The evolution of a complex type B Allende inclusion: An ion microprobe trace element study

0016-7037/89/$3.00

Geochimica n Cosmoehimu Acla Vol. 53, PP. 24 13-2427 Copyright 0 1989 Pergamon Press pk. F’rintcd m U.S.A.

+ .OO

The evolution of a complex type B Allende inclusion: An ion microprobe trace element study GLENN J. MACPHERSON’,GHISLAINECROZAZ’ and LAURA L. LUNDBERG’ ‘Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, U.S.A. 2Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO, 63130, U.S.A. (Received

June 17, 1988; accepted in revisedform June 23, 1989)

5241 is a Type Bl refractory inclusion from Allende, first described by EL GORESY melilite-rich and spinel-poor mantle enclosing a 0.6 cmradius spinel-rich core; the inclusion contains xenoliths of spinel-free fassaite + melilite t anorthite incorporated within the spinel-rich core. Detailed ion microprobe analyses of individual phases in 5241 show that the rare earth element (REE) concentrations in mantle melilite vary irregularly with increasing distance from the rim of the inclusion, at first decreasing immediately below the rim and then remaining constant between -0.4 and 1.0 mm. More than 1.0 mm from the rim, the REE concentrations again decrease. Although counterintuitive in the context of traditional fractional crystallization models, these REE variations are in fact broadly consistent with such a model in light of recent experimental measurements of DREE3+ (me,)by BECKETTet al. (1988) that show a strong inverse correlation of D with the gkermanite content of the melilite. Local variations, over distances of <20 pm, in the akermanite content of mantle melilite-as much as S-10 mol% Ak-with accompanying fluctuations in REE contents, are due to reaction of gehlenitic melilite with tiny inclusions of fassaite, producing narrow zones of more akermanitic melilite surrounding more fassaitic pyroxene. Spinel-free islands have widely varying bulk compositions, in both major and trace elements, and are probably trapped xenoliths. Measurements of Mg isotopic mass fractionation show that the entire inclusion is enriched in heavy magnesium by 6.1 + 1.2%0relative to terrestrial standards, but there are no significant differences between mantle melilite, core melilite + fassaite + spinel, and spinel-free island melilite + fassaite. There is thus no evidence that volatilization played any major role during the formation of the melilite mantle. Significant assimilation of very fassaite-rich spinel-free island material during the post-mantle crystallization of the melt is required to explain the observed decrease of Eu concentration in the core. We interpret 5241 as having formed largely by fractional crystallization during the first -40% of its solidification; this was followed by fractional crystallization + xenolith assimilation during the last 60%. Abstract-USNM

et al. (1985), that consists of a 1.2 mm-thick

I. INTRODUCIION

disagreed with the model of KURAT et al. ( 1975), concluding instead that the Al-Mg composition gradients in melilite crystals in the mantle of an Allende Bl inclusion are the simple consequence of closed system, inward-progressing, fractional crystallization of the progenitor molten droplet that lost heat from its outer surface. STOLPER’S( 1982) experiments demonstrated that the crystallization sequence for a melt of Type B composition is very close to that deduced from pctrographic observations of natural inclusions such as the one described by MACPHERSONand GROSSMAN( 198 1). Stolper concurred with the latter authors that the major features of Type B inclusions are consistent with a simple fractional crystallization model and noted that variable cooling rates would add some complexities. MEEKERet al. ( 1983) proposed a radically different model in which much of the melilite in Type Bl mantles formed by replacement of fassaite during planetary metamorphism. Their model sought to explain the presence of irregularly shaped fassaite grains within mantle melilite in many Bl inclusions. Most recently, EL GORESYet al. (1985) described a Type Bl Allende inclusion (USNM 5241; hereafter, 5241) that contains “islands” having distinctly different bulk compositions than the remainder of the inclusion; specifically, melilite in the islands is richer in Nathan that in the mantle and core, and the islands contain no spine1 in contrast with the remainder of 524 1. El Goresy and co-workers postulated that

THE MOSTTHOROUGHLYstudied and best understood of the various kinds of refractory inclusions in chondritic meteorites are the melilite-pyroxene-spinel-anorthite-rich Type B’s (GROSSMAN,1975) in CV3 chondrites. The physical, textural and mineral-chemical characteristics of these spheroidal, cmsized objects are consistent with experimental evidence (STOLPER,1982; STOLPERand PAQUE, 1986) that they formed by solidification of molten droplets. Nonetheless, there continues to be disagreement about whether all features of these inclusions can be explained by simple closed system fractional crystallization. For example, Type B 1 inclusions differ from Type B2 inclusions in having a melilite-rich outer mantle (WARK and LOVERING, 1982) and the origin of this structure is debated. KURAT et al. (1975), in their study of a Bl inclusion from Bali, noted that its mantle is distinctly more enriched in refractory components (Al, Ti, Ca) and depleted in volatile components (Na, Fe) than its interior. They suggested that the inclusion experienced fractional volatilization from its outer surface while still molten; the resulting local enrichment of more refractory components in the residual melt produced a concentration gradient of components inward from the rim of the droplet that influenced the eventual mineralogy and mineral chemistry of that outer zone. MACPHERSON and GROSSMAN (198 1) 2413

7414

G. J. MacPherson, ci. (Yrozaz and L. I.. Lundberg

the islands are xenoliths, trapped by and partially dissolved in the molten inclusion prior to its final solidification. They also interpreted the melilite mantle as having formed by the late addition of a separate melt that was plastered onto the outside of the already-solidified 524 I core. Inclusion 5241 provides a unique opportunity to assess the relative contributions of fractional crystallization. volatilization and assimilation in a single sample. Using the newlyproven ability (ZINNER and CROZAZ, 1986a) to analyze rare earth elements (REE) with the ion microprobe. we have made detailed trace element and isotopic analyses of the constituent phases in each of the major textural parts of the inclusion (mantle. core, and islands). We were particularly interested in trace element zoning profiles within crystals. Our goal was to apply trace element fractionation models to our data in order to discriminate between possible origins for 524 I. Preliminary data and interpretations were given in MACPHERSON et al. (1987); the more extensive data and numerical modelling presented herein lead to different and more definitive conclusions than those in our earlier work. II. SAMPLE

AND

EXPERIMENTAL

TECHNIQUES

“1 Sumpie description 5241 is a spheroidal inclusion, - I.6 cm m diameter. whose petrology was studied by EL GORESY ef al. (1985).A line drawing of the polished thin section that we analyzed-the same one studied by EL G~RESY ef al. (1985)-is given in Fig. 1 to show the essential structure of the inclusion and the locations of the ion microprobe analyses. The thin section samples a “pie slice” of approximately % of the original inclusion. The outer - 1.2 mm of the inclusion is a nearly monomineralic melilite mantle* in which the long axes of the melilite crystals are oriented at high angles to the outer surface. Numerous but small grains of pyroxene and minor spine1 are present as inclusions within the mantle melilite crystals. The core of 5241 is dominated by fhssaitic pyroxene, spinel, melilite, and minor anorthite. Dispersed within the core, and extending in some cases part way into the mantle, are “islands” of fassaite _t melilite I anorthite that are devoid of spine1 and have sharp well-defined boundaries with the surrounding core material. One reason that EL GORESYer al. ( 19851 postulated separate origins for the islands, the core and the melilite mantle is the marked difference in spine1 contents of these three components.

B Measurement of rare eurrh uhunduncvs Trace element analyses (by neutron activation and isotope dilution) of a bulk sample and of melilite and pyroxene mineral separates of this same inclusion were previously reported by %KiASAWA ct ui (1977). Rare earth elements were analyzed with the Washington Umversity CAMECA IMS-3f ion microprobe, using an experimental procedure similar to that described by ZINNER and CROZAZ(I 986a). Energy filtering at low mass resolving power effectively removes all interferences except for the interfering oxide ions of the light REE (LREE) at the masses of the heavy REE (HREE). The mass spectrum in the REE region is deconvolved into contributions from REE and REE monoxide ions. To shorten the length of time required for a measurement, Gd, Yb, and Lu (which, because oftheir isotopic abundance patterns, require high counting statistical precision for spectral deconvolution) were usually not determined and the number of mea-

__.

-_II

* To avoid confusion, henceforth in this paper the terms “mantle” and “core” will refer to the outer and inner regions of the whole inclusion: “margin” and “center” will refer to the respective parts of individual crysfals. “Rim” refers exclusively to the Wark-Lovering rim sequence (WARK and LOVERIN& 1977) on the outermost surface of the inclusion

USNM

5241

1.0 cm 4

‘V’

_

Melllrte

mantle

‘island

core

FIG. I Line drawing of the thin section of USNM 524 i. s’noum~ the core/mantle structure and spinel-free islands: the numbers I .-.! correspond to the island nomenclature used by EL GOKES‘~~7 ;!, (I 985). Also shown are the locations of ion microprobe analysis spoli “Y”, “8”, and “V” mark the locations ofthe three step scan traverse.. across the mantle that are discussed in the text sured masses in the REE region was reduced to .!> (from the 411u.seJ in the original method) between masses 137 and l6Y. Ion signals were also measured at other masses to monitor the relative abundances of several major and minor elements, for use in normalization (f:i! and evaluation of contamination from alteration veins (Na) and l&saite inclusions in melilite (Ti). Typically, the primar)r beam currem was about 15-30 nA and the beam spots up to - 50 pm. Measuremen: times generally ranged from 20 minutes to one hour Absolute REE concentrations were estimated using sensitivity ta. tots relative to Ca, determined by measurements of synthetic Ca-A:. silicate glasses (ZINNER and CROZAZ, 1986b). 11 is well known 1~ secondary ion mass spectrometry that both the intensity of molecular interferences and the relative ion yields depend on the composition of the analyzed sample (matrix effect). Ideally, therefore, one chooses standards that are close in composition to those of the unknown The problem is that very few REE standards are available at present (although progress in this direction is being made). Fortunately. thr. energy filtering technique greatly reduces matrix effects. Measurcments in the silicate glasses and Angra dos Reis fassaites show tha? the sensitivity factors of REE relative to Ca are almost identical t,those in phosphates (ZINNER and CROZAZ, 1986b). FAHFY e! 11’ (1987) showed that the respective REE sensitivity factors for htbonirt and perovskite differ by only - I. 14’~ and - ! 35 r from those irk phosphates and silicates. In all of these mmerals. the relative REI ion yields are similar. Given this relative uniformity of REE sensmvny factors among the disparate mineral standards now available, we believe that the values determined for the silicate glass standard can confidently ht used to determine the REE concentrations in the melilite and tbc anorthite in 524 1. In particular, the effect of compositional difference:~ in the melilite solid solution series is believed to be negligible.

In order to evaluate the possible role of volatilization in the iormation of the melilite mantle, we measured the intrinsic Mg isotopic mass fractionation along three traverses of mantle melilite for which we also determined REE and major element variations. We alsci measured the Mg isotopic compositions of phases in the spinel-rich core and spinel-free islands, to test whether these components had separate origins as proposed by EL GORESYPI n! ( 1985). The ana.lytical techniques used for all Mg isotopic measurements were the same as those described in FAHEY ei al. (19873. except that run consisted of 90 cycles through the mass sequence 24. 25. 26 and 77 D. Major element analyse~s

Major element concentrations were determined at each ot me SC lected locations. in many cases prior to ion probe measurement ani’

Evolution of a Type 3 inclusion in Allende on exactly the same spot. In those instances where the analyses were

done subsequently to ion probe measurements, two or more spots close to but on opposite sides of the ion sputter hole were averaged to obtain the composition of the area analyzed by the ion probe. Most analyses were acquired using wavelength-dispersivetechniques on the Smithsonian ARLSEMQ, automated, 9-spectrometerelectron microprobe. Operating conditions used for analyses were 15 kV aecelerating potential and 0.15 pA probe current. Data were reduced using the correction procedures of BENCEand ALBEE(1968). A few energy dispersive analyses were acquired on the Smithsonian JEOL JSM-840A scanning electron microscope, which is equipped with a KEVEX X-my detector. Operating conditions for this instrument were I5 kV accelerating potential and 1 nA probe current; data were reduced using ZAF factors generated by the program MAGIC V, a modified version of a program originally written by J. W. COLBY ( 1968).

III. RESULTS

A. Melilitr mantle Representative REE patterns for melilite from just beneath the Wark-Lovering rim (WARK and LOVERING, 1977), and from outer, intermediate and inner “normal” (c$ the melilite “veneer” of EL GORESY et al., 1985; see below) portions of mantle melilite from 5241 are shown in Fig. 2. Step scan traverses across three locations on the melilite mantle of 524 1 are shown in Fig. 3, for bulk &ermanite com~sition (a) and for the elements Ce, Sm, Eu and Er (b-e); for clarity, only a best fit line is shown to the ikermanite data in (a). Analytical data are reported in Table 1. Except for the outermost two points in traverse “V” and the one outermost point in “Y”, the REE traverses are along the lengths (c-axes) of single crystals. The composition of the melilite changes away from the Wark-Lovering rim. ImmediateIy beneath the rim in two of the traverses (“V” and “8”; see also “Rim” in Fig. 2), REE are strongly enriched in the very gehlenite-rich melilite (Ak 2- 10); these melilites are more aluminous than any reported by EL GORESYet al. ( 1985), because those authors specifically avoided the zone close to the rim (A. EL G~RESY, pen commun., 1988). Along traverse Y”‘, the REE are not so enriched

100

j

I

Mantle

Melilite

s

z

2

10

WA6 YM3 YM6 YM4

E

. 1

0 Rim X Outer

B lnlermediate * Inner

I I I I I 1 I r I , I , I I I , La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

FIG. 2. Representative REE patterns for mantle melilite. Outer, inner and intermediate refer to positions in the mantle with respect to the rim of the inclusion, i.e.. “outer” is closest to the rim and “inner” is closest to the center of the inclusion. Note that in this and subsequent REE diagrams herein, the Gd concentrations have been inferred by linear interpolation between Sm and Tb. The Cl chondrite values used for normalization of all reported REE analyses are (in ppm): La 0.245, Ce 0.638, Pr 0.096, Nd 0.474, Sm 0.154, Eu 0.058, Tb 0.037, Dy 0.254, Ho 0.057. Er 0.166, Tm 0.026 (PALMEet ai., 1981).

2415

and the melilite is less gehlenite-rich relative to “V” and ‘“8”. With increasing distance inward from the rim, up to about 0.4 mm, the REE concentrations along “V” and “8” decrease sharply as the lkermanite content rises to -Ak 24; the REE concentrations along “Y” show no such decrease away from the rim. Up to 0.4 mm the melilite has essentially unfractionated REE patterns that, at 0.4 mm, are enriched 7-10X Cl chondrites and have a positive Eu anomaly. In the region between 0.4 mm and 1.0 mm in from the rim, the REE concentrations decrease slightly as the gkermanite content increases gradually from Ak 24 to Ak 32. Note that here and in other parts of the mantle, small scale ~uctuations in both the REE concentrations and in the Ak content of the melihte are superimposed on the overall trends described above. The small variations in the akermanite content, visible optically as differences in the melilite birefringence, are clearly not oscillatory zoning of the kind described by MACPHERSON et al.(1984): rather, they are narrow (typically. less than 20 pm) zones, surrounding tiny fassaite inclusions within the melilite that are enriched by -5 mol% akermanite relative to the surrounding melilite. At> - 1.O mm from the rim, the REE become fractionated (e.g., increasing Ce/Er) as the core/mantle boundary is approached and the akermanite content rises sharply to Ak 4247 at the boundary. In general, the magnitude of the Eu anomaly throughout the mantle increases with distance from the rim. One of the analyzed melilite crystals (traverse “V” on Fig. 3) extends into the spinel-rich core (the two innermost points, at d - 1.2 and 1.4 mm; note that this spinel-rich part of the melilite mantle corresponds to what EL GORESY et al. (1985) referred to as the “melilite veneer”, and which we interpret to be simply mantle melilite extending into the spinel-rich zone). With the apparent exception of Eu, there is no significant discontinuity in the REE abundances across the core/mantle boundary, an observation that is consistent (see below) with the core and mantle being cogenetic. Although broadly similar in some ways, the three traverses in Fig. 3 show significant differences: traverse “Y” shows no REE enrichment just beneath the rim, and the sub-parallel traverses “8” and “V” do not exactly overlap. These differences are not due to analytical imprecision, because the error bars for each measurement, based on counting statistics, are much smaller than the observed differences and cannot explain them. Moreover. since the differences are systematically correlated and not random, it is unlikely that they are due to any analytical errors. The error in the measurement of distance from the rim is conceivably as large as 50 pm, due to irregularities in the surface of the rim that make it difficult to decide where exactly to measure from. Even so, the errors are far too small to account for differences in the zoning trends. A key observation relevant to the above question is that the Ak data in Fig. 3 show significant scatter about the best fit line. In fact, although for reasons of clarity we have not shown individual Ak profiles on Fig. 3, the three traverses exhibit analogous differences to those between the REE trends, suggesting that a probable cause of the differences is a sectioning effect. Ideally, length-parallel traverses of the kind shown in Fig. 3 are measured from one end of a crystal through the exact center to the other end ofthe crystal, parallel

2416

G. .I. MacPherson. G. Crozaz and L. L. Lundberg Mantle

Melilite

in Allende

5241

Traverse Y . Traverse 8 x Traverse V

VI

3-i Q”

i;^

I

,

Od

cl6

I

08

Distance from

1

,

‘2

‘S

Rim

I

+

34

(mm)

FIG. 3. REE step-scan traverses across three locations III the melilite mantle. Also shown at upper left ISa composite of the melilite Hkermanite variations along the same traverses, as determined by detailed electron microprobe step scan analyses. Typical errors for Ce are less than 5% (relative) and 5-10% for the other REE.

to the section plane but above or below rt. Therefore. mos: of our traverses are along the prujecrrorr,~ of’ the cores of the analyzed crystals into the plane of the section and are not

to the c-axis. In practice, this is nearly impossible to achieve; generally, either the c-axis projects out of the plane of the section to some degree or else the core of the crystal is parallel

7Sample

(WI%)

t

I

Mgc? Fe0 cso \‘a*0

:i y;

V

_...~_

-.--.____.-.

0.17

0 9Y

ND ND

25.84 0 lrn J3d UO8 “i 33 0 “I ilO1 0 :u

28.69 25.34 0.06 4.61 0.05 41.06 0.01 0.01 0.04

30.29 32.19 0.06 5.84 0.09 41 32 0.03 001

28.2 24 ? OG 4.:

$;p3 I

2’03

01

I

4: :

n:

I -/--____-..--_-.

‘GO CrA TOtal

Ion !

lMJ2

09 2

99 64

99.81

3 .90 i 0. I 1 9 .65 t 0 24 1 .57 t 0.06 7 .63fO17 2 42iOl2 1 .33 * 0 05 0 .55 * 0.03 3 .59 * 0. IO 0 .75 * 0.0s 2 .18fO.C4 20*0@2

2.51 zt 0.07 6.31 i 0.1s 1.06 * 0.04 4.45*o.ll 74 * 0.08 1.19*0.03 0.38 * 0.02 2.70fOC6 0.50 2 0 03 t 0.04 olR+001

2.07 * 0 I1 5.36 * 0.20 0.91 * 0.06 4 1s*o.1s 143io.10 1.12*0.06 0.33 * 0.03 1 74 * 0.08 0.33 * 0.03 0.85 * 0.05 008R~OOlS

I stl* 0.06 3.79 * 0.14 0.61 f 0.05 3.40 * 0.08 02 * 0.07 1.57io.13 0.21 * 0.03 .07 * 0.07 0.25 * 0.02 0.78 * 0.06 0 074 t 0.023

1.97 i 0.09 4.78 * 0.15 0.75 * 0.03 4.00 f 0.09 1.49 f o.oa 1.58~tO.11 0.24 f 0.03 1.39 * 0.09 0.30 * 0.02 0.91 * 0.07 0.14 * 0.03

.:

,.“

0 (j-2

-

99.80

~~-.--

loo 07

--_.

101 ;i

Ak

La (ppm) CC Pr Nd Sm EU Tb DY HO Er Tm

Traverse

,,, 1

0 04

Rlm(mm)

Averape

I‘able 1. Mantle Melilite --__.._ -.

/

Dlsl. from

SIO,

______---_---._

[ e

I

1.48

I

I

2.16 f 0.09 4.73 f 0.15 0.69 * 0.03 3.41 i 0.09 1.01 f0.06 1.39*0.10 0.20 f 0.02 0.96 f 0.06 0.19 f 0.02 0.51 f 0.05 0.090 * 0.019

2.20 * 0.06 4.73f0.10 0.6-l * 0.02 3.11~0.05 0.83fOo4 1.68 f. 0.07 0.132 f 0.013 0.83 * 0 04 0207iO.013 0.62 + 0.07 0059fOrllS __ _. ._.

: 37 * 0.i:’ 2.83fOC-J 0365iOOi: 1.57*0.02 0345~O.#I1 07SiOO~ O.M6 * o.cc 0288~0009 0.059 * 0 002 0 166 * 0.00”~ n.0 I 5 i 0 w: _. _._

2417

Evolution of a Type B inclusion in Allende Table 1. (continued) Travcrrc

Travcnc

Y 8U4’

8M3

8M2

8MI

1.41

0.16

0.33

0.44

0.82

1.22

31 26 20.73 0.06 6.22 0.11 41.73 0.08 0.01 003

24.2 32.2 0.0 1.4 0.2 41.9 0.1 N.D. N.D.

28.0 25.7 0.0 4.0 0.1 41.9 0.1 N.D. N.D.

27.12 27.65 0.05 3.55 0.06 41.33 0.02 001 0.04

28 56 2558 004 4.32 0.07 41.20 0.00 0.01 0.02

31.15 20.79 0.03 6.20 0.14 41.14 0.03 000 0.03

loo.0

998

99.81

99.81

99.51

I2

30

25

31

43

3.18f0.11 7.55 * 0.19 1.22f005 6.13iO.14 1.85 f 0.09 1.22 * 0.05 0.47 * 002 3.23 ?z 0.09 0.59 f 0.04 1.73 * 0.05 ~.188*0.011

1.77fO.lO 4.70 * 0.20 0.72 f 0.05 3.79io.14 l.o9f0.09 I .07 * 0.06 0.30 i. 0.02 I .69 t 0.08 0.30 * 0.04 1.00 * 0.05 0.130f0.012

YU6

Y?.iI

YM2

Yhr3

YM4

n.45

Dlst. from Rlm(mm)

0.11

0.44

0.76

1.05

I .27

25.66 30.18 0.07 2.62 0.10 4141 0.03 0.01 O.O4

28.77 25.50 0.05 4.53 0.08 40.55 0.08 0.01 0.04

28.53 25.09 0.04 4.47 0.05 41.36 0.02 0.01 0.W

28.18 26.62 0.06 4.10 0.06 41.11 0.01 0.01 0.05

29.48 24.41 0.07 4.84 0.07 41.21 0.03 0.01 0.03

loo 13

99.62

99.60

loo.19

loo.15

31

32

SIO, (wt%) 401 TIO, Me0 Fe0 cno N%O “20 Cr,O, TOtpI Average ip k

b (ppm) CC Pr Nd Sm EU Tb DY Ho Er

Tm

I8 I 87f0.11 3 .% f 0.20 0 55 * 0.M

3 36 * I 35 * I 77 * 0 30 f 1.94 * 0 39 * 1.34 * 0. 12 f

0.10 0.08 0.20 0.03 0.09 0.02 0.08 0.03

2.30 * 0.07 5.63 f 0.15 0.91 f 0.03 5.11 to.08 I.71 f0.06 1.15io.14 0.33 f 0.02 I .82 f 0.06 0.410 * 0.015 1.28 f 0.05 0.l58f0.014

2.82 * 0.09 7.87 f 0.20 I .24 f 0.01 6.39fO.10 2.19 f 0.08 1.64f0.17 0.52 f 0.03 3.32 f 0.09 0.90 * 0.02 3.24 f 0.08 0.53 f 0.02

28 2.08 f 0.08 5.27 f 0.16 0.84iOO3 4.62 * 0.08 1.47 * 0.06 1.53 * 0.14 0.27 f 0.02 l.58fO.O6 0.362 f 0.015 1.16f0.05 0.162 f 0.017

34 I .77 * 0.07 4.54 * 0.14 0.70 * 0.03 3.68 ?c 0 07 1.o6 * 0.05 1.50i0.13 0.216 f 0.018 I.13 * 0.05 0.256 f 0.012 0.87 f 0.04 0.107 * 0.015

true end-center-end zoning profiles. A collection of traverses from different crystals would be expected to show variations in zoning profiles, which is what we observe. This sectioning effect is probably responsible for some of the large scale differences between the trends shown in Fig. 3. Smaller-scale differences among the traverses are due to contamination of individual analysis spots by alteration veins and by tiny inclusions within the melilite crystals. We have already noted the presence of numerous tiny pyroxene crystals enclosed within the mantle melilite crystals, with narrow zones of Hkermanite-rich melilite surrounding them (the origin of such pyroxene inclusions, common in most Type Bs, is not clear: see MEEKER et al., 1983, and MACPHERSONet al., 1984, for discussions). In addition, numerous veins of alteration material crosscut the melilite. We took care to avoid alteration veins and larger pyroxene inclusions by petrographically scrutinizing the crystals prior to analysis to select contamination-free sites; avoiding all of the tiny pyroxene grains and, especially, the Hkennanite-rich zones surrounding them was not possible. We therefore monitored elements such as Ti, Zr and Na in order to evaluate the degree of pyroxene and alteration vein contamination of each analysis. None of the analyses represented in Fig. 3 contain more than approximately 3% contamination by pyroxene, the most likely source of REE, and only 3 out of 19 analyses show contamination in excess of 1%. Thus, many small-scale variations in REE concentrations in Fig. 3 are probably due to the smallscale variations in melilite akermanite content. Some small-scale variations cannot, however, be explained by simple akermanite variations. An example is the point at -0.75 mm from the rim on traverse “Y” (Fig. 3), which is 2X more enriched in Ce, Sm, and Er than any of the other points at comparable distances from the rim and yet shows no excess contamination. JOHNSONet nl. (1988) noted similar “hot spots” in other Type B inclusions for which no textural

8

8M5’

Snmpk

IC024 43 I.50 3.51 0.542 3.02 IO1 I .88 0.220 I 37 0.304 0.929 0.122

* 0.03 * 0.06 f 0.012 * 0.03 io.02 * 0.05 f OD.38 f 0.02 * o.oaf f 0.019 * 0.009

2.30 f 0.07 5.39 * 0.16 0.84 t 0.03 4.68 f 0.09 I .59 f 0.08 1.25iO.ll 0.37 * 0.03 1.82 * 0.08 0.405 * 0.017 1.26 f 0.05 0.189 f 0.017

I.81 f0.M 4.83 * 0.14 0.79 * 0.03 4.15fO.o7 l.l5iO.o6 1.34*0.10 0.233 f 0.019 1.17*0.05 0.265 f 0.013 0.84 f 004 0.115 t 0.014

I

.43 * 2.99 * 0.5 1 * 2.45 f 0.65 f I.46 t 0.09 * 0.39 * 0.074 * 0.23 f 0.029 f

0.05 0.1 I 0.02 0.06 0.04 0.09 0.01 0.03 0.00 0.02 0.01’

explanation could be found, and suggested they might be chemical traces of pre-igneous grains. Finally, there is the possibility of lateral heterogeneity within the inclusion. Such heterogeneity could have resulted from local kinetic effects during crystallization of the original melt droplet from which 524 1 solidified, alternatively, it could be the result of later local reprocessing of the inclusion. The very local development of anomalously gehlenite-rich (Ak 2) melilite near the rim along traverse “V”, and the lack of a pronounced enrichment of REE just beneath the rim in traverse “Y”, appear to represent inhomogeneities developed within 524 1. B. Spinel-rich core Typical REE patterns for melilite and fassaite from the spinel-rich core of 5241 are shown in Fig. 4, and analytical data are reported in Tables 2 and 3. Fassaite has a fractionated REE pattern, Ce being -915X Cl chondrites and the HREE -20-40X Cl, with a large

La

Ce Pr Nd

Sm

Eu Gd Tb Dy Ho Er Tm Yb Cu

FIG. 4. Representative REE patterns for pyroxene and melilite in the spinel-rich core of 524 I. Margin, center and intermediate refer to positions within single melilite and pyroxene crystals.

2418

G. J. MacPherson, G. Crozaz and L. L. Lundberg Table 2. Compositionof Core Melilite Sample

6MI

6M2

6M4

6M3

6M5

6M6

WMI

WM2

WM3

WM6

Dist from Ctr. of (mm)

-0.83

-0.74

-0.43

-0.29

0

0.18

0.36

0.76

1.4

2.2

36.16 13.31 0.02 9.28 0.04 41.01

35.26 14.29 0.04 8.62 0.09 41.14

31.72 20.55 0.04 6.29 0.10 41.22

32.98 17.62 0.05 7.54 0.09 41.15

33.24 17.82 0.02 7.41 0.07 41.28

32.29 19.29 0.06 6.51 0.07 41.59

32.84 18.01 0.06 1.44 0.10 41.30

Na,O IV’ Crz%

0.11 0.01 0.03

0.03 0.01 0.04

0.04 0.00 0.04

0.06 0.01 0.03

0.04 0.01 0.03

0.05 0.01 0.00

0.11 0.01 0.04

34.29 16.13 0.05 8.23 0.16 41.15 0.05 0.02 0.04

34.08 15.72 0.05 8.34 0.20 40.69 0.10 0.01 0.04

34.62 IS.58 0.05 8.33 0.11 41.09 0.15 0.02 0.04

Totkll

99.97

99.52

lcu3.W

99.53

99.93

99.87

99.89

loo.12

99.23

99.98

64

61

52

52

48

51

Crystal

sio,

(wt%)

‘4403 TIO, Me0 Fe0 cso

Average

Lb

0.29iO.03 0.63iO.05 0.1Oet0.019 0.43+0.03 0.071+?3.018 0.87iO.08 0.018iO0.007 0.12OiO.017 0.048iO.009 0.14fo.02 0.025iO.014

La (ppm) Ct Pr Nd Sm EU Tb DY Ho Er Tm

45

0.38M.04 0.63iO.05 0.085M.017 0.33fo.03 0.08Mo.018 1.07M.07 O.OOSM.004 0.027M.010 0.015iO.005 0.045~.011
1.77iO.11 3.3HzO.17 0.43*0.05 1.89M.10 0.38M.06 0.97f0.15 O.OS9f0.022 0.21fo.05 0.043~0.013 0.17HU4 0.058+0.019

1.u*o.o7 2.80-Jz0.11 0.37~04 1.82ztO.07 0.36iO.04 1.81f0.10 0.032+0.010 0.06M0.019 0.017iO.006 O.O.Q!&OlS O.oopH).Ol3

2.04M.10 4.27i0.15 0.63~.05 2.53fo.09 0.81M.07 1.9SM.14 0.09iO.02 0.28fo.04 0.057M.013 0.25M.04 0.015fo.023

negative Eu anomaly. The margins of these fassaite crystals are somewhat more enriched in REE than their centers; in contrast, the major elements Ti and Al are more enriched in the centers of the crystals. The entire complement of core melilite in this thin section of 5241 is contained in a single large (nearly 0.5 cm long) crystal that has a reversely zoned center (Ak 5 1, surrounded by a narrow zone of Ak 44). The REE are fractionated in favor of the LREE throughout this crystal (Fig. 4): Ce/Er ratios (Cl normalized) range from 1.2 + 0.2 to 17 + 6 (1 a), and positive Eu anomalies increase as the absolute abundances of the other REE decrease. A line drawing of the melilite crystal and zoning profiles across it are shown in Fig. 5. Unlike the mantle melilite crystals which presumably grew from the surface of the inclusion inward, this melilite crystal

2.26f0.09 4.91f0.18 0.62H.04 2.67iO.10 0.78iO.07 1.97&0.06 0.124M.015 0.73*0.05 0.13iOo.02 0.49HJ.03 0.042f0.012

1.s5f0.05 3.07M.10 0.4tiO.02 1.98iO.04 0.44M.03 1.28iO.09 0.025i0.006 0.106iO.012 0.022M.003 0.046iO.010 0.01010.007

56

57 0.42H.02 0.70+0.03 0.093iO0.006 0.424M.014 0.053i00.008

I .87M.O6 4.22iO.12 0.51iOo.02 2.1 liO.05 0.46+0:03

0.86iO.07 0.005~0.003 0.028M.006 0.0014~0.003 0.034iOo.C~36 0.004M.006

0.94M.M 0.006~.002 0.018iO0.003 O.Olle3.02 0.022fo.M)3 0.009Hoo4

0.87f0.09 0.04SM.CnY 0.3Mo.02 0.063fo.o(k 0.173f0.01’ 0.024M.00

in the spinel-rich core grew concentrically outward from its center. This conclusion is based on the petrographic observation that birefringence (a direct function of gkermanite content) contours in the melilite are concentric about the center of the crystal. Accordingly, and as shown in Fig. 5a, the ion probe traverse cuts through the region of the center of the crystal, extending in one direction toward a neighboring spinel-free island and in the opposite direction toward the most extreme Ak-rich outer margin of the crystal. All of the REE show parallel trends, being most enriched in the crystal center and decreasing toward its margin. At the extreme margin of the crystal farthest away from the spinel-free island, the REE increase once again to nearly the levels present in the crystal center; however, where the crystal adjoins the spinel-free island, the REE in the melilite margin show no such

Table 3. Composition of Core Fassaite and Spinel-free Islands Fassaite, Anorthite and Melilite T

Core Fassalte

Splncl-free

Islands Amnhirc TAI

Mdllll~ TM1

45.13 15.06 3.19 12.63 0.05 25.33 0.01 0.00 0.09

42.81 36.30 0.03 0.18 0.03 20.33 0.12 0.01 0.00

35.52 13.70 0.03 9.03 0.05 41.07 0.17 0.01 0.04

101.37

101.50

99.88

99.62

1.28f0.06 5.91M.15 1.34fo.06 8.39iO.14 3.73M.13 0.17M.OI I .32iO. IO 8.O.tO.3 1.75io.08 6.02fo. 17 0.76fo.04

2.2’&0.08 9.41M.19 I 94M.08 12.58M.18 5.41?cOo.17 O.IOLtO.06 1.5Lx0.12 10.6M.4 2.8OztO.10 9.1M.2 1.30+0.05

l.wt0.08 2.6SM. 12 0.28OHl.017 1.16iO.04 0.28jzO.03 2.99ctO.15 o.o33K).oo7 0.224M.015 0.088iO008 0.144iO.16 0.030+!3.018

0.283M.016 0.55f0.03 0.075?zO.w6 0.32O~tO.014 0.086H1.01 I l.lM0.06 0.018f0.006 0.088i0.006 0.027~0.004 0.07M0.009 0.013~0.009

F%Jaitc Sample

XF4

XF6

1FZ

IF4

ZF8

ZFlO

sio*

IV Cv%

40.75 20.13 5.75 10.36 0.10 24.86 0.01 0.01 0.10

39.71 18.80 8.86 9.80 0.05 24.72 0.02 0.02 0.08

39.81 17.79 9.32 9.85 0.05 24.29 0.00 0.01 0.06

45.10 15.15 3.18 12.99 0.07 25.04 0.01 0.02 0.11

4257 15.56 6.81 11.41 0.10 24.84 0.01 0.01 0.06

TOtpI

102.08

102.05

101.17

101.67

2.16f0.08 9.58fo.19 1.92f0.08 12.64f0.17 5.32io.17 0.2OiO.04 1.67i0.13 1 l.MO.4 2.42iO.10 8.39f0.25 1.09f0.04

1.38+cO.c.5 5.82f0.14 1.24f0.06 7.52+0.33 2.96fO. 12 0.18M.04 0.98f0.10 6.16fo.28 1.42iO.07 4.66f0.18 0.61iO.03

1.53io.06 6.21ffl.14 1.29zto.06 8.5lM.15 3.53*0.15 0.16fO.W 1.09io.09 6.70f0.28 1.53io.07 5.37f0.19 0.77f0.05

7.6SiO.18 28.1fo.4 5.77M.15 35.M0.4 13.1iO.5 0.35f0.1 I 4.26i0.24 27.7M.7 6.94f0.19 24.6fo.5 3.41M.13

cwt9b:

A’,% TIO, M@ Fe0 cso Na,O

La (ppm) CC Pr Nd Sm EU Tb DY Ho Er Tm

58

0.52iO.03 l.OliO.06 0.107M.011 0.56H3.02 0.091M.014

2419

Evolution of a Type B inclusion in Allende

during the final stages of solidification of the 524 1 melt (see Discussion). C. Spinel-free islands __ dM4,,,,.,_....."..l'......, Spin&rich

core

I

Representative REE patterns for fassaite, anorthite, and melilite from spinel-free islands in 5241 are shown in Figs. 6-8. Analytical data are given in Table 3. Note in Fig. 8 that the irregularities in the HREE patterns of both the melilite and anorthite are due to analytical uncertainties associated with the measurement of relatively low-abundance HREE in the presence of relatively large LREE oxide interferences. Fassaite in spinel-free island #l (see Fig. 1 for numbering of islands; also, EL GORESY et al., 1985) is strongly zoned with respect to REE (Fig. 6) and Ti and Al abundances. The center of this crystal has a fractionated REE pattern, with Ce and the HREE enriched by - 1OX and -30X C 1 chondrites. respectively. REE concentrations in the margin are higher (Ce -30X and HREE - 100X Cl chondrites). Conversely, the margin is depleted in Ti (by -4X) and Al relative to the center. All fassaite REE patterns have a large negative Eu anomaly. Another island (+I; Fig. 7) contains a fassaite crystal whose boundaries extend into the spinel-rich core of 524 1. Figure 7b clearly shows, however, that the crystal is essentially homogeneous with respect to REE. Ce is -10-15X Cl chondrites, and the chondrite-normalized Ce/Er ratio is - 3. The REE patterns of melilite in the spinel-free islands (e.g., the one shown in Fig. 8) are distinct from those of melilite in either the spinel-rich core or the mantle: REE are relatively unfractionated and are uniformly low in absolute abundance, 5 - 1X C 1 chondrites. The large melilite crystal in the spinelrich core, discussed above, extends for several hundred microns into the interior of the adjoining spinel-free island (# 1); within -200 pm of the island/core boundary the REE composition of the core melilite becomes very low and unfractionated, remaining so across the boundary into the island and resembling all island melilite. Anorthite crystals are present in three of the spinel-free islands. Their REE patterns (e.g., Fig. 8) are similar, and are characterized by low abundances of all REE except ELI,which typically is 40-50X Cl chondrites. HREE (-1X Cl chondrites) are depleted relative to LREE (2-5X Cl). The trace element bulk compositions of the islands differ

b 7(x

Core

Melilite

q 60 spineI

. free 50

Island

3 3 26 24 2 1 I IL,

I

-1.5

-1.0

I

-0.5

I

0.0

I

0.5

3

1.0

.

1.5

I

2.0

.

2.5

SpmeCFree Island Fassaite

#l

:

Distance from Center of Crystal (mm) FIG. 5. (a) Line drawing of and (b) REE step-scan traverses across a single melilite crystal in the spinel-rich core of 5241. The crystal extends part way into the interior of a spinel-free island. The labels to each spot shown in (a) correspond to specific analyses reported in Table 2. The dotted line marks the location of the most gehlenite-rich zone in the reversely zoned interior of the crystal. adjacent

La Ce PC Nd

enrichment. This asymmetrical zoning is a reflection of the varying degree of efficiency with which island assimilation was able to contaminate isolated, residual pockets of melt

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

FIG. 6. Representative REE patterns of a single fassaite crystal in

spinel-free island # 1. Margin and center refer to positions within the crystal.

2420

G. J. MacPherson, G. Crozaz and L. L. Lundberg

SpineCFree Island

, , , , ;‘,

.lI SpinG_.ch

core\

;

j

--v”“5’

I’“-

island ___

La Ce Pr Nd

, , , , , , , , ,

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

FIG. 8. Representative REE patterns for melilite and anorthite in spinel-free islands.

Spinel-Free Island #4:

this feature cannot be the result of alteration and must reflect primary differences between the islands and the rest of 524 1.

Fassaite

D. Isotopic results

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

FIG. 7. (a) Line drawing and (b) representative REE patterns of a single fassaite crystal in spinel-free island #4. This crystal cuts across the boundary between the spinel-free island and the spine]-rich core. Dotted lines mark the 7 wt% and 10 wt% Ti02 contourS in the crystal. Analyses corresponding to the labelled spots are given in Table 3.

markedly from one another and from the bulk composition of 5241 given by NAGASAWAet al. (1977). The modal mineralogy estimate+ for island #1 is 3 I % melilite, 64% fassaite, and 5% anorthite; for #2 is 56% melilite, 44% fassaite, and trace anorthite; and for #4 is 93% fassaite and 7% anorthite. We necessarily assume that the modal differences among the islands, with respect both to each other and to the bulk inclusion, are real, because we have no way of evaluating whether the plane of the thin section through each island is truly representative. Reconstruction of bulk REE patterns from modes and mineral REE analyses shows all three islands to have LREE-depleted patterns and negative Eu anomalies (both due to the abundance of fassaite). Island #l is -40X Cl enriched in HREE, whereas island #2 is only 20X Cl. Therefore, the trace element data indicate that the islands differ significantly from one another in bulk composition; a further inference is that the bulk compositions of at least some of the islands are different from that of the melt from which the mantle and spinel-rich core solidified. This is supported by data (EL GORESYet al., 1985) showing that melilite in the spinel-free islands contains -0.1-0.35 wt% Na20, whereas the core and mantle melilite contain undetectable quantities (~0.05%). As EL GORESY et al. (1985) point out,

Magnesium isotopic data for melilite in the mantle, core, and spine]-free islands are given in Table 4; magnesium in core fassaite and spinel, and in spinel-free island fassaite, is fractionated to the same extent as in melilite. The inclusion is isotopically heavy and, within analytical uncertainty, uniform throughout its entirety: the intrinsic mass fractionation, F = 6.1 + 1.20/w(1 u). These data allow an upper limit to lx?&aced on the amount of differential volatilization that could have been involved in the formation of the melilite mantle of 5241 (see below). IV. DISCUSSION

A. General comments There is little dispute (although see MEEKER et al., 1983) among researchers who work on refractory inclusions that many of the features of Type B (especially Type B 1) inclusions resulted from melt solidification. All of the discussion that follows accepts this interpretation for 5241 which (except for

Table

4.

in Allende

5241 Melilite

Fhlg@’ (%J zt 1 a) Manrl@”

Area V

7.8 f 1.1 7 I f

1.4

7.6f

I.1

67f

I.1

4.7 It I .3 40*

I.1

5.2 f 0.9 Area 8

5.2 f I 5 4.5 f I.2 59t1.3

Area Y

Core

(a) Difference

)

79kl2

/

6.9f1.3

and on the terrestrial

(Tres Hermanos (b)Data

5.8i

I.6

between the measured A”Mg

the meteorite

t Modes were estimated by cutting and weighing tracings from back-scattered electron photomicrographs of each island. Area % is assumed to be approximately equal to volume %.

Mp.MassFractionation

Intrinsic (F,,)

meIlk).

reponed in order of ~“creasmg

distance from nm.

on

standard

242 I

Evolution of a TypeB inclusion in Allende the islands) is a fairly typical B I. However, we noted earlier that a continuing point of controversy regarding Type Bl inclusions in general, and 5241 in particular (EL GORESY et al., 1985), is the origin of the melilite-rich and relatively spinel-poor mantle. Specificaliy, did it originate by simple fmctional crystallization of the same melt that ultimately formed the core, or were other processes (such as surface volatilization or later addition of a second melt) involved? In addition, the presence of the spinel-free islands in 5241 complicates the situation and must be. accounted for in any petrogenetic model. From the outset of this project, we specifically aimed at collecting sufhciently detailed mineralogic and trace element data (e.g. zoning profiles) to permit an accurate reconstruction of the crystallization history of 524 1. In this section we compare our trace element and isotopic data with the model predictions of simple Rayleigh fractional crystallization, combined fractional c~s~i~tion/xenoli~ assimilation, and volatilization. Two prerequisites for quantitative calculations are to construct a simplified and numerically tractable physical model for 524 1, and to synthesize the mineral-chemical observations of 524 1. Constructing numerical models for fractional crystallization of 5241 requires relating the mathematical parameter F-the degree of crystallization-to a measurable physical property. In our first and simplest model, the inclusion is assumed to have been a spherical molten droplet 1.6 cm in diameter (based on the radius of curvature of 524 1 as seen in the thin section; see Fig. I), that cooled by radiating heat from its surface (MACPL~ERSONand GROSSMAN, 198 1; MACPHERSONet ai., 1984). As a consequence, it crystallized from the outside inward to form the mantle first and then the core. Therefore, at least in the case of the mantle, distance from the rim is an index of fractionation. The numerical value of F at a given location in the mantle is calculated by dividing the volume of the portion of the inclusion (a spherical “shell”) that is exterior to the spot by the volume of the entire inclusion; for example, the melilite mantle comprises roughly 40% of the total volume of 524 1, so the mantle/core interface represents F = 0.4. The outermost surface of 5241, located approximately where the Wark-Lovering rim now is, corresponds to F = 0. Unlike the mantle, however, there is no evidence that the core crystallized from the outside inward. Indeed, once a relatively solid mantle had formed, heat loss was probably controlled by conduction through the mantle. The core probably cooled more slowly than the mantle, and crystallization may have proceeded more homogeneously than in the mantle. Hence, we do not make the assumption that the exact center of the sphere represents F = I.0 or complete solidification. Instead, we have used the concentric zonation of the large melilite crystal in the core of 524 1 as a “time stratigraphic” record of the sequence of events during the later stages of solidification of 5241: we assume that the crystal’s center started crystallizing at the same time that melilite in the mantle of 5241 stopped crystallizing, a reasonabfe assumption because both are Ak 50-60. Thus, the crystal center corresponds to F = 0.4 and its outermost margin corresponds to F = 1.0. It follows from the above that a complete trace element evolution history of 5241 can be reconstructed by combining the traverses across the melilite mantle with the single-crystal

40 l

a

00

Europwm

cI Cenum

ii*

Degree

04

06

of Crystallization

OS

10

(F)

FIG.9. (a) Generalized REE patterns for mantle + core melilite in 5241, compared with (b) calculated REE trends from simple Rayleigh fractional crystallization models. Note that the horizontal axis here and inFig. 10is F. the fraction crystallized, corresponding to f -fin Eqn [I] (seetext). zoning profile of the core melilite. Three such composite profiles (Ce, Eu, Er), generalized to account for the differences between individual mantle melilite traverses and in which the abscissas are not distance as in Figs. 3 and 5b, but, rather, F, are presented below (Fig 9a) and compared with calculated models. The experimentally determined crystallization sequence of a melt of Type B composition (STOLPER,1982) is spine1 first, followed closely by melilite; both phases then crystallize together over a significant temperature interval to form the mantle, prior to the appearance of fassaite and anorthite and the beginning of core formation. The textures of 5241 conform reasonably well to such a sequence: rare spine1 crystals enclosed within the largely melilite mantle indicate that spine1 was followed by melilite, and the fact that the mantle occupies 40 ~01% of the inclusion shows that little else crystallized during a substantial interval (cJ: EL GORESY et al., 1985, and MACPHERSON and GROSSMAN,~ 981,fordi~ussionsofwhy spine1 might be so depleted in Bl mantles). The spinel-rich core consists mostly of fassaite (80%), with lesser spine], melilite (- 10% each) and trace anorthite. center-to-rim

B. A fractionalcrystallization model for 5241 The simplest model that might explain the structure and features of 5241 is fractional (Rayleigh) crystallization, in which the spinel-free islands are treated as inert bodies that do not participate chemically in the evolution of the core or mantle of the inclusion. The mantle and core are assumed to represent two stages in the solidification of a single liquid undergoing fmctionation, their boundary corresponding simply to a change in the crystallizing phase assemblage. The degree of success in modelling the observed REE trends across the core/mantle boundary is a test of the validity of this assumption vs. the model of EL GORESY et al. (1985) in which the mantle formed from a separate, later melt unrelated to that from which the core formed.

G. J. MacPherson, G. Crozaz and L. L. Lundberg

2422

The concentration, Cj (melt), of a trace element i in a melt that is undergoing fractional crystallization, can be described by the Rayleigh equation:

Table 6. Fassaite Partition Element Ce

C, (melt) = C, (melt),*f’+”

[II

Sm EU

where C, (melt), is the concentration of element i in the original liquid, fis the fraction of melt remaining, and Di is the solid/liquid partition coefficient. For a multi-stage process involving changes in the crystallizing phase assemblage, Di during each stage is the modally weighted bulk partition coefficient (e.g., ALL&GREand MINSTER, 1978). Note that in this paper we use the notation F, where F=l-/

I21

is the fraction crystallized. Because we are interested in m~elling the evolution of REE zoning profiles in crystallizing melilite, the instantaneous concentration C’,in melilite is calculated by simply inverting the partition coefficient for melilite at each step: Ci (melilite) = I& (mel/liq) * Ci (liq). Thus, a critical foundation of our model calculations the choice of partition coefficients.

131 is

C. Partition coeficients Melilite. Four experimental determinations of melilite REE partition coefficients, made on melilites of differing akermanite contents by RINGWOOD (1975), NAGASAWA efal. ( 1980), WWLUM et al. (1988) and KUEHNER et al. (1988), suggested a strong compositional dependence of the DEE (mel) on Ak. BECKEIT etal. (1988) recently demonstrated that DREEs+(mel) vary according to: DREEx+(mel) = constant*XG,/XAk

[41 where X, and Xnt are the gehlenite and akermanite mole fractions of the melilite, respectively. As a result, DREE3+ (mel) is a very sensitive function of the akermanite content of the melilite for XA~ < 40, but is much less so for XA~ > 40. We have used the mehlite partition coefficients for La, Sm, Y (an analog of Ho) and Yb, as measured by KUEHNER et al. (1988) for Ak 12 to calculate the appropriate constants for Eqn. [3] (see Table 5). The Ce constant (0.082) was taken to be the same as that for La and Sm, as determined from the data by KUEHNER et al. (1988). BECKETT et al. (1988) measured a much lower value for the Ce constant (0.054), Table

5. Melilitc

Partition

Coefficienls

La

0.082

(4)

CC

0.082”

(4)

0 054

(5)

Sm

0.082

14)

Y(Ho) Er

0.06

Tm

0.042

(5)

Yb

0.038

(4)

(1)Calculated

D in diopside’”

D’J’

D’”

0.13 0.42 0.26 0.45

0.098 0.26 0.31 0.30

0.12 0.33 0.06

0.09 0.34 0.12 0.74

I

(I) McKay et al (1988) (2) Gmtzeck er al. (1974) (3) Calculated from our data assuming that the centers of the core mehlite @MS) and of the core fassalte (XF6) where in equilib~um with the same melt (4) Calculated from OUTdata assuming that melilite just Inside the corn along traverse V (VM5) and the centerof core fassane (XF6) were in equilibrium wttb tbe same melt.

probably due to the presence of Ce4’ in their experimental melts (J. R. BECKETT,pets. commun., 1988). The partition coefficient for divalent Eu is expected to be approximately independent of Ak (BECKETT et al., 1988, 1989, in prep.). We have used DE” (mel) - 0.6 (NAGASAWA et al., 1980), a value identical to that determined for the partitioning of Sr in melilite by WOOLUM et al. (1988). The Er constant (0.05) was obtained by interpolation of data for Ho (Y) and Yb, taken from KUEHNER et al. (1988). Fassaite. There are no experimental determinations of REE partition coefficients in fassaites of similar composition to those in this inclusion. However, available experimental data for other pyroxene compositions indicate that the partition coefficients for a given REE are remarkably insensitive to major element compositional differences. This is shown in Table 6, where data for fassaitic pyroxenes in a melt of Angra dos Reis composition (MCKAY et al., 1988) are compared with those of GRUTZECK et ai. (1974) for diopside. The last two columns of Table 6 are empirical coefficients, calculated from our data by assuming that the center of core fassaite was in equilibrium with the same melt as either the center of the core melilite or the core melilite adjacent to the core-mantle boundary. The experimental and empirical values for Ce and Sm are in good agreement (i.e., within a factor of 1S), whereas differences for Er (factor of 3) and Eu (factor of 5) are much larger. Faced with the problem-all too common in terrestrial igneous petrology-of choosing between poorly constrained empirical partition coefficients from ap propriate bulk compositions and well-constrained experimental partition coefficients from inappropriate synthetic bulk compositions, we have chosen the synthetic values. The reason is the good agreement between the two sets of experimentally determined partition coefficients for very different bulk compositions, contrasted with the much poorer agreement between the two sets of empirical values. We used the DREW(fassaite) from MCKAY et al. (1988) to model the evolution of inclusion 524 1.

(4)

D. Cr~~staIi~~ationmodel

0.09*

from data an Reference Interpolated from data in (4) (3) DE” does not stg~i~cantiy vary with meliiite composttton (4.5) (4) Kuehner c, aI (1988) (5) Becken etaI (1988) (2)

D in fassaitet”

(4) 0.6”’

EU

EI

Coefficients

We assume that melilite was the only crystallizing phase during formation ofthe mantle, since the minor mantle spine1 has no significant effect on REE evolution. The crystallization of the mantle was modelled in three steps in order to take into account the variations in the Ak content of the melilite: from F = 0 (at the rim) to F = 0.14 (at d - 0.4 mm), Ak

2423

Evolution of a Type B inclusion in Allende varies from 10 to 24; from F = 0.14 to 0.22 (at d - 1.Omm), Ak varies from 24 to 32; from F = 0.22 to 0.4 (d - 1.2 mm). .&k varies from 32 to 42. In each interval the ikermanite variation is assumed to be a linear function of F. Although the crystallizing assemblage during solidification of the core was 10% melilite + 80% pyroxene -t 10% spine1 + trace plagioclase, we use the approximation

Mantle Melrkte

D, (core, bulk) - 0.1 *D, (mel) + O.S*Q (pyx) because lo.1 *L)i (spine11 + -0.02*0, (plag)] is effectively zero and therefore has been neglected in our calculations. The crystallizing proportions are assumed to remain constant. The melilite composition is assumed to vary linearly from Ak 42 at F = 0.4 to Ak 60 at F = 1.0 and, again, Ci (mel) and Dj (mel) are calculated according to f3] and [4] respectively. I

E. Initial melt composition Assuming that the outermost mantle melilite represents the very first crystallizing solid from the 5241 Iiquid, dividing the measured REE composition of this melilite (e.g., Table 1, samples VM8 and 8M5) by the appropriate partition coefficient, D,, should give the REE composition of the starting (bulk) liquid. The result is a calculated liquid with uniform REE enrichment of -20-25X Cl chondrites and a positive Eu anomaly. in good agreement with the measured REE bulk composition of 5241 (-18-21X Cl) given by NAGASAWA et al. (1977), except that the latter data show little if any Eu anomaly; the probable contribution of some Eu-depleted spinel-free island material in the NAGASAWA et al. sample can easily reconcile the difference in Eu. For the numerical models described below, we assumed that Eu = 35 X Cl and other REE = 23 X Cl in the initial melt. The fact that the REE content of the outermost mantle melilite is in agreement with that predicted by [Di X bulk 52411 is supporting evidence that 5241 did indeed solidify from a melt and argues against a fassaite-replacement origin for the melilite (e.g., MEEKER et al., 1983). F. Resulis The results of calculations for closed system fractional crystallization are shown in Fig. 9b, along with composite trends (Fig. 9a) showing the observed REE variations in core and mantle of 524 1 as described above. Figure 10 shows two actual Ce trends from the melilite mantle, traverses “8” and “V”, compared with calcuiated trends for the mantle only (I; I 0.4) (results for Sm and Er are similar to those for Ce, but are not shown in Fig. 10 for the sake of clarity). The fit of the calculated trends to the large-scale observed variations is good for Ce and Er (and, similarly, Sm); no attempt was made to model the small scale variations in the gkermanite content of the melilite, such as the zones surrounding the tiny fassaite inclusions, so the reader is cautioned against expecting any success at modelling small-scale REE variations. A counterintuitive prediction of the mode1 is that the REE contents of crystallizing melilite decrease with increasing f~ct~onation during the early stages of c~s~~i~tion, because the increase in melt REE concentrations is more than o&t by the decrease of D, (met) with increasing Ak. However, this decrease in C, (mel) is precisely what is observed in the mantle

00

01

02

03

04

05

Degree of Crystallization (F) FIG. 10. Calculated fractional crystallization (Rayleigh) model for the evolution of Ce and Eu in 5241 mantle melilite, compared with observed variation patterns.

melilite of 524 1: the steep increase in the gkermanite content of the melilite away from the rim mirrors a steep decrease in REE concentrations, because of the negative dependence of I&++ (met) on Ak. The calculations are relatively insensitive to the timing of the onset of pyroxene crystallization, except insofar as pyroxene crystallization affects the Ak composition of the crystallizing melilite (MACPHERSON et al., 1984). As would be expected for Rayleigh-type fractionation, the final stages of solidification result in a dramatic increase. of REE in the metilite as the reservoir (residuaf melt) dwindles, The outermost margin of the melilite crystal in the spineirich core of 524 1, on the side opposite from the spinel-free island, shows precisely such enrichments in all REE, but the margin in contact with the spinel-free island shows no enrichment. We interpret (see below) this asymmetry in the zoning of the core melilite crystal to be the result of continued iocai assimilation of the REE-poor islands after communication with more remote pockets of residual melt had ceased due to sealing off of intercrystalline pore space. The fit of the Rayleigh fractionation model for the trivalent REE is by no means perfect, however. The Rayleigh model and the a~imilation models described below predict REE3” contents in melilite for 0.5 < F c 0.8 that are consistently higher than those observed in the outer portions of the core melilite crystal; this discrepancy is discussed in the section on assimilation models. The situation for Eu with respect to a Rayleigh model is very different from that of the trivalent REE. There is indeed good agreement between calculated and observed Eu trends and Eu anomalies in the melilite mantle, as shown in Fig. IO. However, the calculated trend (Fig. 9b) for Eu in the spinel-rich core (F > 0.4) is not even close to the observed one. Whereas the observations show Eu decreasing into the spinel-rich core of 524 1, calculations predict a continued rise in the Eu concentration in melilite with increasing F. One possible but unlikely explanation is that L&u(mei) is strongiy dependent on Ak as are the other rare earths. That effect should be most clearly seen during the crystallization of geh-

2424

G. J. MacPherson, G. Crozaz and L. L. Lundberg

lenite-rich melilite such as that in the mantle, yet it is in the mantle melilite that the model is in best agreement with observed trends. An alternative explanation for the discrepancy is the crystallization, in the core only, of a Eu-preferring phase. We made calculations that explicitly included anot-thite, but even crystallization of voluminous anorthite (for which there is no evidence) is not a solution because L& (An) is -0.3 (e.g. MCKAY et al., 1988): significant plagioclase removai from the melt would thus not deplete the reservoir in Eu and cannot explain the discrepancy between the observed Eu abundance variations in 524 1 and a fractional crystallization model. A more likely cause is assimilation of the spinel-free islands, discussed in more detail below. Finally, we tested the “two melt” model of EL GORES eI 01. (1985) in which the mantle solidifies out of a separate, later liquid that collides with and coats the outer surface ofwhat is now the spinel-rich core. We assume in this case, just as in the case of “single melt” models as discussed earlier. that the mantle crystallized from the outer margin inward: we specifically reject the possibility that the putative mantle melt quenched on the cool surface of the already-solidified core. The reasons justifying this assumption are twofold: ( 1) in 5241, just as in other Type Bl inclusions, the mantle melilites are oriented radially inward from the outer surface of the inclusion and show growth interference textures (see MACPHERSONand GROSSMAN, 198 l), suggesting that they nucleated on and grew inward from the outer surface of a radiatively cooling melt droplet; and (2) the Hkermanite content of the mantle melilite increases inward instead of outward as would be the case if the mantle solidified from the inside out. All numerical models based on the EL GORESYd ul. (1985) two melt proposal predict a rise in REE concentrations in the mantle melilite near the mantle/core interface, because that interface corresponds to F = 1 for the putative mantle liquid. No such increase is observed in 524 1 mantle melilite, so we conclude that the melilite mantle of 524 1 did not form from a separate later liquid. Ci. Evaluation @the role c$xenohth as.s~miiatwr~ The two features (noted above) of the REE distribution patterns in 5241 that are not easily explanable by simple fractional crystallization-the decreasing abundance of Eu in melilite with increasing fractionation, and the curious asymmetricaI zoning of REE in the large melilite in the spinelrich core of 5241-suggest the possibility of another active process. Assimilation of the spinel-free islands is an obvious candidate for such a process, for three reasons: ( 1) one of the features that makes 5241 unusual relative to other Type B inclusions is the presence of the “islands”; (2) the islands themselves all have very rounded and irregular shapes, consistent with resorption; and, (3) the asymmetrical REE zoning in the core melilite seems to be directly related to the presence of a spinel-free island next to one side of the melilite. Numerical models for describing the effect on melt composition of combined wallrock assimilation/fractional crystallization (“AFC” models) have been given in the literature (e.g., DEPAOLO, 198 1, and references therein). We have used the formalism given by DePaolo to evaluate the role of assimilation of the spinel-free islands in controlling the REE

compositions of the evolving liquid and solid phases m 524 I For the case where D, -c 1. and where the mass of material assimilated per unit time M. is not equal to the mass of material crystallized per unit time Me, the concentration C,, o! a trace element in the evolving melt relative to the rnitiaI melt concentration C”, is given by:

‘m

5,’

--

L 1

‘.L_

where: r = MJM, = the “assimilation factor“ ./‘= Mm/~, = the fraction of melt Mm remaining rrjufm* 1~1 the initial melt mass MO, and is not the same as,fin ml: Rayleigh fractionation Eqn. [I] :=(riDl)/(rI). As DEPAOLO(198 1) emphasized, an important resuh nom such assimilation models is that the liquid composition does not generally evolve in the manner that might be expected from a simple binary mixing process. In adapting DePaolo’s model to evaluate the possible roic: of island assimilation in the REE evolution of 524 1, we make the assumption that the islands melt by surface dissolution rather than eutectic melting; this is probably reasonable, hecause there is no evidence for preferentiai resorption near polyphase boundaries or in the island intenors. Aiso, because the phase relationships for spinel--ee Type B compositions are not well known (J. R. BECKETT,pers. commun., 19899. the assumption ofsurface CL,., modal) dissolution is necessark by default. If the phase relationships are anything at all like those in the spinel-rich system (STOLPER. 1982). then the surface melting model introduces no large errors because the fassaite-rich island mineralogy approximates the fassaite-rich composition of the first melt of a Type B bulk composition. Initially, the composition of the “assimilant” is taken to br the average of the REE compositions of islands I. 2. and -1 (the REE composition ofeach individual island is the modall! weighted sum of the compositions of all the island phases. island #3 has been omitted from the island average because it adjoins the melilite mantle. making the modal proportions of the island difficult to determine with conhdencej. The fahsaite in some of the islands is strongly zoned with respect XL+ REE, so we have simply used an intermediate value for fassaite REE as the “average” Two other hmiting cases are considered: one in which the assimilant consists mamly r:#t REE-poor island fassaite (the dominant REE-bearing phase). and one in which observed mineralogic constraints are ~g nored and best-fit values for the individual elements are determined. These are in fact the only relevant cases. because experimentation quickly showed that only compositions with low overall REE, negative Eu anomalies, and LREE < l-lREt would result in trends approximating the observed ones; II? other words, compositions rich in low-REE fasssitc. The first-order results of the calculations indicate rather clearly that (1) models in which the assimilated matenal 1~ “average island” are not notably more successful at repro ducing the observed REE trends than simple fractional crystallization, and (2) models in which REE-poor island fassaite is the main assimilant aredistinctly more successful, especialI!, in being able to reproduce the decrease in Eu in melilite following mantle formation. Figure II shows the results tor island assimilation usmg

2425

Evolution of a Type B inclusion in Allende “average island” composition and an assimilation factor (r) of 0.95 (i.e.. nearly as much material was being assimilated as was being crystallized). Note that, in this and in all other assimilation models shown, we assume that r = 0 for F < 0.4 (i.e., no significant assimilation occurs until after solidification of the melilite mantle). This is because no model in which assimilation begins at F = 0 can reasonably reproduce the observed REE (especially, Eu) trends in the melilite. The marked decrease of Eu, both in mantle melilite crystals where they cross the mantle-core interface and in the core melilite. indicates a fundamental change at F - 0.4 in the composition of the evolving liquid that cannot be explained by fractional crystallization alone. It is not clear why assimilation should be significant only in the later stages of solidification. The xenoliths obviously had to be added prior to mantle formation, otherwise they would have had to break through and disrupt the mantle “crust” enclosing the molten interior: there is no evidence that such fragmentation of the mantle occurred. Apparently. the chemical effects of xenolith resorption were simply not “seen” by the crystallizing mantle melilite to any extent that can be recognized in the trace element data. One possible explanation is that quench zones, formed from the melt around the relatively cold xenoliths when the latter came into contact with it, did not completely remelt until after the mantle had largely solidified; only then would xenolith material have contaminated the melt. The trends for Ce and Er in Fig. 11 are not appreciably different than for pure fractional crystallization except for slight increases beyond F > 0.4 and no late stage extreme enrichments in the final crystallizing melilite. Eu increases continuously with F, as in the pure fractional crystallization case, but begins to level off after F > 0.5. In all cases, regardless of assimilant composition, decreasing the assimilation factor, r, results in a progressively closer approach to pure fractional crystallization. Thus, in the case of Fig. 11, decreasing r exacerbates the disagreement between calculated vs. observed trends for Eu (e.g., compare Fig. 9 with Figs. 11 and 12). The only models in which calculated and observed Eu trends are in reasonable agreement are those in which the assimilant is very fassaite-rich and in which fassaite Eu concentrations (chondrite normalized) are represented by the lowest values (< -5) given in Table 3. Figure 12 shows the

fiT

40

AFC model: fassalte-rich island

5

01

1

00

04

0.6

0.8

04

06

08

10

FIG. 12. Calculated assimilation/fractional crystallization (AFC) model for REE in 5241 melilite, using “fassaite-rich island” mineralogy as the assimilant: C. (Ce) = 9.2x Cl chondrites, C, (Eu) = 5.6x Cl, C. (Er) = 29.8X Cl. The assimilation factor r = 0.95. results for the case where the assimilant is more pyroxenerich (-88 modal %) than “average island”, again using r = 0.95. In this case the Eu trend shows a dropoff after F > 0.4, similar to the observed trends. Ce and Er are not as well fit in this model as in the case of pure fractional crystallization, but are still in reasonable qualitative agreement. If mineralogic constraints are arbitrarily dropped. it is possible to fit all three elements very well by using assimilant compositions (chondrite normalized) of Eu < 5, Ce < 8. and Er < 20 using assimilation factors of r - 0.9 or less (Fig. 13). Although these individual REE values are not unreasonable in terms of analyses given in Table 3, unfortunately no actual fassaite. anorthite, or melilite analyses, or any combination of the three, will allow simultanmus good fits for Eu, Ce and Er. All three of the assimilation models illustrated here share a problem with the Rayleigh models described earlier, in that they predict markedly higher trivalent REE for 0.5 < F < 0.8 than are actually observed in the core melilite crystal. The reason for this may have to do with our imperfect understanding of the crystallization and/or assimilation processes. but there is one other possible explanation. The core of 524 I contains several large Fremdlinge, described in detail by ARMSTRONG et al. (1985). These Fremdlinge in turn contain, among many other phases, Ca-phosphate. BLUM ct al. ( 1989)

40

,

I

AFC model: best fit

I 02

I

I

02

Relative Fraction Solidified

u

p:

0, I 00

,,l

AFC model: average island

.=

;J-----j

f

1.0

Relative Fraction Solidified FIG. 11. Calculated assimilation/fractional crystallization (AFC) model for REE in 5241 melilite, using “average island” mineralogy as the assimilant: C. (Ce) = 7.3X Cl chondrites, C. (Eu) = 9.8X Cl, C, (Er) = 22.8X Cl. The assimilation factor r = 0.95. Here and in Figs. 12 and 13, F is the relative fraction solidified (see text).

I

00

I

02

04

06

08

10

Relative Fraction Solidified FIG. 13. Calculated assimilation/fractional crystallization (AFC) model for REE in 5241 melilite, using “best fit” values for the individual element assimilation compositions (no mineralogic constraints) C, (Ce) = 4.0X Cl chondrites, C. (Eu) = 4.0X Cl, C. (Er) = 20.0~ C I The assimilation factor r = 0.90.

2426

G. J. MacPherson, G. Crozaz and L. L. Lundberg

suggest that Fremdlinge originate as relatively homogeneous metal liquids within the molten refectory silicate liquid host. it is possible that these metallic assemblages remove some portion of refractory lithophile elements in addition to the siderophiles and, thus, contribute to the overall depletion of the RJ3E during the middle stages of crystallization. Certainly, phosphates can be major carriers of REE. It should be noted, however, that BLUM et al. (1989) regard the phosphate as forming at subsolidus temperatures; if so, the phosphate cannot itself have been involved in the REE fractionation during melt solidification. One other difference between the assimilation models and observed trends is worth pointing out. The REE concentrations vs. Ftrends (Fig. 9a) show late-stage dramatic increases, a feature that is clearly reproduced in the pure fractional crystallization models but missing from the assimilation models. A likely explanation is provided by examining, in Fig. 5, the pronounced asymmetry in the trace element zoning pattern near the margins of the large melilite crystal in the spinel-rich core. Where the melilite adjoins spinel-free island #I, the melilite’s outermost margin has precisely the same low REE composition as the melilite in the island and the trend resembles that of the assimilation models. However. the margin on the opposite side of the same core melilite crystal, away from the island, shows pronounced late stage REE enrichment as predicted by closed system fractional crystallization. We interpret this asymme~ to be the result of the isolation of residual melt pockets during the final stages of solidification. Melt pockets located at any significant distance from the nearest spinel-free island were no longer in “‘communication” with ongoing xenolith assimilation and finished solidification under conditions of essentially pure fractional crystallization. Areas directly adjacent to an island remained in contact with the ~on~minant throughout the final period of crystallization. Clearly, the assimilation models presented above do not pe&ctly reproduce the observed REE trends in 524 1 melilite. Nonetheless, they are reasonable approximations to the observed trends and, in our opinion, island assimilation is the on& likely explanation for the evolution of Eu during the middle and late stages of fractionation. Simple fractional crystallization alone cannot explain the ELIdata. li! Evafuation @the role qf voiatilization KURAT ef af. ( 1975) invoked surface volatili~tion from a melt to explain the origin of the refractory-enriched melilite mantles of Type Bl inclusions. An explicit requirement of their model is that such a volatilization event occurred over a sufficiently short time span relative to cation diffusion rates in the melt to allow a concentration gradient to be established and preserved after solidification of the inclusion. The fact that our trace element data for the melilite mantle in 524 1 are well fit by a simple fractional crystallization model provides no support for the volatilization model. However, lacking specific knowledge of REE diffusion properties in a melt of Type B1 composition, trace element distributions in 5241 do not provide a quantitative test for the volatilization model. A more fruitful approach is to look for signs of isotopic mass fractionation that might accompany a volatilization event, in this case in the melilite mantle of 524 I. As noted

earlier (see Table 4) the magnesium isotopic composmons of melilite from the mantle are in~tinguishable from those of melilite from the core and the spinel-f?ee islands, The air sence ofany significant differences between the melilite mautle and the spinel-rich core allows some limits to be placed on the amount of differential volatilization that might have taken place during mantle formation (via the model of KU%\ I et al., 1975). Experimentally determined fractionation factors ~~S~rMOTO etaf., 1989) for rn~n~urn in a sihcate melt indicate that 25% volatilization would have resulted in a -5% difference in mass fractionation (of ‘5Mg) between thr mantle and core, well within our limits of detection. We cat therefore place an upper limit of 25% on the amount of vo!atilization of Mg that the mantle may have experienced rel, ative to the core in 524 f V. CONCLUSIONS Although numerical modelling of bulk REE data for ~gs neous rocks has been done for many years in order to un, derstand the genesis of these rocks, it has only recently become possible to examine the dist~bution of REE on a few-micron scale within the phases of an individual rock by use of an ion microprobe. Our application of this new technique to the unique Allende Type B inclusion USNM 5241 has given k:$ insights into the evolution of this representative of an important group of refractory inclusions. The REE data are largely consistent with a model rn whrch the mantle and core of 5241 formed sequentially out of d single melt by fractional crystallization. However, numeric& models of REE evolution in the 5241 melt. especially that of europium, require that a significant mass of spinei-free island material was assimilated into the evolving melt during the last half of the ~lidifi~tion history of 5241_ Our resuitz thus strongly support the interpretation of EL GORESY’ cm/hour fo: diopside, depending on the melt composition and super” heating). All of the islands in 5241 are 20.5 mm in radius now, meaning that larger ones would have survived longer than 200 hours if the slower dissolution rates are appropriate ffthe faster dissolution rates are a~rop~te~ even much larger islands (several mm) would have dissolved in only 1-2 hours. Finally, our results provide no support for two models thar have heen proposed for Type RI inclusions. first, the absence

Evolution of a Type B inclusion in Allende

mass fractionation in the melilite mantle relative to the rest of 5241 indicates that the surface volatilization model of KURAT et al. (1975) is probably not required to explain the melilite mantle of 5241 or of any other Type Bl inclusion. The isotopic data place an upper limit of -2590 volatilization of the magnesium, corresponding to a much lower degree of volatilization for the total bulk composition if fractional volatilization occurs. Second, the trace element concentrations in melilite from the outermost portions of the melilite mantle are close to what is calculated by multiplying the bulk REE content of 5241 (NAGASAWA et al., 1977) by the appropriate melilite partition coefficient. This result is consistent with formation of 5241 by melt solidification but inconsistent with the model of MEEKERet al. ( 1983) in which melilite in Type Bl mantles formed by metasomatic replacement of fassaite. of any significant

isotopic

Acknowledgements-GC. thanks L. Haskin for introducing her to REE modelling, and G.J.M. thanks R. Nielson for help in setting up the assimilation/fractional crystallization models. We have greatly benefitted from discussions with J. R. Beckett, A. M. Davis, J. N. Grossman, S. Sorensen, E. Stolper and D. S. Woolum. The paper was significantly improved by the careful and thought-provoking reviews of A. M. Davis, J. R. Beckett and G. McKay. Technical support from the Washington University ion probe team under the direction of E. Zinner is appreciated. This work was supported under NASA grants NAG 9-55 (G.C.) and NAG 9-230 (G.J.M.). L.L.L. acknowledges financial support from the National Science Foundation (EAR 8719528, to G.C.). Editorial handling: G. A. McKay REFERENCES ALLZGREC. J. and MINSTERJ. F. (1978) Quantitative models of trace element behavior in magmatic processes. Earth Planet. Sci. Lett. 38, l-25. ARMSTRONGJ. T., EL GORESYA. and WASSERBURGG. J. (1985) Willy: A prize noble Ur-Fremdling, its history and implications for the formation of Fremdlinge in CAIs. Geochim. Cosmochim. Acta 49, 1001-1022. BECKETTJ. R., SPIVACKA. J., HUTCHEON1. D., WASSERBURG G. J. and STOLPERE. M. (1988) The partitioning of trace elements between melilite and liquid: An experimental study with applications to Type B CAIs. Lunar Planet. Sci. XIX, 49-50. BECKETTJ. R., SPIVACKA. J., HUTCHEONI. D., WASSERBURG G. J. and STOLPERE. M. (1989) Crystal chemical effects on the partitioning of trace elements between mineral and melt: An experimental study of melilite with applications to refractory inclusions from carbonaceous chondrites. Geochim. Cosmochim. Acta (submitted). BENCEA. E. and ALBEEA. L. (1968) Empirical correction factors for the electron microanalysis of silicates and oxides. J Geol. 76, 382-403. BLUMJ. D., WASSERBURGG. J., HUTCHEONI. D., BECKETTJ. R. and STOLPERE. M. (1989) Origin of opaque assemblages in C3V meteorites: Implications for nebular and planetary processes. Geochim. Cosmochim. Acta 53, 543-556. COLBYJ. W. (1968) Quantitative microprobe analysis of thin insulating films. In Advances in X-Ray Analysis, Vol. 11, pp. 287-305. Plenum; New York. DEPAOLOD. J. ( 1981) Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth Planef. Sci. Lett. 53, 189-202. EL G~RESY A., ARMSTRONGJ. T. and WASSERBURGG. J. (1985) Anatomy of an Allende coarse-grained inclusion. Geochim. Cosmochim. Acta 49, 2433-2444. FAHEYA., GOSWAMIJ. N., MCKEEGANK. D. and ZINNERE. (1987) 26A1,2”Pu, %, REE and trace element abundances in hibonite grains from CM and CV meteorites. Geochim. Cosmochim. Acta 51,329-350.

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GRO~.%AN L. (1975) Petrography and mineral chemistry of Ca-rich

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pp. 444-446. Springer-Verlag.