A layered-crust model of a Howardite parent body

lCARUS40, 455--470 (1979)

A Layered-Crust Model of a Howardite Parent Body HIROSHI TAKEDA Mineralogical Institute, Faculty of Science, University of Tokyo, Hongo, Tokyo 113, Japan Received March 19, 1979; revised May 21, 1979 A Howardite parent body is a Vesta-like hypothetical asteroid composed of diogenites, eucrites, and howardites (polymict breccias of various diogenites and eucrites). Combined single-crystal X-ray diffraction and microprobe studies of their pyroxenes indicate that their exsolution and inversion textures vary systematically with respect to their crystallization trend deduced from their Mg and Ca concentrations. Mg-Rich, early crystallized (presumably deep-seated) members revealed slowly cooled textures, except Mg-rich pyroxene fragments in eucritic polymict breccias. Present study of such pyroxenes in Yamato-74450 and -75015 found in Antarctica confirmed that they were originally cores of the very rapidly cooled Pasamonte-like pigeonites. Based on these data, we reconstructed a layered-crust model from bottom to top as: (A) Mg-rich diogenite layer with orthopyroxenes with or without exsolution lamellae of augite with common (100) plane; (B) Fe-rich diogenite layer with inverted low-Ca pigeonites and orthopyroxenes; (C) cumulate eucrite layer with low-Ca inverted pigeonites with blebby augite inclusions with (100) in common generally, and plagioclase (Binda is the most Mg-rich member of this layer); (D) Moore County-like layer with partially inverted pigeonites with (001) augite lamellae and plagioclase; (E) common eucrite layer with the Juvinas-like pigeonites with fine (001) augite lamellae and plagiocalse; (F) surface eucrite layer with the Pasamonte-like pigeonites which are chemically zoned. INTRODUCTION C o m p a r i s o n o f the s p e c t r a l r e f l e c t a n c e s o f a s t e r o i d s a n d m e t e o r i t e s has c o n t r i b u t e d to t h e i d e n t i f i c a t i o n o f t h e i r p a r e n t b o d i e s (e.g., Gaffey and McCord, 1977). A h y p o t h e t i c a l p l a n e t for g e n e t i c a l l y r e l a t e d a c h o n d r i t e s s u c h as d i o g e n i t e s , h o w a r d i t e s , a n d e u c r i t e s p r o p o s e d b y M a s o n (1967) w a s t h e first m o d e l o f s u c h an a p p r o a c h r e l a t i n g m e t e o r i t e s to a p l a n e t with the differentiated crust. The spectral studies of 4 V e s t a r e v e a l e d t h a t it is r e l a t e d to t h e a b o v e meteorites, especially basaltic achondrites. S i n c e then m a n y i n v e s t i g a t o r s h a v e p r o p o s e d t h a t 4 V e s t a a p p e a r s to b e the m o s t l i k e l y c a n d i d a t e in t h e solar s y s t e m for the p a r e n t b o d y o f t h e s e a c h o n d r i t e s ( M a t s o n et al., 1976; T a k e d a et al., 1976; C o n s o l m a g n o a n d D r a k e , 1977). T h e f a c t t h a t t h e s e meteorites are polymict breccias (Duke & S i l v e r , 1967) p r o v i d e s a d d i t i o n a l e v i d e n c e t h a t t h e y a r e the s u r f a c e m a t e r i a l s o f a p l a n e t . L e B e r t r e a n d Z e l l n e r (1978) conc l u d e d t h a t the p o l a r i z a t i o n - p h a s e c u r v e o f

Vesta can be explained by crushed basaltic achondrites with a substantial component of v e r y fine d u s t . P o l y m i c t b r e c c i a s o f the eucritic c o m p o s i t i o n f o u n d in t h e Y a m a t o meteorites (Takeda et al., 1978a,b; M i y a m o t o et al., 1978; T a k e d a et al., 1979a) m a y p r o v i d e i n f o r m a t i o n on t h e s u r f a c e materials of such an asteroid. Because the howardites have been known to b e p o l y m i c t b r e c c i a s o f v a r i o u s c r y s t a l line e u c r i t e s a n d d i o g e n i t e s , " h o w a r d i t e " is not a g o o d n a m e for a m e t e o r i t e class. We p r o p o s e to k e e p the t e r m h o w a r d i t e for the n a m e o f a V e s t a - l i k e p a r e n t b o d y o f genetically related achondrites. Although Mason (1967) a s s u m e d a p a l l a s i t e c o r e for his parent b o d y m o d e l , w e h a v e no d i r e c t e v i d e n c e s u p p o r t i n g this m o d e l . T h e o n l y i n d i c a t i o n w e h a v e at p r e s e n t is the s i m i l a r i t y o f o x y gen i s o t o p e r a t i o s for t h e s e m e t e o r i t e s . T h e h o w a r d i t e p a r e n t b o d y u s e d in this p a p e r is a s s u m e d to be c o m p o s e d o f e u c r i t e s , d i o g e nites, a n d the silicate p o r t i o n o f m e s o s i d e r ites a n d t h e i r p o l y m i c t b r e c c i a s . Many paper proposing Vesta-like bodies 455 0019-1035/79/120455-16502.00/0 Copyright © 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

456

HIROSHI TAKEDA

for the howardite parent bodies (e.g., Consolmagno and Drake, 1977) deal with only chemistry, especially of eucrites. We have been reconstructing a detailed layered structure of the crust of the howardite parent body from the crystallization trend and the exsolution and inversion textures of their pyroxenes (Takeda et al., 1976). The model has been proposed as a working hypothesis to correlate the above meteorite classes by applying typical processes of exsolution, decomposition, and inversion of pigeonite in the crystallization trends of the terrestrial layered intrusions (Ishii and Takeda, 1974). We developed a method to estimate cooling rates that relate them to the depth beneath the surface of a planet from the width of exsolved augite lamellae (Miyamoto and Takeda, 1977). During the course of this study, the recognition of two types of Mg-rich pyroxene found in polymict breccias of eucritic composition found in Antarctica has presented the complexity of the layered-crust model. This paper resolves this problem by the X-ray crystallographic investigation of these pyroxenes. This paper gives detailed accounts of the crystallographic characteristics of the pyroxenes in each layer of the crust together with the reasoning of placing the examined meteorites in each layer. A brief description of Antarctic meteorites newly found and related to these topics is also given as supporting evidence of the layered-crust model. EXPERIMENTAL

TECHNIQUES

The methods we used are the same as those described in our previous paper (Takedaet al., 1976). Several single crystals of pyroxene were separated from small meteorite chips of the Allan Hills diogenite ( A L H A 77256), Shalka, Tatahouine, Ibbenbiiren, Johnstown, Yamato-75032, and from Binda, Moama, Moore County, Mt. Padbury, Haraiya, Yamato-74356, Yamato74450, and Yamato-75015. Single crystals of the Yamato-74450 pyroxenes were separated from coarse-grained clasts more than

5 mm in diameter exposed on small rock chips. The pyroxene crystals were mounted approximately along the c axis, and were aligned with spindle axis parallel to the c* direction. Precession photographs of hO! nets were taken using Zr-filtered MoK radiation. The crystals were subsequently mounted in e x p o x y resin with the b axis perpendicular to the polished surface and were investigated by an electron microprobe analyzer. A polished thin section of ALHA77256 (Yanai et al., 1978) supplied by the Antarctic Meteorite Working Group and four new thin sections produced from Yamato-74450 by the curator at the National Institute of Polar Research (NIPR) have been investigated by an optical microscope and electron microprobe. A polished thin grain mount of Yamato-74356 was investigated by the same method. Quantitative chemical analyses were made with a J E O L JXA-5 electron probe X-ray microanalyzer with a 40 ° takeoff angle. The method is the same as that of Nakamura and Kushiro (1970). The bulk chemistries of the Antarctic diogenites, ALHA77256 and Yamato-

TABLE I BULK CHEMICAL COMPOSITIONS OF ANTARCTIC DIOGENITES n

SiOz TiOz AI2Oa Fe203 FeO

FeS MnO MgO CaO Na~O K20 H,O~-) H~O~+)~ P20,~ NiO Cr20~ Co

Total

Yamato-74013

ALHA77256

Yamato-75032

51.35 0.13 0.89 -16.35 0.82 0.48 26.04 1.10 0,04 0.02 0.00 0.4 0.09 0.00e 2.49 0.003 100.21

49.03 0.26 1.59 0.60 16.69 0.62 0.45 27.66 1.29 0.05 0.02 0.10 0.95 0.21 O.O04t 0.76 -100.28

51.92 0.40 2.28 -18.85 0.30 0.55 20.99 3.31 0.12 0.04 0.00 0.32 0.03 0.003 0.72 0.003 99.84

o Analyst: H. Haramura. Data of Yamato-75032 given in Takeda et al. (1978b). b Volatiles released at 1100°C.

HOWARDITE PARENT BODY MODEL 74013, were analyzed by the standard wet chemical method (Analyst: H. Haramura, Geological Institute, Faculty of Science, University of Tokyo), and are compared with that of the Yamato-75032 diogenite with low-Ca inverted pigeonites (Table I, Fig. 1). RESULTS

Diogenitic Pyroxenes The crystallographic features of orthopyroxenes in diogenites differ very much from those in other achondrites, especially enstatite achondrites. Many of the enstatites, except that of Shallowater, show disorder produced by shock metamorphism (Reid and Cohen, 1967). Although the major phase in diogenites is orthopyroxene, they do not show the above features. No diffuse streaks observed in the enstatite achondrites have been detected. Extreme shock effects were not found. H o w e v e r , single crystal studies of a bronzite from the Johnstown diogenite show diffuse streaks similar to those of disordered orthoenstatite. H o w e v e r , the positions and intensities of the streaks are different from those of the disordered enstatite. The diffuse streaks along a* in some parts of the h02 rows have broad maxima

where twinned clinopyroxene (augite) spots would be expected. There is petrographic evidence for the presence of very fine clinopyroxene lamellae. The X-ray evidence suggests that the structure of the host pyroxene may in part have yielded slabs of the monoclinic sequence. Judging from the position of the broad diffuse maxima, the presence of 1.4% CaO in this bronzite, and the absence of the extreme shock effect, we interpret these slabs to be augite-like, which resemble extremely thin incomplete exsolution lamellae. The diogenites apparently formed by slow near-equilibrium crystallization producing large unzoned homogeneous crystals. No diogenitic orthopyroxene other than that in Johnstown shows the above diffuse streaks. The crystallographic features and the chemistries (Fig. 2) can be classified into two groups. (1) Orthopyroxenes with CaO weight percentage higher than about 1.4% (Ibbenbiiren) show reflections o f exsolved augites with the (100) plane in common with the host orthopyroxene (Fig. 3). The intensities of the augite reflection are almost identical with respect to the (100) plane. The presence of the augite lamellae was not detected by microprobe, but their CaO content varies from one place to another. The above facts suggest that the au-

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FIG. 1. Bulk chemical compositions of Yamato-75032, Binda, Yamato-74159, Yamato-74450, Yamato-75015, and Moama, plotted in a CaO-AIzO3 diagram. Squares: diogenites; solid circles: eucrites; and open circles: intermediate members.

458

HIROSHI TAKEDA CQ

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FIG. 2. Chemical compositions of pyroxenes related to the Howardite parent body. (a) Pyroxene quadrilateral for the monomict diogenites and eucfites. S: Steinbach; AH: Allan Hills ALHA77256; Y: Yamato-75032; B: Binda; M: Moama; MC: Moore County; JV: Juvinas; HR: Haraiya; and MP: eucritic clast in Mt. Padbury mesosiderite. Open circles: bulk compositions; solid circles: host phase; open triangles: exsolved augite; solid triangles: bulk augite. A part of data after Takeda et a/. (1976, 1978b). (b) Enlarged portion of the shaded small quadrilateral in (a). Squares: known diogenites; IB: IbbenbiJren; G: Garland. Dotted lines connect the chemical variation within TH(Tatahuine) and SL(Shalka). The dotted loop indicates variation in Y (Yamato recrystallized diogenites). Open circles: ALHA77256.

gite lamellae are very fine and may be twinned on (100). These exsolution features produced in primary orthopyroxene are very different from those of secondary orthopyroxene inverted from pigeonites and may be used as a signature to recognize diogenitic pyroxenes in polymict breccias. (2) The low-Ca orthopyroxenes (CaO < 1.4%) did not show the above crystallographic features. They are represented by Shalka (Fig. 3) and Tatahouine. The Yamato diogenites show recrystallized textures (Takeda et al., 1978b). The Allan Hills diogenite also belongs to the low-Ca variety and the chemistry lies between those of the above two diogenites (Fig. 2, Table II). The X-ray diffraction photograph did not show any evidence of

the augite exsolution. The CaO content is also consistent with the absence of the exsolved augite. The reflections of orthopyroxenes are diffuse due to the shock effect, but they are different from the diffuse streaks of the shocked orthoenstatite. The textures of Allan Hills (ALHA77256) diogenite is different from the other Antarctic diogenites (e.g., Yamato-6902, -74013, -74136, etc.), which show recrystallized texture. The A L H A diogenite is composed of large fragments with coarse-grained (> 1 mm) orthopyroxene crystals joined by a 120° triple-point juncture and vein-like matrices with fine-grained fragments of pyroxenes filling the interstices of the crystalline clasts. Small grains of augite Ca4sMg44Fe8 are present at some junction,

HOWARDITE PARENT BODY MODEL

I

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459

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FIG. 3. X-Ray precession photographs of the IbbenbOren orthopyroxene (IB) with exsolved augites on (100), and of the Shalka orthopyroxene (SL) with no exsolution of augite. The directions of the reciprocal lattice axes are indicated by arrows. The diffraction spots present in IB but not present in the orthopyroxene (Opx) of SL are those of augite (Aug). and numerous small inclusions of augite are present in some orthopyroxene crystals Ca1.rMgr4.3Fe24.0. The bulk CaO content of ALHA77256 is little higher than that of the Yamato recrystallized diogenites (Table I). An aluminum-rich chromite and troilite are the minor phases.

Eucritic Pyroxenes On the basis of their crystallographic features, eucritic pyroxenes can be classified into four groups. In order to interpret the polymict breccias of eucritic compositions found in the Yamato and Allan Hills eu1978b, 1979a; crites (Takeda et al., Miyamoto et al., 1978), it is essential to dif-

ferentiate two groups of chemically similar low-Ca, low-Fe pyroxenes. One group is what has been called Binda-type inverted pigeonite from the most Mg-rich cumulate eucrites Binda and Moama, and t h e other group is a uniform core of phenocryst pigeonites which occur in rapidly crystallized and cooled pigeonites from Yamato74450 and Pasamonte. The X-ray precession photographs of the Binda-type pyroxenes are characterized by the following patterns. The intensity of the 10.0.2, reflection of the (100) augite which is located between the 17.0.2 and 18.0.2. reflections o f the orthopyroxene is intermediate between those o f the two orthopyroxene

460

HIROSHI TAKEDA TABLE II

SELECXEDCOMPOSITIONSOF MINEaALS1NALHA77256DIOGENITEANDYAMATO-74356AND-74450EUCRITESa ALHA77256

Y-74356

Y-74450

Opx

Opx

Aug

Chromite

Pig

Aug

Plag

Pig

SiO2 AI2Oa TiO2 Cr20.~ FeO MnO MgO CaO Na20 K20 Total

53.8 1.27 0.16 0.25 15.13 0.51 26.6 0.82 0.00 0.00 98.54

54.0 1.22 0.17 0.24 15.28 0.57 27.2 1.16 0.01 0.00 99.85

52.6 1.18 0.19 0.37 5.90 0.26 15.99 22.3 0.13 0.00 98.92

0.01 21.4 0.87 43.4 25.8 0.66 6.71 0.00 0.00 0.00 98.85

48.3 0.37 0.37 0.74 33.2 1.04 12.02 3.41 0.00 0.00 99.45

49.3 0.71 0.48 0.28 19.17 0.64 10.06 17.55 0.00 0.00 98.19

43.9 35.0 0.00 0.05 0.20 0.01 0.03 18.64 1.08 0.04 98.95

49.5 0.98 0.24 0.63 26.8 0.82 15.51 4.18 0.00 0.00 98.66

Ca ° Mg Fe

1.7 74.5 23.8

2.3 74.3 23.4

45.3 45.3 9.4

7.4 36.3 56.3

37.7 30.1 32.2

9.0 46.2 44.8

Takeda, Ishii, and Yanai, unpublished data. b Atomic percentage.

reflections and is much stronger than those of augite exsolved from the primary orthopyroxene (Fig. 4). Comparisons of the intensity of the 802 augite reflection which is located between 17.0.2 and 18.0.2 of orthopyroxene with that of 10.0.2 indicate that the majority of the augite is not twinned on (100). The phenomena are different from the (100) augite in the primary orthopyroxene which shows the twinned intensity distribution of augite. This type of inverted pigeonites is characterized by its low Ca content and the blebby shape of the augite inclusions elongated along the c axis. One pale yellowish pigeonite crystal 0.5 mm in diameter in Yamato-74450, which has a chemical composition (Fig. 5) as Mgrich as the above Binda-type pyroxenes shows an X-ray diffraction pattern very different from the Binda-type. It is a pigeonite with minor exsolution of aug!te with the c o m m o n (001) plane with the host pigeonite and the outer thin rim shows chemical zoning toward the Fe-rich direction (Fig. 5). No exsolution of augite has been detected by the microprobe scan.

Another brownish crystal, which is more Fe rich than the above (Table II) shows a uniform composition and more extensive exsolution of augite on (001). The width of the augite lamellae is an order of the resolution of the microprobe beam (a few micrometers) (Fig. 5). This pattern is different from that of the Moore County pigeonite, which has similar chemical composition (Ishii and Takeda, 1974). Our interpretation is that this Yamato-74450 pigeonite is another variety of the rapidly cooled eucritic pigeonites, but the crystal growth and the subsequent cooling were slower than the above more Mg-rich Yamato-74450 pigeonite and it is different from the early crystallized cumulate eucrites, which cooled very slowly within the .crust. The Moore County pyroxene is characterized by partially inverted high-Ca pigeonite. The single crystal X-ray diffraction pattern (Fig. 6) showed that the (001) coarse lamellae of augite have their c* rotated 1.5 ° toward +a* from the c* of the pigeonite. This relationship was confirmed by separating the augite lamalla from the host mate-

HOWARDITE PARENT BODY MODEL

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FlG. 4. X-Ray precession photographs of the low-Ca inverted pigeonites with blebby inclusions of augite. Augite (Aug) and orthopyroxene (Opx) share the a* direction and (100) in common. BD: Binda; and MA: Moama.

rial. The other fine (001) augite lamellae were also detected by the X-ray single crystal method (Fig. 6a). The b* and c* directions of this fine (001) augite and the host uninverted pigeonite are identical. The b* and a* axes (that is b and c) of pigeonite and orthopyroxene partly inverted from pigeonite are common. All other common eucritic pigeonites which have similar chemical composition (Fig. 2) may be represented by the Juvinas pigeonite, although Nuevo Laredo and Sioux County show some modified features (Takedaet al., 1978a). Figure 6b is a precession photograph of a pyroxene from the Juvinas eucrite. The crystal selected for analysis is a pigeonite, i.e., the low-Ca

pyroxene that coexists with ferroaugite. The primary pigeonite now comprises a monoclinic, very-low-calcium host with exsolution lamellae of ferroaugite on (00l). The separation of the a* axes of pigeonite and augite are close to the maximum values that have been reported. The exsolution lamellae are 7-/zm wide (i.e., wide enough for microprobe analysis), indicating a moderate cooling rate, slow in comparison to lunar basalts. The pattern shows no orthopyroxene spots but does show diffuse streaks along the a* direction in parts of the h02 rows. The intensity distribution of streaks is even and does not show maxima. Strong streaks appear where strong orthopyroxene reflections would be expected,

462

HIROSHI TAKEDA Col

Di

I

Hd

0 o Mg

Fe

FIG. 5, Pyroxene quadrilateral for three typical pigeonites in Yamato-74450. Line connecting open circles indicates chemical zoning within the crystal. Open circles: bulk compositions, lines connect the host (solid circles) and the exsolved augite iamellae (triangles) pairs. Dotted line indicates incomplete resolution of augite lamellae.

between 102 and 202, between 202 and 302. Such streaks have not been reported for lunar pyroxenes. Yamato-74356 is the only crystalline eucrite of the Juvinas type found in Antarctica. This eucrite contains a pigeonite-augite pair with uniform compositions (Table II), but it is shocked. The exsolved augite with (001) in common with the host pigeonite was detected by the X-ray method, but the crystal did not show distinct lamellae-like texture. Pyroxenes in Polymict Breccias

Preliminary examinations of eucritic polymict breccias found in the Yamato and Allan Hills collection have been reported (Miyamoto et al., 1978, Takeda et al., 1978b, 1979a, Miyamoto et al., 1979). On the basis of the crystallographic and chemical characteristics of the pyroxenes, the rapidly cooled Yamato-74450-1ike pyroxenes, which have been called Pasamonte type, have been eliminated from the pyroxene fragments in Yamato-75015, and the bulk chemistries of the primary pigeonites have been plotted in a diagram showing a model crystallization trend (Fig. 7). Most of the pyroxenes in Fig. 7 show extensive exsolution of augite, and the host and lamellae pairs may delineate the shape of solvus at an apparently equilibrated temperature (Fig. 7). The X-ray diffraction study indicated that a pyroxene as Mg-rich

as the Binda inverted pigeonite has also been inverted to orthopyroxene. The pyroxenes close to the Moore County pigeonite also show widely spaced lamellae. The Juvinas-type pyroxenes show fine regular exsolution of augite like in that of Juvinas. Therefore it is natural to consider that the pyroxene chemical trend in Fig. 7 may represent the crystallization trend proposed for a model crust of the howardite parent body (Takeda et al., 1979a). The position beneath the surface of the parent body may also be the sequence of the trends, the Mg-rich one being in the deepest crust. A detailed discussion will be given after an explanation of our layered-crust model. A LAYERED-CRUST MODEL FOR A HOWARDITE PARENT BODY

Crystallographic features of the achondritic pyroxenes described in the previous section such as exsolution and inversion textures vary systematically in accordance with their Fe/(Mg + Fe) and Ca/(Ca + Mg + Fe) ratios. This texturecomposition variation was so simplistic in comparison with that of pyroxenes in the lunar crust that these pyroxenes have been assumed to be produced in a single crystallization trend of the primordial crust formation on the parent body (Takeda et al., 1976). If we can place individual achondrites at various depths within the primordial crust, this model could give a real

463

HOWARDITE PARENT BODY MODEL

Ic*opx

C'pig

Aog\

MC

I~ ¸

a'pig

Opx

a*A~ Cpig

,.IV

Aug

m

o

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•

-

LJ .

t.

m

,w---><

a Pig a Aug

~

t

FiG. 6. X-Ray precession photographs of eucritic pigeonites, c* and (001) are common for both exsolved Aug and Pig. Opx and Pig share a* and (100) in common. (a) Partially inverted pigeonite from Moore County (MC). (b) Juvinas (JX7) pigeonite with (001) augite lamellae.

image of a hypothetical chemical model proposed by Mason (1967) for a differentiated asteroid with eucritic crust, diogenitic mantle, and pallasitic core. The details of data and reasoning in support of the following model have been published in part in our previous papers (Takeda et al., 1976, 1979a; Takeda and Miyamoto, 1977; Takeda, 1977). This paper presents detailed crystallographic features of their pyroxenes. A reference documenting how the various exsolution textures can be correlated to p - T conditions has been given by Ishii and Takeda (1974). However, minor revisions on the origin of the low-Ca inverted pigeonite may be required on the basis of Fig. 7 (Takeda and Miyamoto,

1977). A roughly estimated depth within the crust for each meteorite has been derived from the widths of exsolved pyroxenes by application of diffusion theories and computer simulation (Miyamoto and Takeda, 1977). On the basis of the above data, we first classify the diogenitic and eucritic pyroxenes into seven groups or layers. Then, we will state the reasoning for the order in which we position each layer in the sequence from bottom to top within the proposed layered-crust model. (PA) Very-low-calcium, magnesium-rich orthopyroxenes. The composition of some pyroxenes may approach those in Steinbach, although Steinbach is not a diogenite.

464

HIROSHI TAKEDA Ctl

T

/ Mg

o Fe

FIG. 7. Hypothetical crystallization trend of the Howardite parent body delineated from the bulk chemical compositions of the exsolved unzoned pyroxene clasts in the Yamato-75015eucritic polymict breccia (Takeda et al., 1979a). The trend is projected on an isothermal plane at 1185°Cwith the phase boundaries of protoenstatite (Pr), orthopyroxene (Opx), pigeonite (Pig), and augite (Aug) EE': projection of the pigeonite eutectoid reaction (PER) line (Ishii, 1975). Dotted lines are the solvus separation of the host and lamellae pairs. (A) Medium-calcium and magnesium-rich o r t h o p y r o x e n e s with no exsolution of augite (e,g., Shalka), or more calcium-rich and iron-rich o r t h o p y r o x e n e s with augite exsolution lamellae with (100) in c o m m o n (e.g., Ibbenb0ren), or both. D e v e l o p m e n t s of exsolution lamellae in these p y r o x e n e s of rather low calcium content c o m p a r e d to terrestrial plutonic p y r o x e n e s suggest that the exsolution was developed deeper in the crust than other varieties (Ishii and Takeda, 1974). (B) The last diogenite crystallized in the sequence may contain o r t h o p y r o x e n e s with blebby augites or low-calcium inverted or d e c o m p o s e d pigeonites, such as found in the Yamato-75032 (Takeda et al., 1978a). (C) Inverted or d e c o m p o s e d pigeonites of " l o w c a l c i u m " content with blebby augites with (100) in c o m m o n as were found in cumulate eucrites (e.g., Binda). Plagioclase starts to crystallize at this stage. The temperature of the first crystallization of pigeonite is a b o v e the lower stability limit of pigeonite or pigeonite eutectoid reaction (PER) line (Ishii, 1975). Pigeonite starts to crystallize at 1180°C for the Juvinas composition (Stolper, 1977). (D) Inverted pigeonites of high bulk calcium content with coarse exsolution lamellae of augite with (001) in c o m m o n (e.g., M o o r e County). In some cases, pigeonites have only partially inverted to ortho-

pyroxene, T h e y coexist with equigranular plagioclase grains. (E) C o m m o n eucritic pigeonites with uniform composition and with fine exsolved augite lamellae with (001) in c o m m o n (e.g., Juvinas). The host phase is clinohypersthene. Plagioclase and p y r o x e n e show ophitic to subophitic texture. (F) Pasamonte-like pigeonites with extensive chemical zoning from Mg-rich pigeonite to Fe-rich pigeonite and to ferrohedenbergite. Exsolution is detected only by the X-ray diffraction (Takeda e t al., 1976). Plagioclase is needle-like or radiated together with the pyroxenes. Because this kind of zoning is expected to be produced by crystallization from a supercooled melt, this type of eucrite m a y represent a surface lava or impact melt. Among the seven groups we classified, group PA has no representative meteorite, and is eliminated f r o m Table I I I and the abstract. This type of p y r o x e n e occurs frequently as a mineral fragment in diogeniterich howardites such as K a p o e t a ( D y m e k et al., 1976) and Yamato-7308 (Takeda et al., 1976). Because this p y r o x e n e is the most Mg-rich and Ca-poor among the howarditic pyroxenes, we assume that PA is the first pyroxene to crystallize from the parent magma. In order to position each layer in the sequence, we a c c e p t e d a general principle of

HOWARDITE PARENT BODY MODEL

465

TABLE III LIST OF EACH LAYEROF THE MODEL HOWARDITECRUST, THE KEY DIAGNOSTICSFOR THAT LAYER, AND THE I~AMEOF THE METEORITES WHICH SEEM TO SAMPLETHAT LAYER Layer meteorites

Inversion of Pig to Opx

Ex solution lamellae (plane) width (tzm)

Surface eucrites (PM-type, layer F) ¥-74450 No Pasamonte No

(001) (001)

Common eucrites (JV-type, layer E) Nuevo Laredo No Sioux County No Haraiya No Juvinas No Mount Padbury No Crab Orchyard Partly

(001) (001) (001) (001) (001) (001)

Cumulate eucrites MC-type (layer D) Moore County Serra de Mag6a BD-type (layer C) Moama Y-75015 Binda Diogenites Fe-fich (layer B) Y-75032 Y-75032 Common diogenites (layer A) IbbenbiJren Johnstown Shalka ALHA77256 Tatahouine Y-74013

-0.5

Remarks

Chemical zoning Chemical zoning Possibly annealed Prim. Opx, Pig, Aug Euc. clast in Mes. Euc. clast in Mes.

Partly Yes

(001) (001), (100)

55 25

Yes Yes Yes

(100), (001)

40

(100)

Low-Ca Pig, Aug blebs Miner. clast, Aug blebs Low-Ca Pig, Aug blebs

Yes Opx

(100) (100)

Low-Ca Pig, Aug blebs Prim. Opx

Opx Opx Opx Opx Opx Opx

(100) (100) No No No No

Fine G. P. Zone

Partly BD-type

Prim. Opx Prim. Opx Prim. Opx Aug inclusion Crystalline Recrystallized

After Harlow et al. (1977).

the igneous differentiation of terrestrial magmas, and the processes of exsolution, decomposition, and inversion of terrestrial p i g e o n i t e s (Ishii a n d T a k e d a , 1974). T h e c o n d i t i o n s a r e s u m m a r i z e d as: (1) B e c a u s e c r y s t a l l i z a t i o n o f a M g - r i c h p h a s e l e a v e s r e s i d u a l liquid m o r e F e - a n d C a - r i c h t h a n the initial m e l t a n d t h e e a r l y crystallized phase sinks toward the bottom, we position the Mg-rich and Ca-poor layer lower than the Fe- and Ca-rich layer. The

m o s t t e r r e s t r i a l c r y s t a l l i z a t i o n t r e n d is f r o m O p x + A u g to O p x + Pig + A u g to Pig + A u g , b u t t h e p r o p o s e d h o w a r d i t i c t r e n d is f r o m O p x to O p x + Pig to l o w - C a Pig to h i g h - C a Pig to Pig + A u g (Fig. 7) ( T a k e d a et al., 1979a). (2) B e c a u s e t h e i n v e r s i o n o f p i g e o n i t e to o r t h o p y r o x e n e is k n o w n to t a k e p l a c e o n l y b y v e r y s l o w c o o l i n g , w e p o s i t i o n an inv e r t e d p i g e o n i t e b e l o w an u n i n v e r t e d one. M o o r e C o u n t y is p a r t i a l l y i n v e r t e d , f o r

466

HIROSHI TAKEDA

which the cooling rate 1.5°C/104 years has been proposed (Miyamoto and Takeda, 1977). (3) The blebby augite inclusions with (100) in common generally with the host orthopyroxene inverted from pigeonite have been proposed to be produced by slow cooling of pigeonite whose Ca content is close to or lower than that at the PER line (Fig. 7), whereas the (001) lamellae have been interpreted to be produced at the stable Pig-Aug solvus (Ishii and Takeda, 1974; Takeda and Miyamoto, 1977). The detailed arguments are beyond the scope of this paper but we use the characteristic texture as indicative of the above crystallization and cooling condition. (4) The width of the lamella is the function not only of cooling rate but also of bulk Ca content in pigeonite. By assuming appropriate temperature-time variation curves at any depth beneath the surface of the crust, the time necessary to develop the lamellae can be converted into the depth. (5) As was proposed by Takeda et al. (1979a) and confirmed by the dynamic crystallization experiment of Walker et al. (1978), a rapidly cooled (0.1-100°C/hr) eucritic melt will produce chemical zoning as was observed in the Pasamonte pigeonite. Condition (1) may position the above layers from bottom to top in the increasing sequence of Fe and Ca contents: PA (S in Fig. 2), A (TH, AH, SL, IB, G), B (Y), C (B, M), D (MC), E (JV, MP, HR) as are shown in Fig. 2. Because Moore County in layer D is partly inverted to orthopyroxene and all pigeonite in layer C is inverted to orthopyroxene, D can be positioned on top of C according to condition (2), and similarly layer E on D. Condition (3) also supports the above sequence, because the Ca content of D is high enough to produce (001) augite lamellae. The low-Ca pigeonite in layer B or C is one of the earliest pigeonites to crystallize in the sequence close the PER line. The width of the lamellae given in Table III and condition (4) also agree with the above sequence. The

width of Moama in C is smaller than that of Moore County in D, but C can be placed below D because the bulk Ca content of Moama is lower than that of Moore County (Miyamoto and Takeda, 1977). The absence of microscopically visible lamellae in layer F also positions F on top of E. Condition (5) indicates that chemically zoned pigeonite in F is produced near the surface. The presence of minor plagioclase in B and of a considerable amount in Binda suggests that plagioclase starts to crystallize in C. Thus, layers A, B, C, D, E, and F may be arranged from bottom to top in this sequence. The representative meteorites for each layer are given in Table III. The Ibitira eucrite has been proposed to be in layer F (Steele and Smith, 1976), but its exsolution texture is that in E. DISCUSSION The layered-crust model proposed in the previous section explains the pyroxene crystallization sequence and the relative locations of the pyroxenes within the proposed crust, but does not necessarily imply that they are produced by large-scale crystal fractionation from a totally molten crust with a hypothetical composition. Our model is compatible with the partial melting model for eucrites proposed by Stolper (1977), since the melt produced by the large-scale partial melting could be our initial molten crust. The distribution of the bulk chemical compositions of pyroxenes in Yamato75015, except those of the Pasamonte type, delineates the crystallization sequence as was shown in Fig. 7. Although the location of the PER line (EE') is not precisely known, pigeonites crystallized around or below the PER line in Fig. 7 contain blebby augites of the Binda (BD) type. The Moore County (MC)-like pigeonites immediately to the Fe-rich side of the BD-type pyroxenes show much coarser exsolution textures than the more Fe-rich members in common eucrites. One question that needs to be answered is why the true Moore County-like

HOWARDITE PARENT BODY MODEL pyroxenes with very coarse exsolution lamellae (ca. 50 /zm) in the partially inverted host phase have not been found in any eucritic polymict breccia. Also, the residual materials of the partial melting model of Stolper (1977) have not been found in any polymict breccia. Perhaps these " r e s i dues " if they ever existed may be located too deep to be incorporated into most breccias. (See also Consolmagno and Drake, 1977.) The finding of a diogenite-like meteorite (Yamato-75032) with a pyroxene composition close to that of Binda emphasizes the important genetic link between eucrites and diogenites. There is no apparent gap (difference) in the pyroxene crystallization trends between eucrite and diogenite. The occurrence of pigeonite in diogenites implies that transitional crystallization of pigeonite from orthopyroxene took place before the simultaneous crystallization of plagioclase and pigeonite (as in Binda) and that the occurrence of pigeonite cannot be a diagnostic criterion of eucrites as we proposed previously (Takeda et al., 1979a). Again diogenites and eucrites represent two portions, fortuitously divided, of a continuous spectrum of pyroxenes in a pyroxene crystallization trend of these genetically related achondrites. There have been two unclarified points in our layered-crust model in the past: the relationship among diogenites and the occurrence of pyroxenes intermediate between diogenite and eucrite in polymict breccias. The pyroxene crystallization trends of various diogenites in Fig. 2b are not as straightforward as those of the eucrites in Fig. 2a. A part of these variations may be attributed to some prolonged annealings or recrystallization or both after the initial crystallization. ALHA77256 plots between Tatahouine and Salka and the extension of IbbenbiJren and recrystallized Yamato diogenites. The occurrence of augite inclusions in orthopyroxene crystals and in grain boundaries found in the ALHA diogenites indicates the possibility that the crystals might be an originally low-Ca pigeonite.

467

However the bulk CaO content of ALHA77256 in Table I is not as high as that of Yamato-75032. The wide separation of the Ca contents of the Opx-Aug pair and the absence of exsolved augite in orthopyroxene indicate a longtime annealing at a temperature lower than that of the crystallization. The temperature of the last equilibration was estimated to be around 950°C by the Opx-Aug geothermometer of Ishii et al. (1976). This fact and texture suggest that they may be located deeper than the Y-75032 diogenite. As was mentioned previously, there are two different Mg-rich pyroxene clasts in eucritic polymict breccias. Because the two kinds of pyroxene occur as large fragments and their chemical compositions are nearly uniform and are intermediate between diogenites and eucrites, they may be misidentified as fragments of Fe-rich diogenitic pyroxenes. However, one type of pyroxene is crystallographically orthopyroxene and has blebby augite inclusions in the host pyroxene with a uniform bulk composition, and the other type of pyroxene with slight Fe enrichment at the very margin is similar to large pyroxene phenocrysts found in Yamato-74450. We confirmed that the latter pyroxenes are Mg-rich, low-Ca pigeonite Ca~MgrrFe2a by the X-ray method. We interpret the fragments in the polymict breccias as originally representing a part of the core of pigeonites similar to those found in Yamato-74450. The more Fe-rich pigeonite CasMg4nFe4.~ in Table II with less chemical zoning found in Yamato-74450 shows very fine exsolution of augite (Fig. 5). The other pyroxene clasts in the brecciated matrix in Yamato-74450 (Fig. 5) are even more Fe-rich and show coarser exsolution lamellae identical to those found in Juvinas. The above variation may represent the fine structure within the surface eucrite layer (layer F). Besides the good correlation of the crystallographic characteristics and their composition of the proposed layered-crust model, the presence of polymict breccias

468

HIROSHI TAKEDA

composed of diogenites and a variety of eucrites in the proposed model in various proportions gives additional evidence of the existence of such layered crust on their parent body. A deep excavation by a large impacting body may give diogenetic polymict breccias with few eucritic clasts such as Yamato-7308 (Miyamoto et al., 1978). A shallow excavation by a small impacting body may produce polymict breccias rich in the Pasamonte-like components. All polymict breccias found up to date are tabulated in Table IV. The predominance of diogenitic polymict breccias (howardites) may be attributed to a presence of largescale impact craters on the parent body comparable to Mare Imbrium on the Moon or the Caloris basin on the surface of Mercury. All previous models did not take into account a process of actual crust formation on the parent body. We have proposed a new model (Takeda, 1977 and Takeda et al., T A B L E IV ACHONDRITIC POLYMICT BRECCIAS ARRANGED IN ORDER OF EXCAVATION BY HYPOTHETICAL IMPACTS Excavation Shallow

Intermediate

Deep

Meteorite sample Eucrite p o l y m i c t breccias Pasamonte Y-74450 Y-74159 Y-75295 Y-75296 Y-75307 A L H 7605 ALHA77302 Macibini Y-75011 Y-75015 Howardite Diogenite-rich polymict breccias Frankfort Y-7308

Reference ~

a a, b a c c c c c d e e f

g b, g

a a: Takeda et al. (1978a); b: M i y a m o t o e t al. (1978); c: M i y a m o t o et al. (1979); d: Reid, private communication (1974); e: Takeda e t al. (1979a); f: Wilkening e t al. (1971); g: Takeda et al. (1976).

1979a) involving accretional processes for producing the layered crust, by taking into account all reasonable processes proposed for both the partial melting and the crystal fractionation models. The crystallization sequence and cooling histories of pyroxenes of the layered-crust model may be postulated as below. Because the depths and extensions of the molten layer are assumed to be planetary in scale, we have to admit that there are some local modifications. However, since the correlation between the chemical compositions of pyroxenes and their exsolution and inversion textures is high both for individual meteorites (Fig. 2) and for pyroxene fragments in a polymict breccia (Fig. 7), we expect the sequences may really be represented by those given in Fig. 7. This schematic figure was originally given as a hypothetical model and shows a section of phase boundaries and a projection of the crystallization trends onto an equithermal plane at about 1180°C. The EE' line is the projection of the pyroxene eutectoid reaction (PER) line (Ishii, 1975). The depth of each layer below the surface of the parent body proposed in this paper has been estimated from the width of exsolved augites in the pigeonites and their bulk chemistry by Miyamoto and Takeda (1977). Although their attempt is preliminary in nature and they assumed simplified diffusion and cooling models, it would give us some idea of the thickness of the crust. For a body of 1000 km in diameter, the estimated depth for the cumulate eucrites is about 10 km and for the common eucrites 1 km. Since the eucritic polymict breccias newly found in Antarctica may represent brecciated products of our proposed layered crust, their reflectance spectrum should match that of the asteroid 4 Vesta. This conclusion is consistent with that reached by LeBertre and Zellner (1978) who reported the resemblance of Vesta's reflectance spectrum with that of powdered mixtures of various eucrites. Thus the de-

HOWARDITE PARENT BODY MODEL tailed studies of the eucritic and diogenitic polymict breccias will provide information on the surface materials o f the differentiated asteroids. It is interesting to observe the reflectance spectra o f Vesta for every quarter revolution to see whether there is a spectral difference from diogenetic to eucritic trend. Such an observation would give us evidence of the large impact crater which excavated diogenitic materials if Vesta were intact since its formation.

ACKNOWLEDGMENTS This work was initiated in 1970 at NASA Johnson Space Center in collaboration with Drs. Arch M. Reid and M. B. Duke as a Senior Research Associate of NRC, and was continued at the present Institute with Drs. M. Miyamoto and T. Ishii. The author is indebted to them for microprobe analyses and to all members of the center and the Institute for discussion and encouragement. The author thanks Dr. K. Yanai and Professor Y. Matsumoto and their parties, and National Institute of Polar Research and NSF for the Antarctic meteorite samples, Dr. Brian Mason, Professor C. B. Moore, Professor J. F. Lovering for achondrites samples, Mr. H. Haramura for the chemical analysis, and Miss M. Sato for assistance in microprobe analysis.

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ISHll, T., AND TAKEDA, H. (1974). Inversion, decomposition and exsolution phenomena of terrestrial and extraterrestrial pigeonites. Mere. Geol. Soc. Japan II, 19-36. ISHII, T., MIYAMOTO, M., AND TAKEDA, H. (1976). Pyroxene geothermometry and crystallization, subsolidus equilibration temperatures of lunar and achondritic pyroxenes. In Lunar Science VII, pp. 408-410. The Lunar Science Institute, Houston. LEBERTRE, T., AND ZELLNER, B. (1978). The surface texture of Vesta. In Lunar and Planetary Science IX, pp. 642-644. Lunar and Planetary Institute, Houston. MASON, B. (1967). Meteorites. Amer. Sci. 55, 429-455. MATSON, n . L., FANALE,F. P., JOHNSON, T. V., AND VEEDER, G. L. (1976). Asteroids and comparative planetology. Proc. Lunar Sci. Conf. 7th, 3603-3627. MIYAMOTO, M., AND TAKEDA, H. (1977). Evaluation of a crust model of eucrites from the width of exsolved pyroxenes. Geochem. J. II, 161-169. MIYAMOTO, M., TAKEDA, H., AND YANAI, K. (1978). Yamato achondrite polymict breccias. Mere. Nat. Inst. Polar Res. Spec. Issue 8, 185-197. MIYAMOTO, M., TAKEDA, H., AND YANAI, K. (1979). Eucritic polymict breccias from Allan Hills and Yamato Mountains. In Lunar and Planetary Science X, pp. 847-849. The Lunar and Planetary Institute, Houston. NAKAMURA, Y., AND KUSHIRO, I. (1970). Compositional relations of coexisting orthopyroxene, pigeonite and augite in a tholeiitic andesite from Hakone Volcano. Contrib. Mineral. Petrol 26, 265-275. REID, A. M., AND COHEN, A. J. (1967). Some characteristics of enstatite from enstatite achondrites. Geochim. Cosmochim. Acta 31, 661-672. STEELE, I. M., AND SMITH, J. V. (1976). Mineralogy of the Ibitira eucrite and comparison with other eucrites and lunar samples. Earth Planet. Sci. Lett. 33, 67-78. STOLPER, E. (1977). Experimental petrology of eucritic meteorites. Geochim. Cosmochim. Acta 41, 587-611. TAKEDA, H. (1977). Genetic relationship of diogenites and eucrites and its parent body. Proc. lOth Lunar Planet. Syrup., p. 47-52, Inst. Space Aero. Sci. Univ. of Tokyo, Tokyo. TAKEDA, H., AND MIYAMOTO, M. (1977). Inverted pigeonites from lunar breccia 76255 and pyroxenecrystallization trends in lunar and achondritic crusts. Proc. Lunar Sci. Conf. 8th, 2617-2626. TAKEDA, H., DUKE, M. B., ISHll, T., YANAI, K., AND HARAMURA, H. (1979b). Some unique meteorites found in Antarctica and its relation to asteroids. Mem. Nat. Inst. Polar Res. Spec. Issue 15, in press. TAKEDA, S . , MIYAMOTO, M., DUKE, M. B., AND ISHII, T. (1978a). Crystallization of pyroxenes in lunar KREEP basalt 15386 and meteoritic basalts. Proc. Lunar Planet. Sci. Conf. 9th, 1157-1171.

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TAKEDA, H., MIYAMOTO, M., ISHll, T., AND REID, A. M. (1976). Characterization of crust formation on a parent body of achondrites and the moon by pyroxene crystallography and chemistry. Proc. Lunar Sci. Conf. 7th, 3535-3548. TAKEDA, n . , MIYAMOTO, M., ISHII, T., AND YANAI, K. (1979a). Mineralogical examination of the Yamato-75 achondrites and their layered crust model. Mere. Nat. Inst. Polar Res. Spec. Issue 13, 82-108. TAKEDA, H,, MIYAMOTO, M., YANAI, K., AND HARAMURA, H. (1978b). A preliminary mineralogical examination of the Yamato-74 achondrites.

Mere. Nat. Inst. Polar Res. Spec. Issue 8, 170184. WALKER, D., POWELL, M. A., LOFGREr~, G. E., AND HAYS, J. F. (1978). Dynamic crystallization of a eucritic basalt. Proc. Lunar Planet. Sci. Conf. 9th, 1369-1391. WILKENING, L., LAL, D., AND REID, A. M. (1971). The evolution of Kapoeta howardite based on fossil track studies. Earth Planet. Sci. Lett. 10, 334-340. YANAI, K., CASSIDY, W. A., FUNAKI, M., AND GLASS, B. P. (1978). Meteorite recoveries in Antarctica during field season 1977-78. Proc. Lunar Planet. Sci. Conf. 9th, 977-987.