The demonstration of the presence of ^(41)Ca in the early solar system, based on the observation of excess ^(41)K in Efremovka CAIs [1,2], has led to several attempts that sought to identify plausible stellar sites and processes

that could have produced and subsequently injected a host of short-lived radionuclides (e.g., ^(41)Ca, ^(26)Al, ^(60)Fe, ^(53)Mn, and ^(107)Pd) into the protosolar cloud [3-5]

Goswami, J. N.

Sahijpal, S.

Srinivasan, G.

Wasserburg, G. J.

Caltech Authors - Main

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570 Abstracts 
bonaceous chondrites with properties that are between the CM and CO me-
teorite groups, though they are more closely related to the latter [I ,2]. 
One hundred seventy CA!s were observed in a 102-mm2 thin section of 
MAC 87300, all <500 pm and most <300 pm in size. Most are composed of 
pyroxene and spinel, with textures similar to spinel-pyroxene inclusions in 
CM meteorites [3]. Melilite-bearing inclusions are uncommon. One unusual 
hibonite-bearing inclusion, #43, is composed of three large (<;,75pm) hibonite 
laths surrounded by thin rims of hibonite +spinel+ (outermost) diopside. 
The hibonite has low Ti02 ( 1.8 wt%) and MgO (0.8 wt% ), reminiscent of 
Murchison PLACS [ 4]. Hibonite in #43 has an unusual smoothly fraction-
ated concave-upward REE pattern, with Lu enriched -3x relative to the light 
REE, a small negative Eu anomaly, and a much larger negative Yb anomaly. 
The largest inclusion ( 480 pm across) in this section, #44, is a type A con-
sisting ofmelilite (Ak7_17) enclosing 1-5-pm-sized grains of spinel. The in-
clusion has a well-defined diopside rim and an accretionary mantle. REE in 
#44 are unfractionated, -I0-30x Cl, with a small negative Yb anomaly. 
No excess 26Mg was detected in either #43 or #44: for #43, inferred initial 
26AJf27AJ <I x I0-5 at 27AJf24Mg- 105-125; for #44, initiaJ26AJf27AJ = 
(0.5 ± 2.0) x I 0-5 (±2cr) at 27 AJf24Mg up to 32 (see Fig. I). 
One hundred sixty-two CA!s were located in a 172-mm2 thin section of 
MAC 88107, similar in size distribution to those in MAC 87300. The most 
common CA!s are again spinel- and pyroxene-rich, but irregularly shaped 
melilite-rich type-A inclusions (A k3_19 ; FeO in spinel <;,0.6 wt%) similar to 
those in the least-metamorphosed C03 chondrites [5] are more common than 
in MAC 87300. Inclusion# 14 is a compact inter growth of melilite, spinel, 
perovskite, and hibonite laths, with an incomplete diopside rim. Inclusion 
# 16 is unrimmed and extensively cracked, consisting of melilite enclosing 
spinel and perovskite. Inclusion #20 consists of melilite, spinel, and 
perovskite (the latter two phases rimmed with Ti-rich pyroxene), and has 
numerous round voids. The inclusion has a complete diopside rim. Inclu-
sion #35 is a compact, unrimmed intergrowth of melilite and spinel only. 
Apart from pyroxene, no secondary minerals were detected in these four 
inclusions. Each contains an excess of 26Mg that corresponds to an initial 
26AJf27AJ of -4.5 x 10-5. REE patterns for #14, #16, and #20 are unfrac-
tionated with REE -SOx CI (#14, #16) and -lOx CI (#20). Inclusion #35 
has a modified group II REE pattern. 
Differences in CAl populations from MAC 87300 and MAC 88107 sug-
gest that the two meteorites are not paired. The CA!s from MAC 87300 and 
MAC 88107 are most closely related to those from the lowest petrologic 
type C03s, and demonstrate that these two meteorites are extremely primi-
tive carbonaceous chondrites. The canonical initial 26 Al/27 AI of CA!s ( 4-5 x 
10-5), defined by the major carbonaceous chondrite groups [6], also applies 
to inclusions from MAC 88107, whereas no evidence for 26 AI in MAC 87300 
has yet been found. 
References: [I] Kallemeyn G. (l992)LPSXXlll, 649-650. [2] Sears 
D. et al. (1990) LPS XXI, 1121-1122. [3] MacPherson G. and Davis A. 
(1994) GCA, 58, 5599-5625. [ 4] Ireland T. et al. (1988) GCA, 52, 2827-
2839. [5] Greenwood R. et al. (l992)Meteoritics, 27,229. [6] MacPherson 
G. et al. (1995) Meteoritics, 30, in press. 
30 
25 
'(;'20 
~ 
-15 
0'1 
:2 10 
"' 
'Zo 5 
0 
-5 
0 20 
• MACS7300 H3 "' MAC87300 144 
c MAC88107 114 • MAC88107 116 
)II MAC88107120 a MAC88107 135 
~ w 00 100 1~ 1~ 
21 AI/24Mg 
Fig. I. 
QUANTITATIVE MODELS OF CAl RIM LAYER GROWTH. A. 
Ruzicka and W. V. Boynton, University of Arizona, Tucson AZ 85721, USA. 
Many hypotheses have been proposed to account for the -50-pm-thick 
layer sequences (Wark-Lovering rims) that typically surround coarse-grained 
CA!s, but to date no consensus has emerged on how these rims formed. A 
two-step process- flash heating of CA!s to produce a refractory residue on 
the margins of CA!s [ 1-3], followed by reaction and diffusion between CA!s 
or the refractory residue and an external medium rich in Mg, Si, and other 
ferromagnesian and volatile elements to form the layers [3-5]- may have 
formed the rims. 
We have tested the second step of this process quantitatively, and show 
that many, but not all, of the layering characteristics of CAl rims in the 
Vigarano, Leoville, and Efremovka CV3 chondrites can be explained by 
steady-state reaction and diffusion between CA!s and an external medium 
rich in Mg and Si. Moreover, observed variations in the details of the layer-
ing from one CAl to another can be explained primarily by differences in the 
identity and composition of the external medium, which appears to have 
included vapor alone, vapor+ olivine, and olivine± clinopyroxene± vapor. 
An idealized layer sequence for CAl rims in Vigarano, Leoville, and 
Efremovka can be represented as MSF I S I AM I D I 0, where MSF = melilite 
(M) +spinel (S) + fassaite (F) in the interior ofCAis; S =spinel-rich layer; 
AM= a layer consisting either of anorthite (A) alone, or Malone, or both A 
and M; D =a clinopyroxene layer consisting mainly of aluminous diopside 
(D) that is zoned to fassaite toward the CAl; and 0 = olivine-rich layer, 
composed mainly of individually zoned olivine grains that apparently pre-
existed layer formation [3]. A or Mare absent between the Sand D layers in 
roughly half the rims. The 0 layer varies considerably in thickness (0-60 pm 
thick) and in porosity from rim to rim, with olivine grains either tightly 
intergrown to form a compact layer or arranged loosely on the outer sur-
faces of the CA!s. None of these variations in rim layers are correlated with 
the modal compositions of the CA!s. 
In our models, we investigated the reaction of CAl interiors (containing 
M + S + F) with various proportions of vapor (V), 0, and D in the five-
component system Mg0-AI0312- Ca0-Si02-Ti02. Representative composi-
tions were assumed for the solids. Most likely, a vapor reacting with CA!s 
would have small (e.g., solar) or trivial abundances of AI, Ca, and Ti com-
pared to Si and Mg, and such AI-, Ca-. and Ti-poor compositions were as-
sumed for the vapor. The model zone sequence MSF I S I A I D IV can form 
when Mg/(Mg + Si) = 0.28-0.47 in the vapor, and is consistent with rims 
that contain an A layer but lack an 0 layer. The zone sequence MSF I S I 
D I VO, which can form when Mg/(Mg + Si) = 0-0.47 in the vapor, may 
explain rims that lack an A (and M) layer and that have a porous (or poorly 
compacted) 0 layer. Finally, the model zone sequence MSF IS I A I D I 0 ± 
Dis consistent with rims that contain both an A layer and a compact 0 layer, 
but this sequence can form only if the system experienced open-system loss 
of Ca at the D-0 contact, with Ca-poor vapor being a possible open-system 
sink for Ca. The occasional presence of M in a mono- or bimineralic layer 
within rims apparently cannot be explained by the models, possibly indicat-
ing that the rims did not fully attain a steady-state condition. 
References: [I] Boynton W. V. and Wark D. A. ( 1985) Meteoritics, 
20, 117-118. [2] Murrell M. T. and Burnett D. S. (1987) GCA, 51, 
985-999. [3] Ruzicka A. and Boynton W. V. (1994) Meteoritics, 29, 529. 
[4] MacPherson G. 1. et al. (1981) Proc. LPS 12B, 1079-l091. [5] Wark 
D. A. et al. (1988) LPSXIX, 1230-1231. 
OBSERVATION OF CORRELATED CALCIUM-41 AND ALUMI-
NUM-26 IN CV3 HIBONITES. S. SahijpaJI, G. Srinivasan', G. 1. 
Wasserburg2, and 1. N. Goswami 1, 'Physical Research Laboratory, 
Ahmedabad 380 009, India, 2Division of Geological and Planetary Sciences, 
California Institute of Technology, Pasadena CA 91125, USA. 
The demonstration of the presence of 41Ca in the early solar system, 
based on the observation of excess 41 K in Efremovka CA!s [1,2], has led to 
several attempts that sought to identify plausible stellar sites and processes 
that could have produced and subsequently injected a host of short-lived 
radionuclides (e.g., 4ICa, 26AJ, 60Fe, 53Mn, and I07Pd) into the protosolar 
cloud [3-5]. Radiogenic excess of26Mg (from 26AJ decay) was also found in 
© Meteoritical Society • Provided by the NASA Astrophysics Data System 
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the Efremovka CAis where excess 41 K was present [6,7]. This association 
suggests possible coproduction and subsequent injection of these two short-
lived nuclides to the solar nebula. We have measured K and Mg isotopic 
composition in a single mineral phase (hibonite), having contrasting 26Mg 
excesses to further substantiate this hypothesis. 
We have chosen hibonites from two CV3 meteorites, Efremovka and 
Allende, for the present study. The Efremovka hibonite is from a CAl (E50) 
having a well-behaved Mg-Al isotopic systematics with initiaJ26AI!27 AI of 
( 4.87 ± 0.87) x I 0-5 [6]. The Allende sample (HAL hibonite), on the other 
hand, has a very low initial 26AJJ27 AI of (5.2 ± 1.7) x I 0-8 [8]. We have 
carried out K and Mg isotopic studies ofE50 hibonite and K isotopic study 
of HAL hibonite using procedures described previously [ 1,2,6]. Because of 
the small size of E50 hibonites, K isotopic composition could be measured 
in only a few hibonite grains, where the contribution from the neighboring 
K-rich phase (melilite) toward the measured K signal was negligible. In the 
case of HAL hibonite there is a large variation inK content, and the signal at 
39K is often close to the system background, while the signal at mass 41 is 
dominated by the (40Ca42Ca)++ interference. The magnitude of this interfer-
ence was estimated by measuring the (40Ca43Ca)++J43Ca+ ratio in terrestrial 
(Madagascar) hibonite as well as in the HAL hibonite. We obtained a value 
of (7.3 ± 0.6) x I 0-6, which is about 4x lower than the corresponding value 
for pyroxene [ 1,2]. 
Definite excess of 41 K has been found in E50 hibonites with Ca/K ratio 
exceeding 5 x I 05. A forced linear fit of the data through the solar system 
41KJ39K ratio (0.072) yields an initiaJ 41CaJ40Ca of about 1.4 x I0-8, which 
is close to the value reported for Efremovka pyroxene [I]. The Al-Mg iso-
topic data for the E50 hibonites reveaJ26Mg excess, consistent with there-
sults reported earlier [6]. In contrast, no excess 41 K was found (within the 
limits of our experimental uncertainties) in HAL hibonite, even though its 
Ca/K ratio is much higher than in Efremovka hibonite. Our data provide an 
upper limit of 3 x J0-9 for initia1 41 Ca/40Ca in HAL hibonite. These results 
and data obtained from studies of other Efremovka CA!s [ 1,2,6, 7] show that 
(!) 41Ca and 26AJ are present at the same level (41CaJ40Ca- 1.4 x J0-8 and 
26AIJ27 AI- 5 x I 0-5) in several Efremovka CA!s and (2) 41Ca is very low or 
absent when 26AJ is at very low levels (HAL hibonite).lt therefore appears 
that 41Ca and 26AJ are coupled either in their original stellar source(s) or 
were well mixed in some parcels of interstellar material before they were 
injected to the solar nebula. The lower initials in HAL hibonite may reflect 
either a heterogeneity of 26AJ and 41 Ca in the solar nebula (due to variable 
mixing of the source material with nebular material) or processing of mate-
rial and/or redistribution ofMg and K isotopes at a later time. Although we 
cannot decide between these alternatives based on our data, extensive stud-
ies of HAL and other refractory objects indicate that many CAis evolved 
through multiple events in the early solar system, spreading over a signifi-
cant time interval [e.g., 9-12]. 
References: [I] Srinivasan G. eta!. ( 1994) Astrophys. J. Lett., 43 I, 
L67-L 70. [2] Srinivasan G. eta!. ( 1995) GCA, submitted. [3] Wasserburg 
G. J. eta!. (1995) Astrophys. J. Lett., 440, LIOI-L104. [4] Cameron 
A. G. W. et a!. (1995) Astrophys. J. Lett., in press. [5] Ramaty R. et a!. 
(!995)Astrophys. J., submitted. [6] Goswami J. N. and Srinivasan G. (1994) 
Proc. IndianAcad. Sci. (EPS), 103,57-82. [7] Goswami J. N. eta!. (1994) 
GCA, 58, 431-447. [8] Fahey A. eta!. (1987) GCA, 51, 329-350. [9] Lee 
T. eta!. (1980) GRL, 7, 493-496. [10] Brigham C. A. (1990) Ph.D. thesis, 
Caltech. [II] Sheng Y. J. eta!. (1991) GCA, 55, 581-599. [12] Ireland 
T. R. eta!. (1992) GCA, 56, 2503-2520. 
ORDINARY CHONDRITES VIEWED AS REASSEMBLED 
"SPLASH EJECT A." I. S. Sanders, Department of Geology, Trinity Col-
lege, Dublin 2, Ireland. 
A case has already been made favoring chondrites as reassembled "splash 
ejecta" following low-velocity collisions between molten planetesimals[!]. 
Here I briefly review this hypothesis, then develop further arguments in its 
support. 
The scenario envisaged may be summarized as follows. Planetesimals 
grew to radii greater than 30 km in less than 1 Ma after the formation of 
CA!s, and they were heated rapidly by the decay of 26Al. By 2 Ma each 
planetesimal, had a molten interior insulated by a cool, dusty carapace. Low-
velocity collisions at this stage released enormous, turbulent, expanding 
clouds of incandescent spray mixed with dust and solid grains from the cara-
pace. The cloud constituted a rather special, transient nebular environment; 
as it cooled, the melt droplets became chondrules. Much of the cloud's con-
tents reassembled under gravity onto the surface of the hot, residual plan-
etesimal, and the accumulated debris became reheated and metamorphosed. 
Collisions recurred over the few miflion years that relative velocities re-
mained low and planetesimals remained molten. Thus, the cumulative de-
bris contained many recycled and broken chondrules. This scenario is 
apparently reconcilable with chondrule cooling rates, the preservation of 
clasts of "planetary" rock in chondrites, the retention of volatiles in chon-
drules, the preservation of solar chemistry, and more than a dozen other 
features. 
Is it reasonable to claim that 30-km-radius bodies existed by I Ma, and 
were substantially molten by 2 Ma? Cameron [2] argued that CA!s were 
saved from drifting into the Sun by their incorporation, soon after forma-
tion, into planetesimals whose mass was sufficient to hold them in orbits, 
decoupled from the drag of nebular gas. Wetherill's models [3] show that 
many bodies >I 00 km radius may have formed on a timescale of I 05 yr. In 
these terms, the proposed 30 km by I Ma is quite conservative. Regarding 
26Ai heating, the remarkably constant initial ratio of26A]/27 AI (5 x I 0-5) in 
CA!s from different classes of meteorite [ 4] suggests that 26 AI was uniformly 
distributed in the dust that eventually formed the chondrite parent bodies. 
This amount of26Al translates to some 7000 J g-1. A simple finite element 
calculation was made to assess the likely thermal evolution of planetesimals 
of different sizes, starting from 300K at different times. The proposed body 
of 30 km ·radius at I Ma was found to be a limit for substantial internal 
melting. Its interior would have remained molten for several million years. 
Earlier accretion, or larger planetesimals, would have generated even more 
melt. It seems, therefore, that molten planetesimals were abundant in the 
early solar system. Moreover, they evidently suffered collision and accre-
tion. If their collision products were not chondrules, then what were they? 
Two further arguments favoring the proposed scenario concern the age 
difference of CA!s and chondrules and the existence of macrochondrules. 
Cameron's Leonard A ward address [2] was stimulated by the inferred time 
interval of several million years between the formation of CAis and chon-
drules. Chondrules were interpreted as dust melted by solar flare activity, 
the dust having been produced by late collisions between planetesimals. If, 
as is argued here, the planetesimals were already internally molten, chon-
drules would have been produced directly, without needing to invoke a solar 
flare heat source. A separate issue is the existence of porphyritic olivine 
macrochondrules up to 4 em across [5]. Macrochondrules are not easily 
reconcilable with chondrule formation by radiative heating in a nebular set-
ting. Such a mechanism predicts an inverse relationship between chondrule 
diameter and temperature rise, which is not observed. However, in the present 
scenario, macrochondrules are interpreted simply as examples of large blobs 
of frozen melt. 
References: [I] Sanders I. S. (1994) Meteoritics, 29, 527. 
[2] Cameron A. G. W. (1995)Meteoritics, 30, 133-161. [3] Wetherill G. W. 
(1989) in Asteroids// (R. P. Binzel eta!., eds.), 66!-680, Univ. of Ari-
zona, Tucson. [ 4] MacPherson G. J. eta!. (!992)Meteoritics, 27,253-254. 
[5] Binns R. A. (1967) Mineral. Mag., 37, 319-324. 
OXYGEN ISOTOPES IN FORSTERITE GRAINS FROM JULES-
BURG AND ALLENDE: OXYGEN-16-RICH MATERIAL IN AN OR-
DINARY CHONDRITE. J. M. Saxton, I. C. Lyon, and G. Turner, 
Department of Earth Sciences, University of Manchester, Manchester M 13 
9PL, UK. 
Grains offorsterite form a significant fraction of the matrices of the CM, 
CV, CO, and UOC meteorites; some of them consist of nearly pure forsterite 
(Fa <I) and display blue cathodoluminescence. Steele [I] has made a com-
prehensive study of the minor elements in these grains, and found that the 
most Fe poor(< Fao_7) display blue CL and seem to be common to all the 
aforementioned chondrite groups. Bulk samples of, and chondrules from, 
the UOCs and the carbonaceous chondrites have different 0 isotopic signa-
tures [2,3]. It is therefore desirable to determine the 0 isotopic signatures of 
the forsterite grains from the different meteorite groups to determine whether 
© Meteoritical Society • Provided by the NASA Astrophysics Data System 


Observation of Correlated Calcium-41 and Aluminum-26 in CV3 Hibonites

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Observation of Correlated Calcium-41 and Aluminum-26 in CV3 Hibonites

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