir/Au ratios of 3.3, 2.7 and 2.2 for the H-, L- and LL-groups respectively, a variation of a factor of about 1.5 throughout the ordinary chondrite groups. Their and our data combine to give Au/Ni ratios rising from 3.8 x 1O-s in the H-group to 4.5 x 1O-6 in the LL-group. The significance of these rather surprising variations in the ratios of one strongly siderophillic element to another will be discussed in the next section. As has been shown by other authors for other siderophilic elements, Ir and Ni abundances tend to be positively correlated in each of the three groups of meteorites. The correlation coefficients are 0.67, 0.70 and 0.19 for the H-, L- and LL-groups The first two are significant at levels of about 0.998, the last only a.t respectively. the level of 0.38. Since these correlations could be brought about by an inhomogeneous distribution of metal and silicate, they are of relatively little diagnostic value for investigating models for the origin of ordinary chondrites. f’actor of about 1.25 during the course of the formational
Relationship between sidero&lic of ferromaglzesian minerals
element concentrations
and the iron content
A prime goal in our investigation was the determination of the relationship between the siderophilic element content a,nd ferromagnesian mineral composition
OTTO M&LER,
1130
PHILIP
A. BAEDECKER and JOHN T. WASS~N
Table 4. Maan bulk concentrations of Ni, Ir, Si, Fe and metallic Fe and ferromagnesian silicate compositions in ordinary chondrites, arranged in order of incrsasing Fe content of the olivins
Meteorite
Ir (ppm)
Abbr.
8i20 (%)
Metallic Total Fe (%)
Fe in olivine (%)
H-Group K0sen Richardton Msrua Mount Browne k&low Ochansk Monroe Hossle Stiilldalen Ahogan Pultusk Beard&y Beaver Creek Fortwt City Djati-Pengilon Ekeby Butsura Z hovtnevyi
K0 Ri
BC
Zh
187 1.97 1.69 1.91 1.66 189 1.82 1.82 1.73 l-62 1.76 1.79 1.87 1-72 l-64 1.91 1.80 l-52
0.71 0.77 0.78 0.77 0*75 0.78 0.70 0.75 o-77 O-64 0.74 0.72 O-82 0.67 0.68 0.76 0.73 o-70
1.65 l-38 la38 I.26 l-30 1.20 l-23 l-28 1.55 l-36 l-01 l-35 1.24 1.27 l-24 l-18 l-44
0.53 O-42 0.46 0.48 0.50 o-47 0.45 0.49 0.56 0.44 O-32 O-50 O-42 o-49 o-43 o-41 0.54
o-94 0.93 1.11 l-18 l-03 l-09
16.63.8 16-24.8
27.g3.8.16
16.9 16.9 16.9 16.9**% 16.9 16.9 16.9 17.25s.16
28.1i6
17*07** 16.98 16.9 17*04** 16.9 X6.9 16.9 17.5”
28.54,&l%
27.84.6.16
18-23 18.34
17.84~~
17.65,” 16.07
27.14,*.16
17.2*
26.86.16
13.5%
11.95 12.08 12.33 12Y532 12.80 12.99 13.06 13.12 13.12 13.18 13.26 13.39 13.46 13.52 13.84 13.84 14.03 14.04
L-Group Fukutomi Bald Mountain Homestead L’AigIe Aztec Kyushu Peace River Farmington Bath Furnace Kuttippuram Knyahinya Aueson Holbrook Bruderheim Moos Bjurbdle Ha&ton
Fu BM
Kn
Bj
18.39 18.7 18~4~~ 18.7 18.7 18.74,8 18+6’o 19.04,8
18.7 18.7 19.04 18.7 18.84% 18.61%,1% 18.54.17
16.05 16.06 9.7111 16.51 16.59 16.78 6.274 17.04 8.1810 17.042 17.11 6.PO* 17.30 17.37 3.364 17.45 17-45 17.63 7-184 7.g512.19 17.68 8.654~17 17.70 17.82 7-2613 17.82 9.549
21.84.8.16 22.31% 2l.O4.%*‘% 22.816 20.24 21.24.8.16 22*712,1%,19 22.14.16.17
19.38 18.613
2&IPAl%
O-32 O-36
19-o* 19+?
1g.44.8.16 19-04
4.374
0.38 0.40 O-36 0.25
19.15 18.64 18-7l4 19.0
20*45 21.24 20.7r4
2.286 2.82’ 2.5114
22~6’~
LL-Group Dhwmsala Guidder Cherokee Springs Ottawa Ensisheim Benton
Dh GU
0t Bn
19-54 19.75 20.33 20.53 20.58 21.29
Siderophilic-element content and oxidation state of ordinary chondrites
1131
Table 4. (continued)
Meteorite Manbhoom Karatu Appley Bridge Jelica
Ni
Ir
SiZo
Abbr.
(%)
(ppm)
(%)
Ma
1.00 0.93 0.89 1.04
0.27 0.35 0.36 0.35
18.94 19515 19.04 19.0
Tote1 Fe
Metallic Fe
Fe in olivine
(%)
(%)
(%)
0.704 0.6815 0.334
19.64.16
18.615 20.14 20.216
21.44 21.60 21.62 22.31
l Fc,iiv * from Feoliv and Fe,,, data of KEIL K. and FREDRIKSSONK. (1964) J. Geophys. Res. 66, 3487, and FREDRIKSSONK. et al. (1968) In O&gin and Distribution of the Elements, pp. 457-466, as described in text. 2 Based on olivine compositions of MASON B. (1963) Geochim. Cosmochim. Acta 87, 1011 and ibid. (1967) 31, 1100. Tabulated values are weighted means (see text) based on mean Feoliv + Fcpyr values found by KEIL and FREDRIKSSON(1964) for other chondrites for which Mason has reported the same olivine compositions. 3 MIYASRIRO A. (1962) Jap. J. Geol. Geogr. 36, 73. 4 WIIK H. B. (1969) Comment. Phys.-Math. 34, 135. 5 JAROSEWICHE. and MASON B. (1969) Geochim. Cosmochim. Acta 66, 411. 6 DYAKONOVA M. E. and KHARITONOVA V. YA (1959) Meteoritika 18,48. 7 DYAKONOVA M. E. and KHARITONOVA V. YA (1961) Meteoritika 21, 52. * VON MICHAELIS H., ARRENS L. H. and WILLIS J. P. (1969) Earth PZanet. Sci. Lett. 5,387. s MIYASHIRO A., in MASON B. (1965) Amer. Mus. Now. No. 2223. lo BAADSCAARDH., FOLINSBEE R. E. and CUMMINGG. L. (1964) J. Geophys. Res. 69,4197. 11 PRIOR G. T. (1918) Mineral. Mag. 18, 173. 12 BAADSGAARD H., CAMPBELL F. A., FOLINSBEE R. E. and CUMMINGS G. L. (1961) J. Geophys. Res. 66, 3574. 13CLARKE R. S., in MASON B. (1965) Amer. Mus. Nov. No. 2223. l4 PRIOR G. T. (1921) Mineral. Mug. 19, 163. l5 JAROSEWICHE. (1966) Geochim. Cosmochim. Acta 30, 1261. 16 NICHIPORUK W., CHODOSA., HELIN E. and BROWN H. (1967) Geochim. Cosmochim. Acta 31, 1911. 17KRARITONOVA V. YA (1965) Meteotitika 26, 146. I* MASON B. and MAYNES A. D. (1967) Proc. U.S. Nat. MM. 124, 1. ls DUKE M. B., MAYNES A. D. and BROWN H. (1961) J. Geophys. Res. 66, 3557. 2o Italicized values are averages of the other values listed for the group. in each of the three groups
of ordinary
chondrites.
it was for were chosen from
As we have noted,
this reason that all save two of our H- and L-group those studied by KEIL and FREDRIKSSON (1964),
chondrites and
all LL-group
were among those studied by FREDRIKSSON et aZ. (1968). In these studies the compositions of both olivine and
chondrites
orthopyroxene
were
Of the two major cations in these minerals, Fe is determined with the smaller relative error. For that reason we have used Fe content as a parameter rather than the Fe/(Fe + Mg) ratio, since the latter quantity involves an unnecessary compounding of errors. On the other hand, a sum of the Fe contents of these two minerals has a smaller relative error than the Fe content of either mineral considered alone. In order to obtain the sum in the form of a meaningful parameter determined.
we have
first converted
the pyroxene data to equivalent olivine concentrations. for the combined H- and L-group data of KEIL and
A regression line was calculated FREDRIKSSON
(1964).
All
meteorites
from
these
groups
were
included.
For the
OTTO MUELLER, PHILIP A.
1132
and
BAF,DECKER
JOHN
T. WASSON
equation Feouv_-pyr = a FepYr + b, the values of a and b were found to be 1.878 A similar treatment of the LL-group data of FREDand -2.426 respectively. RIKSSON et al. (1968) yielded values of a and b of l-767 and - 0.834 respectively. Improved iron contents of the meteoritic olivine were then calculated from the following formula: Feoliv* = (2 Feouv + Feo!i,_-Psr)/3. These are tabulated in Table
. 0
I
‘11
I
12
I
13
I
I
l4
Fe
I
16 16 content of
t
17 olivine
1
I
16 19 (weight
H group L group
f
I
20
21
I
22
23
%I
Fig. 3. Plot of Ni atomic abundance vs. Fe content of olivine. The dashed curve is s hypothetical continuous-fractionstion sequence (CFS) p&h linking the H-, L- and LL-groups. Significant negative correlations between the two parameters are found in the H- and L-groups. Solid regression lines (and lines representing lo errors in the slopes) are drawn through these two groups. Centroids of the three groups are each marked with an X. 4, and used in the discussion which follows. The lower weight assigned the Fe OIiV--pVvalue in the calculation of the mean reflects the fact that pyroxene has a somewhat smaller content of Fe than olivine, leading to higher relative errors in the pyroxene data. Figure 3 is a plot of the NilSi abundance ratio (on a logarithmic axis) vs. FeoliV. for the 45 ordinary chondrites. Correlation coefficients have been calculated for the three groups. That for the H-group is -0.47, significant at a 0.95 confidence level; that for the L-group is - 0.44, significant at a 0.91 confidence level; and that for the LL-group is +0*05, indicating a lack of correlation. Because of its very low content of reduced metal, Knyahinya has been eliminated from all L-group calculations. The negative correlations in the H- and L-groups cannot be understood in terms of the OR, OR-inh or MSF models of TANDON and WASSON (1968), but are in keeping with their MSF-OR model. Regression lines (and lines representing lb errors in the slopes) are drawn through the H- and L-group points in Fig. 3. These (and those involving Ir/Si)
Siderophilic-element
content and oxidation state of ordinary chondrites
1133
were calculated according to the procedure of YORK (1966), using weighting factors based on the errors in the various parameters. The assigned errors were 0.053 in log Ni/Si and 0.0375 in log Ir/Si values, and 0.20, 0.26 and 0.32 wt.% Fe for the Pe,aV* values in the H-, L- and LL-groups respectively. A smooth curve (shown dashed) is drawn through the centroids of the three groups, and represents the distribution of data points expected for the continuous-fractionation-sequence
. 0
H group L group
A
LL group
t
rBn
I 211
I
I
12
B
I
I
I
I
I
I
19 19 17 1.3 19 Fe c!ntcnt of olivinc (weight 7.)
20
21
22
23
Fig. 4. Plot of Ir atomic abundance vs. Fe content of olivine. As in Fig. 4, the dashed curve represents a hypothetical CFS path linking the ordinary chondrite groups. Regression lines (and lines representing 1~ errors in the slopes) are also drawn through the three groups. Signifkant negative correlations are found for each group, though only marginally so in the L- and LL-groups. The negative correlations here and for the data shown in Fig. 3 rule out the possibility that the fractionations in each group are of the Prior type, since in this case the abundances of Ni and Ir should be independent of the oxidation state of the iron.
model. The regression lines in the H- and L-groups lie near the CFS curve, which tends to support the CFS model. Although the lack of correlation in the LL-group may result from sampling problems, this lack nonetheless weakens the argument for the CFS model, Figure 4 is a plot of log Jr/Si vs. Fe,,,,*. Correlation coefficients are -0.40, -0.14 and -0.24 for the H-, L- and LL-groups respectively, significant at levels of 0.90, 0.51 and 0.48. The latter two are of only marginal significance. As stated above, negative correlations between siderophilic element concentrations and the olivine composition can only be understood in terms of the MSF-OR model. This is certainly the correct choice for the H-group, and is consistent with the apparent trends in the other groups.
1134
OTTO MILLER, PHILIP A. BAEDECKERand JOHN T. WASSON
A hypothetical CFS-fractionation curve is also drawn through the centroids of the groups in Fig. 4. Regression lines (and lb error estimates in their slopes) are also shown. The regression lines in the L- and LL-groups are in reasonable agreement with the CFS curve. The slope of the line in the H-group, for which the data are more strongly correlated, is considerably less (by more than 20) than that of the CFS curve, and would seem to be more consistent with an independent, MSFOR origin for this group. The slopes of the regression lines in Figs. 3 and 4 are consistently low relative to those of the CFS curves. There is a distinct possibility that this reflects a bias introduced by our selection of meteorites with “extreme” properties from the KEIL and FREDRIKSSON (1964) data. By deliberately choosing meteorites with low or high Fe contents in their ferromagnesian minerals relative to the mean of the group, we have also included determinations which erred on the high side together with the high-Fe subgroup, and a similar situation prevails in the low-Fe subgroup. This is precisely the direction of bias which is needed to explain our consistently lower slopes. Whether the magnitude of this effect is sufficient to account for the difference in slopes should be checked by new, bias-free microprobe determinations in which chondrites of the lH, hN, IL, etc. subgroups (as defined in the experimental section) are combined in single runs. We have also calculated correlation coefficients for log Fe,,#i ratios vs. Feofivs, using the data tabulated in Table 4. The correlation coefficients are -0.92, -0.44 and -0.06 for the H-, L- and LL-groups respectively, significant at levels of 0.99, 0.82 and 0.11. The first two are significant and support the MSF-OR model. Correlation coefficients were calculated for Fe,,&i vs. FeoliV+,and are -0.84, -0.47 and -0.97 for the H-, L- and LL-groups respectively, significant at levels of O-99, 0.79 and > 0.99. These results are in keeping with either the MSF-OR or OR (Prior-type) models for each group. The remarkably high correlation coefficient observed in the LL-group relates to the fact that small changes in the oxidation state of the silicates cause large changes in the total metal content. These changes are apparently much larger than the analytical errors associated with their determination. We have not attempted to use the Fetot and Femet data to test the CFS model, since there are relatively few available, and they are the work of several different researchers. For these reasons we doubt that slopes defined from them would provide convincing evidence either for or against the CFS model. An interesting study for the future would be to determine with high precision the composition of ferromagnesian minerals in a very large number of meteorites previously studied by a single research group-for example, for most of the ordinary chondrites studied by WIIK (1969). If the CFS model is correct, we would expect that plots of any (variable) element vs. FeoliV+would show the data falling along smooth curves connecting the three groups. It is clear from the data we have presented that metal-silicate fractionations have occurred within the H- and L-groups, and that in each case these fractionations have been accompanied by a fractionation of elements or compounds which controlled the final oxidation state of the chondritic material (stronger oxidizing conditions correlating with smaller metal contents). There is also positive evidence
Siderophilic-element
content and oxidation state of ordinary chondrites
1135
that such a NSF-OR process was involved in the formation of the LL-groups, although a simple OR model is also consistent with the data. The choice between the two models is academic for the discussion in the following paragraph, however, since the CFS curves have slopes in the LL-group (in Figs. 1, 3 and 4) which differ only slightly from those expected from the OR model. It appears that there are only two simple models which can account for the data we have presented. Either (1) each of the three groups of ordinary chondrites has experienced separate MSF-OR fraetionations, or (2) they have all experienced the same MSF-OR fractionation, which we call the continuous-fra~tionationsequence model. For the ultimate choice (until substantially better data are available) we must rely on Occam’s Principle. Since a single fractionation event can, in the second case, account for what in the first case is handled by three events, the latter, CFS model is to be favored. The hiatus probably indicate that the H, L and LL groups ori~nated in different parent bodies. The point will be discussed in more detail in WASSON(1971). DISCUSSION
There are a number of arguments which indicate that the fractionation of metal from silicate (and of one siderophilic element from another) took place in the solar nebula, as opposed to the interior of a parent body. The simplest arguments have to do with the primitive and “unequilibrated” nature of petrologic type 3 ordinary chondrites (VAN SCHMUSand WOOD, 1967). They are rich in volatiles, including planetary-type rare gases, and show evidence of rapid cooling from a relatively high temperature in their unannealed glasses and wide compositional ranges within a single mineral. The relatively rapid cooling so indicated demands that the material be dispersed. Although some have proposed that this might occur on the surface of a parent body, a solar-nebula setting offers a better explanation of all properties. The only planetary process which could cause metalsilicate fractionation is gravitational separation of immiscible phases from a melt. In fact, however, this prooess in planets tends to give total separation of metal from a silicate melt; it is very difficult to formulate a model which would, for example, remove about 5 wt. o/o metal from the L-group chondrites while leaving a comparable amount in the melt. For these reasons we feel that the fractionation must have occurred in the solar nebula. Metal-silicate fractionation and the evolution
5f
the solar nebula
We must then ask the question, was the metal-silicate fractionation associated with the condensation, agglomeration or accretion phases in the evolution of the solar nebula? The answer seems to be provided by the observations that the Ir/Ni ratio decreases, whereas the Au/K ratio increases as one proceeds from the metalrich extreme of the I3 group towards the metal-poor extreme of the LL group. LARIMER and ANDERS(1970) pointed out that such variations can be used as the basis for determining the temperature at the time of fractionation. They interpreted the data available to them to indicate that all siderophilic elements as
OTTO MILLER, PHILIP A. BAEDECKERand JOHN T. WASSON
1136
refractory as Ge are present in constant ratios throughout the suite of ordinary chondrites, and concluded that the metal-silicate fractionation occurred at a temperature lower than 1050’K. In contrast, our data and those of EHMANN et al. (1970) show appreciable fractionation of elements as refractory as Ir and Ni. For the elements in their Cl-chondritic ratios, LARIMER and ANDERS (1970) show that, in a cooling solar nebula, Ir, Ni and Au should condense in the order listed.
On this
observed
basis,
variations
E.
ANDERS
in the Ir/Ni
(private
and Au/Ni
communication)
suggests
that
ratios can be readily understood
the in
terms of the loss of an early metal condensate
which had an Ir/Ni ratio somewhat higher than the mean ratio in the nebula, and an Au/Ni ratio somewhat lower than the nebular ratio (fractional condensation is an alternative explanation). LARIMER and ANDERS (1970) give 90 per cent condensation
temperatures
(at a H, pressure
Although one encounters difficulties when one attempts to treat these values as exact, it appears reasonable to think that the temperature at this stage was about 1300°K (at the assumed pressure), which indicates that the condensation of the elements presently to be found in the ordinary chondrites was far from complete. In fact, it is necessary that not all of the metal had condensed. Thus, the metal-silicate fractionation seems to have occurred during the condensation phase of meteorite formation in the solar nebula. A detailed model for the fractionation of the siderophilic elements is given by WASSON (1971). The conclusion which is drawn is that fractional condensation provides a more reasonable mechanism than does loss of an early condensate. of 10”
atm) of 1575, 1280 and 1220°K for Ir, Ni and Au, respectively.
are indebted to E. ANDERS, J. A. V. DOUGLAS,R. E. FOLINSBEE, 0. GABRIELSON, E. P. HENDERSON,M. H. HEY, S. HJELMQVIST, J. W. LARIMER,V. MANSON, B. MASON,C. B. MOORE,E. OLSEN,J. ORCEL,P. PELLASand K. TUREKIANfor provision of the
Aclcnowledgement+-We
samples. Significant improvements in the manuscript are to be credited to the stimulating reviews provided by W. R. VAN SCHX~S, E. ANDERS, J. W. LARIMERand J. W. MORGAN. Neutron irradiations were carried out in the UCLA Engineering Nuclear Reactor under the supervision of J. W. HORNOR. This research was supported in part by NSF grants GA-1347 and GA-15731 and by NASA contract NAS 9-8096.
REFERENCES AHRENSL. H., VONMICHAELIS H., ERLANKA. J. and WILLISJ. P. (1969) Fractionation of some abundant lithophile element ratios in chondrites. &feteoriCeResearch, (editor P. M. Millman). pp. 166-173. D. Reidel. CRAIG H. (1964) Petrological and compositional relationships in meteorites. Isotopic and Cosmic Chemistry, (editors H. Craig, S. L. Miller and G. J. Wasserburg), pp. 401-451. NorthHolland. EHMANNW. D., BAEDECEERP. A. and MCKOWND. M. (1970) Gold and iridium in meteorites and some selected rocks. Geochim. Cosmochim. Acta 34, 493-507. FREDRIKSSON K., NELEN J. and FREDRIKSSON B. J. (1968) The LL group chondrites. Origilc and Distribution of the Elements, (editor L. H. Ahrens). Pergamon. GREENLAND L. and LOVERINCJ. F. (1965) Minor and trace element abundances in chondritic meteorites. Geochim. Cosmochim. Acta 29, 821-858. KEIL K. and FREDRIKSSON K. (1964) The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites. J. Geophys. Res. 69, 3487-3515. LARIMER J. W. and ANDERSE. (1967) Chemical fractionations in meteorites-II. Abundance patterns and their interpretation. Geochim. Cosmochim. dcta 31, 1239-1270.
Siderophilic-element LARIMER element MASON B. MASON B.
content
and oxidation
state of ordinary
chondrites
1137
J. W. and ANDERS E. (1970) Chemical fractionations in meteorites-III. Major fractionations in chondrites. Geochim. Coamochim. Acta 34, 367-387. (1963) Olivine composition in chondrites. Geochim. Cosmochim. Acta 27, 1011-1023. (1967) Olivine composition in chondrites-a supplement. Geochim. Cosmochim. Acta
31,1100-1103. NICRIPORUK W., CHODOS A., HELIN E. and BROWN H. (1967) Determination of Fe, Ni, Co, Ca, Cr and Mn in stony meteorites by X-ray fluorescence. Beochim. Cosmochim. Acta 31,19111930. PRIOR G. T. (1920) The classification of meteorites. Mineral. Mag. 19,51-63. TANDON S. N. and WASSON J. T. (1968) Gallium, germanium, indium and iridium variations in a suite of L-group chondrites. Geochim. Cosmochim. Acta 32, 1087-1110. UREY H. C. and CRAIG H. (1953) The composition of the stone meteorites and the origin of the meteorites. Geochim. Comochim. Acta 4, 36-82. VAN SCHMUS W. R. and WOOD J. A. (1967) A chemical-petrologic classification for the chondritic meteorites. Geochim. Cosmochim. Acta 31, 747-765. VON MICHAELIS H. (1969) Fractionation of lithophile elements in chondritic meteorites. Ph.D. Dissertation, University of Cape Town, South Africa. WASSQN J. T. (1971) Formation of ordinary chondrites. In preparation. WIIK H. B. (1956) The chemical composition of some stony meteorites. Geochim. Cosmochim. Acta, 9, 279-289. WIIK H. B. (1969) On regular discontinuities in the composition of meteorites. Comm. Phys.Math. (Helsinki) 34, 135-145. YAVNEL’ A. A. (1968) Regarding the balance of the contents of iron-magnesium silicates in the ordinary chondrites. Medeoritika 28, 19-29. YORK D. (1966) Least squares fitting of a straight line. Can. J. Phys. 44, 1079-1086.
4
Re~tio~~~
Press.
Printed inNorthern Ireland
between ~dero~~c-element content and oxidation state of ordinary chondrites
OTTO &&LLER,* PHILIP A. BAEDECKER and JOHN T. WASSON Institute of Geophysics and Planetary Physics, Department of Chemistry University of California, Los Angeles, California 90024 (Receiaed 22 July 1970; ~cc~~te~ ila revised
form 28 Jurae 1971)
&&r~&--The concentrations of Ni and Ir have been determined by neutron activation in a suite of ordinary chondrites for which accurate ferromagnesian-mineral compositional data were available. Although hiatus between the individual groups exist, the trends within the groups a,re in keeping with the hypothesis that the ordinary chondrites form a continuous fractionation sequence. A significant negativo correlation is observed between the abundance of Ni or Ir and the Fe content of the ferromagnesian minerals in the H and L groups, and for Ir in the LL group, as expected if the metal-silicate fractionation and the variation in oxidation states were produced by the same or related processes. The Ir//h’i ratio decreases by a factor of l-2 between the II and LL groups. This fractionation must have occurred at an early stage in the condensation phase of the solar nebula. INTRODUCTION THE H-, L- and LL-groups of chondrites are the largest three classes of falls, accounting for about 32, 39 and 6% respectively of falls. The three groups are very similar in many respects and have long been grouped together as the ordinary chondrites. They differ ~ompositionally in their contents of total iron, in their relative amounts of oxidized iron and in their abundances of siderophilic elements. AHRENS et al. (1969) have shown that the Mg, Al and Ca abundances in the three classes are remarkably similar, and quite different from those in the two other major chondrite classes (carbonaceous and enstatite). LARIMER and ANDERS (1967) have noted that the trace-element fractionation patterns are similar in the three groups, and different from those in other classes. PRIOR (1920) recognized the differences in degree of oxidation between the bronzite (H) and hypersthene (L and LL) chondrites, and attributed them to an origin (together with the other chondrite classes) from a single magma subjected to differing degrees of oxidation in different locations. A single-magma origin was refuted by UREY and CRAIG (1953), who showed that these classes differed greatly in total iron contents (i.e. in bulk Fe/Si ratios), and proposed that the two classes must have originated in different parent bodies. MASON (1963) showed that there was also a distinct hiatus in olivine composition between the same groups. CRAIG (1964) gave a careful discussion of prior-type relationships, and argued for the existence of a distinct LL-group (Soko-Banjites) on compositional grounds. KEIL and FREDRIKSSON (1964) showed that the H-L hiatus was also observable in pyroxene compositions, and that the “hypersthene” chondrites could be divided into distinct L- and LL-groups on the basis of smaller hiatus in the compositions of their ferromagnesian minerals. * Present address: 3
E/I~~-Pla,nck-Inst.itut fiir Kernphysik, Heidelberg, Germany. 1121
1122
OTTO MILLER, PHILIPA. BAEDECKERand JOHN T. WASSON
There is general agreement among meteorite researchers that the three groups of ordinary chondrites had very similar origins, and that they probably formed from some primordial material by a process involving both metal-silicate fractionation and oxidation-reduction. The situation is illustrated graphically in Fig. 1, a Urey-Craig-Yavnel’ plot* in which the abundance of Fe present as metal (Fe,& and sulfide (Fe,,,) phases is plotted against the amount of Fe present in silicates
‘\ 0
I
0.1
1
0.2 Fe,ttfSi
I
I
0.3 Mom
0.4
\
‘\ \
0.5
’
0.6
ratio)
Fig. 1. Urey-Craig-Yavnel’ plot showing the variations in the chemical state of iron in the ordinary chondrites. The dashed lines represent constant bulk Fe/Si ratios; the solid line represents a possible differentiationpath linking the three groups of ordinary chondrites.
(Fe,,); both axes are normalized by dividing through by total Si. The plotted data include all published analyses (save one) of falls of ordinary chondrites by the following analysts: H. B. Wiik, E. Jarosewich, M. E. Dyakonova, V. Ya. Kharitonova, D. Maynes and H. Haramura. Where two analyses of the same meteorite were available an average value was plotted. A single analysis has been discarded, Wiik’s result for the highly shocked, very inhomogeneous Rose City chondrite. The variation in oxidation state and bulk Fe/Si (Fe&i) ratio among the ordinary chondrites is apparent on the Urey-Craig-Yavnel’ diagram. On such a plot chondrites having the same Fe&&i ratios but different Fesi,/Fetot ratios * UREY and CRAIG (1953) originally plotted these values in weight per cent, a procedure followed by numerous other authors until YAVNEL’ (1968) recognizedthat plotting Fe/Si ratios gives diagrams which have a simpler graphical interpretation.
Siderophilic-element content and oxidation state of ordinary chondrites
1123
Dashed lines representing the mean Fe,,,/Si ratios fall on lines of -1 slope. for each of the three groups of ordinary chondrites are shown on Fig. 1. That the decrease in mean Fe,,JSi ratio as one goes from H- to L- to LL-groups is accompanied by an increase in the mean Feail/Fetot ratio is evident. The properties of the ordinary chondrites, which are reflected by the UreyCraig-Yavnel’ diagram, can be discussed with reference to a number of models. The existence of hiatus between the H- and L-groups and (less well defined) between the L- and LL-groups suggests that the different chondrite groups formed TANDON and WASSON (1968) have summarized four possible independently. models for the formation of the individual groups. These are: (1) the OR modelthe members of a group exhibit a range in oxidation states, but the abundances (relative to Si) of Fe and the siderophilic elements remain constant; (2) the MSF model-metal-silicate fractionation occurred but the oxidation states of the two fractions remain constant; (3) the OR-inh-oxidation or reduction occurred, the abundances of Fe and the siderophilic elements remain constant over a scale of meters, but local inhomogeneities in the distribution of metal and silicate are found on the cm scale which is involved in typical sampling of specimens (such a model would predict no correlation between the Fe&,JSi and Fe,u/Fet,t ratios within the chondrite groups) ; (4) the MSF-OR model-metal-silicate fractionation was accompanied by the fractionation of substances affecting the final redox state of the chondritic mixture (and specifically, a decrease in the amount of metal relative to silicate was accompanied by an increase in the final oxidation state). TANDON and WASSON (1968) showed that the final model was the only one which could account for the combination of (a) strong, positive correlations between siderophilic elements, and (b) negative correlations between the contents of siderophilic elements and the Fe content of the olivine, as observed in their data on L-group chondrites. The present study was undertaken in order to determine whether the MSF-OR model appeared to hold in the other groups of ordinary chondrites, and, if so, whether the fractionation trends observed within the chondrite groups were part of a continuum extending throughout the ordinary chondrites as a whole. The formation of the chondrites might then be discussed in terms of a continuous-fractionation-sequence (CFS) model, whereby the chondrites formed following condensation of an initially homogeneous gas cloud, or one varying continuously in composition. The different chondrite groups would then represent samplings of different portions of such a sequence, represented by the solid curve (visual fit) in Fig. 1. The tendency of some analyses to scatter along lines of -1 slope on Urey-CraigYavnel’ plots might be explained by the uncertainties in the analytical data used to derive such plots. Errors in the determination of Fe,,,/Si ratios are relatively small, whereas larger errors in the determination of Fe,iJFe,,t (which are not well determined: see, e.g. WIIK, 1956) would tend to simulate such trends. Support for the CFS model is provided by the apparent relationship between bulk FeJSi ratios and the mean Fe,il/Fet,, ratios between chondrite groups. However, there is too much scatter in the Fig. 1 data to allow definition of an accurate model. A better set of data for testing various models is obtained by investigating the relationship between siderophilic-element concentration and the
1124
OTTO MUELLER,PHILIP A. BAEDEC~ER and JOHN T. WASSON
Fe content (1968)
via X-ray nesian study this
of the ferromagnesian
used
the olivine
diffraction.
minerals
we have chosen technique
studied
by
data
Microprobe
appear
17 H-group and
a factor
chondrites
belonging
accurate.
chondrifes
(1964),
and
were
types
obtained
of ferromag-
For the present
which
were studied
10 LL-group
In all cases we have fo petrologic
which
of the composition
of 2 more
and 16 L-group
FREDRIKSSON
by FREDRIKSSON et al. (1968).
“recrystallized”
of MASON (1963),
determinations
to be about
KEIL
In their work, TANDON and WASSON
minerals.
composition
attempted
4-6,
by
chondrites
as defined
to choose by VAN
SCHMUSand WOOD (1967). EXPERIMENTAL Samples
and sample
preparation
The source and catalog numbers for the 45 chondrites which were analyzed in this work are listed in Table 3. We have analyzed fresh interior fragments whenever possible. In many cases only small samples were available, and we were forced to clean existing surfaces with an Al,O, wheel, following this by rinsing in reagent grade acetone. Separate chips weighing about 500 mg were taken for each analysis. Each sample was powdered in a Diamond percussion mortar, and packaged in 2-ml polyethylene vials for irradiation. Duplicate flux monitors were prepared by evaporating Mg(NO,), solutions containing approximately 230 yg Ni and 0.4 pg Ir onto separate high purity aluminum foils. The samples and flux monitors were irradiated for three hours at a flux of about 2 x lOla neutrons crnm2see-1 in either the UCLA or Northrop reactors.
Analytical
and radiometric
procedures
Nickel and Ir were determined simultaneously in each sample. About 100 mg of Ni carrier and 10 mg of Ir carrier were pipetted into Zr crucibles, and evaporated to dryness. The irradiated samples were added and fused with about 5 g of Na,O,. After cooling the fusion cake was dissolved in H,O, the resulting solution made acid with HCl, HNO, added and the mixture boiled to insure complete dissolution of the fusion cake. An aliquot containing about 10 per cent of the total solution was transferred to a centrifuge tube for Ni analysis, and the remainder of the solution was retained for the Ir determination. One drop of 3 per cent H202 was added to the solution in the centrifuge tube, and the mixture made basic with 5 M NH,OH. The resulting precipitate [mainly Fe(OH)s] was centrifuged and discarded. The Ni containing supernatant was acidified, Fe and Mn carriers were added, and the precipitation with NH,OH repeated. The supernatant was treated with dimethylglyoxime, and the resulting precipitate collected by centrifugation. The Ni(dmg), precipitate was dissolved in 2 N HCl, and any undissolved residue removed by centrifugation. The supernatant was transferred to a beaker, 100 ml of water added, and the solution heated to 60°C. Dimethylgloxime was again added, and the Ni precipitated by the dropwise addition of 3 N NH,OH. The precipitate was collected on glass-fiber filter paper, washed with water, dried to 105”C, weighed and counted. The chemical yield ranged from 60 to 80 per cent. The Ir procedure is similar to that employed by TANDON and WASSON (1968). The Ir
containing solution was evaporated to approximately 50 ml, and the silicic acid removed by ccntrifugation. The solution was diluted to 200 ml with H,O and the IrCle2- anion absorbed onto a Dowex-1 x 8 anion-exchange column. The column was washed with O-1 N HCl (200250 ml) until the effluent was free of iron (i.e. showed no color with NH,SCN solution). The upper half of the column was transferred to a beaker, 10 ml of a 10 per cent solution of NH,OH* HCl added, and the mixture warmed on a hot plate to reduce the Ir(IV) to Ir(II1). The resin was filtered and Ir(II1) eluted with 150 ml of 8 N HCl. The eluate was evaporated to 10 ml and the Ir(II1) oxidized to the +4 state with HNO,. About 100 mg of NaBrOs was added, the pH made about 8 with NaOH and NaHCO,, and the mixture boiled to precipitate hydrous IrO,. The precipitate was centrifuged, washed with H,O, and dissolved in 2 ml cont. HCl.
Siderophilic-element The resulting
solution
content
was transferred
and oxidation
to a plastic
chondrites
1125
The yield determination
was
state of ordinary
counting
tube.
by reactivation, and was generally from 40 to 60 per cent. Gamma counting was carried out using a 3 x 3 in. NaI well detector coupled to a 400-channel analyzer. Nickel was determined using the 1.43MeV photopeak of 65Ni, while Ir was determined using the 0.31-MeV photopeak of ls21r.
Precision
ad
accuracy
In order to measure the precision of our method, replicate determinations have been carried out on six chips from the Peace River chondrite and four samples of the Holbrook chondrite. The results are shown in Table 1. The relative standard deviation was about 9 per cent for Ir, and about 13 per cent for Ni. Although this precision was obtained in most of our analyses, the data for some of the meteorites listed in Table 3 show greater scatter. This is apparently the result of occasional fluctuations in the amount of metallic fraction present on the 0.5-g sampling level, since Ni and Ir generally vary coherently. Table
1. Replicate Peace
Mass (g)
Mean
(%)
on two chondrites Holbrook Ir
Nl
Mass
%
(g)
(ppm)
(%)
0.41 0.50 0.42 0.47 0.46 0.45
1.04 1.28 1.29 1.36 1.31 1.10
0.511 0.509 0.338 0.421
0.45 0.36 0.40 0.45
1.51 1.09 1.16 1.18
0.45 0.033 7.3
1.23 0.128 10.4
0.42 0.044 10.5
1.24 0.187 15.1%
(ppm)
0.511 0.497 0504 0.512 0.544 0.491
o/Me:
River Ir
Ir and Ni determinations
Ni
Errors involved in the measurement of flux-monitor specific activities can cause systematic errors in the values obtained from a given irradiation. We have taken steps to prevent such biases from affecting the statistical arguments to be presented later in the paper. The samples were first divided into hH, EH, hL, IL, hLL and ILL groups, where the small letters indicate whether the meteorite was reported by KEIL and FREDRIESSON (1964) to have a high or low Fe content in the ferromagnesian minerals as compared to the mean of that group. Six samples were included in each neutron irradiation, and consisted of three pairs, each made up of an h and 1 member from a single group. At least two groups were represented in each irradiation, and two samples of the same meteorite were never irradiated together. We believe that these techniques have completely avoided biases in the total data, and in the data for all members of each group. The accuracy of our results can be judged by comparison of our analyses with those of other research groups, where comparisons are possible for specific meteorites. Table 2 provides a comparison of our data with those of other researchers for eight chondrites. NICHIPORUK et al. (1967) have determined Ni in 18 of the same chondrites by X-ray fluorescence, while GREENLAND and LOVERING (1965) employed a spectrophotometric method in analyzing Ni in eight of the same meteorites. Our Ni results are found to be very similar to or slightly higher than those of NICHIPORUK et al. (1967), GREENLAND and LOVERING (1965), and various analysts using standard wet chemical methods. It appears that our results are systematically 5-8 per cent higher than those of other investigators. However, this will have no effect on the conclusions which we draw on the basis of our data. We have determined Ir in 14 meteorites studied by EHMANN et ~2. (1970), who utilized a non-destructive neutron-activation analysis procedure. Again, no systematic differences can be observed between the results obtained by EHICANN and coworkers and ourselves. The scatter of the Ni and Ir data obtained by other research groups appears to be slightly greater than ours.
1126
OTTO MILLER, PHILIP A. BAEDECKERand JOHN T. WASSON
Table 2. Ni and Ir concentrations in ordinary chondrites as determined by different research groups Meteorite Allegan Dhurmsala B jurbiile Farmington Forest City Holbrook Mount Browne Ochansk
* 1.62 0.94 l-18 1.28 1.72 1.24 1.91 I*89
Ni (%) t
$
1.76 0.93 I.01 1.10 1.67 1.28
2.03
1.768
1.02 1.28
1*05** l-06** l-69** 1.09**
I.79
1.22 1.93
§
1&3** 1.687?
*
Ir @pm) II
0.64 0.32 0.41 0.49 0.67 0.42 0.77
0.75 0.35 0.38 0.49 0.78
0.78
0.79
0.89
* This work, t NICHIPORUKet al. (1967). $ GREENLANDand LOVERING (1965). 5;Wet chemical analyses. 1)EHMANN et al. (1970). T[JAROSEWICHE. and MASON B. (1969) Geochim. Cosmochim. Acta 33, 411-416. ** WIIK H. B. (1969) Conzmeratationes Physico-Mathenzaticae34, 135-145. tt DYAKONOVAM. E. and KHARITONOVAV. YA. (1959) Meteoritikal&48-67. RESULTS AND INTERPRETATION
Individual determinations of Ni and Ir in the investigated chondrites are listed in Table 3. The chondrites are listed alphabetically within groups. Most of the replicated analyses agree to within about 10 per cent. A few analyses show relatively large, correlated variations in Ni and Ir, which can be attributed to sampling variations on a scale of O-5 g. The most striking example of this behavior is provided by Allegan. In one case, Fukutomi, there were large fluctuations in the observed Ni concentration, while the Ir content measured remained relatively constant. This apparently reflects experimental errors, although differences in the distribution of Ni and Ir between different metallic phases is not inconceivable. In Table 4 are listed mean Ni and Ir concentrations, and Si, Mg, metallic Fe (Fe,,& total Fe (Fe& and the Fe contents of the olivine of the investigated chondrites, in order of decreasing Fe content of the olivine. The latter values were taken from the work of KEIL and FREDRIKSSON (1964) for the H- and L-group stones, and from the work of FREDRIKSSON et al. (1968) for the LL-group chondrites. As will be described later, the values have been corrected slightly on the basis of pyroxene determinations reported by these groups. No attempt was made to correct for the fact that in the 1968 work the L- and LL-group values are systematically about O-4 wt.% higher than those reported in the earlier study, as this will have no appreciable effect on the correlations investigated in the following section. The Mount Browne and Peace River values were calculated by averaging the composition of the ferromagnesian minerals as reported by KEIL and FREDRIKSSON (1964) for those meteorites which MASON (1963, 1967) reported to have the same olivine compositions as Mount Browne and Peace River. The Si, Femet and Fetot values are taken exclusively from investigations carried
Siderophilic-element Table
Meteorite H-group Allegan
content
3. Individual Source* and cat. no.
and oxidation
state of ordinary
Ir and Ni determinations
Type
Ir (g/lo6 Replicates
0.85, 0.45, 0.76, 0.79, 0.73, 0.69, 0.72, 0.64, 0.76, 0.72, 0.78, 0.72, 0.62, 0.76, 0.78, 0.71, 0.74, 0.81, 0.68, 0.74
0.51, 0.74 0.68 0.85 0.73 0.67 0.80 0.69 0.74 0.69 0.71 0.84 0.77 0.78 0.77 0.77 0.79 0.73 0.67,
chondrites
on 45 ordinary g) Mean
0.64
chondrites
Ni (%I Replicates
2.01, 1.04, 1.86, 1.75, 1.80, 1.65, 1.84, 1.73, 1.80, 1.82, 1.59, 1.57, 1.64, 2.07, 1.98, 1.82, 1.91, 1.68, 1.68, 1.49
1127
1.24, 2.18 1.72 1.98 1.80 1.63 1.97 1.70 1.83 1.92 1.73 1.81 1.99 1.75 1.80 1.70 2.02 1.77 1.39,
Mean
1.62
SI 15
H5
UChiFMNH Me352 BM 34795 MHNP 1494 GIULAMNH 2421 NMS 69:0354 AMNH 3940 FMNH 1389 BM 1924,134 UCLA 266 AM DR2494 FMNH Me1441 UCLA 322 AMNH 662 NMS 061893 ASU 743
H5 H4 H6 H6 H H5 H5 H4 H4 H H4 H6 H4 H5 H5 H5 H
MHNP FMNH FMNH
106 Me2298 Me2392
L5 L L4
Bath Furnace Bjurbole Bruderheim Farmington Fukutomi
ASU 243A FMNH Mel426 UAlb B3 FMNH Me345 FMNH Me1491
L6 L4 L6 L5
Harleton
SI 2576
L6
Holbrook Homestead Knyahinya Kuttipuram Kyushu L’Aigle Mocs Peace River
AMNH 586 YalUAMNH 1068 BM 191588 AMNH 1072 UCLA 248 AMNH 1079 UAlb PR4
L6 L5 L5 L L6 L6 L6 L6
BM 1920,40 GSC 218101
LL6 LL6
0.32, 0.40 0.18, 0.27, 0.31
0.36 0.25
0.94, 1.01 0.98, 1.35, 0.94
0.98 1.09
ASU
LL6
0.39, 0.42, 0.32
0.38
1.10, 1.35, 0.87
1.11
Beardsley Beaver Creek Butsura Djati-Pengilon Ekeby Forest City Hessle Kesen Menow Merua Monroe Mount Browne Ochansk Pultusk Richardton Stalldalen Zhovtnevyi L-group Ausson Aztec Bald Mountain
LL-group Appley Benton
Bridge
Cherokee Springs
319.1 x
L4t
0.50, 0.49 0.52, 0.48 0.33, 0.46, 0.48 0.57, 0.55 0.42, 0.40 0.45, 0.52 0.47, 0.51 0.50, 0.50, 0.54, 0.56 0.52, 0.59, 0.51 see Table 1 0.48, 0.43 0.31, 0.32 0.44, 0.43 0.45, 0.48 0.46, 0.49 0.44, 0,42 see Table 1
0.72 0.82 0.73 0.68 0.76 0.67 0.75 0.71 0.75 0.78 0.70 0.77 0.78 0.74 0.77 0.77 0.70
0.50 0.50 0.42 0.56 0.41 0.49 0.49 053 0.54 0.42 0.46 0.32 0.44 0.47 0.48 0.43 0.45
1.36, 1.33 1.31, 1.28 155, 1.21, 1.17 1.52, 1.57 1.16, 1.20 1.26, 1.28 1.20, 1.35 1.71, 2.22, 1.35, 1.32 1.66, 1.29, 1.36 see Table 1 1.46, 1.30 0.98, 1.04 1.41, 1.30 1.16, 1.24 1.29, 1.22 1.28, 1.19 see Table I
1.79 1.87 1.80 1.64 1.91 1.72 1.82 1.87 1.66 1.69 1.82 1.91 1.89 1.76 1.97 1.73 1.52
1.35 1.30 1.38 1.55 1.18 l-27 1.28 1.65 1.44 1.24 1.38 I.01 1.36 1.20 1.26 1.24 1.23
1128
OTTO MILLER, PHILIP A. BAEDECKER and JOHW T. WASSON Table 3. (continued) source*
Ir k/lo6 g)
and cat. no.
Type
Replicates
Mean
Dhurmsala Ensisheim Guidder Jelica Karatu Manbhoom
FMNH Mel348 MHNP 1 MHNP 2262 AMNH 464 SI 2492 AMNH-
LL6 LL6 LL LL6 LL LL6
0.31 0.35 0.32 0.35 0.31 0.44,
0.32 0.36 0.36 0.35 0.35 0.27
Ottawa
AMNH 366
LL6
0.32, 0.36, 0.39, 0.34, 0.38, 0.19, 0.19 0.36,
0.43
0.40
Meteorite
Ni (%I
Replicates
0.92, 0.95 1.09, 0.97
0.91, 0.95, 1.01, 0.86, 0.96 1.13,
Mean 0.94
0.95 1.12 0.85 1.19,
1.03 0.93 1.04 0.93 1.00
1.22
1.18
* The sources are abbreviated as follows: AMNH-American Museum of Natural History; ASU-Arizona State University; AM-Australian Museum; BM-British Museum; GSCGeological Survey of Canada; FMNH-Field Museum of Natural History; GIUL-Geologic Institute, University of Lund; UCLA-University of California, Los Angeles; MHNP-Museum d’Histoire Nature110 Paris; NMS-Naturhistoriska Museet, Stockholm; UAlb--University YalU-Yale Uniof Alberta, UChi-University of Chicago; SI-Smithsonian Institution; versity. t W. R. VAN SCHMUShas informed us that he has seen a FMNH sample of Fukutomi which is L3,4. We have studied our sample by electron microprobe, and fmd it to be an “equilibrated” L-group chondrite. Thus the correct classification seems to be L4, rather than L5, as given by Vm SCHD~S and WOOD (1967).
out in the last 11 years, with the exception of two meteorites analyzed by Prior ha.lf a century ago. Where more than one analysis wa,s available an arithmetic mean of the values of the referenced authors is listed. In the case of Si, if no recent value by the chosen researchers was available, the mean value for the group is listed and italicized. Covariation
of siderophilic
elements
An important test for possible models for the formation of the ordinary chondrites is their ability to explain the variation in the relative concentrations of siderophilic metals. Figure 2 shows a plot of Ir vs. Ni. Both elements are normalized to the concentration of Si observed in the individual chondrites, as tabulated in Table 4. It makes little difference whether the normalization is to Si or Mg, since the data of VON MICHAELIS (1969-AHRENS et al., 1969) show that the mean Si/Mg ratio increases by only 2 per cent in going from the H- to L-groups, an amount which is unlikely to have any observable effect on our results. Figure 2 illustrates very nicely the large fractionation of the NilSi and Ir/Si ratios among the various groups. Either of these parameters would be sufficient to classify any of the meteorites studied, with the exception of Ottawa and Knyahinya. We also observe that the mean Ir/Ni ratio drops from 1.25 x 10e5 in the H-group to about 1.00x 1O-5 in the LL-group chondrites. The difference is so great that in most cases, our Ir/Ni ratios can be used to distinguish between H-group chondrites and the more oxidized ordinary chondrites. The index ratio of 1.18 x 10es is illustrated by the dashed line in Fig. 2. The solid line approximates a possible differentiation path if the ordinary chondrites are part of a continuous fractionation sequence. If this interpretation is correct, the Ir/Ni ratio changed by a
Siderophilic-element
content and oxidation state of ordinary ohondrites
1129
AMa
2
, 2
,
.Sn, 3
I
4
I
! 5
1
I 6
Ni/Si bxns/1020toms)
Fig. 2. A plot of Ir abundance vs. Ni abundance in a suite of ordinary chondrites. The dashed line corresponds to a constant Ir/Ni ratio of 1.18 x 10e5, and tends to separate the H-group points, which have Ir/Ni ratios higher than this value from the L- and LL-group points, which have lower values. The solid line is a possible differentiation path linking the three groups.
process. It is important to note that, because of the precautions which were taken (and described in the experimental section), these changes cannot have resulted from systematic errors. EHMANN et al. (1970) report Ir/Au ratios of 3.3, 2.7 and 2.2 for the H-, L- and LL-groups respectively, a variation of a factor of about 1.5 throughout the ordinary chondrite groups. Their and our data combine to give Au/Ni ratios rising from 3.8 x 1O-s in the H-group to 4.5 x 1O-6 in the LL-group. The significance of these rather surprising variations in the ratios of one strongly siderophillic element to another will be discussed in the next section. As has been shown by other authors for other siderophilic elements, Ir and Ni abundances tend to be positively correlated in each of the three groups of meteorites. The correlation coefficients are 0.67, 0.70 and 0.19 for the H-, L- and LL-groups The first two are significant at levels of about 0.998, the last only a.t respectively. the level of 0.38. Since these correlations could be brought about by an inhomogeneous distribution of metal and silicate, they are of relatively little diagnostic value for investigating models for the origin of ordinary chondrites. f’actor of about 1.25 during the course of the formational
Relationship between sidero&lic of ferromaglzesian minerals
element concentrations
and the iron content
A prime goal in our investigation was the determination of the relationship between the siderophilic element content a,nd ferromagnesian mineral composition
OTTO M&LER,
1130
PHILIP
A. BAEDECKER and JOHN T. WASS~N
Table 4. Maan bulk concentrations of Ni, Ir, Si, Fe and metallic Fe and ferromagnesian silicate compositions in ordinary chondrites, arranged in order of incrsasing Fe content of the olivins
Meteorite
Ir (ppm)
Abbr.
8i20 (%)
Metallic Total Fe (%)
Fe in olivine (%)
H-Group K0sen Richardton Msrua Mount Browne k&low Ochansk Monroe Hossle Stiilldalen Ahogan Pultusk Beard&y Beaver Creek Fortwt City Djati-Pengilon Ekeby Butsura Z hovtnevyi
K0 Ri
BC
Zh
187 1.97 1.69 1.91 1.66 189 1.82 1.82 1.73 l-62 1.76 1.79 1.87 1-72 l-64 1.91 1.80 l-52
0.71 0.77 0.78 0.77 0*75 0.78 0.70 0.75 o-77 O-64 0.74 0.72 O-82 0.67 0.68 0.76 0.73 o-70
1.65 l-38 la38 I.26 l-30 1.20 l-23 l-28 1.55 l-36 l-01 l-35 1.24 1.27 l-24 l-18 l-44
0.53 O-42 0.46 0.48 0.50 o-47 0.45 0.49 0.56 0.44 O-32 O-50 O-42 o-49 o-43 o-41 0.54
o-94 0.93 1.11 l-18 l-03 l-09
16.63.8 16-24.8
27.g3.8.16
16.9 16.9 16.9 16.9**% 16.9 16.9 16.9 17.25s.16
28.1i6
17*07** 16.98 16.9 17*04** 16.9 X6.9 16.9 17.5”
28.54,&l%
27.84.6.16
18-23 18.34
17.84~~
17.65,” 16.07
27.14,*.16
17.2*
26.86.16
13.5%
11.95 12.08 12.33 12Y532 12.80 12.99 13.06 13.12 13.12 13.18 13.26 13.39 13.46 13.52 13.84 13.84 14.03 14.04
L-Group Fukutomi Bald Mountain Homestead L’AigIe Aztec Kyushu Peace River Farmington Bath Furnace Kuttippuram Knyahinya Aueson Holbrook Bruderheim Moos Bjurbdle Ha&ton
Fu BM
Kn
Bj
18.39 18.7 18~4~~ 18.7 18.7 18.74,8 18+6’o 19.04,8
18.7 18.7 19.04 18.7 18.84% 18.61%,1% 18.54.17
16.05 16.06 9.7111 16.51 16.59 16.78 6.274 17.04 8.1810 17.042 17.11 6.PO* 17.30 17.37 3.364 17.45 17-45 17.63 7-184 7.g512.19 17.68 8.654~17 17.70 17.82 7-2613 17.82 9.549
21.84.8.16 22.31% 2l.O4.%*‘% 22.816 20.24 21.24.8.16 22*712,1%,19 22.14.16.17
19.38 18.613
2&IPAl%
O-32 O-36
19-o* 19+?
1g.44.8.16 19-04
4.374
0.38 0.40 O-36 0.25
19.15 18.64 18-7l4 19.0
20*45 21.24 20.7r4
2.286 2.82’ 2.5114
22~6’~
LL-Group Dhwmsala Guidder Cherokee Springs Ottawa Ensisheim Benton
Dh GU
0t Bn
19-54 19.75 20.33 20.53 20.58 21.29
Siderophilic-element content and oxidation state of ordinary chondrites
1131
Table 4. (continued)
Meteorite Manbhoom Karatu Appley Bridge Jelica
Ni
Ir
SiZo
Abbr.
(%)
(ppm)
(%)
Ma
1.00 0.93 0.89 1.04
0.27 0.35 0.36 0.35
18.94 19515 19.04 19.0
Tote1 Fe
Metallic Fe
Fe in olivine
(%)
(%)
(%)
0.704 0.6815 0.334
19.64.16
18.615 20.14 20.216
21.44 21.60 21.62 22.31
l Fc,iiv * from Feoliv and Fe,,, data of KEIL K. and FREDRIKSSONK. (1964) J. Geophys. Res. 66, 3487, and FREDRIKSSONK. et al. (1968) In O&gin and Distribution of the Elements, pp. 457-466, as described in text. 2 Based on olivine compositions of MASON B. (1963) Geochim. Cosmochim. Acta 87, 1011 and ibid. (1967) 31, 1100. Tabulated values are weighted means (see text) based on mean Feoliv + Fcpyr values found by KEIL and FREDRIKSSON(1964) for other chondrites for which Mason has reported the same olivine compositions. 3 MIYASRIRO A. (1962) Jap. J. Geol. Geogr. 36, 73. 4 WIIK H. B. (1969) Comment. Phys.-Math. 34, 135. 5 JAROSEWICHE. and MASON B. (1969) Geochim. Cosmochim. Acta 66, 411. 6 DYAKONOVA M. E. and KHARITONOVA V. YA (1959) Meteoritika 18,48. 7 DYAKONOVA M. E. and KHARITONOVA V. YA (1961) Meteoritika 21, 52. * VON MICHAELIS H., ARRENS L. H. and WILLIS J. P. (1969) Earth PZanet. Sci. Lett. 5,387. s MIYASHIRO A., in MASON B. (1965) Amer. Mus. Now. No. 2223. lo BAADSCAARDH., FOLINSBEE R. E. and CUMMINGG. L. (1964) J. Geophys. Res. 69,4197. 11 PRIOR G. T. (1918) Mineral. Mag. 18, 173. 12 BAADSGAARD H., CAMPBELL F. A., FOLINSBEE R. E. and CUMMINGS G. L. (1961) J. Geophys. Res. 66, 3574. 13CLARKE R. S., in MASON B. (1965) Amer. Mus. Nov. No. 2223. l4 PRIOR G. T. (1921) Mineral. Mug. 19, 163. l5 JAROSEWICHE. (1966) Geochim. Cosmochim. Acta 30, 1261. 16 NICHIPORUK W., CHODOSA., HELIN E. and BROWN H. (1967) Geochim. Cosmochim. Acta 31, 1911. 17KRARITONOVA V. YA (1965) Meteotitika 26, 146. I* MASON B. and MAYNES A. D. (1967) Proc. U.S. Nat. MM. 124, 1. ls DUKE M. B., MAYNES A. D. and BROWN H. (1961) J. Geophys. Res. 66, 3557. 2o Italicized values are averages of the other values listed for the group. in each of the three groups
of ordinary
chondrites.
it was for were chosen from
As we have noted,
this reason that all save two of our H- and L-group those studied by KEIL and FREDRIKSSON (1964),
chondrites and
all LL-group
were among those studied by FREDRIKSSON et aZ. (1968). In these studies the compositions of both olivine and
chondrites
orthopyroxene
were
Of the two major cations in these minerals, Fe is determined with the smaller relative error. For that reason we have used Fe content as a parameter rather than the Fe/(Fe + Mg) ratio, since the latter quantity involves an unnecessary compounding of errors. On the other hand, a sum of the Fe contents of these two minerals has a smaller relative error than the Fe content of either mineral considered alone. In order to obtain the sum in the form of a meaningful parameter determined.
we have
first converted
the pyroxene data to equivalent olivine concentrations. for the combined H- and L-group data of KEIL and
A regression line was calculated FREDRIKSSON
(1964).
All
meteorites
from
these
groups
were
included.
For the
OTTO MUELLER, PHILIP A.
1132
and
BAF,DECKER
JOHN
T. WASSON
equation Feouv_-pyr = a FepYr + b, the values of a and b were found to be 1.878 A similar treatment of the LL-group data of FREDand -2.426 respectively. RIKSSON et al. (1968) yielded values of a and b of l-767 and - 0.834 respectively. Improved iron contents of the meteoritic olivine were then calculated from the following formula: Feoliv* = (2 Feouv + Feo!i,_-Psr)/3. These are tabulated in Table
. 0
I
‘11
I
12
I
13
I
I
l4
Fe
I
16 16 content of
t
17 olivine
1
I
16 19 (weight
H group L group
f
I
20
21
I
22
23
%I
Fig. 3. Plot of Ni atomic abundance vs. Fe content of olivine. The dashed curve is s hypothetical continuous-fractionstion sequence (CFS) p&h linking the H-, L- and LL-groups. Significant negative correlations between the two parameters are found in the H- and L-groups. Solid regression lines (and lines representing lo errors in the slopes) are drawn through these two groups. Centroids of the three groups are each marked with an X. 4, and used in the discussion which follows. The lower weight assigned the Fe OIiV--pVvalue in the calculation of the mean reflects the fact that pyroxene has a somewhat smaller content of Fe than olivine, leading to higher relative errors in the pyroxene data. Figure 3 is a plot of the NilSi abundance ratio (on a logarithmic axis) vs. FeoliV. for the 45 ordinary chondrites. Correlation coefficients have been calculated for the three groups. That for the H-group is -0.47, significant at a 0.95 confidence level; that for the L-group is - 0.44, significant at a 0.91 confidence level; and that for the LL-group is +0*05, indicating a lack of correlation. Because of its very low content of reduced metal, Knyahinya has been eliminated from all L-group calculations. The negative correlations in the H- and L-groups cannot be understood in terms of the OR, OR-inh or MSF models of TANDON and WASSON (1968), but are in keeping with their MSF-OR model. Regression lines (and lines representing lb errors in the slopes) are drawn through the H- and L-group points in Fig. 3. These (and those involving Ir/Si)
Siderophilic-element
content and oxidation state of ordinary chondrites
1133
were calculated according to the procedure of YORK (1966), using weighting factors based on the errors in the various parameters. The assigned errors were 0.053 in log Ni/Si and 0.0375 in log Ir/Si values, and 0.20, 0.26 and 0.32 wt.% Fe for the Pe,aV* values in the H-, L- and LL-groups respectively. A smooth curve (shown dashed) is drawn through the centroids of the three groups, and represents the distribution of data points expected for the continuous-fractionation-sequence
. 0
H group L group
A
LL group
t
rBn
I 211
I
I
12
B
I
I
I
I
I
I
19 19 17 1.3 19 Fe c!ntcnt of olivinc (weight 7.)
20
21
22
23
Fig. 4. Plot of Ir atomic abundance vs. Fe content of olivine. As in Fig. 4, the dashed curve represents a hypothetical CFS path linking the ordinary chondrite groups. Regression lines (and lines representing 1~ errors in the slopes) are also drawn through the three groups. Signifkant negative correlations are found for each group, though only marginally so in the L- and LL-groups. The negative correlations here and for the data shown in Fig. 3 rule out the possibility that the fractionations in each group are of the Prior type, since in this case the abundances of Ni and Ir should be independent of the oxidation state of the iron.
model. The regression lines in the H- and L-groups lie near the CFS curve, which tends to support the CFS model. Although the lack of correlation in the LL-group may result from sampling problems, this lack nonetheless weakens the argument for the CFS model, Figure 4 is a plot of log Jr/Si vs. Fe,,,,*. Correlation coefficients are -0.40, -0.14 and -0.24 for the H-, L- and LL-groups respectively, significant at levels of 0.90, 0.51 and 0.48. The latter two are of only marginal significance. As stated above, negative correlations between siderophilic element concentrations and the olivine composition can only be understood in terms of the MSF-OR model. This is certainly the correct choice for the H-group, and is consistent with the apparent trends in the other groups.
1134
OTTO MILLER, PHILIP A. BAEDECKERand JOHN T. WASSON
A hypothetical CFS-fractionation curve is also drawn through the centroids of the groups in Fig. 4. Regression lines (and lb error estimates in their slopes) are also shown. The regression lines in the L- and LL-groups are in reasonable agreement with the CFS curve. The slope of the line in the H-group, for which the data are more strongly correlated, is considerably less (by more than 20) than that of the CFS curve, and would seem to be more consistent with an independent, MSFOR origin for this group. The slopes of the regression lines in Figs. 3 and 4 are consistently low relative to those of the CFS curves. There is a distinct possibility that this reflects a bias introduced by our selection of meteorites with “extreme” properties from the KEIL and FREDRIKSSON (1964) data. By deliberately choosing meteorites with low or high Fe contents in their ferromagnesian minerals relative to the mean of the group, we have also included determinations which erred on the high side together with the high-Fe subgroup, and a similar situation prevails in the low-Fe subgroup. This is precisely the direction of bias which is needed to explain our consistently lower slopes. Whether the magnitude of this effect is sufficient to account for the difference in slopes should be checked by new, bias-free microprobe determinations in which chondrites of the lH, hN, IL, etc. subgroups (as defined in the experimental section) are combined in single runs. We have also calculated correlation coefficients for log Fe,,#i ratios vs. Feofivs, using the data tabulated in Table 4. The correlation coefficients are -0.92, -0.44 and -0.06 for the H-, L- and LL-groups respectively, significant at levels of 0.99, 0.82 and 0.11. The first two are significant and support the MSF-OR model. Correlation coefficients were calculated for Fe,,&i vs. FeoliV+,and are -0.84, -0.47 and -0.97 for the H-, L- and LL-groups respectively, significant at levels of O-99, 0.79 and > 0.99. These results are in keeping with either the MSF-OR or OR (Prior-type) models for each group. The remarkably high correlation coefficient observed in the LL-group relates to the fact that small changes in the oxidation state of the silicates cause large changes in the total metal content. These changes are apparently much larger than the analytical errors associated with their determination. We have not attempted to use the Fetot and Femet data to test the CFS model, since there are relatively few available, and they are the work of several different researchers. For these reasons we doubt that slopes defined from them would provide convincing evidence either for or against the CFS model. An interesting study for the future would be to determine with high precision the composition of ferromagnesian minerals in a very large number of meteorites previously studied by a single research group-for example, for most of the ordinary chondrites studied by WIIK (1969). If the CFS model is correct, we would expect that plots of any (variable) element vs. FeoliV+would show the data falling along smooth curves connecting the three groups. It is clear from the data we have presented that metal-silicate fractionations have occurred within the H- and L-groups, and that in each case these fractionations have been accompanied by a fractionation of elements or compounds which controlled the final oxidation state of the chondritic material (stronger oxidizing conditions correlating with smaller metal contents). There is also positive evidence
Siderophilic-element
content and oxidation state of ordinary chondrites
1135
that such a NSF-OR process was involved in the formation of the LL-groups, although a simple OR model is also consistent with the data. The choice between the two models is academic for the discussion in the following paragraph, however, since the CFS curves have slopes in the LL-group (in Figs. 1, 3 and 4) which differ only slightly from those expected from the OR model. It appears that there are only two simple models which can account for the data we have presented. Either (1) each of the three groups of ordinary chondrites has experienced separate MSF-OR fraetionations, or (2) they have all experienced the same MSF-OR fractionation, which we call the continuous-fra~tionationsequence model. For the ultimate choice (until substantially better data are available) we must rely on Occam’s Principle. Since a single fractionation event can, in the second case, account for what in the first case is handled by three events, the latter, CFS model is to be favored. The hiatus probably indicate that the H, L and LL groups ori~nated in different parent bodies. The point will be discussed in more detail in WASSON(1971). DISCUSSION
There are a number of arguments which indicate that the fractionation of metal from silicate (and of one siderophilic element from another) took place in the solar nebula, as opposed to the interior of a parent body. The simplest arguments have to do with the primitive and “unequilibrated” nature of petrologic type 3 ordinary chondrites (VAN SCHMUSand WOOD, 1967). They are rich in volatiles, including planetary-type rare gases, and show evidence of rapid cooling from a relatively high temperature in their unannealed glasses and wide compositional ranges within a single mineral. The relatively rapid cooling so indicated demands that the material be dispersed. Although some have proposed that this might occur on the surface of a parent body, a solar-nebula setting offers a better explanation of all properties. The only planetary process which could cause metalsilicate fractionation is gravitational separation of immiscible phases from a melt. In fact, however, this prooess in planets tends to give total separation of metal from a silicate melt; it is very difficult to formulate a model which would, for example, remove about 5 wt. o/o metal from the L-group chondrites while leaving a comparable amount in the melt. For these reasons we feel that the fractionation must have occurred in the solar nebula. Metal-silicate fractionation and the evolution
5f
the solar nebula
We must then ask the question, was the metal-silicate fractionation associated with the condensation, agglomeration or accretion phases in the evolution of the solar nebula? The answer seems to be provided by the observations that the Ir/Ni ratio decreases, whereas the Au/K ratio increases as one proceeds from the metalrich extreme of the I3 group towards the metal-poor extreme of the LL group. LARIMER and ANDERS(1970) pointed out that such variations can be used as the basis for determining the temperature at the time of fractionation. They interpreted the data available to them to indicate that all siderophilic elements as
OTTO MILLER, PHILIP A. BAEDECKERand JOHN T. WASSON
1136
refractory as Ge are present in constant ratios throughout the suite of ordinary chondrites, and concluded that the metal-silicate fractionation occurred at a temperature lower than 1050’K. In contrast, our data and those of EHMANN et al. (1970) show appreciable fractionation of elements as refractory as Ir and Ni. For the elements in their Cl-chondritic ratios, LARIMER and ANDERS (1970) show that, in a cooling solar nebula, Ir, Ni and Au should condense in the order listed.
On this
observed
basis,
variations
E.
ANDERS
in the Ir/Ni
(private
and Au/Ni
communication)
suggests
that
ratios can be readily understood
the in
terms of the loss of an early metal condensate
which had an Ir/Ni ratio somewhat higher than the mean ratio in the nebula, and an Au/Ni ratio somewhat lower than the nebular ratio (fractional condensation is an alternative explanation). LARIMER and ANDERS (1970) give 90 per cent condensation
temperatures
(at a H, pressure
Although one encounters difficulties when one attempts to treat these values as exact, it appears reasonable to think that the temperature at this stage was about 1300°K (at the assumed pressure), which indicates that the condensation of the elements presently to be found in the ordinary chondrites was far from complete. In fact, it is necessary that not all of the metal had condensed. Thus, the metal-silicate fractionation seems to have occurred during the condensation phase of meteorite formation in the solar nebula. A detailed model for the fractionation of the siderophilic elements is given by WASSON (1971). The conclusion which is drawn is that fractional condensation provides a more reasonable mechanism than does loss of an early condensate. of 10”
atm) of 1575, 1280 and 1220°K for Ir, Ni and Au, respectively.
are indebted to E. ANDERS, J. A. V. DOUGLAS,R. E. FOLINSBEE, 0. GABRIELSON, E. P. HENDERSON,M. H. HEY, S. HJELMQVIST, J. W. LARIMER,V. MANSON, B. MASON,C. B. MOORE,E. OLSEN,J. ORCEL,P. PELLASand K. TUREKIANfor provision of the
Aclcnowledgement+-We
samples. Significant improvements in the manuscript are to be credited to the stimulating reviews provided by W. R. VAN SCHX~S, E. ANDERS, J. W. LARIMERand J. W. MORGAN. Neutron irradiations were carried out in the UCLA Engineering Nuclear Reactor under the supervision of J. W. HORNOR. This research was supported in part by NSF grants GA-1347 and GA-15731 and by NASA contract NAS 9-8096.
REFERENCES AHRENSL. H., VONMICHAELIS H., ERLANKA. J. and WILLISJ. P. (1969) Fractionation of some abundant lithophile element ratios in chondrites. &feteoriCeResearch, (editor P. M. Millman). pp. 166-173. D. Reidel. CRAIG H. (1964) Petrological and compositional relationships in meteorites. Isotopic and Cosmic Chemistry, (editors H. Craig, S. L. Miller and G. J. Wasserburg), pp. 401-451. NorthHolland. EHMANNW. D., BAEDECEERP. A. and MCKOWND. M. (1970) Gold and iridium in meteorites and some selected rocks. Geochim. Cosmochim. Acta 34, 493-507. FREDRIKSSON K., NELEN J. and FREDRIKSSON B. J. (1968) The LL group chondrites. Origilc and Distribution of the Elements, (editor L. H. Ahrens). Pergamon. GREENLAND L. and LOVERINCJ. F. (1965) Minor and trace element abundances in chondritic meteorites. Geochim. Cosmochim. Acta 29, 821-858. KEIL K. and FREDRIKSSON K. (1964) The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites. J. Geophys. Res. 69, 3487-3515. LARIMER J. W. and ANDERSE. (1967) Chemical fractionations in meteorites-II. Abundance patterns and their interpretation. Geochim. Cosmochim. dcta 31, 1239-1270.
Siderophilic-element LARIMER element MASON B. MASON B.
content
and oxidation
state of ordinary
chondrites
1137
J. W. and ANDERS E. (1970) Chemical fractionations in meteorites-III. Major fractionations in chondrites. Geochim. Coamochim. Acta 34, 367-387. (1963) Olivine composition in chondrites. Geochim. Cosmochim. Acta 27, 1011-1023. (1967) Olivine composition in chondrites-a supplement. Geochim. Cosmochim. Acta
31,1100-1103. NICRIPORUK W., CHODOS A., HELIN E. and BROWN H. (1967) Determination of Fe, Ni, Co, Ca, Cr and Mn in stony meteorites by X-ray fluorescence. Beochim. Cosmochim. Acta 31,19111930. PRIOR G. T. (1920) The classification of meteorites. Mineral. Mag. 19,51-63. TANDON S. N. and WASSON J. T. (1968) Gallium, germanium, indium and iridium variations in a suite of L-group chondrites. Geochim. Cosmochim. Acta 32, 1087-1110. UREY H. C. and CRAIG H. (1953) The composition of the stone meteorites and the origin of the meteorites. Geochim. Comochim. Acta 4, 36-82. VAN SCHMUS W. R. and WOOD J. A. (1967) A chemical-petrologic classification for the chondritic meteorites. Geochim. Cosmochim. Acta 31, 747-765. VON MICHAELIS H. (1969) Fractionation of lithophile elements in chondritic meteorites. Ph.D. Dissertation, University of Cape Town, South Africa. WASSQN J. T. (1971) Formation of ordinary chondrites. In preparation. WIIK H. B. (1956) The chemical composition of some stony meteorites. Geochim. Cosmochim. Acta, 9, 279-289. WIIK H. B. (1969) On regular discontinuities in the composition of meteorites. Comm. Phys.Math. (Helsinki) 34, 135-145. YAVNEL’ A. A. (1968) Regarding the balance of the contents of iron-magnesium silicates in the ordinary chondrites. Medeoritika 28, 19-29. YORK D. (1966) Least squares fitting of a straight line. Can. J. Phys. 44, 1079-1086.
4