Impact melting of the Cachari eucrite 3.0 Gy ago

Impact melting of the Cachari eucrite 3.0 Gy ago G. J. TAYKBR~,K. KEIL~, M. R SMITHIES and R. A. SCXMIT? ‘NASA, Johnson Space Gnter, Iiouston, TX 77058 21nstituteof Meteoritics, Dept. of Geology, University of New Mexico, Albuquerque, NM 87131 chemistry Dept. & Radiation Center, Oregon State University, Corvallis, OR 97331 ‘Present addre&nRattelle Northwest Laboratories, Richland WA 99352 D. D. BOGAM',

(ReceivedMay 25, 1984; oecepredin revisedform Jmwy

7, 1985)

Abstract-Roth the host phase and glass veins of the Cachari eucrite have been analyxed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their 39Ar40Argas retention ages. Cachari is chemically similar to other non-cumulate eucrites. The vesicular glass veins vary from pure glass, to devittified glass, to areas that are substantially crysmlhne. Tbe glassy areas have nearly the same concentrations of major and tmce elements as the unmelted portions of Cachari, but some differences,probably due to prcfmntial dissolution,occur along melt contacts The @assformed by shock melting of Cachari host or of rock identical to it. -Ar-@Ar data for the host and glass suggest distinctly different ages of 3.04 + -07 Gy and 3.47 f .O4CJy,mspectively. Tbe time of glass fwmption, which may also be the time of bmcciation, is most likely given by the 3.0 Gy age of the host. Tbe bigber age for the glass is interpreted to represent incomplete Ar degas&g during the 3.0 Gy event due to the greater r&stance to Ar di&ion shown by the glass compared to the host. Event ages si@cantly younger than 4.5 Gy have now been determined for several eucrites and howardims and suggest a long dynamic regolith history for the parent body.

THE 26 KG CACHARIeucrite was found in the Buenos Aims province of Argentina in 192 1. Early descriptions (LACROIX, 1926; DUCLOUX,1928) drew attention to the presence of dark, glassy veins in the meteorite. FREDRKZICSWNand KRAUT( 1967) showed that these glassy areas have the same ~rn~tion as the host eucrite and demonstrated that the @ass formed by hyperve&ity impact in space or on the eucrite parent body. Other baa&tic achondrites have also su&ed impacts on their parent bodies, and some are known to have had their ~orn~c ages at least partially reset (e.g., BOGARD, 1979). However, well defined times of specific events have been determined for only a few eucritea and howardites. Similarly, the formation times of individual clasts or of specific breccias have been determined for few chondrites. To increase the data base on specific impact events in the histories of meteorite parent bodies, we have made 19Ar-*oArage rn~~rnen~ on both the glass and host rock in Ca&ari. We also made additional mi measurements of the glass and dammined the bulk composition of glass and host by instrumental neutron activation analysis. Gur results indicate that the glass formed by impact melting on the Cachari parent body 3.0 Gy ago ( 1 Gy = 109years).

visible glass vein and were analyr.ed by INAA (122 mg) and by mass spectrometry (198 mg). Petrologic work was done on thin sections in the Institute’s &ection. Electron microprobe analyses of tbe glassy areas were done with an ARL-EMX electron microprobe using wavelength spectrometers. The analyses were cormcud for matrix effects by the- method developed by RJZFBZand ALBEE (1968). To obtain a good estimate of average composition and to prevent loss of volatik elements from the glass, we used a 25 pm electron beam. INAA procedures were similar to those described by LAUL and SUIMF~T(1973). The samples for age dating were irmdiated with fast neutrorm Their gases were extracted by stepwise heating, and the isotopic composition of Ar was measured mass spectrometritally. Detaikd procedures are given in I(EIL et al. (1980). Two hombknde NL25 tluence monitors irradiated with the Cnchari samples gave *Ar/BAr ratios of 33.3 and 32.7. RJBUL’IS AND DISCUSSION Petrology and chemistry

The Cachari host has a basahic texture and is heavily shocked. ~~0~ plagioclase has not been converted to maskelynite, it displays mottled extinction when viewed in cross polarixem. Orthopyroxene and augite occur, and each contains exsolution lamellae of the other (Fig 1); both are severely fia&ned and mosaicized. A zone of intense brecciation, about 1 mm wide, separates the Cachari host from the impact-melted veins. In this zone, pyroxene grams SAMPLES AND ANALYTICAL TFXHNIQUW are fragmented and are only about 50 pm across. In contrast, pyroxene grains in the host average 0.7 mm SamPks of dark vesicularglass and of the hght0010red long Also in this zone of bmcciation, plagioclase hostbasaltwemobtainedfromaspecimenofCachariin the meteorite collection of the Institute of Me%co&h, occurs as small patches and has mottled extinction. University of New Mexico. A~ Cachari is riddled with In some areas the brecciated zone was breached by glassy veiny the dark material in our specimen represents mohen vein material, leaving islands of highly shocked only one vein. Adjacent chips of glass were provided for INAA at Orqon State University (94 mg) and for )pAr-*Ar basalt surrounded by glass. The presence of this analysis at the Johnson Space Center (192 mg). Samples of bmcciated zone suggests that the glass was forcibly the host rock were taken about ! cm away from the nearest injected as a melt into the Cachari host basalt. The 941

942

D. D. Bogard CI ai

FIG. I. Back-scattered etectron image of a portion of a thin section of Cachari. Upper left half shows host rock, in which plagioclase is dark gray, orthopyroxene is gray, and augite is light gray. Lower right half is an impact-melted vein. Note the quenched zone containing white metallic iron grains near the host rock. Black pits are vesicles. White scale bar is 100 pm long.

fractional crystallization of 60% pigeonite and 40% plagioclase. respectiveIy. However. PALMA C'Id‘s (1978) whole rock Cachari data (Table 2) show a slight positive Eu anomaly and a depleted LREE pattern, i.e. (La/Sm). < 1.0. Their measured pattern is similar to Sioux County’s REE profile but - 1.3X greater. We ascribe the difference between their results and ours to sampling. If PALME etal.3 (1978) sample was more repre~ntative than the one we analyzed. then Cachari may represent a primary unfractionated magma produced by - 15% partiai melting of 3 slightly depleted chondritic source composed of -60% olivine, 30% o~hopyroxene, 6% plagioclase (with all plagioclase being melted) and 4% c~inopyroxene, The glassy areas of Cachari are fairly uniform in composition (note the small standard deviations in Table 1) and have nearly the same concentrations of major, minor, and trace elements as the unmelted portions (Tables I and 2. and Fig. 2). The glass, partly devitrified glass, and quenched-crystallized melt are slightly higher in Al&& and CaO and lower in Fe0 than the host, perhaps due to incomplete melting of pyroxene and opaque oxides. The presence of relic pyroxene and chromite grains in the glass supports this interpretation. The distinct, micr~~sta~iine border that occurs where the melt is in contact with unmelted rock is richer in AlzO, and CaO and poorer in Fe0 and MgO compared to the bulk melt and the unmelted host (Table I). This difference is probably

impact-melted veins are not totally glass. Some areas are pure glass (~eenish-yeIlow in color), othem are partly devitrified. and most areas are substantially crystalline. The crystalline areas consist of pyroxene (about I by 10 ym in size), embedded in a feldspathic glass (Fig. 2). and obviously formed by rapid crystallization when the melt was quenched. The largely crystalline areas also contain small, irregular-shaped chromite clasts and metallic iron spheres. All areas are vesicular. At the border between the unmelted rock and the melt, the latter is uniformly fine-grained and is decorated with spheres of metallic iron 1 to 10 I.rrn in size (Fig. I). Micropro~ and INAA major element data appear in Table t; trace element data are given in Table 2. Abundances of selected trace eiements, normalized to chondrites, are plotted in Fig. 3. Cachari’s major and trace element composition is like that of other normal non~umuiate eucrites. More specifically, Cachari is closely related to the eucrites Sioux County, Juvinas, Bereba, Haraiya, Ibitira, and Pasamonte on the basis of their similar chondrite normalized Sm/ Eu ratios, REE, SC, and Fe/(Fe + Mg) values (SMITH, 1982). Using REE, SC and Fe/(Fe + Mg) values as FIG. 2. Back+cattemd electron image of portion of quenchconstraints for geochemical models, Smith showed crysta&ed impact melt in Cachari. Scale bar is IO pm long. that the compositions of Haraiya, Rereba, Cachari Thin crystats are pyroxene, which are surrounded by dark, and Pasamonte could be reiated to a primary magma feidspathic glass. Black areas are vugs; the white object is a metallic sphere. like Sioux County by -15, -23. -26 and -33%

943

Impact melting of Cachari TABLE 1:

Major

Elaent

eleaent

concentrations B

A

in glass

and host

of Caohari

C

D

E

P

c

H

48.8 0.6 12.9 0.34 19.2 0.60

-0.73 12.5 0.33 10.0 0.54

BAD2 TAO2 Al2O3 cr203 Fe0 HIlO

49.5 0.77 14.0 0.34 18.4 0.54

A.6 A.04 A.3 f.05 A.3 *.Ol

49.4 0.14 15.5 0.31 17.3 0.51

A.3 ~.06 A.5 k.02 ~.6 A.04

-0.6 13.4 0.30 18.8 0.50

-0.6 12.7 0.27 19.6 0.51

48.6 0.78 14.0 0.06 19.9 0.74

48.3 0.63 12.8 0.32 19.1 0.59

zi Na20 g2D p2D5 rota1

10.7 6.3 0.49 0.06 0.08 101.18

~.l k.1 f-04 :.Ol A.01

11.4 5.6 0.52 0.08 0.08 101.44

fi2 f.1 k.03 A.01 k.02

6.2 9.4 0.40 0.13 -__

10.0 6.5 0.47 0.06 ---

6.7 8.9 0.96 0.13 0.06 100.4

10.2 7.1 0.51 0.04 0.08 99.67

6.9 9.9

0.48 0.04 --

~.16 t.5 2.03 *.5 A.03

10.3 6.7 ~.6 t.3 0.47 ~.06 0.045k.013 --

A) Glass, partly derftrified glass, and quench-crystallized melt in U&i sections 402 and 403 (13 analyses with 25 um been). 'Jnoertaintisa given in columns A, B, and H represent one standard deviation of each aat of analyses. 8) Microcrystalline areas between glass and rook in ONM sections 402 and 403 (11 analyses wltb 25 urn beam). Numbers in parentheses represent one standard deviation. C) INAA of 94 mg glass sample (this work). D) INAA of 122 mg sample of unmelted host rock (this work). B) Whole rock chemical analysis (Ducloux, 1929). P) Whole rock chemical analyals (McCarthy et al, 1973). 0) Whole rook obemlcal analysis (Palme et al, 1978). H) Average of 12 non-cumulate eucrites (Smith, 1982).

due to preferential dissolution into the melt of plagioclase (the low-melting component) when the melt was injected into the rock. Because shock melts are usually superheated, the dissolution of plagioclase may have been quite rapid. The compositional similarities of Cachari with respect to the main group eucrites implies a similar origin and an initial 4.5 X 10’ yr crystallization age. The chemical data support FREDRICKSON and KURAT’S (1967) interpretation that the glassy areas in Cachari were formed by an impact in space. We believe that this impact took place on the Cachari parent body, which may have been the same parent body of other eucrites and howardites. The impact melted basaltic rock and injected it into unmelted basalt, now represented by the Cachari host. This is likely to have occurred on the floor of a crater on the Cachari parent body.

concentration ratios as a function of fractional 39Ar release are presented in Fig. 4. Ar was clearly more tightly bound in the glass compared to the host. Whereas the host released most of its Ar over the relatively broad temperature range of - 700- 1200°C. the glass released -72% of its Ar in the 1300°C extraction alone, presumably as a result of melting. The host sample shows a reasonably good age plateau of 3.04 f .07 Gy over 48% of the 39Ar released. Relative to this age - 14% of the total 40Ar in the host has been lost from lower temperature sites. These sites have a higher K/Ca ratio and probably a different mineralogy. The 1300°C extraction of the glass sample shows a distinctly higher “Ar-@Ar age of 3.47 + .04 Gy, with slightly lower ages at lower extraction temperatures. The 13OO’C extraction of the host gave a nearly identical age of 3.43 Gy and may represent a small amount of glass in the host sample. Conversely, the ages of -2.9 Gy for the

‘9Ar-MAr dating Results of the Ar isotopic analyses are given in Table 3, and the calculated ‘gAr-40Ar ages and K/Ca

TABLg

2:

Trace glans

glunt

Noat

Sa V

32 69 4.1 3.2

(PP)

co

Ll C* 3=

b

2.1 0.64

Dlaaa

in tbc host and

S Brmr

A

B 31.3 73 6.6 3.2 9.5 2.0 0.65

5

6';

:,

6.3 2.9 7 0.64 1.8

4 2 15 :

; 4 7 10 15 25

30 __ 4.5 2.61 B 1.56 0.63 0.40 1.70 0.26 1.1 0.14 -__

10

8

Tb Tb LU w Ta Tb I? (Ppb)

1.27 0.31 0.34
0.47 1.8 0.28 0.62 0.21 0.49 6

Au (ppb)

<6

<6

A B

0.52 i:X

element abundances pb*s.e. or Caobari

0.51 1.95 0.31 1.6 0.23 0.5 __

Ubol. rook malyais by Pair et al (1978) Av*rua of 12 non-c-late awrites (blth.

ti.7 +.9 d.5 l .D a2.2 * .5 a.09 a.13 t.30 f.05 +. .6 f.12 f .3

-

ti

1982)

FIG. 3. Plot of concentrations of selected trace elements in whole tuck and glass phases of the Cachari euctite, normali& to non-volatik (minus C and Hfi) co~trations in Cl chondrites (ANDERS ad EBIHARA, 1982).

944

D. D. Bogard e/ ul TABLE 3: host and produced

Heasured argon glans phases of interferences.

measurements

and

isotopic Cachari.

from

755.

*183

87.5

132.2

0.842 0.538 1.471

21.77 32.06

rt.22 ~.34

3.49 3.61

A.49 k.44

2.267

31.82 36.95

k.42 k.66

3.09 4.29

f.47 ~.51

42.53

2.09

5.40

k.55

3.617 4.308 2.621 2.122

42.51 40.73 44.40

1300

1.826

54.3

t3.2

1500 CACHARI

0.551 CLASS

90.7

9.2

1125 1200

2.131

~1.34 Al.00 d.0

15.05 4.93

*.50 kt.60

5.91 k.59 7.49 lt.61 13.8 fl.0 171.

*14

23.6

kl.4

350

0.026

450

0.101

1330. 494.

550

0.102

252.

D

20.7

f2.0

650

0.197 0.082

196.

i3

15.1

21.1

7a::

0.275 1.287

k4.2 Al.0

1000 1150 1300 1450 1600

3.192 14.93

*18 ~68

127. d 34.1 k6

09.2 39.0

kl.4 2.5

3.4 4.0

32.2

2.4

4.11

k.7

7.31 6.89 7.47

2.62 2.6 21.17

;;.a7 z;.; 60:3 e:5

0.309 0.164

141.

*4

37Ar/39Ar

9.5

26.2

A.41

36.80 40.44

rt.55 A.57

44.30 2.64 60.2 *l.O 81.0

tl.7

95.7 94.6

*2.2 +3.1

113. 209.

d k10

269.

216

155.

d

0

II

0.2

8

”

0.4

” 0.6

36AtV39Ar

” 0.6

233. 53 8.08 A.53

1.0

CuMuLATlVE PRACTION PAI

FIG. 4. wArpAr ages and K/Ca ratios as a function of cumulative fraction of -Ar released for host and glass phases of the Cachari eucrite.

L.49

1.68

lt.02

7.89 2.34

k.43 k-20

j_.o2

1.424

*.016

2.130

2.020

3.77 1.22

lt.58 2.55

L.02 A.02

0.88

A.01

2.676 2.631

2.023 2.025

1.85 2.54

2.66 2.78

0.64

2.01

2.60

2.80

0.55 0.56

A.01 lt.02

3.57 6.94

i.94 21.7

0.47 0.25

22.2

9.08

ei.5

157. 92 74.2 fi.2

24

2.01

1.30 1.19

420.

115. d 123. i3.5 141. 26

109YP

1.43

k.03

2.7 ~.8 ~1.0

AGE

x100

~1.16

t.08

38.0 52.0 61.4

K/Ca

~.76

1.48

2.3 2.5

neutron-irradiated

1.92

15.9

20.9 28.8

Of

0.64

0.55

125.

d.8

26.4

31.40

850°C and 1150°C glass extractions could represent minor amounts of crystalline material in the glass sample. The high apparent ages at low extraction temperatures of the glass represent only -2% of the total Ar released and are probably due to adsorbed atmospheric Ar and/or inherited radiogenic Ar containedwithin vugs. The K concentrations determined in the glass (262 ppm) and host (300 ppm) samples used for dating are somewhat lower than other determinations reported in Table 1. Which of the two ages, -3.0 Gy for the host or -3.5 Gy for the glass, more likely represents the time of formation of the glass and its dissemination

l.0

release

for system blanks and reactorderived from uncertainties in

x100

550

900

temWrature

x100

650

975 1050

~.22 k.13

38Ar/39Ar

25.64 11.59

750 825

stepwise

Corrections have been made Uncertainties in isotopic ratios are in applied corrections.

TEHP. 39Ar &OAr/39Ar co lo-8cn3/g CACHARI HOST 0.035 350 1.205 450

data

a

59.7 17.9 6.8 3.56

T2.9 4.4 &?.o ~t.7

5.01 2.9 4.16 51.0 2.4 ~4.0 17. 211

2.842 3.046

*.032 2.036

3.045

T.050

*.Ol k.01

2.982 3.109

~.040 ~.069

0.20

*..Ol

0.34

*.Ol

3.431 4.225

A.091 2.061

35.5 2.7 95.8 514 2.52 *.03 1.83

*.03

1.39 1.01 0.86

A.03 A.02 *.Ol

0.46 0.43

2.01 k.01

0.37 0.42

*.02 k.01

8.90 7.13

k.033 2.25

5.95

~~027

5.52 2.034 4.20 k.034 2.921 ~.026 2.648 2.026 2.947

t-042

3.476 3.574

~.046 k.068

4.957

~.056

through the rock? To answer this question requires a detailed consideration of relative Ar diffusion prop erties of glass and host. An appreciable difference in Ar diffusion properties of glass and host is apparent from the large difference in extraction temperature at which glass and host released most of their 39Ar (Table 3). An Arrhenius plot (not shown) for 39Ar diffusion in the host is typical for meteoritic feldspars (BOGARDand HIRSCH, 1980) and suggests D/a2 values of - lo-’ set-’ at 1000°C and - IO-’ set-’ at 600°C. Ar diffusion from the glass is slower by about a factor of 30 in this temperature range. For a given temperature the host phase will lose Ar much more readily than will the glass. We suggest that the time of glass formation, which was probably the time of breccia formation, was -3.0 Gy ago. That is, the glass was only partially degassed and the crystalline host was completely degassed 3 Gy ago, in spite of the obviously higher post-shock temperature of the glass. The glass veins in Cachari have thicknesses of the order of 1 cm or less and, when injected into the much cooler host, could be expected to have appreciably cooled on a time scale of the order of tens of seconds. Thus, within minutes the glass veins and host material would have significantly approached toward thermal equilibrium. The value of that equilibrium is unknown, but probably was not higher Even at temperatures as high as than -600°C. - 1OOO’Ccooling times on the order of days would be required to produce substantial Ar loss in the @ass with a D/a2 value at this temperature of - 10e6 (BOGARD and HWCH, 1980). At -600°C the glass would require times of the order of months and the host times of the order of days to lose a substantial

945

Impact melting of Cachari

fraction of their @Ar. A reasonable conclusion, therefore, is that essentially all of the loss of radiogenic 40Ar from both glass and host occurred, not during the formation and injection of the glass, but after aproximate thermal equilibrium had been reached between glass and host. After this point the rock would cool at a much slower rate largely dependent upon the physical size of the ejecta produced by the impact event which formed Cachari. If this ejecta was at least several meters in thickness and was initially at a temperature of +OO°C, cooling would be slow enough to cause major *Ar diffusive loss from the host. Cooling was apparently too rapid, however, for complete loss of radiogenic Ar from the glass. Several strongly shock-heated chondrites apparently show analogous effects of thermal quenching by relatively cooler phases which prevented complete loss of aAr (BOGARD and HIRSCH, 1980). The alternative explanation, that the glass and host formed separately at 3.5 Gy and 3.0 Gy, respectively, is unlikely because of the evidence that the glass was injected molten into the host. These wAr-aAr ages are much older than the most recent collisional event which led to initiation of cosmic ray exposure of Cachari. Cosmogenic “Ar concentrations (calculated from Table 3) are - 1.5 X 10-I and - 1.8 X IO-* cm?iTP/g for the glass and host, mspectively, and these values are similar to data reported by HEYMANNet al. (1968). The cosmic ray exposure age for Cachari is about 7-12 My (million years), depending on the “Ar production rate assumed (CRESSYand BOGARD, 1976; FREUNDELet al., 1983). Discussion of “Ar-*‘Ar ages Several eucrites and howardites are known to have experienced some degree of resetting of their radiometric ages (e.g., BOGARD, 1979); however, reasonably well-defined times of specific events have been determined for only a few of these achondrites. Two clasts in the Kapoeta howardite gave Rb-Sr isochron ages of 3.6-3.9 Gy, but their ‘9Ar-“‘Ar plateau ages were -4.55 Gy. A third Kapoeta clast gave a Rb-Sr age of 3.9 Gy and an 3gAr-QAr age of -3.5 Gy (HUNEKE et al., 1977; RAJAN et al., 1979). The Kapoeta breccia was probably formed more recently than the youngest dated clast. The Bununu howardite gave 3gAr-40Ar ages of 4.24 Gy and 4.42 Gy for glass and plagioclase phases, mspectively (RAJAN er al.. 1975). The presence of solar gases in Bununu indicate compaction with minimum heating, and the glass fragments were probably produced by impact into the regolith before formation of the breccia (IWAN et al., 1975). Bununu appears to be just opposite the case for Cachari, where the glass gives an older age than the host material and appears to have formed during the brecciation event. Separated glass and a clast from the shocked Malvem howardite both gave 39Ar-40Ar plateau ages of 3.6-3.7 Gy (KRSTEN and HORN, 1975; RAJAN ef al., 1975). For Malvem the glass

probably formed in situ by the same shock event which caused resetting of the K-Ar age of the clast. A few eucrites, e.g., Pasamonte, Yamato 74159, and Yamato 74450, indicate 39Ar-40Ar degassing ages for whole rock samples of 4.0-4.1 Gy (PODOSEK and HUNEKE, 1973; KANEOKA, 1981). Two eucrites, Sioux County and Bereba, gave Rb-Sr mineral isochrons of -4.1 Gy, and two other eucrites, Stannem and Pasamonte, show disturbance of their Rb-Sr systems possibly as recently as 3 Gy ago (BRICKand ALLEGRE, 1978). Disturbance of radiometric dating systems in basaltic achonclrites in the time interval of 3-4.1 Gy ago appears to be commonplace, although the nature of the metamorphic events are not generally known. The eucrite parent body may have experienced dynamic regolith processes over a long period of time. For Cachari (and also for Malvem) the event dated at 3.0 Gy most probably represents a collisional impact on its parent body which produced some glass and injected it into the Cachari host. We cannot be sure whether this parent body was the original eucrite parent asteroid or some smaller object that was ejected from the parent asteroid prior to 3.0 Gy ago. The former explanation is simpler and would suggest that the dated events in various eucrites and howardites may all have occurred on the same parent asteroid. The second explanation would require an extra event for Cachari prior to 3.0 Gy ago which ejected a sufficiently large body to both shield Cachari from cosmic rays and to survive for >3 Gy in the asteroid belt. By the second explanation the dated impact events in various basaltic achondrites could have occurred on separate, intermediate parent bodies. If it could be shown that two texturally or chemically distinct achondrites likely experienced a common impact event, this might constitute evidence for the event having occurred on the original parent. Unfortunately, the relatively few dated events among eucrites and howardites are too diverse to currently decide between the two explanations. Acknowledgemems-We thank P. Johnson for support in the Ar analyses, T. D. Dickinson for insightful discussions and suggestions, and H. Y. McSwecn Jr. and an anonymous nviewet for useful comments. A portion of this work was supported by NASA grants NAG 9-30 (K. Keil) and NAG 9-63 (R. A. Schmitt). Editorial handling:M. J. Drake REFERENCES

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