An ion microprobe study of corundum in the Murchison meteorite: Implications for 26A1 and 16O in the early solar system

0016.7037/91/$3.00 + .I0

(ieochrmrcrr PIC<,smochimlca AcluVol. 55.~.2045-2062 copyright 0 1991Pergamon Press plc.Printed in U.S.A.

An ion microprobe study of corundum in the Murchison meteorite: Implications for 26A1and 160 in the early solar system ALOIS VIRAG,“* ERNST ZINNER,' SACHIKOAMARI,'.~

and EDWARDANDERS~

’ McDonnell Center for the Space Sciences and the Physics Department, Washington University, St. Louis, MO 63 130-4899, USA * Enrico Fermi Institute and Department of Chemistry, University of Chicago, Chicago, IL 60637-1433, USA (Received December 26, 1990; accepted in revised form April 25, 199 1)

Abstract-Twenty-six A1203 grains from the Murchison CM2 chondrite have been analyzed by ion microprobe mass spectrometry for the isotopes of 0, Mg, and Ti and the abundances of Mg, Ca, SC, Ti, V, Sr, Y, Zr, La, and Ce. Being the most refractory major phase in solar matter, A1203 retains a particularly durable record of the early solar system. 26Mg/24Mg ranges up to 56X the solar-system ratio, owing to decay of extinct 26A1,but the initial 26A1/27A1ratios do not exceed the canonical maximum of 5 X lo-’ established in earlier work. There is no evidence for fossil radiogenic 26Mg surviving from presolar times. Oxygen isotope compositions cluster mainly near b180 = -50%0 (lighter than bulk spinel), but range from -94 to - 11%o.The grains divide into three groups on the basis of 26A1,160, Ti, and V content, and 26A1and 0 show distinctive correlations (in contrast to all previous studies), suggesting an origin from the following components. Group 1 (high 26A1,Ti, V): mixture of material with 26A1/27A1= 5 X lo-’ and 6’*0 = -45%0 with dead Al of 6180 = - 100%~ Group 2 (low 26A1,Ti, V): mixture of material with 26A1/ *‘Al = 5 X 1O-6 with dead Al, with complex fractionation and exchange of 0 resembling that of FUN inclusions. Group 3 (no 26A1;high Ti, V): dead Al from various sources. In terms of this model, the corundum formed from two components with live 26Al and a mass fraction of 43% dead Al, but we do not know whether this figure is typical of carbonaceous chondrites in toto, let alone the entire solar nebula. Trace element abundances in corundum are generally at less than Cl levels relative to Al, and decline with increasing volatility, from Zr to Ca. 1.

INTRODUCTION

2. SAMPLE PREPARATION AND ANALYSIS TECHNIQUES

(A120x) IS THE FIRST refractory phase predicted to condense from a gas of solar composition, followed by hibonite (CaAlI,019) (GROSSMAN, 1972; KORNACKI and FEGLEY, 1984; WOOD and HASHIMOTO,1988). Whereas hibonite is commonly found in primitive meteorites (e.g., MACPHERSONet al., 1988) and is an important carrier of large isotopic anomalies (e.g., FAHEY et al., 1987a; HINTON et al., 1987; IRELAND,1988,1990), corundum is exceedingly rare (BAR-MATTHEWSet al., 1982; MACPHERSONet al., 1984; KUEHNER and GROSSMAN, 1987; EL GORESY et al., 1989) and only few isotopic measurements, for Mg and Ti, have been made to date on corundum samples (BAR-MATTHEWS et al., 1982; HINTON et al., 1987; FAHEY, 1988). The presence of corundum grains in a chemical separate from the Murchison carbonaceous chondrite, produced with the prime purpose to isolate large Sic grains (AMARI and LEWIS, 1989), presented a unique opportunity to study this rare meteoritic mineral in the ion microprobe. Although the petrographic context of corundum is lost by the chemical processing, this lack of information is more than balanced by the large number of grains that could be studied. Here we report measurements of oxygen, magnesium, and titanium isotopes as well as trace elements in 26 individual corundum grains. CORUNDUM

The separation procedure was a variant of earlier Chicago procedures (TANG and ANDERS, 1988;ZINNER et al., 1989), modified to permit recovery of Ca, the graphitic carrier of Ne-E(L). It will be described in detail in a forthcoming paper (AMARI et al., in prep.). An 88 g sample of fusion-crust-bearing chips from Murchison was dissolved by several alternating treatments with HF-HCI and HCl. After sulfur removal by CS2 the residue was treated with Na2Cr207 to destroy the most reactive part of the kerogen. After another cycle of HSB03, HF, and HSBO,, the remaining residue underwent colloidal separation for removal of microdiamonds, another sulfur extraction with methanol, propanol, and toluene, and finally density separation. The greater than 2.5 g/cm3 density fraction was treated with NaOCl6M NaOH (SO”C, 19 h) and HClO, (203”C, 2 h) to remove organic and graphitic carbon. After another treatment with 10M HF-1M HCI (6O”C, 10 h) it was size separated by sedimentation in NHJ H20. Grains of nominal sizes 2-10 pm and greater than IO pm were treated with H,SO, (185”C, 34 h) to dissolve spinel, followed by HC104 (+H,SO,) to oxidize black carbon released from the spine1 and again H$04. The resulting separates LS (2-10 pm) and LU (Z 10 pm) contain silicon carbide but also zircon, hibonite, and corundum (A&OJSiC = 0.5). Single grains were mounted with the help of a micromanipulator onto gold foil along with appropriate standards and pressed into the foil with a clean quartz disk. The terrestrial standards used for the analysis of corundum grains were Burma spine1 (USNM #135273) and Madagascar hibonite (CURIEN et al., 1956). Grains were documented in the SEM and identified as corundum by EDX analysis. Corundum grains ranged in size from 3 to 15 pm. Most of them broke into pieces during pressing into the gold foil but show smooth crystal faces and sharply fractured outlines of the original pieces, indicating that they were themselves fragments of larger crystals (Fig. 1). Oxygen isotopic measurements were also made on nine selected Murchison spine1 grains from the spinel-SiC residue CFOc (YANG

* Present address.. Institut ftir analytische Chemie, Technische Universittit Wien, A-1060 Wien, Austria.

2045

2046

A. Virag et al. mass fractionation of corundum and the spine1 standard we used. However, MCKEEGAN(1987) showed that no such effect exists between two types of spine1 (New York and Burma spinel) and between spine1 and hibonite. and WEIN~RUCHet al. (1989) found no ~~ifi~nt difference in the instrumental mass fractionation between terrestrial spine1 and olivine standards of known isotopic compositions. We are thus confident that the use of Burma spine1 is justified. This expectation has indirectly been confirmed by the fact that the corundum grains ofgroup 1 and Murchison spine1 grains occupy the same region in an oxygen 3-isotope plot (Fig. 2a and b). Fu~e~o~, a systematic difference in the instrumental mass fractionation between corundum and spine1 would merely shiff all data points in Fig. 2a along a slope l/2 mass fractionation line, but would not change any of our conclusions. As for the second question, ion probe analyses on different grains pressed into gold foil reveal variations in the inst~mental mass fractionation that are much greater than the individuat measurement errors. On a 3-isotope plot, data points lie along the mass fractionation line (see, for Si, Fig. 2 of ZINNERet al., 1989). Since variations are much smaller for polished samples, under careful conditions ag proaching the measurement errors, the reason for the larger variations on grains ap~ntly is the variation in the angular dist~bution of secondary ions emitted from irregular samples with different morphologies. This represents a limit to the accuracy of isotopic measurements on isolated grains. Rather than showing the range of the instrumental mass fractionation separately (Fig. 8 of ZINNERet al., 1989), we have combined, in the graphical representation of the Oisotopic data, the indi~du~ me~urement errors with the range in instrumental mass fractionation as expressed by the spread of the data points along the mass fractionation line in a 3-isotope plot. The resulting error bars are still normal to one another, representing the major and minor axes of the final Gaussian error ellipses, but have different orientations for different data points. Magnesium isotopes

FIG. I. Scanning electron micrographs of Murchison corundum grain 20 (graph a) and 25 (graph b) pressed into gold foil for ion microprobe analysis. Several grains broke into pieces during mounting (a), but the sharp angular features of most grains (b) indicate that they themselves were fragments of larger crystals.

and EPSTEIN, 1984). These grains were mounted and analyzed in the same way as the co~ndum grains. They were more carefully characterized by SEM-EDX than the CFOc oxide grains whose O-isotopic compositions had been measured by MCKEEGAN( 1987) and ZINNER and EPSTEIN(1987), in an effort to ensure that only Mg-spinels of relatively high purity were analyzed in the present work. The isotopic and trace elemental measurements were made with the W~in~on University ion micropro~, a modified CAMECA IMS 3F. The techniques for the analysis of 0, Mg, and Ti isotopes have been described previously (FAHEYet al., 1987a,b; MCKEEGAN, 1987) and only modifications of the established methods are given here.

The three Mg isotopes were measured along with Al. Since Mg concentrations are extremely low in most corundum grains, the peak centem of the Mg” signals and Al’ were determined on the Burma spine1standards, but only the Al+ peak center was dete~in~ on the corundum grains and used for correcting shifts of the Mg+ peaks. Al/2”Mg ratios were calculated from the Alf/24Mgi ratios and the relative sensitivity factor of 1.17 obtained from the Burma spine1 standard.

Trace element measurements were made under energy filtering conditions (ZINNER and CROZAZ, 1986), with Al as the reference element. Sensitivity factors relative to Al were obtained from those for hibonite relative to Ca (FAHEY et al., 1987a; IRELAND et al., 1988) and the sensitivity factor of Al relative to Ca measured in the Madagascar hibonite standard. 3. ISOTOPIC AND CHEMICAL COMPOSITIONS MURCIIISON CORUNDUM GRAINS

OF

The isotopic measurements are given in Tables l-3 and Figs. 2 and 3, and the trace element measurements in Table 4 and Figs. S-7. 3.1. Oxygen Isotopes

For ion probe isotopic analysis of elements with only two isotopes, or elements with more than two isotopes where no internal mass fractionation correction is made, the instrumental mass fractionation must be determined by measurements on standards (e.g., ZINNER, 1989). Two questions have to be addressed: (1) is the standard ap propriate and (2) how much does the instrumental mass fractionation vary during different measurements on standard samples? There could, in principle, be a systematic difference in the instrumental

The oxygen isotopic compositions of the corundum grains are given in Table 1 and plotted in Fig. 2a. In two cases (1213 and 15-16) the distances between grains were so small that they could not be safely separated in the O-isotopic measurement and two grains were measured together in each case. Each pair possibiy consists of fragments of the same parents. For the second pair (15 and 16) this is largely con-

2047

Isotopic and geochemical study of corundum in Murchison TABLE 1 Oxygen isotopic compositions of Murchison LU & LS corundum grains Grain 1

2 3 4 5 6 7 8 9 10 11 12&13 14 15&16 17 18 19 20 21 22 23 24 25

6”0sM0w

c*2 0)

-24.1 f 3.7 42.0 f 5.2 -25.8 * 1.8 -48.3 f 5.5 -55.9 f 4.7 -73.1 IL5.5 -39.0 zt 5.8 -36.6 f 8.1 -48.3 f 6.2 -10.8 + 5.3 -52.1+ 5.5 43.1 f 4.8 48.9 f 4.5 -35.3 + 5.2 47.4 f 7.1 -44.2 * 7.5 48.4 f 8.9 -50.1 f 8.1 49.9 + 5.4 -44.6 f 5.5 -56.5 f 8.5 -53.1 k 6.2 -53.6 k 3.1

PO

SMOW

(*20) -26.5 + 4.2 -44.2 f 4.4 -14.4 f 3.0 43.8 f 6.2 -51.8f3.1 -94.1 + 2.7 -37.0 f 3.8 -18.4 k 5.0 47.6 f 5.2 -10.6 k 6.0 -58.0 zt 3.0 -44.9 f 3.7 -46.7 f 3.5 -16.4 f 3.8 -57.8 f 4.1 -47.2 f 4.5 -53.3 f 3.5 49.6 f 4.8 -59.2 f 3.0 49.8 f 4.4 -56.5 f 2.8 -68.3 + 4.2 -57.9 f 4.2

has the most normal 0 (Fig. 2a), is distinguished by having the highest Ti and Zr concentrations. As can be seen from Fig. 2a, several data points fall off the ‘60-rich mixing line. The points to the right are reminiscent of FUN inclusions. Recently, DAVIS et al. ( 1990) have shown

10

-10

-30 -50 -70 -90 -90

The errors are 2oman of individual measurements and do not include the variation of the instrumentalmass fractionation.

-70

-50

-30

-10

10

s l *OS,,, (old 10

&tent with the Mg-Al and trace element data. Grains 12 and 13 have the same 26A1/27A1ratios and similar SC, Ti, V, Y, and Zr, but differ in their Mg, Ca, and Sr. However, because of possible contamination contributing to the latter elemental signals, we deem it likely that these two grains also are fragments of the same parent. The O-isotopic compositions (Fig. 2a) exhibit large but variable I60 enrichments and significant deviations from the i60-rich mixing line. Most of the data points cluster in the same region of the 3-isotope plot as the Murchison spine1 grains (Fig. 2b). The latter are (with one exception) more 160-rich than the spine1 fraction from Murchison measured by CLAYTON and MAYEDA (1984). There has been additional evidence from the 0 data on Murchison hibonites (FAHEY et al., 1987b; IRELAND and ZINNER, 1989) and Vigarano spinels (ZINNER et al., 1991a) that the bulk Murchison spine1 separate does not represent the ‘60-rich endmember (e.g., THIEMENS, 1988). Apparently it is a mixture between a more ‘60-rich component represented by corundum, hibonite, and spine1 from refractory inclusions (IRELAND and ZINNER, 1989) and an isotopically normal component represented by the larger spine1 crystals analyzed by GROSSMAN et al. (1988). It should be pointed out that these larger spinels have high concentrations of Cr (several % of Cr,O,) and Fe, and indeed, the exceptional CFOc grain in Fig. 2b has higher Cr and Fe than the others. Because of a Cr background originating from the gold foil we could not measure Cr reliably and for that reason do not list Cr in Table 4. However, we can set upper limits of -50 ppm on the Cr concentrations in all grains. Grain 10, which

1”

b Murchison CFOc

-10 -30 -50

om Murchison ayton and Mayeda, 1984)

-70 -90 -90

-70

-50

-30

-10

10

6 l *OS,,, (old FIG. 2. The O-isotopic compositions of Murchison corundum grains (a) differ significantly from Murchison spine1 grains (b). Corundum grains which lie outside the main cluster at 6’*0 = -SO%0 are labeled by their number and (in parentheses) the (26A1/27Al)0 ratio in units of IO-‘. The groups in (a) refer to the (26A1/27Al)o vs. Ti (V) relationships displayed in Fig. 9a and b. Besides the terrestrial fractionation line and ‘60-rich mixing line, plot (a) also shows a fractionation line through an oxygen composition enriched in I60 by approximately 55L. The inset is an enlarged plot of the group 1 data points, which also shows the endmembers (solid circles) and correlation line derived from the 26A1/27A1 vs. 8”,“0 relationship of Fig. 10. Errors are 1q,,_“, and include both the individual measurement errors and the range of instrumental mass fractionation measured on the standard.

A. Virag et al.

2048

that evaporation from a melt of forsterite composition produces heavy isotope enrichments of 0, Mg, and Si in the residue, consistent with earlier suggestions (e.g., CLAYTONet al., 1985) that refractory inclusions with positive isotopic mass fractionation effects, in particular FUN inclusions, are the result of distillation. IRELAND et al. ( 199 1) have argued that four hibonite inclusions, including HAL, with fractionated 0, Ca, and Ti experienced distillation during their formation. One common feature of these four hibonite samples is a large depletion in Ce, probably implying an oxidizing environment. Unfortunately, La and Ce are below detection in grains 3, 15, and 16, but in grain 8 the CI-normalized Cc/La < 0.15, similar to the depletions shown by the HAL type hibonites. In addition, the Ti is much below average in these grains, again in agreement with the hibonites, and too low to allow isotopic measurement. The Mg is also low and shows no fractionation effects in the 2sMg/24Mg ratios within the precision of our measurements. However, we argue below that most of the normal Mg probably originates from contamination and does not represent original Mg in the corundum grains. Several corundum points fall to the left of the 160-rich mixing line, especially grain 6. Such compositions have not been seen before in refractory inclusions. ZINNER and TANG ( 1988) reported that fine-grained ( 1000-2000 A) spine1 from Murray has 6’*0 values in the range from -50 to -307~ but a”0 values above the terrestrial mass fractionation line, and thus could not have been derived by mass fractionation from any known O-isotopic composition. It is conceivable that the 0 in corundum grain 6 and a few others on the left of the ‘60-rich mixing line originated by fractionation of 160-rich compositions. If this were the case, it would be no coincidence that grains 6, 8, and 15 and 16 lie on a fractionation line (see Fig. 2a) going through the most extreme composition on the

mixing line (a”0 = -54%0; 6”O = -53%). Condensation from the first fraction of evaporated material would yield an isotopically light composition. Fine-grained CAIs from Allende usually have isotopically light Mg (NIEDERERand PAPANASTASSIOU,1984), but corresponding isotopic fractionation effects favoring the light isotopes have not yet been observed for oxygen. If the O-isotopic composition of grain 6 is derived from an original composition on the ‘60-rich mixing line, the fractionation in ‘8O/‘6O is approximately -4OL. This is more than the theoretical value of -30% for kinetic ‘8O/‘6O fractionation if O2 is the evaporated species. DAVIS et al. (1990) obtained a value of -23% for the “O/ I60 fractionation during evaporation of forsterite. Furthermore, only the very first puff of evaporated material will be that much fractionated relative to the original composition; any subsequent material will be isotopically heavier. It would require more than one evaporation step to produce the Ocomposition of grain 6 by fractionation (in contrast, the composition of the residue can become arbitrarily heavy in just one evaporation step). Finally, it will be shown in Section 5.2 that there exists a correlation between the 6’*0 and the (26A1/27Al)o ratio for corundum grains with (26Al/27Al)o> 10e5 (group 1 in Fig. 2a) with grain 6 as a potential extreme endmember. Thus it is unlikely that grain 6 owes its 0 composition to kinetic fractionation. 3.2. Magnesium Isotopes The Mg-Al measurements are listed in Table 2. The Mg concentrations in most grains are extremely low ( 1- 10 ppm) and vary greatly from run to run on a given grain. Probably most of the (isotopically normal) magnesium originates not from corundum but from contamination, either from Mgrich material adhering to the grains or from the Au-foil. An-

TABLE 2 Mg and Al in Murchiion LS and LU corundum grains Grain

hf#‘%Xo,

(*2(J) 1 2

0.1679 f O.OO40

S2kg

(* 2 0)

(*2@

(26Al/nAl)* (f20) o (4.67 f 0.64) x 1O-s

2’All”M.g

205 f 29

585 f 36

0.3810 f 0.0738

1735 f 530

5979 f 332

4.273 f 0.850

29670 f 6100

8OOOOf12180

2.557 f 0.571

17350 f 4100

49240 f 5240

2.755 f 0.599

18774 f 4300

49970 f 5890

6.680 f 2.146

46947 f 15400

13O4OO f 17380 14o14of5220

6.672 f 0.571

46890 f 4100

5.772 f 0.594

40430 f 4260

6.278 f 1.382

44060 f 9920

112O9Of 12840

(4.81 f0.17) x Ws

1191oof6o20

5.663 f 0.580

39650f416O

110580 f 12720

6.177f0.217

43340 f 1560

1274OOf374O

7.858 f 0.864

55400 f 6200

147160f 6010

3

0.6797 f 0.0279

3879 f 200

100240 f 8260

(5.25 f 0.51) x IO-6

5

3.568 f 0.724 5.265 f 0.515

24610 f 5200 3679Of37OO

168650 f 27600 280000 f 12900

(1.81 f0.16) x 1O-5

5.567 f 0.580

38960f416O

303600 f 19300

4.917 f 0.451

34296 f 3240

2614OOf 16120

5.272 f 0.283

36840 f 2030

6

0.1428 f 0.0109

25f78

286100 f 8300 103700f 13160

<2.4 x lo-’

2049

Isotopic and geochemical study of corundum in Murchison Table 2. (Continued) 7

0.1421 f 0.0043

20f31

5483 f 438

0.1420 f 0.0020

19f 14

7385 f 242

a

0.5523 f 0.0794

2964 f 570

10219Of39120

9

0.3555 f 0.0279

1552 f 200

4134 f a22

10

0.1382 f 0.0035 0.1396 f 0.0035

12

13

14

15

16

716f9 766f

10

1 f 15

846f

14

0.6088 f 0.3102

3370 f 2226

1.270 f 0.325

al 16 f 2333

38340 f 10430

1.213 f 0.540

7708 f 3876

38650 f 17350

2ao6f317

9111 f 1268

0.6698 f 0.0547

3808 f 393

14970 f 980

0.75 16 f 0.0624

4395 f 448

17820 f 1230

1.636 f 0.356

10740 f 2560

30420 f 5260

1.441 f 0.404

9346 f 2903

271OOf969O

1.960 f 0.239

13070 f 1720

1.382 f 0.445

8916f

1.756 f 0.570

11610f4O90

647300 f 2ooooo

1.173 f 0.368

7421f2641

419900 f 158900

3O4f 241

0.2898 f 0.0666

lo80 f 478

45750f14320

0.4993 f 0.1216

2584 f a73

121200 f 26800

0.1918

i.i20*0.2la

20

85af5la

9441 f 3606

39930f11160

2314 f a82

88080 f 27640

2801 f 1376

152400 f 42600

7042 f 1563

452OOf 11180

5305 f 2116

50460 f 12420 73470 f 16720

1.220 f 0.315

7757 * 2263

3.526 f 0.475

24310 f 3410

70450 f 9530

6.020 f 1.225

42210 f a790

l44600 f 28440

5.477 f 1.363

38310 f 9780

156800 f 38200

3.924 f 0.813

2716of584o

102600 f 2o6oo

4.707f

32790 f a450

154700 f 36900

1.177

0.5289 f 0.0325

2796 f 234

0.6492 f 0.1365

3659 f 980

(3.02 f 0.95) x W5

(3.63 f 0.42) x 1O-5

(4.65 f 0.78)

X

lo@

40660 f 7330 469800 f 129300

0.1816* 0.0336

0.8784 f 0.2948

19

1196Of5270

0.5302 f 0.0441

0.2589 f 0.0721

(5.1 f 1.2) x lo-5 <1.2x lo-6

-1 f 13

3194

(3.93 f 1.68) x lo-6

685 f. 7

-6f24

0.5296f

ia

2 f 25

0.1392 f 0.0018

0.4617 f 0.1228 17

652f8

0.1385 f 0.0034 0.1394*0.0021 11

-a*25

(3.7 f 2.5) x lo-’

9132fa3i

(2.54 f 0.70) x lOA

(3.17 f 0.97) x lo-6

(3.06 f 0.98)

x 10-6

(1.73 f 0.74) x lo-5

(4.34 f 0.80)

x 10-5

(3.36 f 0.76) x W5

(4.05 f 0.39) x 10-S

12900 f 3070

1.990 f 0.248

13282 f 1781

1.373 f 0.248

a852 f 1778

39060 f 8210

3.581 f 0.755

247 10 f 5420

l5oaoo f 304oo

2.170 f 0.707

14580 f 5080

125ooO f 3aOO0

22

2.587 f 0.583 0.1371 f 0.0337

17570 f 4180 -16 f 241

93laof 19830 5516Of9520

23

2.227 f 0.411

14980 f 2950

79660 f 16970

2.374 f 0.633

16040 f 4544

96290 f 24180

3.837 f 1.334

26540 f 9604

196480 f 62400

i .020 f 0.098

6322 f 702 4168 f 1300

49800 f 3424 41620 f a200

(1.72 f 0.27) x UT5 (3.42 f 0.48) x 1O-5

21

24

0.7201 f 0.1812 25

26

5ooaof5130

1.351 f 0.227

a694 f 1627

34890 rt 4920

2.032 f 0.396

13580 f 2840

53670 f 9490

l.a16f0.296

12030f2120

50660 f 7680

0.1579 f o.o66o

133 f43

0.1573 f 0.0833

129f6O

423f6

(2.52 f 0.59) x NT5

c6.6 x lo-’ (2.38 f 0.49)

(4.14f

x 10-5

1.11) x lo-5

477 f 30

*Calculated from a least UB~Sfit through the data point(s) for a given grain and normal 2%fg/24Mg (=O.13932) at“4 ‘AYHMg = 0 .

A. Virag et al.

2050

lo a

7oooo Corundum Grain #2

6oaMl

8-

Al l”Mg 7

’

6

b t

Corundum Grain #5

4oooo

a

M5

E

x

3oooo

4

E

M3 El Fi 2

1

0’0

0 lOOOo0

2oooo0

3ooooO

Al l”Mg FIG. 3. Al-Mgdiagrams for two corundum grains on which multiple measurements were made. The extremely low Mg concentrations in these grains resulted in large 26Mg excesses, with 26Mg/24Mg ratios reaching 56 X normal in grain 2 and 40 X normal in grain 5. The correlation lines shown are best-fit lines through the data points and normal Mg at Al/*‘Mg = 0. They are most likely not internal isochrons but mixing lines between radiogenic 26Mgin the corundum and normal Mg from contamination. Errors are 1CT.

measurement was made) lines through the data points and normal Mg (26Mg/24Mg = 0.13932) at 27A1/24Mg= 0. They vary from an upper limit of 2.4 X 10e7 in grain 6 to the canonical value of 5 X 10m5found in Type B Ca-Al-rich inclusions from carbonaceous chondrites (e.g., HUTCHEON, 1982). Their distribution is shown in the histogram of Fig. 4. In their wide range of 26A1/27A1values the Murchison corundum grains are very similar to hibonites from carbonaceous chondrites (IRELANDet al., 1986; FAHEYet al., 1987a; IRELAND,1988). The only two Mg-isotopic analyses reported so far on corundum grains already spanned the whole range observed here: BAR-MATTHEWSet al. (1982) found no 26Mg excesses in corundum of the Murchison hibonite-corundum inclusion BE5, setting an upper limit of -3 X lo-’ for (26Al/ 27Al)o, while FAHEY (1988) obtained a value of (26A1/27Al),, = 4.1 X lo-’ in the hibonite-corundum inclusion F5 from the Murray CM2 chondrite. 3.3. Titanium Isotopes Titanium concentrations vary greatly in individual corundum grains but are generally very low. Moreover, as the samples were small and were partly consumed in isotopic or elemental analyses of 0, Mg, and trace elements, Ti isotopes could be measured in only five grains, and with low precision (Table 3). There are no clearly resolved anomalies and only a hint of 5@Tiexcess in grain 10. Even if this excess were better resolved, in its absolute size it falls far short of the largest “Ti anomalies observed in hibonites (FAHEY et al., 1987a; IRELAND, 1990) and in hibonite and corundum of Murchison inclusion BB-5 (HINTON et al., 1987).

Murchison

Corundum

8

other possibility is that Mg moved from Mg-rich gas in the nebula into the surface of corundum grains or, less likely, from surrounding Mg-rich silicates in the meteorite matrix during metamorphism on the meteorite parent body. A clear sign of surface contamination is the increase in Al/24Mg ratios from run to run (Table 2), as small contaminants are sputtered away. These ratios can be as high as 6.5 X 10’. Most grains contain (apparently radiogenic) excesses of 26Mg,and, because of the low concentrations of normal Mg, this radiogenic 26Mg dominates in many grains (column 3 in Table 2). 26Mg/24Mg ratios range up to 7.9, corresponding to the highest relative 26Mg excess (5.5 X 104%) ever observed. Expressed in different words, 98% of the 26Mg in this sample is radiogenic. The ratio 26Mg(excess)/Al is constant in a given grain, demonstrated by the fact that when several measurements were made the 26Mg/24Mg is correlated with the 2’A1/24Mg.Such correlation diagrams are shown for two grains in Fig. 3. As mentioned above, most of the normal Mg probably is due to extraneous sources, and thus the correlation lines in these. plots are not true internal isochrons but mixing lines. The initial 26Al/27Al ratios in the last column were calculated by fitting (or connecting, for grains on which only one

Group3

lo-’

1o-6 ( %AIl‘Xl)O

FIG. 4. The distribution of initial 26Al/27Alratios in Murchison corundum grams shows a sharp cutoff at a value of 5 X IO-’ and consists of two well-separated populations of grains witb ratios > 10m5 (group 1) and ~6 X 10m6.The second population can be further separated into two groups according to the Ti and V concentrations of their members: low for group 2 and high for group 3 (see Fig. 9). Since three of the (26A1/27Al)o ratios (shaded boxes) in group 3 are only upper limits, the separation between groups 2 and 3 in the histogram might in fact be much more pronounced.

205 1

Isotopic and geochemical study of corundum in Murchison TABLE 3 Ti istopic compositions of Murchiin

Grain

F %OlUllU

corundum grains

S41Ti (f20)

S4% (* 2 o)

PTi (*2o)

1

15.7 f 13.3

9.5 f 35.4

23.7 f 40.9

16.5 f 46.5

2

20.9 f 25.5

0.3 f 41.2

27.3 f 42.6

30.5 f 63.3

5

2.2 f

6.2

5.0 f 12.4

-0.7f

16.3

-2.9 f 15.9

7

2.3 f 12.1

9.6 f 18.4

-4.3 f 17.5

-5.1 f 25.7

10

-2.Of

4.2

3.4f

4. REFRACTORY

5.2

6.7f

8.2

12.9i

8.7

TRACE ELEMENTS

The trace element results are given in Table 4. Problems encountered in these measurements are the low concentrations of most trace elements, the small sample size, and the presence of trace contaminants (e.g., Ca, Cr, Ba) in the gold substrate. This limited the number of elements we were able to measure and allowed us to set only upper limits in some cases. There are several factors that generally determine the distribution of trace elements in the corundum grains.

1) The temperature of formation affects the various elements according to their volatility relative to Al during either distillation (preferential loss of volatile elements) or condensation (preferential uptake of refractory elements). 2) The oxygen fugacity will determine the oxidation state of elements, and this in turn will al&t their mineral-chemical behavior. For example, Ce depletions in CAIs have been interpreted as being due to the high volatility of CeOz relative to Cez03 under oxidizing conditions (BOYNTON, 1978; DAVIS et al., 1982). Another example is Ti and Zr in corundum which are probably present as Ti3+ and Z? since in practically all grains Mg and Fe that could compensate the excess charge of Ti4+ and Zr4’ are much lower than these two elements. 3) The partitioning of trace elements between gas and corundum if the corundum formed by condensation and between liquid and corundum if it formed by crystallization from a melt. In either case the concentrations in the corundum will be affected by the composition of gas/ melt. In contrast to hibonite, the corundum crystal lattice does not permit the easy substitution of trace elements; especially the rare earth elements, whose distribution in hibonites yields important information on high temperature processes in the solar nebula (IRELANDet al., 1988; HINTON et al., 1988), are not taken up in significant amounts. We measured only La and Ce but in many cases even these were below detection limits. 4) Finally, there exists the possibility that refractory trace elements are carried by minor or trace phases associated with corundum. The small size of the grains and the multitude of measurements we performed did not allow any detailed petrographic studies including the search for trace phases. Because of the limited number of elements for which we could obtain reliable data, the lack of data on activity coefficients in corundum, and the lack of knowledge of the pet-

rographic context of the analyzed corundum grains, we cannot hope to disentangle the information contained in the trace element abundances. Still, there are certain observations to be made. Figure 5 shows the CI-normalized (ANDERS and GREVESSE, 1989) abundances of trace elements in all measured grains relative to that of Al, except for grains 7 and 9. The vertical bars indicate ranges of abundances. The elements are ordered according to increasing relative abundances. This order also corresponds to increasing refractoriness of the elements (DAVIS and GROSSMAN, 1979; KORNACKI and FEGLEY, 1986) except for Ca. In fact, since many values for Ca are only upper limits because of Ca contamination of the gold foil, the range of Ca abundances could extend to much lower values than is shown in the figure. On this plot Al would be comparable to SC as at 10m3atm A1203 condenses at about the same temperature as Sc203. The first two conclusions to be made from this overall plot are that (I), with the exception of Zr in a few grains and Y in one, all elements are substantially depleted relative to Al and (2) relative refractoriness is the most important factor governing the distribution of these selected trace elements in corundum. The obvious exception is Ca. One possible explanation for the low Ca concentrations is that this element partitioned into co-condensing hibonite. However, it seems more likely that Ca is systematically excluded from the corundum structure. 4.1. Hihonite-Bearing

Grains

There is a large variety of patterns if we consider individual grains (Fig. 6). The two grains (7 and 9) that were not included in the composite plot of Fig. 5 are characterized by higher

1o-6.

, . , . , . , . , . , . , . , Ca

V

Ce La

Ti

Y

SC Zr

FIG.5. Overallrangesof abundances of trace elements in Murchison corundum grains. The CI-normalized abundances are plotted as fractions of the CLnormalized Al abundance. With few exceptions for Zr and one for Y, all elements are increasingly depleted (i.e., plot below the broken lines) with increasing relative volatility (from right to left).

A. Virag et al.

2052

TABLE 4a Trace element cancentrationa in Murcidi 3

4

13 i3

3.0 ti.7

7.1 k0.7

Ca

59 199 f5 fll

<21

42 f2

cl4

sc

28 *3

24 *3

25 f1

80 k2

195 *5

Ti

575 518 f30 f35

22 f3

7.5 f1.2

1812 +35

V

6.0 7.1 0.33 0.27 ti.7 f1.0 zWJ9 iO.05

Sr

0.95 3.7 0.30 0.49 0.37 0.90 8.0 3.1 0.38 Kf.42 +I.0 40.11 %I.10 MO.18 3.54 No.5 M.4 So.07

Y

2:;

Grain Element

1

2

21 54

2:;

4:;

5

11 f2

7a

9.1 f3.0

141 M

7b

8

9

11

12

3.5 13424 kO.7 f152

681 +17

5.0 zk2.5

652 %9

50 5916 1183 f6 KXI f6

148 1913 zt4 M8


~6.2

cl1

172 *3

121 k.2

137 617 412 ct19 f13 +17

114 f3

23 k5

I 11 rtl

66 M

33 -?rl

2.2 1333 10155 k86 zkl.0 +78

219 +22

1196 fll

7.3 il.1

9.4 kO.4

4.5 1.3 7.3 6.5 0.20 zkO.4 rto.4 Ho.3 Ho.8 kO.08

147 if2

10

78 i2

201 110

&O:Oi 2:;

35 lt8

6

LS and LU eonmdum grtdtts

87 +5

9.0 M.7

61 0.31 f6 k0.19
5.4 H.4


7.4 fl 51 f1.3

1;


ztO.4 1.5 k0.15 0.05 kO.06 0.13

113 82 a12 +12

1374 554

56 k36

602 M6

zr

193 282 k37 f58

86 f16

26 rtl8

La

0.8 0.03 CO.4 fo.6 co.11 fo*o2

11 fll

10 M

57 7.8 0.40 1.6 <0 06
I

Ce

All uncertaintiesare 2crcounting statisticalerrors.

TABLE 4b Trace element concentrations in Murchhn Grain EIernent

I3

14

15

16

17

Mg

37 f3

2.2 k0.5

2.5 ti.5

8.6 fl.1

6.7 k2.0

Ca

~2.3

~8.2

~2.3

<2.6

cl3

SC

32 i2

44 fl

139 k3

122 +3

35 +3

18

LS and LU corundum graina 19

19 f5

6.6 k1.8

6.6 f1.9

;

<4.5

22 %2

<14

28 rt5

113 *6

<15

2

9.5 21.3

36 rt2

61 *3

18 13

43 f3

4.0 fl.O

1446 f25

977 S!8

609 +38

280 116

435 f21

5.4 3.9 k0.4 So.5

52 rt3

34 f5

778 &23

V

8.7 0.07 kO.6 M.03

0.02 0.09 HI.02 M.04

0.61 0.23

2.7 iO.4

3.5 rtO.4

zko.09M.12 kO.31

0.53

1.1 kO.3
0.14 0.18 4.9 rto.06 kOo.08 M.8

0.93 0.21 Ht.29 i0.14

Y zr La Ce

0.17 0.19 fro.11 MO6 9.4 f5.1

10 f3

8.8 22.8

0.36

8.7 f3.3

J+;; _ .


z;

a.03

a.06

25 f12 4.10


L All uncertaintie.s are 20 counting statisticalexrotx

25

5.4 k1.5

317 +19

0.29

24

20 ti

0.81 0.60 kO.6.5 M.40

Co.06

23

4.0 lt1.2

1011 0.19 124 M.16

:$ - .

22

36 M

Ti

sr

2O

235 *29

15 *7

<0.05


0.73 k0.22

c0.5

z:::

2.0 0.16 rto.3 0.13

-&4

0.29 0.30 ~3.19 ti.19

137 f17

20 fl5

2:;;

<0.33 CO.10 g:

co.2

5.0 0.50 kO.6 So.19

12 k7

9.6 M.4


2.3 f3.4

co.1 CO.2 co.2

CO.1

-%I.2 (0.4

4.4

co.4

2053

Isotopic and geochemical study of corundum in Murchison

Ca V Ce La Ti Y SC Zr Ca V Ce La Ti

l”+-----Ta

Y SC Zr 10’

b loo 10-l 10” 10”

-

Grain 78 (3)

---e

Grain 7b (3)

*

Grain9

lo4

(1)

1o-5 GrainB(2)

-I-

d

10’ loo 10-l 10-l 10” 10’

% -

Grain 25 (1)

1o’5 1o.6

10”; Ca V Ce La Ti Y SC Zr Ca V Ce La Ti

Y SC Zr

FIG. 6. Trace element abundance patterns of individual corundum grains. The doubly normalized (to Cl chondrites and Al) data are grouped into families of grains with similar patterns. The patterns in (a) are apparently dominated by hibonite inclusions. Numbers in parentheses after the grain number indicate membership in the (26A1/27Al)0 vs. Ti(V) groups of Fig. 9.

Ca concentrations that suggest the presence of hibonite. That Ca is contained in small subgrains was revealed by ion images of Ca and Al. Another phase, possibly spinel, must also have been included during the trace element analysis of grain 9 (Mg is 1.3% as compared to 165 ppm during the Mg isotopic measurement-see Tables 2 and 4). In grain 7 it is very likely that hibonite intergrown with corundum contributed to the unusually high La and Ce concentrations measured during the first analysis of this grain (7a in Table 4). We therefore measured all REEs in a second analysis (7b). During the second run, Ca was lower by a factor of 5 (Fig. 6a); the abundances of Y, La, Ce, and Pr decreased by similar factors (Table 4 and Fig. 6a, except for Pr), indicating that these elements are residing mainly in hibonite. In contrast, the abundances of V, Ti, and SCdecrease by much less; these elements must be carried mainly by the corundum, as is the case in the other grains.

If we assume that all the Ca is located in hibonite, hibonite constituted 7.6% of the total amount of material analyzed during Run a and 1.5% during Run b. These fractions allow us to calculate the concentrations of the elements measured in both runs in hibonite and corundum, respectively, albeit only in an approximate way, since Y, La, Ce, and Pr decrease by somewhat larger factors than Ca, indicating that they are not uniformly distributed in the hibonite. Still, assuming uniformity for the other elements, we obtain 108 ppm SC, 361 ppm Ti, 6.3 ppm V, 1.9 ppm Sr, and 74 ppm Zr for corundum, well within the concentration ranges in the other grains. It is these values for Ti and V that are plotted, together with Ti and V of the other grains, in Fig. 9a and b. For hibonite we calculate concentrations of 657 ppm SC, 2630 ppm Ti, 10 ppm V, 63 ppm Sr, and 400 ppm Zr. Figure 7 shows these values normalized to CI abundances together with the abundances of Y, Hf, and the REEs in hibonite.

A. Virag et al.

2054

10)

Ca V Ce La Ti Y SC Zr Ca V Ce La Ti Y SC Zr

I

. - - ’ . ’ - ’ . - . - . - m - -* e f

r

-----o-

Grain 12 (1)

Ca V Ce La Ti Y SC Zr Ca V Ce La Ti Y SC Zr FIG. 6. (Continued)

The REE pattern in Fig. 7 is typical for PLAC hibonites (PLAty Crystals as defined by IRELAND, 1988) in the large Eu and Yb depletions and the overall decrease and roundoff in the HREEs from Dy to Tm (IRELANDet al., 1988). The Ce depletion (Ce/Ce* = 0.34)+ is larger than in PLACs but smaller than in the HAL-type hibonites Allende HAL (DAVIS et al., 1982), Dhajala DH-HI (HINTON et al., 1988), and Murchison 7-404 and 7-97 1 (IRELANDet al., 1988). The abundance pattern of the REEs and the other trace elements in hibonite of grain 7 is dominated by volatility, indicating either condensation or distillation at high temperatures. However, there is no evidence for the latter mechanism from the 0 isotopes, as they do not show any fractionation in this grain. In addition to volatility-controlled effects, there is ev-

’ Depletions and excesses are defined as the ratio of the measured (Ce) to the geometrically interpolated (Ce*) CI-normalized abundance.

idence for partitioning of the trace elements between different phases in grain 7. Partitioning of REEs between hibonite and perovskite has been invoked to explain the decrease of HREEs from Gd to Lu (HINTON et al., 1988), but it is likely that at least an additional phase is involved since the LREEs, especially La, are clearly higher than the HREEs. Even lower than the REEs (with the exception of Eu and Yb) are SCand Zr. One of the reasons must be that both these elements were taken up by the corundum. However, partitioning between corundum and hibonite only does not yield mass balance for SC and Zr on the one hand and La on the other, since a corundum/hibonite ratio of approximately 3 is required to achieve the same CI-normalized overall abundance for SC and Zr as for Al, while this ratio is 40 for La. This also indicates that another phase must be involved into which the SC and Zr can partition. What is unusual are the absolute concentrations of the REEs in the hibonite inclusion of grain 7. In hibonites REE concentrations normally range up to 100 X CI (HINTON et

2055

Isotopic and geochemical study of corundum in Murchison Ca V Ce La Ti Y SC Zr Ca V Ce La Ti Y SC Zr . . . . . . . . _ . . . . . . . . rl(J’ . ii

10% -Q-

Grain 22 (3) Grain 6 (3) Grain 11(l)

k

-

-

Grain 14 (2)

---_t

Grain 15 (2)

+

Grain 16 (2)

Grain 3 (2)

Ca V Ce La Ti Y SC Zr Ca V Ce La Ti Y SC Zr FIG. 6. (Continued) al., 1988; IRELANDet al., 1988), approximately the same as that for Al, and only the core hibonite in Murchison GR-1 has much higher concentrations (HINTON et al., 1988). It is interesting that GR- 1 also is a corundum-hibonite inclusion. The high REE concentrations in the hibonite core of CR-1 and Murchison grain 7 thus probably result from partitioning between corundum and hibonite. For example, if corundum condensed from a gas or crystallized from a refractory-rich aluminous melt as the first phase, the gas/melt would get progressively enriched in REEs not taken up by the corundum until hibonite starts to crystallize and takes up the REEs. If all the Ca in grain 9 (Fig. 6a) is in hibonite, then this phase must have comprised 2.5% of the analyzed material. The La and Ce concentrations in this hibonite would then be 64 ppm (270 X CI) and less than 35 ppm (56 X CI), both lower than in grain 7, but with La still substantially higher than in most hibonites. Yttrium is even lower relative to La than in grain 7. If Y is characteristic of the behavior of HREEs such as Ho and Er, this would mean an even steeper fallingoff REE pattern than that shown in Fig. 7 for grain 7.

4.2. Abundance Patterns Another grain in which we probably measured material of an associated phase is grain 8. Its pattern (Fig. 6b) is unusual because of its high Y and Zr concentrations. Although the fractions LS and LU contained a substantial number of zircons (apparently terrestrial contamination) the neighborhood of grain 8 on the Au mount was free of any other grains so that contamination by zircons seems unlikely. If the Y is an indication of the HREEs then we would expect an ultrarefractory REE pattern in this grain, similar to that found by HINTONet al. (1988) in a tiny inclusion within the corundum of Murchison BB-5 (see pattern BB-50 of Fig. 2 in their paper). These authors argued that the REEs were contained in two refractory phases, one mostly Zr oxide, the other one hibonite. Zirconium-rich grains were also observed by HINTON et al. (1988) at the interface between corundum and hibonite in Murchison GR-I. Such an explanation is not unlikely for the present case. As already pointed out, grain 8 has a FUN-type oxygen isotopic composition; i.e., it is 160-

A. Virag et al.

2056 lo4

Hibonite in grain 7

t

1

o calculated from both runs

10”

1 i,f Sr { Y’ , Hf’ , V

Ti

Ce Nd , ’ , ’

SE Zr La Pr

Eu

Tb Ho Tm Lu

, ’ , ’ , ’ ( ’ , ’ Sm Gd t?y Er Yb

FIG. 7. The REEs and Y in grain 7 reside mainly in a hibonite inclusion. Plotted are the CI-normalized abundances in the hibanite for these elements and Hf. The other displayed elements are also found in the corundum portion of the grain. Their abundances in hibonite were calculated from two measurements for which the proportions of hibonite differed. Errors are 1u.

rich and fractionated in favor of the heavy isotopes, indicating a distillation history of this grain. Distillation could have resulted in the extreme enrichment of Y and in the loss of elements such as Ti. IRELANDet al. (199 1) noticed very low Ti in Murchison 7-404, a HALtype hibonite with fractionated 0 and Ce depletion. If this process is indeed responsible for the low Ti concentration of only 2 ppm, we would expect extreme isotopic fractionation effects. Unfortunately, the low Ti concentration did not permit the measurement of Ti isotopes in this sample. Grain 10 has the second highest Zr concentration but has fairly low Y. However, it has the highest Ti and Mg concentrations (in contrast to grain 9, the Mg values obtained during the Mg isotopic and the trace element measurements on grain 10 are very similar). It also is unusual in having the lowest I60 enrichment of all the grains analyzed here. These properties suggest formation from a different reservoir than the other grains, having not only a different G-isotopic composition but possibly a different oxygen fugacity. Equilibration with normal oxygen is in principle possible but unlikely as corundum has essentially the same diffusion coefficient as M&Al-spine1 (REDDY and COOPER, 198 1, 1982; ANW and GISHI, 1983). The other grains show a variety of patterns. In Fig. 6 they are grouped according to these patterns. Unfortunately, many Ce and La data points represent only upper limits. Only in a few cases (Fig. 6e and f) do the trace elements behave more or less according to their relative volatility. In most cases we can observe pronounced depletions in Y (Fig. 6g, h, i, and j) and Ti (Fig. 6k and 1). Zirconium is at or near the CI-abundance of Al only in grains 1,2,4, 18, and 20; in all the other grains of Fig. 6e to 1 it is substantially depleted; in many grains below SC.Scandium, which is expected to behave most like Al during condensation and during igneous processes shows the smallest concentration range of 50X (Fig. 5); since many Ce and La points in Fig. 5 are only upper limits, their concentrations undoubtedly extend to much lower values.

In contrast, Zr, which is more refractory, ranges over 590X. One likely reason is that Zr can exist in different oxidation states, and thus its inanition in corundum can be affected by oxygen fugacity whereas the SC concentration is not. However, that cannot be the only explanation since Ti can have the same oxidation states as Zr, but there exists no obvious collation between the Zr and Ti abundance as CInormalized Zr/Ti ratios range from 0.6 (in grain 25) to 74,400 (in grain 8). Another reason for Zr variability has already been discussed, namely the presence of Zr-oxide. It is thus clear that different processes, affecting the refractory trace elements ditferently, must have been responsible for their distributions in individual corundum grains. This is also indicated by the presence, and lack, respectively, of correlations between the concentrations of different elements. If we exclude Ce and La, we can discern some kind of correlations only between V and Ti and between Y and Zr (Fig. 8) for the remaining six elements plotted in Fig. 6. The reason for the correlation between the second pair apparently is partitioning of Y into Zr oxide. One possible process that could be responsible for the extremely large con~n~tion range of Ti is the removal of Ti by the formation of perovskite; however, the three perovskites analyzed by IRELANDet al. (1988), although having Group II-related patterns with high Nb, Ce, and Yb, do not show very high V. There exist some hibonites with high Ti and V, such as Blue Angel (ARMSTRONGet al., 1982) and the BAG (Blue Aggregates) hibonites analyzed by IRELANDet al. (1988), but they are rare and it is doub~ul whether they could account for the lack of Ti and V in this group of corundum grains. IRELANDet al. ( 199 1) noticed the loss of Ti from hibonite by distillation and argued that after removal of Mg the Ti4’ is no longer compatible with hibonite and that the oxidizing environment does not allow the reduction to Ti3+. It is interesting that all corundum grains with positively fmctionat~ 0 (3, 8, 15, and 16 in Fig. 2a) have Ti-depleted patterns (Fig. 6b, k, and I), but some grains with essentially the same patterns (4 and 14 in Fig, 6k and 1)have unfiactioned 0. However, with the exception of grain 4, for which we do not have Mg-isotopic data, all these grains fall into a tight group on (26Al/27Al)ovs. Ti and (26A1/27Al)o vs. V plots (Fig 9a and b). These relationships will be discussed in Section 5.2.

5. DISCUSSION AND CONCLUSIONS 5.1. Solar or Presolar Origin? A fundamental question that has been debated ever since the discovery of isotopic anomalies is whether CAB with large anomalies are presolar or local. Most workers prefer the latter alternative, contending that even hibonite inclusions that show by far the largest anomalies in Ca and Ti were formed in the solar nebula (FAHEY et al., 1987a; IRELANDet al., 1988). The argument was essentially based on the observation that 0 and Mg in these objects show the same isotopic compositions as those found in “normal” (i.e., non-FUN) CaAl-rich inclusions and that trace element fractionations in hibonites, although more extreme, can, as in other CAD, be explained by high temperature condensation or evaporation

Isotopic and geochemical study of corundum in Murchison TVCI 10”

10-l

1O’2

, .“.,..,

loo

10’

. ..‘-q . . ..a.-. . ... .... . ‘j j’. ,’ n Group1 ,h , I 0 Group2 / I I 0 Group3

lo2 :

. :

-

$0’ 2 6

loo

d

k

,$ D’ ,$ % o+ :D I go” <,,“’ . , , :e+ . , q,/ a : ,A+ .I 1oe2 * ‘***-’ ’ c*‘m’a* ‘....I’ * -.‘..-I “*.---’ -

10-l r

10”

loo

10’ lo2 lo3 Ti cont. (ppm)

lo4

ZrlCI

go1

r

% , . ci loo r ,’ :, E - .

*

I

10’‘r

.I c

10’2 loo

/’

I

2057

range as that seen in all other refractory inclusions. In contrast, silicon carbide and graphite have 26Mg excesses with 26Al/ 27A1as high as 0.2 and 0.06, respectively (ZINNER et al., 199 1b). There are a variety of possible stellar production sites of 26Al(see CLAYTONand LEISING,1987) some with predicted 26A1/27A1ratios of approximately 1 (CAMERON, 1984; ARNOULD et al., 1980; N~RGAARD, 1980). The magic upper limit of 5 X 10m5encountered in all refractory inclusions and now also in the most refractory phase, corundum, which exhibits much higher Al/Mg ratios than any other phases found in primitive meteorites, must be taken as evidence for their solar system origin. It also is evidence for live 26A1in the early solar system and evidence against a fossil origin of the observed 26Mgexcesses in Ca-, Al-rich inclusions (CLAYTON, 1977b, 1986). It thus supports the conclusion that 26Al was live, which has been reached from the 26Mgf24Mgvs. “Al/ 24Mg relationship in inclusions of igneous origin (LEE et al., 1977; WASSERBURGand PAPANASTASSIOU,1982; PODOSEK et al., 1991). A similar argument can be made concerning the O-isotopic compositions of the corundum grains. Thus far we have not yet identified any O-bearing interstellar dust grains. However, whether such grains originated from O-rich layers of supernovae or the atmospheres of O-rich red giants, they are expected to show much more variable O-isotopic compositions (HARRIS and LAMBERT, 1984; HARRIS et al., 1985, 1988) than those found in the corundum grains. 5.2. 26A1and 0: Interrelations

_

::

11.1

I

10’

.

I

I

I...,

I

I

I

,

.

..rn.,

lo2 lo3 Zr cont. (ppm)

I

I

I

lo4

FIG. 8. Correlations are seen for only two pairs of trace elements, V-Ti in (a) and Y-Zr in (b). The broken lines designate constant ratios of CLnormalized abundances. Different symbols indicate group membership according to Fig. 9.

in the solar nebula. This question can be reexamined in the light of our data on corundum, as it is predicted to precede hibonite and spine1 in nebular condensation and is the putative carrier of both 26A1and I60 anomalies in stellar condensation (CLAYTON, 1977a). On these two possible origins (though not nebular vaporization), it should contain a clearer record of the earliest isotopic anomalies. What makes the distinction between presolar and solar material easier is the fact that in recent years we have been able to study refractory grains of silicon carbide (e.g., ZINNER et al., 1989) and graphite (AMARI et al., 1990) about whose circumstellar origin there is little doubt. Not only are they carriers of noble gas components whose nuclear origin can be clearly established (e.g., SRINIVASANand ANDERS, 1978; LEWISet al., 1990), but they have extreme isotopic anomalies in their major elements (ZINNER et al., 1989; AMARI et al., 1990) and in all other elements that could be measured so far (OTT and BEGEMANN,1990; ZINNER et al., 199 1b). The corundum grains show variable 26Mgexcesses corresponding to a range in (26A1/27Al)ofrom 0 to -5 X lo-$, the same

Given that the Murchison corundum grains formed in the solar nebula, what information do they contain concerning the distribution of the oxygen and 26Al reservoirs? Let us examine some key data. One significant trend is the correlation of (26Al/27A1)r, with Ti and V (Fig. 9a and b). The grains divide into three wellseparated groups, with identical membership in the two plots. Group 1, though the most populous, is the most compact. Group 3, of similar Ti and V content, has much lower 26A1/ “Al with three out of four grains giving only upper limits. Group 2 has much less Ti and V, but intermediate 26A1/27A1 ratios. These groups also differ in O-isotopic composition. All but one of the 14 Group 1 grains fall in a main cluster at 6’8O = -5O%o (Fig. 2a), with only grain 1 (-26.5%0) away from it. Similarly, four out of five grains of Group 2 (3, 8, 15, and 16) fall in a small cluster at 6”O x -20%0, while the fifth, 14, falls in the main cluster at -50%0. Two of the four Group 3 grains (6 and 10) fall at the extreme ends of the distribution in Fig. 2a, whereas the other two fall in the main cluster. Are these groups independent or derived from each other? Insofar as 26A1alone is concerned, Groups 2 and 3 could be derived from 1, either by decay or by dilution with dead Al. But decay alone cannot account for the low Ti, V in Group 2 or for the 6”O differences among the groups, and requires times that are longer (3 and 25 Ma) than the theoretical lifetime of the solar nebula (-0.1-0.5 Ma; WOOD and MORFILL, 1988) or the estimated lifetimes of circumstellar disks (43 to - 10 Ma; STROM et al., 199 1). Dilution with dead Al

A. Virag et al.

2058

likewise fails to account for S’*O and requires several types of dead Al, with impossibly large dilution factors for Ti in Group 2 ( IO3 vs. only 20 for 26Al). Let us therefore first assume that the groups are independent. (The possibility that groups 1 and 2 are related will be discussed below.) Each is presumably derived from an isotopically distinct parcel of gas and dust that entered the solar nebula, was vaporized, and then recondensed. All fossil radiogenic 26Mg* must have been lost from the Al-bearing phases at that time, else some corundum grains would show (26Ai/27Al)egreater than the canonical ratio of 5 X lO_‘. During recondensation, the corundum would take up Ti, V, and other compatible refractories in solid solution, in amounts that increased with falling temperature. A thermodynamic calculation on the gumption of idea-solution behavior (L. Grossman, priv. comm.) indeed shows ranges of Ti and V comparable to those observed. The lower Ti, V concentrations in Group 2 may reflect lower nebular pressures, because the condensation curves of these elements diverge increasingly from the Al curve at lower P, leading to lower con~ntmtions in corundum. It should be noted that the lack of any petrographic information precludes unambiguous proof of a direct TilAl (CI-norma~i~d)

Murchison Group 1 Corundum Grains

5 h

v1

3 :

4

t

i

6

I -60

-50

-40

-30

I

1 .

b <

6 0 -101

-20

-90

-80

-70

-60

-50

-40

-30

-20

FZG. 10. Group 1 corundum grains exhibit a correlation between their (26A1/2’AI)a ratios and oxygen isotopic compositions, suggesting mixing between a component with isotopically light oxygen (arrow) and little or no *‘jAland a component with a I60 enrichment of approximately 45% and a (26A1/z7Ai)o ratio of 5 X IO-’ (shaded cirties). Errors are 1~.

10”~ * . ‘.....I . . .***” . ..‘.“I . *...._I . ....,‘.I 10-l loo 10’ lo2 lo3 lo4 Ti cone, (ppm) V/AI (CI-normalized) 10”

10 -I

1o’3

lo*

lo1

lo2

V cont. (ppm) FIG. 9. Murchison corundum grains cluster into three fairly tight groups according to their (z6A~/z’A~~ratios and Ti and V concentrations, resp&iveIy. Errors are 1[F.

condensation origin for the corundum. However, it is clear from Fig. 6 that volatility is the controlling factor in the distribution of trace elements. This in turn implies that high temperature processes such as condensation and evaporation played a critical role during the history of the material that finally led to the formation of the corundum grains of this study, whether or not the last step was direct condensation or c~s~li~tion from a melt. The 6r80 values should reflect those of the accompanying gas, as modified by admixtures of other gas-dust parcels and solid-gas exchange. Group 1 shows an interesting trend: (26Al/ 27Al) correlates with d’*O (Fig. lO).$ This can hardly be due to decay, as the time would have to be 1.1 Ma. More likefy, it represents mixing of 26A1-rich material of 6”O = -45%0 with 26Al-poor material of lighter 6’*0 (near - 100%0?).Grain 6, with (26Al/27Al)oI 0.025 X 1O-5 and b’*O = -94.1 %o,is a suitable endmember. Group 2 has only five members, and their (26Al/z7Al~ratios have larger errors and cover a smaller range. Nonetheless, (26A1/27Al)odeclines rather consistently with Ti, V, 6’*0, and * Of the 14 grains, only grain 1 falls out of line. Exchange with nebular gas is a pIausiMe explanation.

Isotopic and geochemical study of corundum in Murchison 6”O (Fig. 9a and b and Fig. 2a). Again, mixing of an endmember at or beyond grain 6 with 26Al-poor, “O- and “Orich material is a likely explanation. This endmember would have to be lower in Ti, V than grain 6, unless the low trace element content results from condensation at low P. However, the position of the grains on the oxygen plot (Fig. 2a) admits the possibility of more complex mixing and exchange relationships, analogous to that of FUN inclusions (CLAYTON and MAYEDA, 1977; CLAYTONet al., 1978; LEE et al., 1980). Analyses of a larger number of corundum grains may help clarify these relationships. Group 3 probably is not a discrete group at all, as its key characteristic-low (26Al/27Al)o-will be shared by all grains older than - lO’a, regardless of source and accompanying oxygen. Such material ought to be rather common, as the lifetimes of refractory interstellar grains (-400 Ma, MCKEE, 1989) are much longer than the mean life of 26Al (1.02 Ma) and loss of fossil radiogenic 26Mg is expected for all A120j. We can estimate the mass fraction of such dead Al on the assumption that the 26Al-rich endmembers of Groups 1 and 2 had (26Al/27Al)o= 5 X 10m5and 5 X 10m6,and that all lower ratios in these two groups represent dilution by old, 26Al-free grains. If the mass fraction in each group is assumed to be equal the fraction by number, the mass fraction of dead Al obtained after inclusion of Group 3 is 43%. Taken at face value, this number suggests that live 26Al in the early solar system came from two main sources, which had decayed to ratios of 5 X 10m5(44%) and 5 X 10m6(13%). The remaining 43% was dead Al. This calculation is based on the uncertain assumption that corundum is representative of all Al in the early solar system, and it also neglects 26A1 contributions from Sic and graphite (ZINNER et al., 199 1b). Although the latter two phases show 26Mg*/27Alratios up to 0.2, it is not clear whether any of this 26A1was alive at the time these grains entered the solar system. An alternative interpretation of Ti, V, and 0 data in Group 2 is that these grains have undergone distillation. Four of the five grains fall distinctly to the right of the ‘60-rich mixing line in Fig. 2a and even grain 14 is more to the right relative to this line than most other grains. Such O-isotopic compositions are the signature of FUN inclusions (CLAYTONand MAYEDA, 1977) and HAL-type hibonites (LEE et al., 1980; IRELANDet al., 199 I), the former of which show fractionated Mg and Si (WASSERBURGet al., 1977), the latter fractionated Ca and Ti (LEE et al., 1979; HINTON et al., 1988; IRELAND et al., 199 1). These features have generally been interpreted as being the result of distillation (CLAYTONet al., 1985; IRELANDet al., I99 1). Distillation of the precursors of the Group 2 corundum grains would lead to a loss of Ti and V. Evidence for such diffusive Ti loss during distillation is seen in four HAL-type hibonites where the isotopic mass fractionation of Ti is inversely correlated with the concentration, ranging from +4.4k/amu in HAL with 0.80 wt% Ti02 to +189L/amu in Murchison 7-404 with 0.035 wt% Ti02 (IRELANDet al., 1989). During high temperature evaporation, Mg can be lost almost completely. No Mg could be detected (upper limit of ~1 ppm) in hibonite (unpublished data from Washington University) that was produced by distillation of kaersutite

2059

(IRELANDand ESAT, 1986). If 26Mg from 26Al in the Group 2 corundum grains was lost by evaporation, this event must have taken place from 2.3 to 3.1 Ma after the formation of Group 1 grains with (26Al/27Al)o= 5 X 10w5.There is ample evidence for thermal events that disturbed the Al-Mg isotope system in Type B CAB from Allende (HUTCHEON, 1982; PODOSEKet al., 199 I), and PODOSEKet al. (199 1) concluded that in at least some cases the disturbances must have occurred 2-3 Ma after original formation of the inclusions. However, these events, producing mostly local recrystallization of melilite and anorthite, must have been relatively mild compared to events that would have led to distillation of corundum. From the O-isotopic fractionations Fo we can calculate the required loss of 0 from the Group 2 grains if their original O-isotopic compositions were on the ‘60-rich mixing line and diffusive Rayleigh fractionation is the responsible process. For grain 3 we obtain -25% loss, and for 8, 15, and 16, -45%. Spine1 would have enough A1203 but is excluded as a possible precursor since it does not contain SC, Y, and Zr, which are present in the corundum. Whatever the exact starting composition, it must already have been very refractory compared to the average solar composition, otherwise Fo would be much higher than observed. This means that the precursor material must itself have formed by condensation or consisted of a selection of more refractory grains. In detail there are, however, some problems with the distillation model for Group 2 corundum grains. For one, the expected inverse relationship between Fo and Ti and V concentrations does not hold very well; in particular, grain 14 with its extremely low Ti and V does not exhibit much 0 fractionation at all. These properties are similar to those observed in one of the HAL-type hibonites, Murchison 7-404, which shows extreme Ti-isotopic mass fractionation ( 189%0/ amu) but much less O-isotopic fractionation than HAL (IRELAND et al., 1991). Difficulties are also encountered if this model is extended to explain the low 26Al or the lack of it in HAL-type hibonites and FUN inclusions. HAL has a (26Al/ 27Al)o of only 5.2 X lo-‘, requiring 26Mg removal 7.2 Ma after precursor formation. This implies an unreasonably long time scale for high temperature events in the solar nebula, for loss of nebular gas, and for accrection of meteorite parent bodies (STROM et al., 1991). FUN inclusions, on the other hand, are less refractory and still contain plenty of Mg. Their lack of 26Al (not well constrained, though) can be explained if the event that led to their 0, Mg, and Si fractionation occurred sufficiently late and completely equilibrated the Mg isotopic system. However, FUN CAIs exhibit the largest isotopic anomalies among silicate-bearing Allende inclusions, and it is difficult to see why they were preferentially subjected to much more severe thermal events than normal Allende inclusions. Although we cannot yet unambiguously decide on the origin of Group 2 grains, an independent source of low 26Al/27Al seems preferable on balance. Two of the three corundum groups have rough analogues among the hibonite classes of IRELAND(1988). Group 1 corundum, with high Ti, V, and (26Al/27Al),,, is analogous to spinel-hibonite inclusions (SHIBs), which typically have (26Al/ “Al)o = 5 X 10e5, and Group 2 corundum is analogous to HAL-type hibonites, which have low Ti, V, and (26Al/27Al),,

2060

A. Virag et al.

and isotopically fractionated oxygen. Group 3, on the other hand, has no hibonite analogue. Although PLAC hibonites share the low (26Al/27Al)0of Group 3 corundum grains, they match neither their high Ti and V, nor the extreme O-isotopic compositions of grains 6 and 10 (PLAC and SHIB hibonites plot essentially in the region of the Group 1 corundum on an oxygen 3-isotope plot; IRELAND et al., 199 1). Corundum apparently carries information on O-isotopic reservoirs not revealed by less refractory objects. We have argued above that the composition of grain 6, which in principle can be derived from an ‘60-rich composition by mass fractionation, is more likely to represent a primary component. This is also indicated by the mixing invoked for Group 1 grains (Fig. 10). Another interesting question is posed by the compositions of grains 1 and 10, which show much smaller I60 enrichments (Fig. 2a), in contrast to the spine1 grains (Fig. 2b). This is not a consequence of the statistics of the number of grains analyzed; in addition to the spinels plotted in Fig, 2b, we measured ‘8O/‘6O ratios in more than 100 spine1 grains and all of them fall into the range displayed by the grains for which we also measured the ‘7O/‘6O ratios. Later equilibration of originally ‘60-rich corundum with normal 0, as invoked for the normal O-isotopic composition of melilite in CAIs (CLAYTONet al., 1977), probably cannot explain the heavy 0 in grains 1 and 10, because the 0 diffusion coefficients in Mg-Al-spine1 and corundum are the same (REDDY and COOPER, 1981, 1982; ANDO and OISHI, 1983). The alternative is that the grains formed from 0 reservoirs with different degrees of I60 enrichments. The equilibration model for CAIs (CLAYTONet al., 1977) implies that the Oisotopic composition of the region in which CAIs formed must have changed from ‘60-rich to normal. Thus, it is not inconceivable that corundum grains had formed at various times with intermediate 0 composition during that period. This scenario is not very satisfying, however, since it is difficult to see why just the most refractory mineral grains formed over an extended period, while less refractory objects, such as various types of CAIs, apparently formed earlier from the ‘60-rich reservoir. The corundum data also shed further light on a controversial question: how much fossil radiogenic 26Mg survived in the early solar system? There is no doubt that the carriers of 26Al entering the early solar system all contained some fossil 26Mg from decay of 26Al in transit. In addition, there is now clear evidence for fossil 26Mg in interstellar silicon carbide and graphite grains (ZINNERet al., 199 1b). CLAYTON (1977b, 1986) has contended that fossil 26Mg has been misinterpreted as evidence for live 26Al in Allende refractory inclusions. However, his model has been shown to be untenable by various workers who have repeatedly pointed out that the correlation of 26Mg*/24Mg with 27Al/24Mgin inclusions which formed by closed-system igneous crystallization (plagioclase does not form from the vapor phase) is proof for the in situ decay of live 26Al (LEE et al., 1977; WASSERBURG and PAPANASTASSIOU,1982; PODOSEKet al., 199 1). Our corundum data provide further evidence for live 26Al in the early solar system. They do give mixing lines, but with exceedingly high 26Mg/24Mg ratios-up to 56X solar rather than 1.1X (the value chosen by CLAYTON(1986) in his model,

corresponding to solar Al/Mg and an initial 26Al/27Al ratio of - I)-and with (26Al/27Al)0no higher than the canonical ratio found in “normal” Allende Type Bl inclusions. The corundum data not only provide further proof that 26Al was alive in the early solar system, but also set strict upper limits on the amounts of fossil 26Mg*. The sharp upper cut-off of (26A1/27Al)ois the same, within errors, as the canonical value of 5 X 10m5established from Allende inclusions. Since production ratios of 26Al/27A1in various stars range from 10e3to 1 (e.g., CLAYTONand LEISING,1987), this means that more than 99% of all fossil 26Mg* must have been vaporized and equilibrated with other Mg during the formation history of the corundum grains. Apparently, the Murchison corundum grains have lived up to their promise, by providing insights into some of the earliest solar-system processes involving 26Al and 0. With these insights, it may be profitable to reexamine earlier data on hibonite and other phases that lie lower in the condensation sequence. Acknowledgments-We thank L. Grossman for the condensation calculations, J. Yang and S. Epstein for providing Murchison separate CFOc, J. Foote for ion probe maintenance, and E. Koenig for manuscript preparation. This work was supported by NASA (S.A. and E.A.), the Fond zur Fijrderung der wissenschaftlichen Formhung, Austria (A.V.), and NSF (E.Z.). S.A. and E.Z. also thank R. M. Walker for his interest and support. This manuscript benefitted from an exemplary detailed review by T. R. Ireland. Editorial handling: J. D. Macdougall

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