Preferred chondrule orientations in meteorites

18

Earth and Planetary Science Letters, 51 (1980) 18-25 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

[5]

PREFERRED CHONDRULE ORIENTATIONS IN METEORITES PHILIP M. MARTIN Department o f Physics, University of Essex, Wivenhoe Park, Colchester, Essex C04 3SQ (U.K.}

and A.A. MILLS Department o f Geology, University of Leicester, Leicester, LE1 7RH (U.K.)

Received April 24, 1980 Revised version received June 28, 1980

The orientation of chondrule long-axes has been examined in relatively friable (Allegan, BjurbSle, Chainpur) and more compacted (Allende, Kediri, Knyahinya, Parnallee) meteorites. Preferential chondrule orientations were detected in the more compacted specimens but were found to be much less well developed in the friable meteorites. We conclude therefore that the chondrule petrofabric was imposed during compaction in the meteorite parent bodies.

1. Introduction Meteoritic chondrules although once molten are not generally spherical [ 1,2]. We have reported the preferential orientation of chondrule long-axes in four type 3 carbonaceous chondrites [3], while Dodd [4] has published the results of an extensive analysis of twenty-two ordinary chondrites. These investigations revealed that most chondrites display an orientation fabric which varies widely in degree of development but does not appear to be related to degree of recrystallisation. This led Dodd to conclude that the fabric element was a relict depositional feature imposed during sedimentation of the chondrules, rather than a result of compaction and metamorphism as inferred by Stacey et al. [5] to explain the observed increase in magnetic anisotropy with decrease in porosity in chondrites. We decided to extend our previous investigation to relatively friable chondrites, because if orientation arose during compaction and metamorphism it might be expected that these particular meteorites would exhibit little or no preferential alignment of chondrule

long-axes. Our studies of the size and shape characteristics of chondrules separated from meteorites [1,2] prompted us to examine the friable Allegan, Bjurb61e and Chainpur meteorites. We were also able to obtain photographs of cut faces of the less friable meteorites Parnallee, Knyahinya, Kediri and Allende for analysis and comparison.

2. Samples To detect the presence of lineation and/or foliation in any matrix, the orientations in three orthogonal faces should be examined. Unfortunately it was not possible to adopt this ideal approach in the present work because of the scarcity of material. However, it was possible to examine museum specimens which had already been sliced to produce relatively smooth, flat surfaces. A number of such specimens were chosen in which well-defined chondrules were also displayed. These included six faces on specimens of the Parnallee (LL3) meteorite (British Museum, Natural History specimen number BM 34792), two faces on specimens of the

0012-821X/80/0000-0000/$02.50 © 1980 Elsevier Scientific Publishing Company

19 Kediri (L) meteorite (BM 1974, M14), one face on a specimen of the Knyahinya (L5) meteorite (Natural History Museum, Paris, specimen number A759) and one face of the Allende (CV3) meteorite (Natural History Museum, Vienna, specimen number L2923). The friability of the Allegan (H5), BjurbSle (L4) and Chainpur (LL3) meteorites makes it difficult to obtain smooth flat surfaces when these specimens are cut. However, it is possible to obtain reasonable thinsections of these meteorites if they are impregnated with a suitable material to bind them together during the grinding and polishing processes. The disadvantage of examining thin-sections for chondrule long-axis orientation is the small area available for study, with the consequent low number of chondrules displayed. To compensate for this, however, chondrules are usually more apparent in thin-sections, enabling more of them per unit area to be identified and their orientations measured with greater accuracy. Thin-sections of the Allegan (BM 1920, 281a; 1920 281b), Bjurb61e (BM 1926, 494; 1927, 11) and Chainpur (BM 1915, 86) meteorites were kindly loaned by Dr. R. Hutchison, British Museum (Natural History) for the purposes of this study.

3. Measurement Measurements of the orientation of chondrule longaxes from cut surfaces and thin-sections necessitated two different approaches. In the case of the sliced surfaces, each face was photographed using finegrained monochrome film and appropriate enlargements were prepared. This avoided the problems posed by the removal and transport of actual specimens and facilitated magnification to a suitable size. Individual photographs were mounted on the table of an orientometer designed to allow movement in translation and rotation [6], and viewed through a binocular microscope fitted with a line reticule in one eyepiece. Individual chondrules were aligned with their long-axis along this fiducial line, and their nominal orientation was read from the degree scale of the orientometer turntable. Each measured image was marked with a spot of ink to avoid confusion and possible duplication. The meteorite thin-sections were placed on a mechanical microscope stage and viewed under low

magnification through an eyepiece containing a line reticule. All elongate chondrules in each section were oriented one-by-one with their long axis along this fiducial line, and their nominal orientation read from the circumferential degree scale of the microscope stage. In both cases the measurements were of the doubleheaded vector type, so two directions could be read one hundred and eighty degrees apart. Only the direction between 0 ° and 180 ° was taken.

4. Analysis and results Chondrule orientations from each of' the specimens were grouped into 10 ° or 20 ° classes, and histograms constructed of number percent in each class versus orientation. The resulting histograms are shown in Fig. 1. Following Dodd [4], Krumbein's [7] method was used to determine the mean azimuth and standard deviation (a) of chondrules in each specimen. This is a simple and rapid technique - though it fails where there is no mode in the data. The standard deviation can be used as a measure of the degree of orientation shown by the chondrules. If all of them are oriented in the modal class (i.e. maximum preferential orientation) tr will have a value of 0 °. A a value of 52 ° indicates an equal number of chondrule orientations in all classes and, therefore, no preferred orientation. Calculated values for each specimen are given in Table 1. The mean azimuths are also shown on the relevant histograms. Examination of these results shows that some degree of preferential chondrule long-axis orientation appears common in the less friable chondritic meteorites. In the case of the Parnallee and Kediri meteorites (where more than one face was examined) the results tend to indicate a wide spread in the degree of preferential alignment. For example, Kediri 2 exhibited random chondrule orientations, whereas the other face (Kediri 1) displayed the strongest petrofabric of the meteorites examined in the present analysis. Likewise, Parnallee specimens 1 and 6 showed strong, 2 and 4 moderate, and 3 and 5 weak fabric orientation. These differences probably arise because those faces exhibiting the weakest petrofabric happened to be cut at greater angles to the direction of maximum fabric orientation. This may also explain our observation that Knyahinya pos-

20

preferential direction of chondrule alignment. No more than a weak petrofabric is indicated. The results for the three relatively friable meteorites

sesses a weak chondrule petrofabric, whereas Dodd found good foliation and weak lineation. The results for Allende show a wide scatter with no prominent

(a) Parnallee

1

Parnallee

30

30

20

2O

10

i0

I

,

-\

2

1

I

•

I

i

,I

0 40

8O

120

,

7

0

20

io0

160

(b)

30 Parnallee

I

1

140

o

I

i

40

80

30

3

20

I

Parnallee

4

2O

i

I

i0

i0

r 0

I

I0O

140

I

0

I

40

80

0 160

I

20

I

60

i00

I

140

Fig. 1. Histograms of the orientations of the long-axes of chondrules in meteorites. Abscissa is orientation angle in degrees; ordinate is number percent; dashed line is the calculated mean azimuth. (a) Parnallee 1 and 2; (b) Parnallee and 3 and 4.

21

-~q

(c)

Parnal lee 40 I

Parn,il l~w %

]0

L._

--11

2O

~ lO

I

I

I)

40

i

8(}

1,0

,I,

I

0 I -I~]

.h~

O

(d) 30

30 Chainpur

Allende

I I

20

20

I

O

I

4O

80

I

120

160

|

20

o loo

I

|

I

140

O

40

Fig. 1 (continued): (c) Parnallee 5 and 6; (d) Chainpur and Allende.

I

22

(e) 30

30

SjurbSle

1926,494

Bjurb61e

1927,11

I

I

2O

20

I t I

iO

I O

!

O

!

!

4O

ol

I

120

8O

0

160

I I

I

!

4O

8O

!

I

120

160

3O

30

Allegan a

Knyahinya

I I

20

2O

J

I

• "1 1(3

IO

I I

] O 120

!

!

160

20

I

60

i

100

O

O

I

I

I

!

40

80

I I

!

120

|

160

Fig. 1 (continued): (e) Bjurb61e specimens BM 1926, 494 and 1927, 11; (f) Knyahinya and Allegan a.

23 3O

3O Allegan

b

(g)

Kediri

I

20

2O

lo

iO

O

I

40

I

80

120

O 160

60

I

F

iOO

140

O

4O

30

(h)

Kediri

2

2O

10

0

O

I

I

i

40

80

120

160

Fig. 1 (continued): (g) Allegan b and Kediri 1; (h) Kediri 2.

are significantly different. The sections o f Bjurbble, Chainpur and Allegan all displayed a wide scatter o f chrondule long-axis orientations, with little indication of a preferential direction o f alignment. In the case of

Bjurb61e BM 1926, 494 random chondrule orientations were deduced from the analysis. Dodd's results for Bjurb61e indicated the presence of only a weak foliation and little or no tineation. Certeinly none o f

24 TABLE 1 Orientation analyses of chondrule long-axes in chondritic meteorites Sample

Type

Number of chondrules examined

Allende Bjurb/51e (BM 1926, 494) Bjurb61e (BM 1927, 11) Chainpur AUegan a -~ AUegan b ~ Parnallee 1 Parnallee 2 ParnaUee 3 Parnallee 4 ParnaUee 5 ParnaUee 6 Knyahinya Kediri 1 ) Kedki 2

CV3

210

L4

43

L4 LL3

43 30 57 74 139 81 136 108 108 81 178 54 54

H5

LL3

L5 L

the friable chondrite sections studied exhibited as prominent an alignment of chondrule long-axes as has been observed in some of the more compacted meteorites. It would seem that chondrule fabric orientation is considerably less developed in the friable chondritic meteorites, although it is possible that the sections examined happened to be oriented to miss showing a preferred orientation (cf. Kediri 2 above).

5. Discussion The results outlined in the previous section complement those from previous investigations in demonstrating that, (a) preferential alignment of chondrule long-axes appears common in, and perhaps characteristic of, the tess friable chondritic meteorites; (b) there is a wide range in degree of development of the chondrule petrofabric; and (c) there appears to be no obvious correlation between degree of chondrule longaxis preferential orientation and petrologic type (a chemical process). Despite the inadequacies in the approach adopted and the limited data, we believe

Mode (o)

10

Mean azimuth (o)

10.1

Standard deviation (o)

41.6

no mode 90 110 90 90 130 10 10 70 50 50 30 170

102 116 88.2 81.6 122.6 10 4.8 63.8 47.8 50.2 30.2 177.4 no mode

39.3 46.5 42.0 44.0 26.6 35.5 47.5 37.9 45.8 21.0 44.6 20.8

that our results indicate a further important observation, namely that (d) there appears to be a correlation between fabric orientation and compaction (a physical process) since the less compacted meteorites display a much weaker degree of development of preferential alignment of chondrule long-axes. Point (c) above argues against the chondrule petrofabric being imposed solely during metamorphism whereas point (d) suggests that deposition during sedimentation might also not have been the prime factor in generating the chondrule petrofabric. A1"though the data are not yet sufficient to rule out these possibilities, we would suggest, from our present analysis, that the chondrule petrofabric was most likely imposed during compaction, probably as a result of overburden pressure, in the meteorite parent bodies. It is generally assumed that degree of recrystallisation and overburden pressure are related, so that a more highly recrystallised chondrite should also have experienced a higher pressure of overburden and should thus exhibit a more strongly developed petrofabric if this was imposed during metamorphism. There is, however, no reason to assume that degree of recrystallisation and pressure of overburden should

25

be so directly related. It seems likely that in parent bodies of different sizes similar temperatures (with resulting broadly similar degrees of recrystallisation) would be reached at different depths. Consequently, meteorites having experienced similar temperatures of reheating could have been subjected to very different pressures of overburden. In such a situation the chondrule petrofabric could have been imposed during compaction yet be unrelated to petrologic type. The problem is further complicated if external heat sources affected the temperature gradient inside the meteorite parent bodies. We therefore conclude that although temperature (degree of recrystallisation) may have played a role in the development of the chondrule petrofabric, the dominant force was compaction, since the friable meteorites display a weaker petrofabric than more compacted representatives. If our conclusion is correct the results from the present analysis agree with other indications that the meteorite parent bodies were of different sizes. It might also be expected that chondrules in the more compacted meteorites should be more aspherical than those in the friable chondrites. Our analysis have shown that the departures from sphericity by the

latter are small [1,2]. To our knowledge no extensive analysis has been conducted on the shape characteristics of chondrules from compacted meteorites.

References 1 P.M. Martin and A.A. Mills, Size and shape of chondrules in the Bjurb61e and Chainpur meteorites, Earth Planet. Sei. Lett. 33 (1976) 239. 2 P.M. Martin and A.A. Mills, Size and shape of near-spherical Allegan chondrules, Earth Planet. Sci. Let. 38 (1978) 385. 3 P.M. Martin, A.A. Mills and Elaine Walker, Preferential orientations in four C3 chondritic meteorites, Nature 257 (1975) 37. 4 R.T. Dodd, Preferred orientation of chondrules in chondrites, Icarus 4 (1965) 308. 5 F.D. Stacey, J.F. Lovering and I. Parry, Thermo-magnetic anisotropies of some chondritic meteorites, J. Geophys. Res. 60 (1961) 1523. 6 J.H. McD. Whitaker and M. Gatrall, Two "orientometers" for measuring lineations on hand specimens, J. Geol. 77

(1969) 710. 7 W.C. Krumbein, Preferred orientation of peppbles in sedimentary deposits, J. Geol. 47 (1939) 673.