Cumberland Falls chondritic inclusions. II - Trace element contents of forsterite chondrites and...

Geochimica et Cosmochrmico Acfn Vol. 47, pp. 1625-1633 0 Pergamon Press Ltd. 1983. Printed in U.S.A

0016.7037/83/091625-09$03.00/O

Cumberland Falls chondritic inclusions-II. Trace element contents of forsterite chondrites and meteorites of similar redox state

R. MICHAEL VERKOUTEREN~ and MICHAEL E. LIPSCHUTZ* Department of Chemistry, Purdue University, W. Lafayette, IN 47907

(Received March 1, 1983; accepted m revisedfirm June 16, 1983)

Abstract-Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. This highly shocked suite, the forsterite (F) chondrites, exhibits mineralogic trends apparently produced during primary nebular condensation and accretion over a broad redox range. We analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb. Sb, Se, Te, Tl. U and Zn, trace elements known to yield important genetic information. The results demonstrate the compositional coherence and distinctiveness of the F chondrite suite relative to other chondrites. The Antarctic aubrite, ALH A78 I 13, may include more F chondrite material. Trace element contents do not vary with mineral compositions hence do not reflect redox variations during formation of F chondrite parental matter. Trace element mobilization-during secondary heating episodes in the F chondrite parent or during its disruptive collision with the enstatite meteorite parent body-is not detectable. Chemical trends in F chondtites apparently reflect primary nebular processes. Cosmochemical fractionation of lithophiles from siderophiles and chalcophiles occurred at moderately high temperatures, certainly higher than those existing during formation of primitive carbonaceous, enstatite and ordinary chondrites of petrologic type 53.

INTRODUCIION

IN PAPER I of this series, NEAL and LIPSCHUTZ ( 198 1) studied 9 black inclusions in the Cumberland Falls enstatite achondrite mineralogically and reported that they constitute a sample suite distinct from other chondritic groups. These inclusions-the forsterite (F) chondrites-are highly unequilibrated, 8 being of petrologic type 3 and 1 of even lower type, and intermediate in redox state between enstatite and ordinary chondrites. Mean Fs contents of low-Ca pyroxene range from 0.3-14.5 mole percent and correlate with olivine, kamacite, schreibersite variability and compositions (i.e., obey Prior’s Rules), arguing that the suite compositionally reflects primary nebular condensation and accretion over a broad redox range. Textural features and jadeitic pyroxene in F chondrites attest to substantial shock-loading, prob- ably produced during collision of the enstatite meteorite parent body’ (WATTERS and F’RINZ, 1979, 1980; BISWAS et al., 1980; DODD, 1981) with the F chondrite parent (Paper I).

Several other meteorites differ from known chondritic groups in redox state, and GRAHAM et al.

(1977) had linked four-including two falls, Kak- angari and Pontlyfni-with F chondrites in a single group. [Kakangari-like F3 chondrites-contains chondrules.] However, oxygen isotope data show this link to be illusory and indicate that members of the putative group derive from at least three nebular ox-

t In partial fulfillment of requirements for Ph.D. degree. * Author to whom correspondence should be addressed. I Others (cJ: SEARS et al., 1982) propose that enstatite

meteorites derive from > 1 parent body.

ygen reservoirs (CLAYTON and MAYEDA, 1978a,b). [Paper I includes references to other studies describ- ing similarities and differences between these meteorites and F chondrites.] Two other meteorites, Aca- pulco and Allan Hills (ALH) A77081 differ mineralogically and chemically from known chondritic groups: these also differ from F chondrites in redox state (BISWAS et al., 1981; PAL.ME rt al., 1981: SCHULTZ et al., 1982). Finally, Cumberland Falls is not the only aubrite containing large dark inclusions: ALH A78 113 contains at least one cm-sized entity macroscopically resembling F chondrites.

Previous studies demonstrated that certain trace elements-Ag, As, Au, Bi, Cd, Co, Cs. Ga, In, Rb, Sb, Se, Te, Tl and Zn-give especially important information on geochemical and thermal fractionations during meteorite formation and evolution (BISWAS et al., 1980; NEAL et al., 1981; WALSH and LIPSCHUTZ, 1982). We decided to measure these elements (and, later, U) in F chondrites and meteorites in some way resembling them. By this, we intended to establish the compositional characteristics of the primitive F chondrite suite, the cause(s) for their compositional trends and the extent to which F chondrites and these other meteorites resemble or differ from each other. Here, we report our results.

EXPERIMENTAL

In addition to aliquants of the inclusions studied in Paper I (and obtained from the sources listed there), we measured trace elements in inclusion X (off BM 1920, 106) referred to in Paper I and described originally by BINNS (1969). We also included Kakangari (off BM 69062). Pontlyfni (off BM 1975 M.6) and the large inclusion ALH A781 13,38 (thin section, 3 1). We cite data for other relevant samples reported previously.

1625

1626 R. M. Verkouteren and M. E. Lipschutz

Prior to running any other meteorite, we assessed experimental precision and accuracy by analyzing replicate 0.2 g samples of homogenize Allende standard reference powder. We prepared 0.2 g samples of other meteorites for irradiation by chipping away potentially contaminated sur- faces. The 3-4 day irradiations at the University of Missouri Research Reactor were at a thermal neutron flux of lOI cm --’ set-‘. Chemical processing, counting and data reduc- tion techniques are as in SMITH ei al. (1983) and references therein except that:

(1) During the Allende measurements only, we found >20% differences between neutron fluence-corrected (using ‘“Cs) specific activities for As and U monitors. Allende results for these were normalized to mean fluence-corrected specific activities from other irradiations.

(2) About half-way through this study we found that we could determine U via fission 13’Te in Te fractions. The run containing 2739-l. 604-9 and 477- 1 included no U monitor but subsequent ones did; we estimated U contents for these three samples using tluence-corrected (via ‘?Zs) 13*Te from subsequent runs. After all samples had been run, we analyzed replicate Allende samples: though precise, our results were systematically high indicating a systematic effect. To assess 13*Te production by fast neutron fission of Th, we included Th monitor in a later irradiation: assuming Th/lJ = 3.6 and insignificant shielding differences between monitor and chondrite, <OS% of total ‘32Te could derive from Th. z35U depletion in our U monitor was not a factor: we compared fission “‘Te in it and in normal U (as UOJ) from the U.S. Department of Energy (New Brunswick Labora- tories reference material 1118) and found no significant difference. Our quest ended with the discovery that more severe treatment was necessary to equilibratefi~ion 13’Te with Te carrier. When heated at 90°C for 10 minutes in 7 N HN03, only 37% of “*Te equilibrated. However, 100% equilibrated on more severe treatment (repetitive boiling to near dryness with cone HCI, Te precipitation using 15% N2H4 followed, after 15 minute digestion, by reoxidation with cone HN03 at 90°C). Under both sets of conditions, fission ‘*Ba equilibmtion with Ba carrier was compiete. The data in Table 1 are corrected for this systematic effect.

We used 5 monitors for each irradiation: A (Bi, Ti), B (Ga, In, Sb, Se), C (Ag, Cd, Te), D (As, Au, Cs) and E (Co, Rb. Zn) in earlier irradiations; U replaced Co (added to B) and Rb (added to C) in monitor E in later runs.

Average chemical yields for samples were Ag @I%), As (40%). Au (46%) Bi (67%), Cd (55%), Co (67%), Cs (37%), Ga (24%), In (41%), Rb (51%), Sb (44%). Se (51%), Te and U (73%), Tl(57%) and Zn (68%). For monitors, yields were Ag (86%) As (62%). Au (95%) Bi (78%), Cd (68%) Co (100%) Cs (75%), Ga (44%), In (54%), Rb (80%), Sb (84%), Se (96%) Te (94%) Tl (85%) U (97%) and Zn (94%).

RESULTS

We can assess the accuracy and precision of our Allende data (Table I) with previous results from our laboratory: BART ef al. (1980) and Nca and LIPSCHUTZ (1980) compared their data with earlier results and found excellent agreement. Experimental precision of our data is generally similar to earlier values, all relative sample standard devia- tions ranging from 2-9% except for Cd (16%). Few means differ signi~~ntly: In and Se, by 2.6~ and 5.0a, respectively, from values by IKRAMCJDDIN and LIP-SCHUTZ (1975); and Sb and Te, by 3.40 and 2.30, respectively, from those of NEAL et al. (1981). Other data for these elements do not differ from ours and except possibly for Se, the difference is not serious. At present, the overall Purdue mean for 32 replicate Se analyses is 8.69 i 1.25 ppm; our results may therefore be systematically low by 20%. Our U data (Table 1) seem satisfactory, judged both by precision and comparison with prior results for Allende: 16.3 and 16.0 ppb

(FIREMAN et al., 1970); 15.5 and 16.5 ppb (FISHER, 1972); 15.3 ppb (TATS~MOTO et al., 1976) and 13.4 and IS.3 ppb (TAKAHASHI et al., 1978).

The convenient classification of F chondrites by mean Fs content prompts the order in Table 1: inclusion X is inter- polated in the position indicated by BINNS’ (1969) results (Paper I). The absence of mineralogic data for inclusion Y (BISWAS et a/., 1980) precludes interpolation: we merely list it in Table 1 with other F chondrites. Apart from U (2.1 2 0.5 ppb) and Te (0.31 i 0.11 ppmf results of CLARK cr nl. (1967) no other data exist for any inclusion in Cum- berland Falls. Values of CLARK ef al. ( 1967) for these elements in other meteorites are suspect (MORGAN, I97 I; PELI.Y and LIP~CH~JTZ, 197 1) and we feel no need to com- ment further on the disagreement with our results. As will be seen, chemical compositions of F chondrites apparently reflect primary (i.e., nebular) processes. We therefore order elements in Table 1 by presumed volatility during nebular condensation and accretion of solids, i.e. by elemental atomic abundance in ordinary chondrites, especially equilibrated ones, relative to cosmic, generally C 1, values (see CAMERON, 1982). Prior results for Kakangari and Pontlyfni (Table 2) generally accord with ours (Table I) except possibly for Zn in Pontlyfni, where the value of DAVIS et al. (1977a) is about halfour first datum. A clear choice between these is impossible and we assume that the difference arises from sampling. Fortunately. this creates no interpretive problems.

DISCUSSION

Even a glance at Table 1 shows that F chondrite data (including those for sample Y) are quite coherent, in fact remarkably so given the range in Fs contents. Of other samples listed, only the chondritic inclusion ALH A78 I 13,38 even generally resembles F chondrites compositiona~ly and we depict in Fig. 1 data for those inclusions as atomic abundances relative to cosmic, essentially C 1 chondrites (CAMERON, 1982). We treated the Si content determined by JA- ROSEWICH ( 1967) for an otherwise undescribed Cum- berland Falls inclusion, 19.53 weight percent, as ap- plicable to samples shown in Fig. 1.

Considering for the moment inclusions in Cum- berland Falls only, 9 elements vary overall by factors <3X; 3 others (Ag, Se and Te) vary by 5-10~ and the most volatile ones (Cd, Bi, Tl, In) vary by 13- 470X (Table 2). These variations generally exceed those in unequihbrated enstatite or carbonaceous chondrites but are lower than or comparable to those in unequilibrated portions of gas rich regohth breccias or ordinary chondrites. We omit equilibrated enstatite or ordinary chondrites in this comparison only because F chondrites are of petrologic types 53: were they included, variations for elements more volatile than Te would be signi~cantly greater in E4- 6 or H, L or LL3-6 chondrites. For example, Tl, the most variable element in F chondrites, exhibits ranges in C3, E4, H3, L3, LL3 and dark portions of H chondrite regolith breccias of 5.6X, 1.4X, 180X, 170X, 170X and 62X, respectively. Other elements are more like In, varying by 13X in F chondrites, and by factors of 2.0X, 6.7X, 7 I X, 79X, 60X and 4000~ in the other chondrites, respectively (cf: BINZ et al.

Cumbedand Falls inclusions 1627

Table 1. Trrce elananta in P chondrite inclusions from Cumbarland Falls. am inclusion

in ALE A78113 aubrite, meteoritea of nimihr redox state and Allende standard reference

powder.

nereorite *

Pa(PW) ” AU Co Rb AII Sb Aa Ca

eeb ppb PP eem pm peb ppb PPm

F chondtite

604-Z 14,5(251 18 149 478 3.16 2.0 54.1 31.4 3 .b8 604-7 13.0(47) 153 197 3.05 1.4 88.1 31.8 4.07 477-l 12.7fZ3) 20 131 502 2.88 1.7 62.5 41.8 3.39 604-5 8.8(42) 21 150 512 2.50 I .6 90.6 27.1 3.49 x 173 452 b.b9 2.) 123 75.7 3.76 2739-l 6.9(44) 19 lb8 614 3.04 1.8 92.5 16.5 4.15 604-6 5.5(60) 80.9 306 3.05 0.97 41.1 8.9 3.74 6$4-S 0.3(101) 24 151 470 2.94 1.7 98.4 68.5 4.69 Y 198 2.88 1.2 56.1 3.70

ALH A78113.38 16 116 404 6.19 1.6 58.8 22.3 65.7

Acapulco . 244 941 0.24 2.8 65 51 .o 11.7 - Al& A77061 100 743 0.47 4.8 56 82.8 10.6

Xekaogarl 2l8 666 I .70 2.9 148 340 6.91. pontlyfni lb 370 1000 1.07 587 276

325 930 0.97 5.2 484 474 19+3 :t

Allende’ 16.3+ 6001 1.09+ 1.56* 0.473)

1442

E(6) 7.2r7) 0.08T6) 0.06r3) 71.6_+ 98.42 6.332

6.4(7) 9.0(8) 0.40(S)

neteorite Pam)* se C8 Te m Cd ai T1 In

PPm ppb Ppm PP ppb PPb wb wb

F chondrite

604-Z 14.%25) 20.2

604-7 13.0(47) 6.05 477-l 12.7(23) 6.20 604-5 8.8(42) 11.7 x 4.08 2739-l 6.9(44) 3.94

604-6 5.5(60) 7.65 6q4-9 0.3(101) 7.96 Y 11.1

Acapulcoi ALH A77061 ’

172 6.26 9.13 193 0.91 8.19

71.4 0.33 41.0

5.45 0.94 1.13 198 9.86 1.65 1.03 219

Rakangarl 17.6 Ponrlyfoi 40.7

50.0

53.3 124 28 24.8 3.01 3. dfl

4.65 ::Y 266 190 27.5 4.64 2. Yl

9.28 10.2 155 77 17.6 3.15 2.90

7.22+ .95.3+ 0.963+ 122+ 475t 50.5, 65.4+ 0.39?7) 3.878) 0.039r4) 376) 7bT6) 2.8(3) 3.576)

325 2.79 7.69 189 6.85 8.08 159 2.71 5.98 126 3.64 7.05 109 5.01 6.50 181 7.57 7.95 203 2.71 9.07

6.9 0.40 44 7.75 54 7.30 9.3 1 .a5 29 1.94

6.1 so.33 3.9 0.41 110 3.04 16 3.31

15 0.36

14 27 .l 3.3

0.10 0.83 48.2 0.44 6.59

0.12 0.16

0.62 0.47

0.44 I.13

5.45 2.22 1.11

0.61 0.54 1 .oo

0.42

2.06 1.40

19.7 2.93

3.75 0.78

30.5_+ 1,7(b)

Wherstogic data from Weal sod Lipschutr (19811.

‘Trace element date published previously: Cmberland Falla inclusion - Biavas & 81. (1980); Acspulco and AM A77061 - Bisuae cd. (1981).

‘Data associated with cbmical yields of _(I22 and which, therefore, should be regarded with more thao the usual suapician.

kmhrs in parenthesee indicate number of replicates meesured. Vneerteinties rre one sample stsadazd daviatioo.

(1976); MASON (1979); LIPSCHUTZ et al. (1983) and references in them). To some extent, F chondrite variations may reflect the inclusions’ small size which inevitably maximizes sample heterogeneity; to com- pensate, interlaboratory bias is absent. Thus, trace element contents-like minemlogic properties of Cumberland Fails inclusions-are consistent with their belonging to a single chondritic suite.

F chondrite inclusions exhibit no systematic variations (Table 1, Fig. 1). For example, 604-9, the unique inclusion of petrologic type <3 from, e.g., its high-M sulfides, absence of chondrules or taenite, etc. (Paper I), is not systematically enriched or depleted relative to other inclusions. Inclusion Y is like all others except for its unusually low Co and Te contents. In fact, apart from unusually high Tl values for

477-l and inclusion X, causing them to stand out, values for each element form seeming continua (Fig. 1).

F chondrite c~rnp~~iti~~a~ distinctiveness

To compare trace element contents in F chondrites with those of other groups, we plot in Fig. 2 geometric mean atomic abundances in several chondritic groups normalized to those in Cl chondrites (EBIHARA et ai., 1982). In addition to F chondrites (Table 1) we include abundance patterns for E and H chondrites, both equilibrated and unequilibrated (data from LAUL et al. ( 1973), BINZ et al. ( 1974, 1976), MASON

(1979) and references cited by them and HUSTON

and LIPSCHUTZ, 1983). We chose H chondrites

(omitting data for gas rich regolith breccias [e.g. LIP-

SCHUTZ et a/., 19831) as representative of ordinary chondrites because this group contains the plurality of unequilibrated ordinary chondrites. Furthermore, of all ordinary chondrites, those of the H group most closely resemble F chondrites in redox state. To min- imize confusion, we chose not to illustrate abundance


& , TJ Ag ( ;o Te in Ed

FIG. 1. Depletion factors or atomic abundances in F chondrite inclusions in Cumberland Falls (tri- angles) and ALH A781 13 (star) normalized to cosmic, i.e. Cl abundances (U, to the present-day Cl value). Elements are ordered from left to right by putative nebular volatility. Overall variations for each element in Cumberland Falls’ inclusions are comparable with or smaller than those in other chondritic groups. Contents of most elements and oxygen isotopes in ALH A781 13,38 are similar to those for Cumberland Falls inclusions; discrepancies for Te, Ga and Zn indicate compositional differences between F chondrite parent regions sampled by the two aubritic impact breccias (see text).

patterns for C2, C30 or C3V chondrites in Fig. 2. Very briefly, these essentially parallel the horizontal

(Cl) line at 1 .O for the most volatile elements (except for Cd in C30) but are displaced to 0.5 1 (for C2) and 0.24-0.28 (for C3); the least volatile elements lie at 1.0 and intermediate elements lie between (TAKA- HASHI et al, 1978; WOLF et al., 1980). These patterns accord with trends expected from a two-component model (see TAKAHASHI et al. (1978) and references cited therein); however, alternate views exist (WAI and WASSON, 1977; WASSON, 1977).

chondrites: Ga, Se, Bi and Tl are lower in F chondrites and Rb is higher.

The abundance pattern for F chondrites differs markedly from those of unequilibrated E and H chondrites although use of geometric means implies large variations, and substantial overlap in concen- tration ranges. We take a 2X difference in means as suggesting a noteworthy difference. Compared with other chondrites (Fig. 2), F chondrites are generally depressed in less volatile siderophiles and chalcophiles-Au, Co, Ag, Ga and Se, Sb being lower only compared with E4 chondrites-and contain much lesser quantities of the 5 most volatile elements. Cu- riously, chalcophile Te is at E4 levels in F chondrites, substantially higher than in H3,4 chondrites (Fig. 2). Of course, in no way does the F chondrite pattern resemble those of Cl-3 chondrites.

Surprisingly-considering F chondrite petrology- trace element, particularly volatile, abundances are more similar to H5,6 chondrites than to others but, again, differences appear. Mean contents of 6 elements differ from those in equilibrated chondrites: Ag, Zn and Co are lower in F chondrites and Cs and Te higher; In is more abundant in E5,6 and less abundant in H5,6 than it is in F chondrites. Contents of

The F chondrite data differ markedly from those of Kakangari and Pontlyfni, on the one hand, and Acapulco and ALH A7708 1, on the other, as even a glance at Table 1 reveals. To quantify these, we used a one sided t test and our data and those of GRAHAM et al. (1977), DAVIS et al. (1977a,b), BIS- WAS et al. (1980, 1981), PALME et al. (1981) and SCHULTZ et al. (1982) for Kakangari, Pontlyfni, Acapulco and ALH A77081 to establish the significance levels of the differences. Nearly every element differs significantly in abundance: at >99% confidence level, Au, Co, As, Sb, Ag, Ga, Se, Zn, Bi and In are lower and Rb and Cs are higher in F chondrites than in Kakangari and Pontlyfni; at >97.5% confidence level, Cd is lower and, at >85%, Te may be lower in F chondrites. The list is shorter when F chondrites and Acapulco and ALH A7708 1 are compared: Au, Co, As, Ga and Zn are lower in F chondrites at >99% confidence level and Rb and Cs are higher: Te is higher (~97.5%) and Ag (>90%) and Sb (285%) may be lower. [Differences between Kakangari and Pontlyfni and Acapulco and ALH A77081 are less striking. Nine elements are higher in the former pair but significance levels are lower, ~99% for Rb, >97.5% for Ag, ~95% for Sb and Se, >90% for Au and Ga, and ~85% for Te, Cd and In.]

While trace element contents of F chondrites and the chondritic inclusion ALH A781 13,38 differ somewhat, the resemblance is striking. Eleven elements in inclusion ALH A781 13,38 fall within the respective ranges for F chondrites (Fig. 1). Differences for Rb and Cs are marginal; Te and especially Ga

5 elements differ from those in E5,6 but not H5,6 and Zn seem discrepant. Mineralogically, inclusion

Cumberland Falls inclusions I629

F

E4 ---- E5.6 ------ H3,4 ----___- H5,6

0.001’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ U Au Co Rb As Sb Ag C+J Se Cs Te Zn Cd BI TI In

FIG. 2. Mean depletion factors or elemental atomic abundances relative to cosmic (C I) for F chondrites, and unequilibrated and equilibrated E and H chondrites. The F chondrite pattern is quite distinct, especially in its low volatile element contents and high Rb, Cs and Te. The most volatile elements in F chondrites are depleted well below values for E and H (and C, L and LL) chondrites of similar low petrologic type (53); in most cases, depletions are even more severe than those of equilibrated E and ordinary chondrites. F chondrite parent material apparently condensed and accreted from the nebula at temperatures higher than the formation temperature of other primitive chondrites.

ALH A781 13,31 contains two sorts of material (DODD, pets. commun.). The most obvious is coarse grained (52 mm), low Ca pyroxene evidencing strong deformation by the presence of some granulated margins and mosaic extinction. Intergranular metal, troilite and an unidentified, fine, black phase are also present. Within this are located rounded masses of the second sort of material, a mixture of reddish- brown microcrystalline phase and abundant irregular olivine and twinned clinopyroxene. Relict barred or excentroradial chondrules are present in some of these masses. No evidence for extensive melting is evident. Until we obtain detailed petrochemical information, we are reluctant to classify inclusion ALH A78 113,38 as an F chondrite; however, it is obvious that the resemblance is not superficial.

Almost certainly, inclusion ALH A781 13,38 is highly unequilibrated, both on textural grounds and from the similarity of its oxygen isotopic composition (al80 = +4.97%0, 6”0= +3.46%0 relative to SMOW) to those of F chondrite samples 604-2 and 604-9 (6”O = +5.53 and +5.79%0, 6”O = +3.74 and +4.00%0, respectively) (CLAYTON, pers. commun.) and an otherwise unspecified Cumberland Falls inclusion (6”O = +5.8%0, 6”O = +3SL) (CLAYTON

and MAYEDA, 1978b). CLAYTON and MAYEDA (1978b) noted that the isotopic composition of this last inclusion fell “on a mass fractionation line through the H-group ordinary chondrites but . displaced toward higher 6i80 by about IS%0 . . unlike any previously analyzed chondritic material.” From more recent, largely unpublished data, CLAY- TON (pets. commun.) finds that highly unequilibrated type 3 ordinary chondrites of all three iron groups have oxygen isotopic compositions generally similar to those of inclusions 604-2, 604-9 and ALH A78 113,38; hence the last part of the quoted state- ment is no longer true. CLAYTON (pers. commun.) reminds us further that chemical group distinctions among such highly unequilibrated ordinary chondrites (UOC) can be ambiguous. The available isotopic data hint that F chondrite inclusions in Cum- berland Falls and the related one from ALH A78 113 are even more displaced toward more positive 6180 values than are the highly unequilibrated type 3 ordinary chondrites but the differences are not defini- tive. It is clear, however, that the oxygen isotopic composition of inclusion ALH A78 113,38-while on the F chondrite (and UOC?) mass-fractionation line-is substantially less positive than are the others.


Trace elepnenf genelic clues

The highly shocked state of F chondrites (Paper I), their very low volatile element contents (Table 1, Fig. 1) and the substantial degree to which trace elements can be lost by shock-loading (NEAL et al., 198 1; WALSH and LIIXHUTZ, 1982; HUSTON and

LIPSCHUTZ, 1983) requires that we consider possible shock-induced compositional alteration. Two lines of evidence suggest this as being unimportant for F chondrites.

First, mobility order, the relative ease of trace element volatilization and loss from heated meteoritic matter, is not identical to volatility order, the relative ease of trace element introduction into solid parent material during nebular condensation and accretion (NGO and LIPSCHUTZ, 1980). To examine whether

shock heating was significant in m~ifying indige- nous trace element contents of F chondrites, we re- plotted Fig. 1, re-ordering elements by mobility (WALSH and LIPSCHUTZ, 1982) at 1000°C as determined in laboratory heating experiments (Fig. 3). Also included are data for the achondritic hosts in Cumberland Falls and ALH A781 13 and the range of data for all aubrites measured by BISWAS et al. (1980). If shock heating had been important, F chondrite variability should increase from Te to Cd (cf: WALSH and LIPSCHUTZ, 1982; HUSTON and LIP- SCHUTZ, 1983) and trace element loss or exchange would tend to homogenize inclusions and hosts in Cum~rland Falls and ALH A78 I 13. Obviously, nei- ther effect is evident in Fig. 3 nor do the mineralogy or petrology of F chondrites signal a prolonged thermal episode (Paper I). [A reviewer, A. M. Davis, pointed out that shock-induced depletion ofall highly mobile elements but Cd (the most sensitive to thermal effects) and Zn in L4-6 chondrites (WALSH and

-

t

1 -

-

i

1 -

LIPSCHUTZ, 1982) fall within the analytical range for F chondrites. This, to us, merely emphasizes the unusually low mobile trace element contents of F chon- d&es of petrologic type 13. Pre-shock mobile element contents in L4-6 chondrites must have been already well below the C t line bqfore shock-induced loss occurred.]

The second type of evidence indicating the absence of appreciable shock heating lies in the shock-produced jadeitic pyroxene in F chondrites (Paper I). Pure jadeite is thermodynamically unstable at pressures less than a few GPa (BELL and ROSEBOOM, 1969; WILLIAMS and KENNEDY, 1970). Thus, its metastable existence in F chondrites apparently reflects slow decomposition kinetics. At temperatures of 600-775°C and pressures of 0.9-1.2 GPa, pure jadeite will not survive l-2 hours heating (ROBERT‘- SON et a!., 1957), so that post-shock time-te~l~rature conditions in F chondrites (at even lcflver pressures) must have been much less severe. Substantial mobile element loss should not occur under these conditions (see, e.g., NGO and LIPSCHUTZ, 1980). DAVIS (pers. commun.) points out that if jadeite is stabilized by other components in pyroxene mixtures at the .<35% level found in F chondrites (Paper I), its utility as a thermometer in the temperature region of interest here might be severely compromised. On the other hand, jadeite was previously unreported in meteorites, even in shock heated L chondrites ( VAN SCHMUS

and RIBBE, 1968; REIMOLD and ST~FFLER, 1978;

DODD and JAROSEWICH, 1979). where trace element

mobilization occurred (NEAL et a/., 198 1; WALSH

and LIPSCHUT~, 1982; H~JSTON and LIPSCF~UTZ, 1983). This suggests that shock conditions severe enough to mobilize trace elements causes shock- formed jade&e to decompose. Its presence in F chon- d&es is consistent with our view that they experi-

8

i

5 * n

n 8 H 1

IO'

IO"

0 IO“ A

i

9 ‘o-2 i lo-’

L -i 10-4

U Au CO As Sb Se So Rb Cs Te Bi In Ag Zn TI Cd

FIG. 3. Depletion factors or atomic abundances in F chondrite inclusions in Cumberland Falls (tri- angles) and ALH A78 113 (star), the achondritie portions of these aubtites (circle and square, respectively) and the overall range for all aubrites (bars). Elements are ordered from left to right by experimentally- determined mobility at IOOOT. While aubrite data for the more mobile (and, generally, more volatile) elements generally lie closer to F chondrite values than for the less mobile ones, subtle trends suggest that the differences are not shock-induced but, rather, are of primary origin (see text).

Cumberland Falls inclusions 1631

enced less severe shock loading, hence alteration, than did L chondrites and that trace element patterns were established during primary nebular processes, not during later parent body collisions.

F chondrite data suggest moderate lithophile en- richment and/or siderophile and chalcophile depletion, especially when compared with other chondritic groups (Figs. 1, 2). The correlation profile or pattern of statistically significant linear and exponential (or power law) relationships--i.e., y = mx + b and y = kx”, respectively-summarizes all direct and anti- correlations involving two elements or physical properties (as pyroxene disequilibrium parameter, PMD, or Fs content). The profile for F chondrites (Fig. 4) predominantly reflects chemical effects. Most of the 12 linear and 14 exponential correlations (9 of both types) involve siderophile and chalcophile elements and/or PMD. Formation temperatures for F chondrites apparently were sufficiently high that little of each highly volatile element could be introduced: hence only a few Cd, Bi, Tl and In correlations are present in Fig. 4. Anti-correlations of Tl and In with Zn suggest that higher temperatures during F chondrite formation were coupled with more reducing conditions; direct correlations of PMD with non-volatile lithophile U, siderophile/chalcophile Ga and chalcophile Zn and anti-correlations of Fs content with Ga and the PMD parameter are also consistent with this notion. Chance can never be disregarded however, e.g. Rb/As (Fig. 4). F chondrites exhibit about the same proportion of significant relationships as do moderately-to-heavily shocked L4-6 chondrites (WALSH and LIPSCHUTZ, 1982), i.e. less than do C

FIG. 4. Correlation profile for 9 Cumberland Falls (F chondrite) inclusions involving 16 elements and the disequilibrium parameter (PP) and ferrosilite content (Fs) for pyroxene. Circles and crosses denote statistically significant (~95% confidence level) 1inear:linear and logarithmic:logarithmic correlations, respectively (except for PP where crosses indicate 1ogarithmic:linear correlations), while diamonds and plus signs designate analogous anti-correlations. The high temperature origin of F chondrites (at least relative to other primitive chondrites) seems responsible for the dearth of correlations involving highly volatile elements (lower right) and an important siderophile and chalcophile fractionation is indicated by correlations to the upper left. Other direct and anti-correlations reflect the coupling of these genetic phenomena and, of course, chance (see text).

e ,Kakongar

*

,ALH A78113, 38

/ALH A77081

Au (atoms/105Co atoms)

! FIG. 5. Co-normalized correlation of siderophilic Au with

So in F chondrites and other meteorites. [The Cl datum (CAMERON, 1982) is shown for reference.] The F chondrite trend (including ALH 78 113,38) apparently reflects moderately high temperatures accompanying nebular cosmochemical frationation: other meteorites of rare type-ALH A7708 1, Acapulco and Kakangari-may also reflect a similar history (see text).

or E chondrites but more than do randomly selected or lightly-to-moderately shocked L chondrites (Ku- RIMOTO et al., 1973; BINZ et al., 1974, 1976; WALSH and LIPSCHUTZ, 1982). Inclusion of data for additional F chondrites should yield a more informative correlation profile.

In F chondrites, siderophilic elements of lesser volatility (Au, As, Sb) systematically and progressively increase in abundance (Fig. 4) even when normalized to geochemically similar Co (Fig. 5). The F chondrite trend, which encompasses inclusion ALH A78 113,38 cannot merely represent fortuitous sampling of metal- rich or -poor regions because the data are siderophile- normalized. Furthermore, the array of points fail to correlate with Fs content implying absence of a redox role. We tentatively attribute the trend to thermal fractionation accompanying separation of siderophiles and chalcophiles from lithophiles at moderately high temperatures during nebular cooling. The presence of Kakangari on the F chondrite trend and Acapulco and ALH A7708 1 (but not Pontlyfni) near it, hints that the fractionation was not unique to F chondrite parent material. Acapulco and ALH A77081 have also been suggested as having a high temperature origin but their large contents of highly volatile/mobile trace elements are puzzling (PALME


et al., 1981; SCHULTZ et al., 1982; Table 2). Aca- Trace elements in primitive meteorites-V. Abundance

pulco, ALH A7708 1, Kakangari and Pontlyfni clearly patterns of thirteen trace elements and interelement re-

derive from parental materials different from the one lationships in enstatite chondrites. Geochim. Cosmochim.

that produced F chondrites. Data now being gathered Acta 38, 1579-1606.

BINZ C. M., IKRAMUDDIN M., REY P. and LIPSCHUTZ on more refractory elements in F chondrites should M. E. (1976) Trace elements in primitive meteorites- provide further genetic information. VI. Abundance patterns of thirteen trace elements and

interelement relationships in unequilibrated ordinary

CONCLUSIONS chondrites. Geochim. Cosmochim. Acta 40, 59-7 1.

BISWAS S., WALSH T., BART G. and LI~~CHUTZ M. E.

Our results demonstrate that the F chondrite suite (1980) Thermal metamorphism of primitive meteor-

represented by the Cumberland Falls inclusions are ites-XI. The enstatite meteorites: origin and evolution

compositionally coherent and unique from other of a parent body. Geochim. Cosmochim. Acta 44, 2097- 21 10.

chondritic groups just as do the mineralogic prop- BISWAS S., WALSH T. M., NGO H. T. and LIP~CHUTZ

erties described in Paper I. Our results also indicate M. E. (198 1) Trace element contents of selected Antarctic

that at least one other meteorite, the Antarctic aubrite meteorites-II. Comparison with non-Antarctic speci-

ALH A78 113, is host to more F chondrite material, mens. Proc. Sixth Symp. Antarctic Meteorites, 22 l-228.

CAMERON A. G. W. (1982) Elementary and nuclidic abun- compositionally different in some respects from the dances in the solar system. In Essays in Nuclear Astro- inclusions in Cumberland Falls. Trace element trends physics (eds. C. A. BARNES, D. N. SCHRAMM and D. D.

apparently reflect primary nebular processes-cos- CLAYTON), Cambridge Univ. Press.

mochemical fractionation of lithophiles from side- CLARK R. S., ROWE M. W., GANAPATHY R. and KURODA

rophiles and chalcophiles at moderately high tem- P. K. (1967) Iodine, uranium and tellurium contents in meteorites. Geochim. Cosmochim. Acta 31, 1605- I6 13.

peratures, certainly higher than those existing during CLAYTON R. N. and MAYEDA T. K. C 1978a) Genetic re-

formation of primitive carbonaceous, enstatite and lations between iron and stony meteorites. Earth Planet.

ordinary chondrites of petrologic type 53. Trace ele- Sci. Lett. 40, 168-174.

ment mobilization-during secondary heating epi- CLAYTON R. N. and MAYEDA T. K. (1978b) Multiple par-

ent bodies of polymict brecciated meteorites. Geochim. sodes in the F chondrite parent or during its disrup- Cosmochim. Acta 42, 325-327. tive collision with the enstatite meteorite parent body DAVIS A. M., GANAPATHY R. and GROSSMAN L. (1977a)

(Paper I)-is not detectable. Variations in redox state Pontlyfni: a differentiated meteorite related to group IAB

during formation of F chondrite parental material, irons. Earth Planet. Sci. Lett. 35, 19-24.

detected by major mineral compositions, are not re- DAVIS A. M., GROSSMAN L. and GANAPATHY R. (1977b)

Yes, Kakangari is a unique chrondite. Nature (London) flected in mobile trace elements. Further study of 265, 230-232.

these newest samples of primitive chondritic material DODD R. T. (198 1) Meteorites: A Petrologic-Chemical Syn-

should give further information on early nebular his- thesis. Cambridge Univ. Press.

tory. DODD R. T. and JAROSEW~CH E. (1979) Incipient melting

in and shock classification of L-group chondrites. Earth Planet. Sci. Lett. 44, 335-340.

Acknowledgments-We thank Dr. R. S. Clarke, Jr. and EBIHARA M., WOLF R. and ANDERS E. (I 982) Are Cl chon- A. F. Noonan of the Smithsonian Institution, Dr. C. A. drites chemically fractionated? A trace element study. Francis of the Harvard Universitv Mineraloaical Museum. Dr. R. Hutchison of the British Museum (Natural History);

Geochim. Cosmochim. Acta 46, 1849-l 86 1. FIREMAN E. L., DE FELICE J. and NORTON E. (1970) Ages

the Meteorite Working Group and the Curatorial Branch of the Allende meteorite. Geochim. Cosmochim. Acta 34, of the Johnson Space Center for providing samples. We also 873-881. thank Drs. A. M. Davis, E. Rambaldi and A. E. Rubin for FISHER D. E. ( 1972) Uranium content and radiogenic ages helpful comments on an earlier version of this paper and of bronzite, amphoterite and carbonaceous chondrites. Profs. R. N. Clayton and R. T. Dodd for permission to use Geochim. Cosmochim. Acta 36, 15-33. unpublished data. We are grateful to the staff of the Uni- GRAHAM A. L., EASTON A. J. and HUTCHISON R. (1977) versity of Missouri Research Reactor for their aid and ir- Forsterite chondrites: the meteorites Kakangari, Mount radiation support, derived partly through DOE grant DEFG Morris (Wisconsin), Pontlyfni, and Winona. Min. Mag. 0280 ER 10725, and to NASA and the NSF for supporting 41.201-210. this research under grants NGL 15-005-140 and DPP- HUSTON T. J. and LIPSCHUTZ M. E. ( 1983) Chemical stud- 8 1115 13, respectively. ies of L chondrites--III. Mobile trace elements and @Ar/

39Ar ages. Geochim. Cosmochim. Acta, in preparation.

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