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Studies of Brazilian meteorites
Earth and Planetary Science Letters, 35 (1977) 317-330 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
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STUDIES O F B R A Z I L I A N M E T E O R I T E S , III. ORIGIN AND HISTORY OF THE ANGRA DOS REIS ACHONDRITE M. P R I N Z Department of Mineral Sciences, American Museum of Natural History, New York, N. Y. 10024 (USA) K. K E I L 1, P.F. H L A V A 2, J.L. B E R K L E Y Department of Geology and Institute of Meteoritics, University of New Mexico, Albuquerque, N.M. 87131 (USA} C.B. G O M E S Instituto de Geociencias, Universidade de Sao Paulo, Sao Paulo (Brasil) and W.S. C U R V E L L O Museu Nacional, Rio de Janeiro (Brasil)
Received November 12, 1976 Revised version received March 14, 1977
The Angra dos Reis meteorite fell in 1869 and is a unique achondrite. It is an ultramafic igneous rock, pyroxenite, with 93% fassaite pyroxene which has 15.7% Ca-Tschermak's molecule, plus calcic olivine (Fo53.1 ; 1.3% CaO), green hercynitic spinel, whitlockite (merrillite), metallic Ni-Fe, troilite, as well as magnesian kirschsteinite (Ks62.3Mo37.7), within olivine grains, and celsian (Cs90.2AnT.7Ab 1.7Or0.4) which are phases reported in a meteorite for the first time, and plagioclase (An86.0), baddeleyite, titanian magnetite (TiO2, 21.9%), and terrestrial hydrous iron oxide which are phases reported for the first time in this meteorite. Petrofabric analysis shows that fassaite has a preferred orientation and lineation which is interpreted as being due to cumulus processes, possibly the effect of post-depositional magmatic current flow or laminar flow of a crystalline mush. The mineral chemistry indicates crystallization from a highly silicaundersaturated melt at low pressure. Since the meteorite formed as a cumulate, pyroxene crystals may have gravitationally settled from a melt which crystallized melilite first. Plagioclase would be uostable in such a highly undersaturated melt, and feldspathoids would be rare or absent due to the very low alkali contents of the melt. The presence of rare grains of plagioclase and celsian may be the result of late-stage crystallization of residual liquids in local segregations. Thus, the Eu anomaly in Angra dos Reis may be the result of pyroxene separation from a melt which crystallized melilite earlier, rather than plagioclase as previously suggested.
1. Introduction The Angra dos Reis ( A D O R ) , Rio de Janeiro, Brazil achondrite fell in the latter half o f J a n u a r y , 1 Consortium Leader. 2 Now at Sandia Laboratories, Albuquerque, N.M. 87115, U.S.A.
1869 ( 2 2 ° 5 8 ' S , 44° 19'W) at a b o u t 0 5 0 0 hours (5 alm.). The b o d y was apparently travelling f r o m n o r t h to s o u t h and some smoke but no light and sound was observed [1 ]. The m e t e o r i t e was observed by a local judge to fall into the bay near the t o w n o f Angra dos Reis. It was found, at a depth o f 2 m by a local diver a day after the fall and consisted o f two pieces; f r o m an u n m a t c h e d fresh surface it was surmised that a
318
third piece was missing. Ludwig and Tschermak [2] report that the recovered mass was originally one stone of about 1.5 kg. St6rzer and Pellas [3] conclude from measurements of galactic cosmic ray track densities that the meteorite had a minimum preatmospheric mass of 80 kg. The ADOR meteorite is a unique ultramafic pyroxenite containing about 93% fassaite, and was shown to have a very primitive 87Sr/S6Sr ratio [4], an old 2°TPb/2°6Pb age of 4.555 b.y. [5], and evidence for extinct 244Pu [6]. The only modem mineral analyses of ADOR are those by Albee and Chodos [7] who analyzed pyroxene, and Hutchison [8] who analyzed pyroxene, olivine, spinel, and also gave a partial analysis of two sulfide grains. A progress report by some members of the present consortium was published earlier [9]. Previous work has suggested that ADOR is an igneous rock of complex history, yet all indications are that it is an ancient rock with many "primitive" properties. Thus, this meteorite is an extremely important one whose comprehensive study is likely to provide clues to the understanding of the early differentiation history of its parent body and the early history of the solar system in general. Therefore, a consortium study of this meteorite was undertaken [9] with the intent of coordinating a wide variety of studies on the samples (see the following papers in this issue). Here, we report on the microscopic study of polished thin sections of meteorite slices and grain mounts of separated phases, and the electron microprobe study of the meteorite's constituents which are fassaite, olivine, spinel, whitlockite (merrillite), metallic Ni-Fe, trollite, as well as phases reported for the first time in a meteorite (magnesian kirschsteinite, celsian) and phases reported for the first time in this meteorite (bytownite, baddeleyite, titanian magnetite, terrestrial hydrous iron oxide), in an attempt to provide petrological clues for the origin and history of this meteorite.
2. Materials and methods
Polished thin sections of the rock and of mineral separates prepared at the University of New Mexico and California Institute of Technology were studied microscopically and by electron microprobe techniques. One polished thin section was made available
for study by the Smithsonian Institution of Washington, D.C., at an early stage of the study. The complex sample preparation of this meteorite is described by Wasserburg et al. [10]. Mineral compositions were determined with an ARL-EMX-SM electron microprobe according to the procedures of Keil [11,12], and corrections for differential matrix effects were made following the method of Bence and Albee [13] and Albee and Ray [14]. Petrofabric analysis was carried out utilizing a 5-axis universal stage with data points plotted on a Schmidt stereographic projection net. Triple junctures were also measured on the universal stage to obtain the true grain boundary intersection angles.
3. Texture and petrofabrics 3.1. Textural relations
The ADOR meteorite consists mainly of pyroxene (fassaite), with minor olivine, and accessory magnesian kirschsteinite, spinel, whitlockite (merrillite), metallic Ni-Fe, troilite (some has been oxidized to "limonite"), and very rare plagioclase, celsian, baddeleyite, and titanian magnetite. Fassaite is present in two textural patterns. The most common pattern (Fig. 1A) is groundmass fassaite consisting of small xenomorphic grains (up to 0.5 mm; average 0.1 mm) which range from equidimensional to an aspect ratio (length/width) of 5 : 1. Grain boundaries of groundmass fassaite meet at triple juncture intersections (Fig. 1B) and can be termed xenomorphic granular, except for a weak mineraLelongation lineation in one thin section with an average aspect ratio of 2 : 1 (described below). The other textural pattern is less common and consists of large (up to 3 mm; average 1.3 mm), optically continuous, xenomorphic poikilitic grains (Fig. 1C) which are inclusion-free or completely enclose groundmass fassaite. The margins of these poikilitic grains commonly partially enclose groundmass pyroxenes. Both types of fassaite exhibit sharp extinction and show no evidence of strain or deformation. Olivine occurs as separate grains within the groundmass fassaite or as granular aggregates of equidimensional grains (up to 1 mm; average 0.75 mm) (Fig. 1D). The structural configuration of these aggregates is not
319
Fig. 1. Textural relationships in the Angra dos Reis meteorite. A. Typical textural relationship of small groundmass fassaite grains. Some poikilitic fassaite grains are also present (dark). Transmitted light, crossed nicols. Field of view 4.8 mm wide. B. Triple juncture relationship of groundmass fassaite. Transmitted light, crossed nicols. Field of view 0.7 mm wide.
320
Fig. 1. Textural relationships in the Angra dos Reis nieteorite (continued). C. Large poikilitic fassaite, with some twinning, enclosing small groundmass fassaite. Transmitted light, crossed nieols. Field o f view 4.8 m m wide. D. Aggregate o f rec~ystallized olivine (center) surrounded on b o t h sides by finer-grained groundmass fassaite. Note that triple junctures in olivine are closer to 120 ° than those in pyroxene. Transmitted light, crossed nicols. Field o f view 4.8 m m wide.
321
Fig. 1. Textural relationships in the Angra dos Reis meteorite (continued). E. Large whitlockite grain (white, diagonal) surrounded by fassaite. Transmitted light. Field of view 2.9 mm wide. F. Olivine grain containing inclusions of magnesian kirschsteinite. Miner. al separate, embedded in epoxy. Reflected light. Field of view 0.7 mm wide.
322
known but they may be discontinuous lenses or thin laminae. Olivine segregations display well-developed triple junctures with gently curving surfaces. Olivine grains display sharp extinction and are unstrained. Small (0.1 mm), xenomorphic grains of green spinel are dispersed throughout the fassaite. Magnesian kirschsteinite is very rare and found only as small inclusions in olivine grains. Whitlockite (merrillite) occurs as sparsely distributed, millimeter-sized grains (Fig. I E) in widely separated areas in one thin section. Very sparse grains of metallic Ni-Fe and somewhat more abundant troilite are randomly distributed. Rare grains of celsian which are very small (up to 30 /am) are found in the groundmass. Plagioclase (bytownite) was not detected in situ in any of the thin sections, but several grains were found in mineral separates. The mineral separations were carried out at the California Institute of Technology with great care and the plagioclase grains do not appear to be contaminants. St6rzer and Pellas [3] also note the presence of rare feldspar in their hand-picked mineral separates. They report finding eight crystals, seven of which have etching characteristics similar to that of bytownite, and one crystal with characteristics of a sodic plagioclase. Lugmair and Marti [15] note the presence of feldspar in their mineral separates but the type is not known. Rare grains of titanian magnetite (up to 50 #m) were found in the groundmass.
3. 2. Petrofabric analysis The X, Y and Z optical directions were measured for 100 groundmass fassaite and 6 poikilitic grains. The groundmass grains and one poikilitic grain were contained in one small thin section which displayed a weakly developed mineral elongation lineation oriented N-S. The orientation of 5 poikilitic fassaites were measured in another fragment along with 12 groundmass grains. Orienting on the groundmass fassaite measurements and a very weak lineation discovered in a small domain of this fragment, the 5 poikilitic grain orientations were rotated approximately into the plane of the lineated fragment. The results of these measurements are shown in Fig. 2 which demonstrates a preferred orientation for groundmass fassaite and a contrasting apparent random orientation for the poikilitic grains. However, the small number of poikilitic grains measured cannot unequivocally be used to prove a random orientation. The preferred orientation of the groundmass fassaite is characterized by a partial girdle of horizontally oriented (relative to the thin section plane) Y axes with a tendency for rotation within the thin section plane from NW-SE to NE-SW. Since the c crystallographic axis is oriented normal to Y, the long axis of the fassaite crystals also rotates in the plane of the
l XCPX
t YcPx
ZCpx
Fig. 2. Lower hemisphere Schmidt net projections of fassaite indicatrix axes for g r o u n d m a s s fassaite in Angra dos Reis. Filled triangles are for poikilitic grains, l = lineation. C o n t o u r s are 1, 5, and 8% o f 1% areas.
323
section and is approximately symmetrical about the lineation (l). The Z patterns for groundmass fassaite is less developed than Y but displays a N-S-trending "hour glass-shaped" girdle with maxima concentrated in the southern sector. The flaring out effect displayed in the northern and southern sectors roughly corresponds in angular magnitude to the limits of Y rotation and, therefore, the Z girdle pattern appears to be strongly dependent on Y orientation, though some rotation about the Y axis is indicated. The weak maxima located in the southern sector suggests the c+ crystallographic axes having a preferred northerly orientation, though in many crystals c+ points in a southerly direction. This can also be seen in the X diagram which displays a weak maxima mainly in the northern sector, whereas otherwise it displays a rather poorly developed pattern.
3.3. Triple juncture analysis In an attempt to evaluate the degree of subsolidus recrystallization, measurements of triple juncture angles were performed on the universal stage, following the procedures of Smith [16], Kretz [17], and Vernon [18,19]. One hundred fifty angles (representing 50 triple junctures) were measured for groundmass fassaite grains and 60 angles (representing 20
triple junctures) were measured for olivine grains. All angles were measured between grains of the same mineral phase. Assuming crystallographic orientation and anisotropy effects to be negligible, the three grain boundary angles formed at the junction of three crystals should approach 120 ° provided interfacial tensions have achieved a static balance [17]. This equilibrium state is approached in highly recrystallized metamorphic rocks, and failure to achieve nearly perfect 120 ° angles may be attributed to a departure from grain boundary equilibrium (again, neglecting crystal orientation and anisotropy effects) as displayed in less completely recrystallized rocks. Although crystal orientation effects may be significant for the groundmass pyroxene in ADOR, the petro fabric analysis (Fig. 2) suggests a high degree of variability in orientation, within certain limits. Therefore, orientation effects for pyroxene are discounted in the present study. Olivine orientations have not been determined; however, since no olivine lineation has been observed, the effect of orientation on olivine triple juncture angles is also discounted. The results of the triple juncture analysis are presented in Fig. 3. The variability of fassaite angles is clearly shown, although measurements do cluster around the mean of 120 ° (the mean will always be 120 ° when all three angles are averaged). The standard deviation (o) for fassaite angles is 39.7 which indicates a marked
,50 40
OL I V I N E ~:2SO
30 20
I,M IO.
¢/)
<¢ ~O
O,
LI. 0 40
FASSAITE :3S7
I1 0
65
85
105
120
135
TRIPLE
155
175
195
JUNCTURE
215
235
255
275
ANGLE
Fig. 3. Triple j u n c t u r e angle variation histogram for g r o u n d m a s s fassaite (lower) and olivine (upper). Standard deviation (o) f r o m t h e mean (120 ° ) is given for each phase.
324 departure from an equilibrium recrystallization state. Kretz [17] determined o = 7.5 for highly recrystallized metamorphic scapolite grains and assumed a somewhat greater o for metamorphic pyroxene due mainly to the higher degree of crystal anisotropy inherent to inosilicates. Vernon [18] determined o = 8.4 for highly recrystallized hedenbergites in a terrestrial metamorphic rock. The standard deviation of 29.0 for olivine (Fig. 3) indicates a closer approach to a grain boundary equilibrium energy state but is clearly too great to be considered characteristic of a highly recrystallized rock.
4. Mineral chemistry Fassaite. Pyroxene (fassaite) is the most abundant mineral but its precise abundance is difficult to esti-
mate because of the small surface area of the polished thin sections and the inhomogeneous presence of relatively coarse olivine and whitlockite (merrillite) crystals. Ludwig and Tschermak [2] calculated about 93% pyroxene for this meteorite, and this appears to be a reasonable value. The report of low-Ca pyroxene in ADOR [9] was in error (the grain was olivine) and there is no evidence for this phase in this meteorite. The fassaite is slightly pleochroic, from near colorless to red-brown, and is enriched in CaO and A1203 (Table 1 ; Fig. 4). Analysis of a large n u m b e r of groundmass and poikilitic crystals indicates that compositions are essentially homogeneous although Hazen and Finger [20] suggest minor variations. No evidence of zoning or exsolution was found. Deer et al. [21 ] characterize fassaite as having a high Ca content, variable but high A1 content, and high ferric to ferrous ratio. However, these characteristics are also
TABLE 1 Electron microprobe analyses (wt.%) of silicate, oxide, and phosphate minerals in the Angra dos Reis meteorite Fassaite
Olivine
Magnesian Spinel kirschsteinite
Whitlockite
Plagioclase
Celsian
Titanian magnetite
SiO 2 TiO 2
45.9 2.16
36.3 0.05
34.6 0.02
0.51 0.65
0.67 n.d.
46.2 n.d.
33.6 n.d.
1.23 21.9
A1203 Cr20 3 V20 3 Fe20 3 1 FeO MnO MgO CaO Na20 K20 P205 BaO
10.0 0.21 n.d. n.d. 6.7 0.06 10.6 24.1 <0.02 n.d. n.d. n.d.
<0.02 0.03 n.d. n.d. 38.3 0.60 24.3 1.29 n.d. n.d. n.d. n.d.
0.33 0.02 n.d. n.d. 26.2 0.42 8.9 28.9 0.05 n.d. n.d. n.d.
54.5 3.3 0.07 3.4 28.4 0.18 8.0 0.76 n.d. n.d. n.d. n.d.
n.d. n.d. n.d. n.d. 1.29 n.d. 2.82 49.4 0.68 n.d. 45.1 n.d.
33.6 n.d. n.d. n.d. 0.21 n.d. n.d. 17.2 1.47 0.11 n.d. n.d.
28.0 n.d. n.d. n.d. 0.43 n.d. n.d. 1.20 0.15 0.05 n.d. 38.2
3.5 1.02 0.26 18.5 48.8 1.17 1.18 1.21 n.d. n.d. n.d. n.d.
100.01
100.91 2
99.44
99.94
100.22 3
98.82
101.63
12
13
18
Fo 53.1 Fa 46.9
Mo 37.7 Ks 62.3
Cm Hc Sp Uv Mt
Total Number of grains
20
1
4
6
98.77 2 9
Molecular end members
Wo 54.6 En 33.5 Fs 11.9
3.8 56.4 33.4 2.8 3.6
An 86.0 Ah 13.3 Or 0.7
An 7.7 Ab 1.7 Or 0.4 Cs 90.2
I Fe203 calculated to achieve stoichiometry. 2 Ni below detection limit. 3 Includes 0.01% Y203 and 0.25% Ce203. Analysis by A.A. Chodos, California Institute of Technology. n.d. = not determined.
Cm 1.8 Hc 1.1 Sp 6.4 Uv 65.1 Mt 25.6
325 CoSiO 3
CoSiO 3
/
I
^
~
~.-Co-P-eSi206 -
,/
MgSi03 I 0 I
I00
FeSiO a I
I
I
I
I
I
80 80
I
II
I
I
60 40 Mole % Forsterite I
I
I
60 40 Mole % Monticellite
•
'
I
I
I
I
ZO 20
I
0 I
0
Fig. 4. Composition of fassaite in three-component diagram CaSiO3-MgSiOa-FeSiO 3 (molecular percent), disregarding other pyroxerie components. Also shown are the Compositions of coexisting olivine and magnesian kirschsteinite.
noted in what they call titanaugites, especially those from highly undersaturated rocks such as teschenites, essexites, and melilite-nepheline basalts. There is no sharp distinction found in metamorphic and metasomatic carbonate rocks, and titanaugite (rich in A1) is used in igneous rocks. The term fassaite will be used in this paper because it emphasizes the unique chemical characteristics of this pyroxene. It should be noted that the term fassaite has been used in some "igneous" assemblages in meteorites such as the light inclusions in Allende ( e.g. [22]), a multi-phase assemblage in an olivine crystal in the Sharps chondrite [23], and ADOR [81. The analysis of fassaite (Table 1) is very similar to analyses by Ludwig and Tschermak [2], Albee and Olodos [7], Hutchison [8], Hazen and Finger [20] and Ma et ai. [25], except for the alkali contents in the Ludwig and Tschermak [2] analysis which are
erroneous. Ludwig and Tschermak [2] reported that the fassaite in ADOR contains 0.33% Fe203 and 7.48% FeO but Mao et al. [24] show that no Fe a+ is present as determined by M6ssbauer spectra. Hutchison [8] reported 7.1% Fe, calculated as FeO. He found that tetrahedral AI did not balance the sum of the remaining trivalent cations and suggested that onethird of the Fe 2+ was present as Fe 3+, to make up the difference. The ADORABLES [9] also calculated that Fe 3+ was present in the pyroxene, following the suggestion of Hutchison [8]. However, in this calculation Ti is treated as if it is a trivalent cation, instead of as a tetravalent cation, and there is a fairly large excess of SiO2. Ti is now treated as a tetravalent cation by calculating the Ca(Mgo.sTio.s)(SiA1)O 6 and Ca(Feo. sTio.sXSiA1)O 6 molecules, along with other pyroxene molecules. A recalculation of the pyroxene analysis (Table I) was carried out by cal-
326 culating the following molecules, sequentially, with the following results (in molecular percent): CaMnSi206, 0.2%; Ca(Mgo.sTio.sXSiA1)O6, 9.0%; Ca(Feo.sTio.s)(SiAl)O 6, 3.2%; CaCr(SiA1)O6, 0.6%; CaAI(SiA1)O6, 15.7%; Ca2SiO6, 34.1%; Mg2Si206, 27.4%; and Fe2Si206, 9.7%. In this calculation a nearly perfectly balanced formula is obtained with only a very slight excess of SiO 2, and without the need for calculating any Fe 3÷. Albee and Chodos [7] noted that the formula proportions of A1 in the ADOR pyroxene is much more than twice the amount of Ti. They found that one-third of the Al is related to the CaTi(SiA1)O 6 molecule and two-thirds to CaAI(SiA1)O 6. Since the CaAI(SiA106) molecule is also high in pyroxene of deep-seated terrestrial igneous rocks they suggested that the unusual pyroxene compositions may be indicative of high pressure crystallization, below the plagioclase stability field. However, plagioclase and/or celsian has now been found in ADOR ([3,15], this study); moreover, this type of pyroxene is present in highly undersaturated basaltic lava flows. Hence, recourse to the high-pressure hypothesis for the origin of the fassaite in ADOR appears unwarranted.
Olivine and magnesian kirschsteinite. Ludwig and Tschermak [2] calculated that about 5.5% olivine is present in ADOR. They had noted that the olivine is unusually Ca-rich and somewhat similar to an Fetich monticellite. The Ca-rich nature of the olivine in ADOR is confirmed by electron microprobe analysis (Table 1). In addition, in some of the olivines a few tiny crystals of magnesian kirschsteinite were found, as first reported by the ADORABLES [9]. The high CaO content of the olivine is indicative of a relatively high solubility of the monticellite (CaMgSiO4)kirschsteinite (CaFeSiO4) molecule in the olivine. Warner and l_~th [26] have studied the maximum solubility of monticeUite in forsterite as a function of temperature and pressure. They showed that at temperatures compatible with magmatic crystallization of Mg-rich olivine, 5 - 1 0 mol.% of monticellite can be dissolved, if the melt is saturated with respect to monticellite. More Fe-rich olivine can dissolve larger amounts, but this was not experimentally determined. They also found that increasing pressure decreases the solubility of monticellite in forsterite. Hence, the high Ca content of the ADOR olivine also
appears to be indicative of low pressure. Warner and Luth [26] find that highly calcic olivines are always from melilite-bearing rocks (highly SiO2 undersaturated), and these rocks are expected to contain olivine with more than 1% Ca, as is the case for olivine in ADOR. Their experimental results are consistent with the electron microprobe data of Simkin and Smith [27]. Stormer [28] studied Ca zoning in olivine phenocrysts from nephelinite and basanite lavas. He finds that the Ca content of olivine is very sensitive to the silica activity of the liquid and that high Ca con tents in olivine are found in highly undersaturated rocks. He also finds that Ca content in olivine decreases as pressure increases, as did Warner and Luth [26]. Warner and Luth [26] determined the solvi bounding the two-phase region along the monticellite-forsterite join, at a variety of temperatures and pressures. Although the addition of Fe to this system would change the position of the solvi it is reasonably certain that a two-phase assemblage such as that present in ADOR would stably coexist. Without work on the more complex experimental system it is not possible to determine the temperature and pressure of crys. tallization of the ADOR olivine-magnesian kirschsteinite assemblage.
Spinel. Minute grains of dark green spinel are homogenously distributed throughout the meteorite. The spinel is compositionally homogeneous and rich in Fe and A1, making it a hercynitic spinel (Table 1). Cr is low, as in the entire mineral assemblage, and it was necessary to calculate a portion of the total Fe as Fe a+ in order to maintain stoichiometry of the phase. This indicates a somewhat higher pO 2 for the melt in comparison to most ordinary chondrites and achondrites. Celsian. Several small grains of celsian were found in the groundmass. This is the first report of celsian in a meteorite. The grains are chemically homogeneous (Table 1), and contain about 90% of the celsian molecule. St6rzer and Pellas [3] found one feldspar crystal in ADOR with etching characteristics which may be indicative of alkalic feldspar. Plagioclase. Several grains of plagioclase (bytownite) were found in the mineral separates of Wasserburg et al. [10]. The plagioclase is homogeneous and highly
327 calcic, An86 (Table 1). It is similar in composition to plagioclase found in other achondritic meteorites and in lunar basalts. Lugmair and Marti [I 5] and St6rzer and Pellas [3] also found some feldspar crystals which might be calcic plagioclase. No plagioclase was detected in polished thin sections studied.
Baddeleyite. Minute grains, very high in Zr, were detected but were too small for qunatitative analysis. The level of Zr was too high for zircon. Whitlockite (merrill#e). This phase is inhomogeneously distributed and may attain several millimeters in size (Fig. 1 F). Ludwig and Tschermak [2] calculate 0.3% of the phosphate phase in this meteorite. Rare earth elements are generally at or below the detection limits of the microprobe and are difficult to measure (Table 1). Dowty [29] has studied the phosphate in ADOR and showed that it is merrillite, a structure similar to/3-Caa(PO4) 2, and that this name is preferable to whitlockite for this type of meteoritic phosphate. Titanian magnetite. Titanian magnetite is reported for the first time in this meteorite. It occurs as very small, rare grains and contains 21.9% TiO2 (Table 1). It also contains significant amounts of Si, A1, Cr, V, Mn and Mg. Titanian magnetite of similar composition is also found in the Nahkla and Lafayette meteorites [30], where the grain size and abundance is greater. Brett et al. [31 ] measured the oxygen fugacity, as a function of temperature, in ADOR. They find that ADOR appears to have crystallized and cooled initially under conditions above the iron-wiistite buffer, and much closer to the buffer for most of its cooling history. The role of the titanian magnetite in the oxidation history of this meteorite is not clear. Metal and troilite. Metallic Ni-Fe is very rare in this meteorite and occurs as tiny blebs in troilite, which is more common. Ludwig and Tschermak [2] calculated 1.26% troilite for the meteorite, and this appears to be a reasonable value. Analyses of two metallic NiFe grains gave the following results: Ni, 1.4%, 3.5%; Co, 0.72%, 1.15%; Cu, 0.03%, 0.04%. The remainder is Fe. Troilite is nearly stoichiometric, with Co ranging from 0.07 to 0.13% and Ni at or below detection limits.
Terrestrial hydrous iron oxide. Very small opaque grains, with high Fe contents, are rarely found. Electron microprobe analysis indicates variable Fe, calculated as FeO, from about 68 to 78%, plus some SiO2 and traces of other minor elements, including S. These grains appear to be hydrous iron oxides which are the result of terrestrial weathering of troilite grains. They are very rare and the meteorite is generally fresh.
5. Discussion and conclusions The groundmass fassaite orientations shown in Fig. 2 are similar to the inferred orientation of pigeonite grains in the Kenna ureilite by Berkley et al. [32] which were interpreted as a cumulate fabric. Brothers [33] described a similar orientation of augite in the Skaergaard intrusion which he attributed to orientation of settled cumulus crystals. In all these studies, including the present one, the c crystallographic axes (long axis) of the crystals parallel the mineral lineation. We suggest, therefore, that the groundmass fassaite in ADOR has a preferred orientation and lineation caused by cumulus processes, possible including the effects of post-depositional magmatic current flow. An alternate, but similar hypothesis, is that crystal orientation is the result of laminar flow of a crystalline mush during intrusion within a narrow conduit. Fig. 2 suggests that groundmass pyroxene shows a preferred flattening parallel to the (100) crystal face which is easily explained as a result of low potential energy positioning of an anisotropic crystal on the floor of a magma chamber. A laminar flow regime could probably effect a similar flattening as illustrated in many trachytic or flow banded igneous rocks. The poikilitic fassaite grains are interpreted as oikocrysts resulting from rapid adcumulus crystal growth within restricted high melt/crystal pockets within the crystallizing body. Hence, these grains show no tendency for preferred orientation. Rapid growth of these grains is suggested by the incorporation of groundmass fassaite grains of the same composition (but different crystal orientation) within the oikocryst without absorbing them into the larger grain. Slower crystal growth, such as the growth of porphyroblasts in metamorphic rocks [ 17 ] might
328
lead to diffusion of material resulting in the incorporation of the inclusion material into the host grain. The porphyroblast would then be optically continuous and inclusion-free (providing phases of different composition than the host were not available), since inclusion material would be completely, or mostly, absorbed into the crystal lattice of the porphyroblast. Hess [34] has suggested that the development of adcumulates is facilitated by slow crystal setting rates and the presence of convective currents, conditions possibly prevailing during the accumulation of ADOR pyroxene. The clustering of triple juncture angles near 120 ° for both olivine and clinopyroxene is probably attributable to adcumulus crystal growth and resuiting interference of crystal grain boundaries (cf. [19]). This is a normal igneous post-consolidation adjustment and no distinct post-igneous thermal metamorphic event need to be called upon to explain the resultant textures. Rounded groundmass clinopyroxene inclusions in clinopyroxene oikocrysts are also attributable to subsolidus grain boundary adjustment as observed in terrestrial adcumulates [19]. The mineral chemistry indicates crystallization from a highly undersaturated melt. The high Ca content of the olivine, coexisting magnesian kirschsteinite, and the fassaite composition of the pyroxene are all characteristic of highly undersaturated assemblages. These assemblages may have coexisting melllite, or the plagioclase component may be incorporated in the pyroxene as CaAI(SiAI)O6, since plagioclase is unstable at such low silica activities of the melt [35]. The high Ca content of the olivine is also indicative of crystallization at low pressures, since pressure decreases the solubility of monticellite molecule in forsterite [26]. Thus, the textural and mineralogical data indicate that the ADOR assemblage crystallized at low pressures from a highly undersaturated basaltic melt, and formed as a pyroxene (olivine) cumulate which has been mildly recrystallized or annealed. The presence of rare celsian and plagioclase grains in this highly undersaturated assemblage appears to be anomalous. Perhaps the feldspar formed at a late stage in the history of the rock in small local segregations in which conditions for its stability existed, but this must be a rare event and feldspar could not have been involved in any major way in the crystallization history of this undersaturated melt.
Since this meteorite is a cumulate, the pyroxene crystals have gravitationally settled from a melt which may have crystallized melilite first. Other feldspathoidal minerals would require alkali elements, but since they are very low, feldspathoidal minerals were probably rare or absent. Schnetzler and Philpotts [36] and Ma et al. [25] show that ADOR has a small negative Eu anomaly. Schnetzler and Philpotts [36] suggested that the Eu anomaly may have been caused by the crystallization of plagioclase, but this does not appear to be reasonable because of the highly undersaturated nature of the assemblage and the highly aluminous nature of the pyroxene. Mason and Martin [37] showed that melilite has a positive Eu anomaly and Ringwood [38] has argued that melilite (and perovskite) crystallization may produce a negative Eu anomaly if it precedes pyroxene crystallization Thus, the negative Eu anomaly may be the result of early melilite crystallization, which has been removed in the cumulate process. However, Carmichael et al. [35] point out that if melilite crystallizes with pyroxene, the pyroxene may have low A1 contents. In contrast, Schairer and Yoder [39] point out that pyroxene may become more aluminous with the onset of melilite crystallization. Pyroxenes in terrestrial mehlite assemblages have variable A1203 contents [21 ], but this is complicated by the presence of other feldspathoidal phases. Schairer and Yoder [39] studied some critical planes in the CaO-MgO-AlzO3-SiO 2 system. They found a number of quaternary invariant points including one (labelled Q) with the assemblage (at low pressure) of diopside, forsterite, spinel and melilite. They note that the pyroxene is not pure diopside, but must have calcium Tschermak's molecule, CaA1(SiAI)O 6, in solid solution. The forsterite should have appreciable monticellite in solid solution, and spinel may not be pure MgA1204. They note that spinel appears to react out early in the fractionation scheme when Na20 is added to the system. The very low Na20 in this meteorite assemblage may be the reason for the presence of spinel. Liquids generated at the quaternary invariant point Q would be represented terrestrially by olivine melihte nephehnites. Lack of alkalis in the ADOR melt would inhibit nepheline crystallization. With loss of olivine the liquid at Q will fractionate toward the quaternary invariant point R (anorthite-melilite-diopside-spinel), as the result of
329 Ca e n r i c h m e n t . The presence o f rare grains o f feldspar and the highly calcic nature o f the assemblage m a y be indicative o f fractionation f r o m Q towards R . If, however, the highly aluminous nature o f the p y r o x e n e implies lack o f coexisting melilite, then the melt itself must have been similar to the b u l k composition o f the m e t e o r i t e . This m a y have been the result o f partial melting (at high pressure) o f a mainly aluminous p y r o x e n e source rock, which has already experienced fractionation. This melt was t h e n b r o u g h t to the surface and crystallized mainly i n t o a single phase c o m b i n i n g the p y r o x e n e and feldspathic components.
Acknowledgements Assistance in the electron m i c r o p r o b e w o r k by Mr. George Conrad is gratefully acknowledged. Mr. A.A. Chodos (California Institute o f T e c h n o l o g y ) made available the whitlockite analysis given in Table 1 and Dr. Roy S. Clarke (U.S. National Museum) provided a polished thin section o f the m e t e o r i t e . Dr. Ralph Kretz was very helpful in discussions o f petrofabrics, as was Dr. R.L. Warner in discussions o f forsteritemonticellite stability relations. This w o r k is supported, in part, by N A S A grants NSG-7258 (M.P.) and N G L 32-004-064 (K.K.).
References 1 0 . A . Derby, Meteoritos Brasileiros, Revista do Observatorio (1888) 1. 2 E. Ludwig and G. Tschermak, Der meteorit von Angra dos Reis, Mineral. Petrogr. Mitt. 8 (1887) 341. E. Ludwig and G. Tschermak, Nachtrag zu der mitteilung fiber den meteoriten yon Angra dos Reis, Mineral. Petrogr. Mitt. 28 (1909) 110. 3 D. St6rzer and P. Pellas, Angra dos Reis: plutonium distribution and cooling history, Earth Planet. Sci. Lett. 35 (1977) 285. 4 D.A. Papanastassiou, The determination of small time differences in the formation of planetary objects, Ph.D. Thesis, California Institute of Technology, Pasadena, Calif. (1970). 5 M. Tatsumoto, R.J. Knight and C.J. All~gre, Time differences in the formation of meteorites as determined from the rati o of lead-207 to lead-206, Science 180 (1973) 1279.
6 C.M. Hohenberg, Xenon from the Angra dos Reis meteorite, Geochim. Cosmochim. Acta 34 (1970) 185. 7 A.L. Albee and A.A. Chodos, Microprobe investigations on Apollo 11 samples, Proc. Apollo 11 Lunar Sci. Conf. 1 (1970) 135. 8 R. Hutchison, The Angra dos Reis (stone) mineral assemblage and the genesis of stony meteorites, Nature Phys. Sci. 240 (1972) 58. 9 ADORABLES: K. Keil, M. Prinz, P.F. Hlava, C.B. Gomes, W.S. Curvello, G.J. Wasserburg, F. Tera, D.A. Papanastassiou, J.C. Huneke, A.V. Murali, M.S. Ma, R.A. Schmitt, G.W. Lugmair, K. Marti, N.B. Scheinin, R.N. Clayton. Progress by the consorts of Angra dos Reis, in: Lunar Science VII (Lunar Science Institute, Houston, Texas, 1976) 443. 10 G.J. Wasserburg, F. Tera, D.A. Papanastassiou and J.C. Huneke, Isotopic and chemical investigations on Angra dos Reis, Earth Planet. Sci. Lett. 35 (1977) 294. 11 K. Keil, The electron microprobe X-ray analyzer and its application in mineralogy, Fortschr. Mineral. 44 (1967) 4. 12 K. Keil, Application of the electron microprobe in geology, in: Microprobe Analysis, C.A. Andersen, ed. (Wiley, New York, N.Y., 1973) 189. 13 A.E. Bence and A.L. Albee, Empirical correction factors for the electron microanalysis of silicates and oxides, J. Geol. 76 (1968) 382. 14 A.L. Albee and L. Ray, Correction factors for electron probe microanalysis of silicates, oxides, carbonates, phosphates, and sulfates, Anal. Chem. 42 (1970) 1408. 15 G. Lugmair and K. Marti, Sm-Nd-Pu timepieces in the Angra dos Reis meteorite, Earth Planet. Sci. Lett. 35 (1977) 273. • 16 C.S. Smith, Grains, phases and interfaces: an interpretation of microstructure, Trans. Am. Inst. Min. Metall. Eng. 175 (1948) 15. 17 R. Kretz, Grain-size distribution for certain metamorphic minerals in relation to nucleation and growth, J. Geol. 74 (1966) 147. 18 R.H. Vernon, Microstructures of high-grade metamorphic rocks at Broken Hill, Australia, J. Petrol. 9 (1968) 1. 19 R.H. Vernon, Comparative grain-boundary studies of some basic and ultrabasic granulites, nodules and cumulates, Scott. J. Geol. 6 (1970) 337. 20 R.M. Hazen and L.W. Finger, Crystal structure and compositional variation of Angra dos Reis fassaite, Earth Planet. Sci. Lett. 35 (1977) 357 21 W.A. Deer, R.A. Howie and J. Zussman, Rock-Forming Minerals, 2 (Longmans, London, 1963). 22 E. Dowty and J.R. Clark, Crystal-structure refinement and optical properties of a Ti 3+ fassaite from the Allende meteorite, Am. Mineral. 58 (1973) 230. 23 R.T. Dodd, Calc-aluminous insets in olivine of the Sharps chondrite, Mineral. Mag. 38 (1971) 451. 24 H.K. Mao, P.M. Bell and D. Virgo, Crystal-field spectra of fassaite from the Angra dos Reis meteorite, Earth Planet. Sci. Lett. 35 (1977) 352.
330 25 M.-S. Ma, A.V. Murali and R.A. Schmitt, Genesis of the Angra dos Reis and other aehondritic meteorites, Earth Planet. Sci. Lett. 35 (1977) 331. 26 R.D. Warner and W.C. Luth, Two-phase data for the join monticellite (CaMgSiO4)-forsterite (Mg2SiO4): experimental results and numerical analysis, Am. Mineral. 58 (1973) 998. 27 T. Simkin and J.V. Smith, Minor element distribution in olivine, J. Geol. 78 (1970) 304. 28 J.C. Stormer, Calcium zoning in olivine and its relationship to silica activity and pressure, Geochim. Cosmochim. Acta 37 (1973) 1815. 29 E. Dowry, Phosphate in Angra dos Reis: structure and composition of the Ca3(PO4) 2 minerals, Earth Planet. Sci. Lett. 35 (1977) 347. 30 T.E. Bunch and A.M. Reid, The nakhlites, 1. Petrography and mineral chemistry, Meteoritics 10 (1975) 303. 31 R. Brett, J.S. Huebner and M. Sato, Measured oxygen fugacities of the Angra dos Reis achondrite as a function of temperature, Earth Planet. Sci. Lett. 35 (1977) 363. 32 J.L. Berkley, H.G. Brown, K. Keil, G. Huss, N.L. Carter and J-C.C. Mercier, The Kenna ureilite: An ultramafic
33 34 35 36
37
38
39
rock with evidence for igneous, metamorphic, and shock origin, Geochim. Cosmochim. Acta 40 (1976) 1429. R.N. Brothers, Petrofabric analyses of Rhum and Skaergaard layered rocks. J. Petrol. 5 (1964) 255. H.H. Hess, Stillwater igneous complex, Montana, Geol. Soc. Am., Mem. 80 (1960) 230 pp. I.S.E. Carmichael, F.J. Turner and J. Verhoogen, Igneous Petrology (McGraw-Hill, New York, N.Y., 1974) 739 pp. C.C. Schnetzler and J.A. Philpotts, Genesis of the calcium rich achondrites in light of rare-earth and barium concentration, in: Meteorite Research, P.M. Millman, ed. (Reidel, Dorecht, 1969) 206. B. Mason and P.M. Martin, Minor and trace element distribution in melilite and pyroxene from the Allende meteorite, Earth Planet. Sci. Lett. 22 (1974) 141. A.E. Ringwood, Minor element chemistry of mare basalts, in: Lunar Science V (Lunar Science Institute, Houston, Texas, 1974) 633 (abstract). J.F. Schairer and H.S. Yoder, Critical planes and flow sheet for portion of the system CaO-MgO-A1203-SiO 2 having petrological applications, Carnegie Inst. Washington Yearbk. 68 (1970) 202.
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[4]
STUDIES O F B R A Z I L I A N M E T E O R I T E S , III. ORIGIN AND HISTORY OF THE ANGRA DOS REIS ACHONDRITE M. P R I N Z Department of Mineral Sciences, American Museum of Natural History, New York, N. Y. 10024 (USA) K. K E I L 1, P.F. H L A V A 2, J.L. B E R K L E Y Department of Geology and Institute of Meteoritics, University of New Mexico, Albuquerque, N.M. 87131 (USA} C.B. G O M E S Instituto de Geociencias, Universidade de Sao Paulo, Sao Paulo (Brasil) and W.S. C U R V E L L O Museu Nacional, Rio de Janeiro (Brasil)
Received November 12, 1976 Revised version received March 14, 1977
The Angra dos Reis meteorite fell in 1869 and is a unique achondrite. It is an ultramafic igneous rock, pyroxenite, with 93% fassaite pyroxene which has 15.7% Ca-Tschermak's molecule, plus calcic olivine (Fo53.1 ; 1.3% CaO), green hercynitic spinel, whitlockite (merrillite), metallic Ni-Fe, troilite, as well as magnesian kirschsteinite (Ks62.3Mo37.7), within olivine grains, and celsian (Cs90.2AnT.7Ab 1.7Or0.4) which are phases reported in a meteorite for the first time, and plagioclase (An86.0), baddeleyite, titanian magnetite (TiO2, 21.9%), and terrestrial hydrous iron oxide which are phases reported for the first time in this meteorite. Petrofabric analysis shows that fassaite has a preferred orientation and lineation which is interpreted as being due to cumulus processes, possibly the effect of post-depositional magmatic current flow or laminar flow of a crystalline mush. The mineral chemistry indicates crystallization from a highly silicaundersaturated melt at low pressure. Since the meteorite formed as a cumulate, pyroxene crystals may have gravitationally settled from a melt which crystallized melilite first. Plagioclase would be uostable in such a highly undersaturated melt, and feldspathoids would be rare or absent due to the very low alkali contents of the melt. The presence of rare grains of plagioclase and celsian may be the result of late-stage crystallization of residual liquids in local segregations. Thus, the Eu anomaly in Angra dos Reis may be the result of pyroxene separation from a melt which crystallized melilite earlier, rather than plagioclase as previously suggested.
1. Introduction The Angra dos Reis ( A D O R ) , Rio de Janeiro, Brazil achondrite fell in the latter half o f J a n u a r y , 1 Consortium Leader. 2 Now at Sandia Laboratories, Albuquerque, N.M. 87115, U.S.A.
1869 ( 2 2 ° 5 8 ' S , 44° 19'W) at a b o u t 0 5 0 0 hours (5 alm.). The b o d y was apparently travelling f r o m n o r t h to s o u t h and some smoke but no light and sound was observed [1 ]. The m e t e o r i t e was observed by a local judge to fall into the bay near the t o w n o f Angra dos Reis. It was found, at a depth o f 2 m by a local diver a day after the fall and consisted o f two pieces; f r o m an u n m a t c h e d fresh surface it was surmised that a
318
third piece was missing. Ludwig and Tschermak [2] report that the recovered mass was originally one stone of about 1.5 kg. St6rzer and Pellas [3] conclude from measurements of galactic cosmic ray track densities that the meteorite had a minimum preatmospheric mass of 80 kg. The ADOR meteorite is a unique ultramafic pyroxenite containing about 93% fassaite, and was shown to have a very primitive 87Sr/S6Sr ratio [4], an old 2°TPb/2°6Pb age of 4.555 b.y. [5], and evidence for extinct 244Pu [6]. The only modem mineral analyses of ADOR are those by Albee and Chodos [7] who analyzed pyroxene, and Hutchison [8] who analyzed pyroxene, olivine, spinel, and also gave a partial analysis of two sulfide grains. A progress report by some members of the present consortium was published earlier [9]. Previous work has suggested that ADOR is an igneous rock of complex history, yet all indications are that it is an ancient rock with many "primitive" properties. Thus, this meteorite is an extremely important one whose comprehensive study is likely to provide clues to the understanding of the early differentiation history of its parent body and the early history of the solar system in general. Therefore, a consortium study of this meteorite was undertaken [9] with the intent of coordinating a wide variety of studies on the samples (see the following papers in this issue). Here, we report on the microscopic study of polished thin sections of meteorite slices and grain mounts of separated phases, and the electron microprobe study of the meteorite's constituents which are fassaite, olivine, spinel, whitlockite (merrillite), metallic Ni-Fe, trollite, as well as phases reported for the first time in a meteorite (magnesian kirschsteinite, celsian) and phases reported for the first time in this meteorite (bytownite, baddeleyite, titanian magnetite, terrestrial hydrous iron oxide), in an attempt to provide petrological clues for the origin and history of this meteorite.
2. Materials and methods
Polished thin sections of the rock and of mineral separates prepared at the University of New Mexico and California Institute of Technology were studied microscopically and by electron microprobe techniques. One polished thin section was made available
for study by the Smithsonian Institution of Washington, D.C., at an early stage of the study. The complex sample preparation of this meteorite is described by Wasserburg et al. [10]. Mineral compositions were determined with an ARL-EMX-SM electron microprobe according to the procedures of Keil [11,12], and corrections for differential matrix effects were made following the method of Bence and Albee [13] and Albee and Ray [14]. Petrofabric analysis was carried out utilizing a 5-axis universal stage with data points plotted on a Schmidt stereographic projection net. Triple junctures were also measured on the universal stage to obtain the true grain boundary intersection angles.
3. Texture and petrofabrics 3.1. Textural relations
The ADOR meteorite consists mainly of pyroxene (fassaite), with minor olivine, and accessory magnesian kirschsteinite, spinel, whitlockite (merrillite), metallic Ni-Fe, troilite (some has been oxidized to "limonite"), and very rare plagioclase, celsian, baddeleyite, and titanian magnetite. Fassaite is present in two textural patterns. The most common pattern (Fig. 1A) is groundmass fassaite consisting of small xenomorphic grains (up to 0.5 mm; average 0.1 mm) which range from equidimensional to an aspect ratio (length/width) of 5 : 1. Grain boundaries of groundmass fassaite meet at triple juncture intersections (Fig. 1B) and can be termed xenomorphic granular, except for a weak mineraLelongation lineation in one thin section with an average aspect ratio of 2 : 1 (described below). The other textural pattern is less common and consists of large (up to 3 mm; average 1.3 mm), optically continuous, xenomorphic poikilitic grains (Fig. 1C) which are inclusion-free or completely enclose groundmass fassaite. The margins of these poikilitic grains commonly partially enclose groundmass pyroxenes. Both types of fassaite exhibit sharp extinction and show no evidence of strain or deformation. Olivine occurs as separate grains within the groundmass fassaite or as granular aggregates of equidimensional grains (up to 1 mm; average 0.75 mm) (Fig. 1D). The structural configuration of these aggregates is not
319
Fig. 1. Textural relationships in the Angra dos Reis meteorite. A. Typical textural relationship of small groundmass fassaite grains. Some poikilitic fassaite grains are also present (dark). Transmitted light, crossed nicols. Field of view 4.8 mm wide. B. Triple juncture relationship of groundmass fassaite. Transmitted light, crossed nicols. Field of view 0.7 mm wide.
320
Fig. 1. Textural relationships in the Angra dos Reis nieteorite (continued). C. Large poikilitic fassaite, with some twinning, enclosing small groundmass fassaite. Transmitted light, crossed nieols. Field o f view 4.8 m m wide. D. Aggregate o f rec~ystallized olivine (center) surrounded on b o t h sides by finer-grained groundmass fassaite. Note that triple junctures in olivine are closer to 120 ° than those in pyroxene. Transmitted light, crossed nicols. Field o f view 4.8 m m wide.
321
Fig. 1. Textural relationships in the Angra dos Reis meteorite (continued). E. Large whitlockite grain (white, diagonal) surrounded by fassaite. Transmitted light. Field of view 2.9 mm wide. F. Olivine grain containing inclusions of magnesian kirschsteinite. Miner. al separate, embedded in epoxy. Reflected light. Field of view 0.7 mm wide.
322
known but they may be discontinuous lenses or thin laminae. Olivine segregations display well-developed triple junctures with gently curving surfaces. Olivine grains display sharp extinction and are unstrained. Small (0.1 mm), xenomorphic grains of green spinel are dispersed throughout the fassaite. Magnesian kirschsteinite is very rare and found only as small inclusions in olivine grains. Whitlockite (merrillite) occurs as sparsely distributed, millimeter-sized grains (Fig. I E) in widely separated areas in one thin section. Very sparse grains of metallic Ni-Fe and somewhat more abundant troilite are randomly distributed. Rare grains of celsian which are very small (up to 30 /am) are found in the groundmass. Plagioclase (bytownite) was not detected in situ in any of the thin sections, but several grains were found in mineral separates. The mineral separations were carried out at the California Institute of Technology with great care and the plagioclase grains do not appear to be contaminants. St6rzer and Pellas [3] also note the presence of rare feldspar in their hand-picked mineral separates. They report finding eight crystals, seven of which have etching characteristics similar to that of bytownite, and one crystal with characteristics of a sodic plagioclase. Lugmair and Marti [15] note the presence of feldspar in their mineral separates but the type is not known. Rare grains of titanian magnetite (up to 50 #m) were found in the groundmass.
3. 2. Petrofabric analysis The X, Y and Z optical directions were measured for 100 groundmass fassaite and 6 poikilitic grains. The groundmass grains and one poikilitic grain were contained in one small thin section which displayed a weakly developed mineral elongation lineation oriented N-S. The orientation of 5 poikilitic fassaites were measured in another fragment along with 12 groundmass grains. Orienting on the groundmass fassaite measurements and a very weak lineation discovered in a small domain of this fragment, the 5 poikilitic grain orientations were rotated approximately into the plane of the lineated fragment. The results of these measurements are shown in Fig. 2 which demonstrates a preferred orientation for groundmass fassaite and a contrasting apparent random orientation for the poikilitic grains. However, the small number of poikilitic grains measured cannot unequivocally be used to prove a random orientation. The preferred orientation of the groundmass fassaite is characterized by a partial girdle of horizontally oriented (relative to the thin section plane) Y axes with a tendency for rotation within the thin section plane from NW-SE to NE-SW. Since the c crystallographic axis is oriented normal to Y, the long axis of the fassaite crystals also rotates in the plane of the
l XCPX
t YcPx
ZCpx
Fig. 2. Lower hemisphere Schmidt net projections of fassaite indicatrix axes for g r o u n d m a s s fassaite in Angra dos Reis. Filled triangles are for poikilitic grains, l = lineation. C o n t o u r s are 1, 5, and 8% o f 1% areas.
323
section and is approximately symmetrical about the lineation (l). The Z patterns for groundmass fassaite is less developed than Y but displays a N-S-trending "hour glass-shaped" girdle with maxima concentrated in the southern sector. The flaring out effect displayed in the northern and southern sectors roughly corresponds in angular magnitude to the limits of Y rotation and, therefore, the Z girdle pattern appears to be strongly dependent on Y orientation, though some rotation about the Y axis is indicated. The weak maxima located in the southern sector suggests the c+ crystallographic axes having a preferred northerly orientation, though in many crystals c+ points in a southerly direction. This can also be seen in the X diagram which displays a weak maxima mainly in the northern sector, whereas otherwise it displays a rather poorly developed pattern.
3.3. Triple juncture analysis In an attempt to evaluate the degree of subsolidus recrystallization, measurements of triple juncture angles were performed on the universal stage, following the procedures of Smith [16], Kretz [17], and Vernon [18,19]. One hundred fifty angles (representing 50 triple junctures) were measured for groundmass fassaite grains and 60 angles (representing 20
triple junctures) were measured for olivine grains. All angles were measured between grains of the same mineral phase. Assuming crystallographic orientation and anisotropy effects to be negligible, the three grain boundary angles formed at the junction of three crystals should approach 120 ° provided interfacial tensions have achieved a static balance [17]. This equilibrium state is approached in highly recrystallized metamorphic rocks, and failure to achieve nearly perfect 120 ° angles may be attributed to a departure from grain boundary equilibrium (again, neglecting crystal orientation and anisotropy effects) as displayed in less completely recrystallized rocks. Although crystal orientation effects may be significant for the groundmass pyroxene in ADOR, the petro fabric analysis (Fig. 2) suggests a high degree of variability in orientation, within certain limits. Therefore, orientation effects for pyroxene are discounted in the present study. Olivine orientations have not been determined; however, since no olivine lineation has been observed, the effect of orientation on olivine triple juncture angles is also discounted. The results of the triple juncture analysis are presented in Fig. 3. The variability of fassaite angles is clearly shown, although measurements do cluster around the mean of 120 ° (the mean will always be 120 ° when all three angles are averaged). The standard deviation (o) for fassaite angles is 39.7 which indicates a marked
,50 40
OL I V I N E ~:2SO
30 20
I,M IO.
¢/)
<¢ ~O
O,
LI. 0 40
FASSAITE :3S7
I1 0
65
85
105
120
135
TRIPLE
155
175
195
JUNCTURE
215
235
255
275
ANGLE
Fig. 3. Triple j u n c t u r e angle variation histogram for g r o u n d m a s s fassaite (lower) and olivine (upper). Standard deviation (o) f r o m t h e mean (120 ° ) is given for each phase.
324 departure from an equilibrium recrystallization state. Kretz [17] determined o = 7.5 for highly recrystallized metamorphic scapolite grains and assumed a somewhat greater o for metamorphic pyroxene due mainly to the higher degree of crystal anisotropy inherent to inosilicates. Vernon [18] determined o = 8.4 for highly recrystallized hedenbergites in a terrestrial metamorphic rock. The standard deviation of 29.0 for olivine (Fig. 3) indicates a closer approach to a grain boundary equilibrium energy state but is clearly too great to be considered characteristic of a highly recrystallized rock.
4. Mineral chemistry Fassaite. Pyroxene (fassaite) is the most abundant mineral but its precise abundance is difficult to esti-
mate because of the small surface area of the polished thin sections and the inhomogeneous presence of relatively coarse olivine and whitlockite (merrillite) crystals. Ludwig and Tschermak [2] calculated about 93% pyroxene for this meteorite, and this appears to be a reasonable value. The report of low-Ca pyroxene in ADOR [9] was in error (the grain was olivine) and there is no evidence for this phase in this meteorite. The fassaite is slightly pleochroic, from near colorless to red-brown, and is enriched in CaO and A1203 (Table 1 ; Fig. 4). Analysis of a large n u m b e r of groundmass and poikilitic crystals indicates that compositions are essentially homogeneous although Hazen and Finger [20] suggest minor variations. No evidence of zoning or exsolution was found. Deer et al. [21 ] characterize fassaite as having a high Ca content, variable but high A1 content, and high ferric to ferrous ratio. However, these characteristics are also
TABLE 1 Electron microprobe analyses (wt.%) of silicate, oxide, and phosphate minerals in the Angra dos Reis meteorite Fassaite
Olivine
Magnesian Spinel kirschsteinite
Whitlockite
Plagioclase
Celsian
Titanian magnetite
SiO 2 TiO 2
45.9 2.16
36.3 0.05
34.6 0.02
0.51 0.65
0.67 n.d.
46.2 n.d.
33.6 n.d.
1.23 21.9
A1203 Cr20 3 V20 3 Fe20 3 1 FeO MnO MgO CaO Na20 K20 P205 BaO
10.0 0.21 n.d. n.d. 6.7 0.06 10.6 24.1 <0.02 n.d. n.d. n.d.
<0.02 0.03 n.d. n.d. 38.3 0.60 24.3 1.29 n.d. n.d. n.d. n.d.
0.33 0.02 n.d. n.d. 26.2 0.42 8.9 28.9 0.05 n.d. n.d. n.d.
54.5 3.3 0.07 3.4 28.4 0.18 8.0 0.76 n.d. n.d. n.d. n.d.
n.d. n.d. n.d. n.d. 1.29 n.d. 2.82 49.4 0.68 n.d. 45.1 n.d.
33.6 n.d. n.d. n.d. 0.21 n.d. n.d. 17.2 1.47 0.11 n.d. n.d.
28.0 n.d. n.d. n.d. 0.43 n.d. n.d. 1.20 0.15 0.05 n.d. 38.2
3.5 1.02 0.26 18.5 48.8 1.17 1.18 1.21 n.d. n.d. n.d. n.d.
100.01
100.91 2
99.44
99.94
100.22 3
98.82
101.63
12
13
18
Fo 53.1 Fa 46.9
Mo 37.7 Ks 62.3
Cm Hc Sp Uv Mt
Total Number of grains
20
1
4
6
98.77 2 9
Molecular end members
Wo 54.6 En 33.5 Fs 11.9
3.8 56.4 33.4 2.8 3.6
An 86.0 Ah 13.3 Or 0.7
An 7.7 Ab 1.7 Or 0.4 Cs 90.2
I Fe203 calculated to achieve stoichiometry. 2 Ni below detection limit. 3 Includes 0.01% Y203 and 0.25% Ce203. Analysis by A.A. Chodos, California Institute of Technology. n.d. = not determined.
Cm 1.8 Hc 1.1 Sp 6.4 Uv 65.1 Mt 25.6
325 CoSiO 3
CoSiO 3
/
I
^
~
~.-Co-P-eSi206 -
,/
MgSi03 I 0 I
I00
FeSiO a I
I
I
I
I
I
80 80
I
II
I
I
60 40 Mole % Forsterite I
I
I
60 40 Mole % Monticellite
•
'
I
I
I
I
ZO 20
I
0 I
0
Fig. 4. Composition of fassaite in three-component diagram CaSiO3-MgSiOa-FeSiO 3 (molecular percent), disregarding other pyroxerie components. Also shown are the Compositions of coexisting olivine and magnesian kirschsteinite.
noted in what they call titanaugites, especially those from highly undersaturated rocks such as teschenites, essexites, and melilite-nepheline basalts. There is no sharp distinction found in metamorphic and metasomatic carbonate rocks, and titanaugite (rich in A1) is used in igneous rocks. The term fassaite will be used in this paper because it emphasizes the unique chemical characteristics of this pyroxene. It should be noted that the term fassaite has been used in some "igneous" assemblages in meteorites such as the light inclusions in Allende ( e.g. [22]), a multi-phase assemblage in an olivine crystal in the Sharps chondrite [23], and ADOR [81. The analysis of fassaite (Table 1) is very similar to analyses by Ludwig and Tschermak [2], Albee and Olodos [7], Hutchison [8], Hazen and Finger [20] and Ma et ai. [25], except for the alkali contents in the Ludwig and Tschermak [2] analysis which are
erroneous. Ludwig and Tschermak [2] reported that the fassaite in ADOR contains 0.33% Fe203 and 7.48% FeO but Mao et al. [24] show that no Fe a+ is present as determined by M6ssbauer spectra. Hutchison [8] reported 7.1% Fe, calculated as FeO. He found that tetrahedral AI did not balance the sum of the remaining trivalent cations and suggested that onethird of the Fe 2+ was present as Fe 3+, to make up the difference. The ADORABLES [9] also calculated that Fe 3+ was present in the pyroxene, following the suggestion of Hutchison [8]. However, in this calculation Ti is treated as if it is a trivalent cation, instead of as a tetravalent cation, and there is a fairly large excess of SiO2. Ti is now treated as a tetravalent cation by calculating the Ca(Mgo.sTio.s)(SiA1)O 6 and Ca(Feo. sTio.sXSiA1)O 6 molecules, along with other pyroxene molecules. A recalculation of the pyroxene analysis (Table I) was carried out by cal-
326 culating the following molecules, sequentially, with the following results (in molecular percent): CaMnSi206, 0.2%; Ca(Mgo.sTio.sXSiA1)O6, 9.0%; Ca(Feo.sTio.s)(SiAl)O 6, 3.2%; CaCr(SiA1)O6, 0.6%; CaAI(SiA1)O6, 15.7%; Ca2SiO6, 34.1%; Mg2Si206, 27.4%; and Fe2Si206, 9.7%. In this calculation a nearly perfectly balanced formula is obtained with only a very slight excess of SiO 2, and without the need for calculating any Fe 3÷. Albee and Chodos [7] noted that the formula proportions of A1 in the ADOR pyroxene is much more than twice the amount of Ti. They found that one-third of the Al is related to the CaTi(SiA1)O 6 molecule and two-thirds to CaAI(SiA1)O 6. Since the CaAI(SiA106) molecule is also high in pyroxene of deep-seated terrestrial igneous rocks they suggested that the unusual pyroxene compositions may be indicative of high pressure crystallization, below the plagioclase stability field. However, plagioclase and/or celsian has now been found in ADOR ([3,15], this study); moreover, this type of pyroxene is present in highly undersaturated basaltic lava flows. Hence, recourse to the high-pressure hypothesis for the origin of the fassaite in ADOR appears unwarranted.
Olivine and magnesian kirschsteinite. Ludwig and Tschermak [2] calculated that about 5.5% olivine is present in ADOR. They had noted that the olivine is unusually Ca-rich and somewhat similar to an Fetich monticellite. The Ca-rich nature of the olivine in ADOR is confirmed by electron microprobe analysis (Table 1). In addition, in some of the olivines a few tiny crystals of magnesian kirschsteinite were found, as first reported by the ADORABLES [9]. The high CaO content of the olivine is indicative of a relatively high solubility of the monticellite (CaMgSiO4)kirschsteinite (CaFeSiO4) molecule in the olivine. Warner and l_~th [26] have studied the maximum solubility of monticeUite in forsterite as a function of temperature and pressure. They showed that at temperatures compatible with magmatic crystallization of Mg-rich olivine, 5 - 1 0 mol.% of monticellite can be dissolved, if the melt is saturated with respect to monticellite. More Fe-rich olivine can dissolve larger amounts, but this was not experimentally determined. They also found that increasing pressure decreases the solubility of monticellite in forsterite. Hence, the high Ca content of the ADOR olivine also
appears to be indicative of low pressure. Warner and Luth [26] find that highly calcic olivines are always from melilite-bearing rocks (highly SiO2 undersaturated), and these rocks are expected to contain olivine with more than 1% Ca, as is the case for olivine in ADOR. Their experimental results are consistent with the electron microprobe data of Simkin and Smith [27]. Stormer [28] studied Ca zoning in olivine phenocrysts from nephelinite and basanite lavas. He finds that the Ca content of olivine is very sensitive to the silica activity of the liquid and that high Ca con tents in olivine are found in highly undersaturated rocks. He also finds that Ca content in olivine decreases as pressure increases, as did Warner and Luth [26]. Warner and Luth [26] determined the solvi bounding the two-phase region along the monticellite-forsterite join, at a variety of temperatures and pressures. Although the addition of Fe to this system would change the position of the solvi it is reasonably certain that a two-phase assemblage such as that present in ADOR would stably coexist. Without work on the more complex experimental system it is not possible to determine the temperature and pressure of crys. tallization of the ADOR olivine-magnesian kirschsteinite assemblage.
Spinel. Minute grains of dark green spinel are homogenously distributed throughout the meteorite. The spinel is compositionally homogeneous and rich in Fe and A1, making it a hercynitic spinel (Table 1). Cr is low, as in the entire mineral assemblage, and it was necessary to calculate a portion of the total Fe as Fe a+ in order to maintain stoichiometry of the phase. This indicates a somewhat higher pO 2 for the melt in comparison to most ordinary chondrites and achondrites. Celsian. Several small grains of celsian were found in the groundmass. This is the first report of celsian in a meteorite. The grains are chemically homogeneous (Table 1), and contain about 90% of the celsian molecule. St6rzer and Pellas [3] found one feldspar crystal in ADOR with etching characteristics which may be indicative of alkalic feldspar. Plagioclase. Several grains of plagioclase (bytownite) were found in the mineral separates of Wasserburg et al. [10]. The plagioclase is homogeneous and highly
327 calcic, An86 (Table 1). It is similar in composition to plagioclase found in other achondritic meteorites and in lunar basalts. Lugmair and Marti [I 5] and St6rzer and Pellas [3] also found some feldspar crystals which might be calcic plagioclase. No plagioclase was detected in polished thin sections studied.
Baddeleyite. Minute grains, very high in Zr, were detected but were too small for qunatitative analysis. The level of Zr was too high for zircon. Whitlockite (merrill#e). This phase is inhomogeneously distributed and may attain several millimeters in size (Fig. 1 F). Ludwig and Tschermak [2] calculate 0.3% of the phosphate phase in this meteorite. Rare earth elements are generally at or below the detection limits of the microprobe and are difficult to measure (Table 1). Dowty [29] has studied the phosphate in ADOR and showed that it is merrillite, a structure similar to/3-Caa(PO4) 2, and that this name is preferable to whitlockite for this type of meteoritic phosphate. Titanian magnetite. Titanian magnetite is reported for the first time in this meteorite. It occurs as very small, rare grains and contains 21.9% TiO2 (Table 1). It also contains significant amounts of Si, A1, Cr, V, Mn and Mg. Titanian magnetite of similar composition is also found in the Nahkla and Lafayette meteorites [30], where the grain size and abundance is greater. Brett et al. [31 ] measured the oxygen fugacity, as a function of temperature, in ADOR. They find that ADOR appears to have crystallized and cooled initially under conditions above the iron-wiistite buffer, and much closer to the buffer for most of its cooling history. The role of the titanian magnetite in the oxidation history of this meteorite is not clear. Metal and troilite. Metallic Ni-Fe is very rare in this meteorite and occurs as tiny blebs in troilite, which is more common. Ludwig and Tschermak [2] calculated 1.26% troilite for the meteorite, and this appears to be a reasonable value. Analyses of two metallic NiFe grains gave the following results: Ni, 1.4%, 3.5%; Co, 0.72%, 1.15%; Cu, 0.03%, 0.04%. The remainder is Fe. Troilite is nearly stoichiometric, with Co ranging from 0.07 to 0.13% and Ni at or below detection limits.
Terrestrial hydrous iron oxide. Very small opaque grains, with high Fe contents, are rarely found. Electron microprobe analysis indicates variable Fe, calculated as FeO, from about 68 to 78%, plus some SiO2 and traces of other minor elements, including S. These grains appear to be hydrous iron oxides which are the result of terrestrial weathering of troilite grains. They are very rare and the meteorite is generally fresh.
5. Discussion and conclusions The groundmass fassaite orientations shown in Fig. 2 are similar to the inferred orientation of pigeonite grains in the Kenna ureilite by Berkley et al. [32] which were interpreted as a cumulate fabric. Brothers [33] described a similar orientation of augite in the Skaergaard intrusion which he attributed to orientation of settled cumulus crystals. In all these studies, including the present one, the c crystallographic axes (long axis) of the crystals parallel the mineral lineation. We suggest, therefore, that the groundmass fassaite in ADOR has a preferred orientation and lineation caused by cumulus processes, possible including the effects of post-depositional magmatic current flow. An alternate, but similar hypothesis, is that crystal orientation is the result of laminar flow of a crystalline mush during intrusion within a narrow conduit. Fig. 2 suggests that groundmass pyroxene shows a preferred flattening parallel to the (100) crystal face which is easily explained as a result of low potential energy positioning of an anisotropic crystal on the floor of a magma chamber. A laminar flow regime could probably effect a similar flattening as illustrated in many trachytic or flow banded igneous rocks. The poikilitic fassaite grains are interpreted as oikocrysts resulting from rapid adcumulus crystal growth within restricted high melt/crystal pockets within the crystallizing body. Hence, these grains show no tendency for preferred orientation. Rapid growth of these grains is suggested by the incorporation of groundmass fassaite grains of the same composition (but different crystal orientation) within the oikocryst without absorbing them into the larger grain. Slower crystal growth, such as the growth of porphyroblasts in metamorphic rocks [ 17 ] might
328
lead to diffusion of material resulting in the incorporation of the inclusion material into the host grain. The porphyroblast would then be optically continuous and inclusion-free (providing phases of different composition than the host were not available), since inclusion material would be completely, or mostly, absorbed into the crystal lattice of the porphyroblast. Hess [34] has suggested that the development of adcumulates is facilitated by slow crystal setting rates and the presence of convective currents, conditions possibly prevailing during the accumulation of ADOR pyroxene. The clustering of triple juncture angles near 120 ° for both olivine and clinopyroxene is probably attributable to adcumulus crystal growth and resuiting interference of crystal grain boundaries (cf. [19]). This is a normal igneous post-consolidation adjustment and no distinct post-igneous thermal metamorphic event need to be called upon to explain the resultant textures. Rounded groundmass clinopyroxene inclusions in clinopyroxene oikocrysts are also attributable to subsolidus grain boundary adjustment as observed in terrestrial adcumulates [19]. The mineral chemistry indicates crystallization from a highly undersaturated melt. The high Ca content of the olivine, coexisting magnesian kirschsteinite, and the fassaite composition of the pyroxene are all characteristic of highly undersaturated assemblages. These assemblages may have coexisting melllite, or the plagioclase component may be incorporated in the pyroxene as CaAI(SiAI)O6, since plagioclase is unstable at such low silica activities of the melt [35]. The high Ca content of the olivine is also indicative of crystallization at low pressures, since pressure decreases the solubility of monticellite molecule in forsterite [26]. Thus, the textural and mineralogical data indicate that the ADOR assemblage crystallized at low pressures from a highly undersaturated basaltic melt, and formed as a pyroxene (olivine) cumulate which has been mildly recrystallized or annealed. The presence of rare celsian and plagioclase grains in this highly undersaturated assemblage appears to be anomalous. Perhaps the feldspar formed at a late stage in the history of the rock in small local segregations in which conditions for its stability existed, but this must be a rare event and feldspar could not have been involved in any major way in the crystallization history of this undersaturated melt.
Since this meteorite is a cumulate, the pyroxene crystals have gravitationally settled from a melt which may have crystallized melilite first. Other feldspathoidal minerals would require alkali elements, but since they are very low, feldspathoidal minerals were probably rare or absent. Schnetzler and Philpotts [36] and Ma et al. [25] show that ADOR has a small negative Eu anomaly. Schnetzler and Philpotts [36] suggested that the Eu anomaly may have been caused by the crystallization of plagioclase, but this does not appear to be reasonable because of the highly undersaturated nature of the assemblage and the highly aluminous nature of the pyroxene. Mason and Martin [37] showed that melilite has a positive Eu anomaly and Ringwood [38] has argued that melilite (and perovskite) crystallization may produce a negative Eu anomaly if it precedes pyroxene crystallization Thus, the negative Eu anomaly may be the result of early melilite crystallization, which has been removed in the cumulate process. However, Carmichael et al. [35] point out that if melilite crystallizes with pyroxene, the pyroxene may have low A1 contents. In contrast, Schairer and Yoder [39] point out that pyroxene may become more aluminous with the onset of melilite crystallization. Pyroxenes in terrestrial mehlite assemblages have variable A1203 contents [21 ], but this is complicated by the presence of other feldspathoidal phases. Schairer and Yoder [39] studied some critical planes in the CaO-MgO-AlzO3-SiO 2 system. They found a number of quaternary invariant points including one (labelled Q) with the assemblage (at low pressure) of diopside, forsterite, spinel and melilite. They note that the pyroxene is not pure diopside, but must have calcium Tschermak's molecule, CaA1(SiAI)O 6, in solid solution. The forsterite should have appreciable monticellite in solid solution, and spinel may not be pure MgA1204. They note that spinel appears to react out early in the fractionation scheme when Na20 is added to the system. The very low Na20 in this meteorite assemblage may be the reason for the presence of spinel. Liquids generated at the quaternary invariant point Q would be represented terrestrially by olivine melihte nephehnites. Lack of alkalis in the ADOR melt would inhibit nepheline crystallization. With loss of olivine the liquid at Q will fractionate toward the quaternary invariant point R (anorthite-melilite-diopside-spinel), as the result of
329 Ca e n r i c h m e n t . The presence o f rare grains o f feldspar and the highly calcic nature o f the assemblage m a y be indicative o f fractionation f r o m Q towards R . If, however, the highly aluminous nature o f the p y r o x e n e implies lack o f coexisting melilite, then the melt itself must have been similar to the b u l k composition o f the m e t e o r i t e . This m a y have been the result o f partial melting (at high pressure) o f a mainly aluminous p y r o x e n e source rock, which has already experienced fractionation. This melt was t h e n b r o u g h t to the surface and crystallized mainly i n t o a single phase c o m b i n i n g the p y r o x e n e and feldspathic components.
Acknowledgements Assistance in the electron m i c r o p r o b e w o r k by Mr. George Conrad is gratefully acknowledged. Mr. A.A. Chodos (California Institute o f T e c h n o l o g y ) made available the whitlockite analysis given in Table 1 and Dr. Roy S. Clarke (U.S. National Museum) provided a polished thin section o f the m e t e o r i t e . Dr. Ralph Kretz was very helpful in discussions o f petrofabrics, as was Dr. R.L. Warner in discussions o f forsteritemonticellite stability relations. This w o r k is supported, in part, by N A S A grants NSG-7258 (M.P.) and N G L 32-004-064 (K.K.).
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39
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