A 3.6-b.y.-old impact-melt rock fragment in the plainview chondrite: Implications for the age of the H-group chondrite parent body regolith formation

Earth and Planetary Science Letters, 51 (1980) 235- 247 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

235

[61

A 3.6-b.y.-OLD IMPACT-MELT ROCK FRAGMENT IN THE PLAINVIEW CHONDRITE: IMPLICATIONS FOR THE AGE OF THE H-GROUP CHONDRITE PARENT BODY REGOLITH FORMATION KLAUS KEIL 1, R.V. FODOR 2, P.M. STARZYK 3,,, R.A. SCHM1TT a, D.D. BOGARD 4 and L. ItUSAIN s 1 Department of Geology and blstitute of Meteoritics, University of New Mexico, Albuquerque, NM 8 7131 (U.S.A.) 2 Department o f Geosciences, North Carolina State Uniw~rsity, Raleigh, NC 27650 (U.S.A.) 3 Department of Chemistr),, Oregon State University, Corvallis, OR 97331 (U.S.A.) 4 Geochemistry Branch, NASA Johnson Space Center, Houston, TX 77058 (O\S.A.) 5 Division o f Laboratories and Research, N. Y. State Department o f Health, Albany, N Y 12201 (U.S.A.)

Received May 16, 1980 Revised version received August 18, 1980

Major, minor, trace and REE abundances as well as mineralogical and textural properties of a light-colored, poikilitic, metallic nickel-iron- and troilite-poor lithic fragment in the brecciated Plainview, Texas, H5 chondrite unambiguously show that the fragment is of H-group chondrite parentage, despite its non-chondritic texture and appearance. 4°Ar-39Ar age dating indicates that the fragment is 3.63 b.y. old, whereas the Plainview host chondrite is 4.4 b.y. old. The fragment's texture and certain mineral compositions, such as relatively high CaO in olivine and or~hopyroxene, the presence of alkali-rich feldspar and interstitial material, and metallic nickel-iron of Ni contents of 10-15 wt.% (martensite), indicate that the fragment formed by relatively fast cooling from a melt. We propose that the fragment crystallized from an impact-produced melt of H-group chondrite parentage (similar in composition to the Plainview host material) from which substantial metallic nickel-iron and troilite separated in the impactmelting process. Fragments of this rock, together with unequilibrated mineral and carbonaceous chondrite lithic fragments, were later impact-implanted into the regolith on the surface of the Plainview H5 group chondrite parent body not longer than 3.63 b.y. ago. This regolith, now represented by the Plainview chondrite, must have been in existence until at least 3.63 b.y. ago, when the lithic fragment formed. Of course, assemblage of host chondrite and lithic fragment in the regolith could have taken place even more recently and, thus, the age of the Plainview parent body regolith may even be younger than 3.63 b.y.

1. Introduction The Plainview, Texas, brecciated chondrite contains a variety of dark (carbonaceous) and light (noncarbonaceous) lithic fragments. The carbonaceous fragments are undoubtedly xenoliths [ 1], but interpretation of the origin of the light-colored fragments is more difficult and complex. Fodor and Keil [1 ] pointed out compositional similarities between the light-colored fragments and the silicate portion of the host chondrite, such as in bulk compositions and the

* Present address: Washington Center for Health Statistics, Olympia, WA 98504, U.S.A.

Fe/Fe+Mg ratios in silicates, and dissimilarities, such as in textures (light-colored lithic fragments are free of chondrules and have poikilitic textures) and in higher minor element contents in olivine and orthopyroxene of the light-colored lithic fragments relative to those phases in the host chondrite. We concluded [ 1 - 8 ] that on the basis of bulk composition, as determined by broad-beam electron probe analysis, as well as on mineral abundance and composition, the light-colored lithic fragments in Plainview and other chondrites are not xenoliths of achondritic meteorites, but most likely formed by impact melting and fractionation on their parent bodies of host-like, ordinary chondritic material. Differences in texture and composition between the fragments and the host

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

236 chondrite resulted from melting and fractionation (notably loss of metallic nickel-rion and troilite from the melt) during impact in the regolith of the parent body, a suggestion that was confirmed by Wilkening [91. In this paper, we present additional mineralogic and petrologic, major, minor and trace element, and age-date evidence for this hypothesis. We were fortunate to obtain from Mr. Glenn Huss a specimen of Plainview with a large (~4 X 1.5 cm) light-colored inclusion (Fig. 1). This large fragment allowed not only preparation of polished thin sections but, for the first time, also provided sufficient material for instrumental and radiochemical neutron activation analyses (INAA and RNAA) of major, minor, and trace elements, and for 4°Ar-S9Ar age dating. Specifically, we show that bulk trace element abundances, including those of the rare earth elements (REE), for the lightcolored lithic fragment are typical of ordinary chondrites, thus substantiating our suggestion that this fragment (and many others like it) is not a new type of achondritic meteorite, but formed by impact melting and fractionation of ordinary chondrite material in the regolith of the parent body [1-8]. In addition, the relatively young (3.63 + 0.07 b.y.) 4°Ar-

Fig. 1. Cut specimen of the Plainview chondrite, exposing the large (~4 × 1.5 cm) light-colored, poikilitic lithic fragment. Also note the vague light-dark structure of the brecciated Plainview host. Photograph courtesy of G. Huss. Sample size is about 7 cm across.

39Ar age of the light-colored lithic fragment suggests that regolith formation processes on meteorite parent bodies took place as recently as ~<3.63 b.y. ago and that the Plainview polymict breccia was assembled not earlier than 3.63 b.y. ago.

2. Analytical procedures A polished thin section (UNM 284) exposing 30 mm 2 of surface area of the fragment, as well as the fragment-host boundary, were studied microscopically in transmitted and reflected light, and electron microprobe mineral analyses were performed according to procedures described by Fodor and Keil [1]. Major, minor, and trace element contents of a 125-mg piece of the fragment were determined by INAA using standard procedures [10-12]. The accuracy of the method was determined by simultaneous analysis of U.S.G.S. standard rocks BCR, GSP and DTS. Abundances of K, Ba, Sr, and REE were determined more accurately by RNAA performed on one sample of the fragment and one of BCR. After Na202NaOH fusion with Ba, Sr, K, and REE carriers, the samples were treated with H2SO4 to separate Ba and Sr and insoluble sulfates from K and REE. NaOH was added to the solution to precipitate REE(OH)s and, thus, separate REE from K. For the separation and purification of Ba and Sr, we used the standard procedures of Sunderman and Townly [131 and Laul and Schmitt [12]. laaBa and sTmsr were counted on a NaI [13] detector coupled to a 400-channel analyzer. The REE were purified as a group by alternate fluoride and hydroxide precipitation [14,15] and then counted with Ge(Li) detector coupled to a 4096channel analyzer. Five counts were taken at various intervals to allow detection of each REE isotope with maximum sensitivity. Yields of Ba, Sr, and REE, measured by reactivation were 27% for Ba, 1% for Sr, and 34-56% for REE, the light REE having a smaller yield than the heavy REE. Purification of K was accomplished by Fe(OH)3 scavenging and precipitation as a tetraphenyl borate at pH 1 [16]. The precipitate was washed with H20 several times to remove 24Na activity, dissolved in acetone, and counted for 42K with a Ge(Li) detector. The yield of K (35%) was determined by counting a 137Cs tracer which had been added along with the K carrier.

237 Age determinations using the 4°Ar-39Ar technique were made of a 178-rag piece of the fragment and of a 205-rag piece of the adjoining host. Fragment and host chondrite were separated mechanically, cleaned ultrasonically in acetone, packaged in high-purity aluminum, and encapsulated along with NL-25-2 hornblende separates in a high-purity quartz tube under reduced pressure. The package was irradiated in the core of the Brookhaven National Laboratory's high flux beam reactor to a nominal fast neutron fluence of 2 × 1018. We had previously determined that sample temperature during irradiation under essentially identical conditions did not exceed 200°C, and consequently no loss of 4°Ar during irradiation is expected to have occurred. For additional experimental details and characteristics of the hornblende standard, see Husain [17] and Bogard et al. [18]. The fast neutron irradiation converted a portion of the 39K to 39Ar and a portion of the 4°Cato 3TAr. The Plainview samples were placed in an extraction furnace which was evacuated to high vacuum. Each sample was heated for 45 minutes at a series of increasing temperature steps. A series of extraction blanks was taken prior to each sample analysis. Temperatures were monitored with an internal thermocouple. Gases released were chemically scrubbed on active metal getters. Argon from each extraction temperature was admitted to a mass spectrometer, and all masses from 35 through 40 were repeatedly measured. Mass discrimination of the spectrometer was monitored with an occasional pipette of atmospheric Ar. Argon from each of the hornblende standards irradiated with the Plainview samples was extracted in a single 1620°C temperature step and measured on the spectrometer. Extraction blanks were also taken prior to the hornblende analyses. Only very small corrections had to be made to the 4°Ar and 39Ar data for interfering reactions during the irradiation [17]. The two hornblende standards irradiated with the Plainview material indicate that, within analytical uncertainties, no neutron flux gradients existed along the length of the sample package during irradiation. The J values for the irradiation, as determined by the two hornblendes were 0.0997 + 0.0007 and 0.0992 + 0.0003. Ages for fragment and host chondrite were calculated using an average J value of 0.0995 + 0.0010.

3. Results

3.1. Mineralogy and petrography o f the lithic fragment The lithic fragment is light in color and measures about 4 × 1.5 cm (Fig. 1). The contact between the fragment and the host chondrite is sharp and welldefined (Figs. 1,2a, b). The fragment has a pronounced poikilitic texture consisting of subhedral to rounded olivine, mainly 0.02-0.1 mm in size, enclosed largely in orthopyroxene and sometimes in plagioclase (chadacrysts of olivine in oikocrysts of pyroxene (and plagioclase, Fig. 2c, d)). Plagioclase occurs mainly as anhedral patches filling interstices between the Fe-Mg silicate phases. The poikilitic texture is similar to that of several lithic fragments in the H-group chondrite Abbott [7] and in some LL-group chondrites [4,5]. The poikilitic texture is also similar to the poikilitic texture of the lightcolored portions of the L-group chondrite Shaw [ 19], except that it lacks the aligned and skeletal olivine crystals that Shaw has. In addition to tile olivine in poikilitic relationships, the fragment also contains a few larger, anhedral olivine grains, ~0.25 mm in size, and a few clinopyroxene grains. Where olivine grains of any size are clustered, there are commonly triplepoint junctures between them, indicating some recrystallization of the lithic fragment (Fig. 2d). Small angular grains (~0.02-0.15 mm) of metallic nickel-iron and troilite are present, mainly as mixtures of both phases and commonly within the anhedral oligoclase. It should be noted, however, that the fragment is highly depleted in metallic nickel-iron and troilite relative to the Plainview host, and H-group chondrites in general. A few chromite grains, ~0.10 mm in size, were also observed throughout the fragment. I

3.2. Bulk composition o f the lithic fragment The bulk composition of the lithic fragment is given in Tables 1 and 2. The major, minor, and trace element concentrations in the fragment are identical, within the precision of the analytical technique, to the concentrations of these elements in the silicate fractions of the Plainview host and average H-group chondrites (Table 1). The only exceptions are Ba and

238

Fig. 2. Photomicrographs of the light-colored, poikiliticlithic fragment in the Plainview chondrite. (a) Microscopic overview of part of the lithic fragment (light) showing its sharp boundaries with the host chondrite (left, dark). Note that the host chondrite appears extremely dark, mainly due to overexposure during printing in an attempt to bring out details in the fragment. The dark area at the right is the thin section glass. Transmitted light, crossed nicols. (b) Nearly metal- and troilite-free light-colored, poikilitic lithic fragment (bottom) shows a sharp boundary against the host which has abundant metallic nickel-iron and troilite (white) and some chromite (dark gray, high relief, center top). Gray are silicates. Reflected light. (e) Poikilitic texture of the light-colored lithic fragment, consisting of subrounded chadacrysts of olivine enclosed in oikocrysts of orthopyroxene. Transmitted light, crossed nicols. (d) Triple-point junctures between olivine grains that are poikiliticallyenclosed by oligoclase in the light-colored lithic fragment. Transmitted, plane polartized light.

the siderophile elements. Ba is about three times higher in the fragment than in the silicate portion o f H-group chondrites and is probably in error in the former due to a gamma ray interference in the spectrum taken b y NaI detector. The siderophile elements Co, Ni, Ir, and Au are depleted in the lithic fragment when compared to bulk average H-group chondrites (Table 1, column 4), in keeping with the depletion in

metallic nickel-iron and troilite in the fragment. In fact, the ratios for Co, Ni, Ir, and Au in the fragment vs. average bulk H group chondrites of 0.047, 0.034, 0.036, and 0.030, respectively, suggest that the metallic nickel-iron content in the fragment is depleted to about 0 . 5 - 0 . 8 wt.%, assuming a bulk metallic nickel-iron content for average H-group chondrites o f 16.8 wt.% [29]. This result is con-

239 TABLE 1 Composition of the light-colored, poikilitic lithic fragment in the Plainview, Texas, chondrite (column 1), determined by INAA and RNAA, compared to the average composition of the silicate fractions of the Plainview host (column 2) and average H-group chondrites (column 3), as well as of bulk average H-group chondrites (column 4). References are given in brackets 1

TiO2 (wt.%) A1203 Cr203 FeO* MnO MgO CaO Na20 K20

<0.3 3.6 +0.61± 15.2 _+ 0.38 _+ 30.8 _+ 2.1 +_ 1.21_+ 0.10+-

Sc(ppm) V Sr Hf Ba Co Ni lr (ppb) Au

10.3 I00 15 0.27 15.0 39 570 26 7

0.4 0.04 0.4 0.01 0.5 0.2 0.05 0.01

2111

3

0.15 2.9 0.71 16.6 0.41 29.6 2.27 0.89 0.13

0.15 2.8 0.66 12.4 0.38 30.8 2.6 1.01 0.23

120] 21] 21] 20] 21] 21] 201 21] 201

10.0 80 14 0.25 5

22] 221 [22] [221 [221

_+ 0.05 +_10 +_ 2 + 0.08 + 0.4 _+ 2 _+20 +_ 3 + 1

4

840 17,000 720 230

[231 [24] [251 [261

* All Fe as FeO.

firmed by a microscopic examination o f the fragment, which reveals very low (<1 wt.%) amounts of metallic nickel-iron (Fig. 2b). The REE concentrations (Table 2) determined for the lithic fragment are identical (within +20) to the REE abundances in the Plainview host silicate fraction and are about 23% higher than the REE concentrations in the silicate fractions o f three ordinary chondrites [28]. Although the abundances of the three light REE, La, Ce and Nd in the lithic fragment overlap tile abundances in the Plainview host [27] within the -+20 errors, there is a suggestion that the light REE are progressively enriched in the fragment from the heavier to the lighter region (Sin to La); e.g. the ratios of the REE (fragment)/REE(host) are Sm 1.03, Nd 1.06, Ce 1.08 and La 1.14. We are not aware of any simple mechanism that would enrich the light REE in an impact-produced melt of original Plainview-like host silicate matter. (see section 4 below). In view of the identical abundances (within error) of Sc, V, Sr, Hf and the medium REE, the apparent progressive enrichment of the light REE may be entirely fortuitous.

3.3. Mineral compositions of the lithie fragment Compositions of minerals in the lithic fragment are compared to those in the host Plainview chondrite in Table 3. The major components, olivine and orthopyroxene, are similar but not identical in average composition in fragment and host (Fa2o.8 vs. Fax9.6; Fs~ 7.4 vs. FSl 6.9)- However, they are both of H-group composition (averages and ranges for H-group chondrites are Fa18.7 ( 1 6 . 9 - 2 0 . 4 ) and Fs17.2 ( 1 5 . 7 - 1 8 . 1 )

[7,301. Other compositional differences exist between the phases in the fragment and those in the host. There is slightly higher CaO in fragment olivine versus host olivine, and somewhat higher CaO, A1203, Cr203, and Na20 in fragment orthopyroxene relative to host orthopyroxene. Compositional differences between clinopyroxene in the fragment versus that in the host include higher FeO, MnO, MgO, A1203 and Cr203 and lower CaO and TiO2 in fragment clinopyroxene. The most notable distinction between oligoclase in the fragment to that from the host is the higher Na20 and K20 contents and the presence of micron-sized

240 TABLE 2 Rare earth element abundances (ppm) in the light-colored poikilitic lithic fragment in the Plainview, Texas, chondrite (column 1), compared to abundances in the silicate fraction of the Plainview chondrite (column 2) [27] and in the silicate fraction of the average H-group chondrites (column 3) [28] 1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0.57 1.45 0.18 0.94 0.30 0.109 0.39 0.074 0.47 0.104 0.34 0.049 0.32 0.045

± 0.02 ± 0.04 ± 0.01 ± 0.04 ± 0.01 ± 0.003 -+ 0.04 ± 0.002 ± 0.002 ± 0.006 ± 0.01 ± 0.002 ± 0.01 ± 0.002

2*

3

0.50 1.34 0.89 0.29 0.099 0.39 0.48 -

0.42 1.12 0.15 0.79 0.24 0.090 0.32 0.058 0.37 0.096 0.25 0.040 0.27 0.042

Average ratio of REE in column 1/REE in column 2 = 1.06 ± 0.06. Average ratio of REE in column 1/REE in column 3 = 1.23 ± 0.08. * Abundances in the silicate fraction of Plainview were obtained by dividing the REE abundances in the Plainview whole rock [27] by the amount of the silicate fraction, 0.79 [1].

alkali-rich areas in the former (Fig. 3) (Table 3). Chromite in the fragment has higher contents of the refractory oxides A12Oa and MgO than chromite in the host and in H-group chondrites in general [31]. Finally, martensite (10-15 wt.% Ni) is present among the nickel-iron metal grains in the fragment and not in the host (Fig. 3).

3.4. Age dates Results of the age determinations for the lightcolored poikilitic lithic fragment and the host chondrite from the immediate vicinity of the fragment are presented in Table 4. The age for each extraction temperature step as calculated from the 4°Ar-39Ar ratio is plotted against the fractional release of 39Ar for the light-colored fragment and the host chondrite in Fig. 4. The width of the plotted data represents the uncertainty in individual 4°Ar-a9Ar ages as determined from measurement uncertainties, corrections for blanks and reactor interferences, and analytical errors in measurements of the hornblende standard. The uncertainty in the age of the NL-25-1 hornblende

mini imllNiimnn 0.3

K

FRAGMENT

IpIIn

'=

~"U

c; 0.03

(a)

(b) 0.003

0.8

~°

C, ,-,e o.6

". •. : ,l,

~

:)

"."

•

4,4

7+'.- . .

Ab

io

2o

3o

4.2 *o

o~

Weight %

,'~

£°"

Ni

Fig. 3. Variation diagrams. (a) Co vs. Ni contents (in wt.%) in metallic nickel-iron, showing compositions for typical host metal (open circles) and individual spot analyses for fragment metal (solid circles). Martensite has Ni contents between 10 and 15 wt.%. (b) Spot analyses for plagioclase and alkali-rich material approaching feldspar stoichiometry in the fragment. Most of the plagioclase is albite to oligoclase, whereas the alkali-rich material trends into the anorthoclase field. The diagram is the albite (Ab; NaA1Si3Os) portion of the albiteorthoclase (Or; KALSiaO8)-anorthite (An; CaA12Si208) ternary diagram (in mo1.%).

r'--n

L

[

1

I

i

[]

730°C

"1

Or

,'o

HOST CHONDRITE

%

4.0 40A r

f---

39Ar 3,6 AGE

189yr

3.6 3.4

......

v111zz]

858oc

4

LIGHT FRAGMENT

3.2 3.8

012

01,

'

016

018

'

10

CUMULATIVE FRACTION 39Ar

Fig. 4. Plot of the ages determined from 4°Ar-a9Ar ratios for extraction temperatures against the fractional release of 39Ar for both the light-colored lithic fragment and the host Plainview chondrite sampled from near the fragment.

24l

0o

""

~

""

L

d ,--; d ,d d ,.,d d ,.~ d ..d ~ d

d £

"-' '-' ~ ~ ..-4 ~ d d ,'q ,-., > ~

~a

~ ~ ,~ ~

•

~ r~

.

<
~a g~ d d d d d ~ d d d d © m ~13 ,.o

,..~ '~,

5~

d d d d d d d ~ d d

~

o~

.d ©

© c

~.~

d ,d d .d d d ~i ,d ,5 .d 0o

o

o ©

<
©

8 ~'~ © ,.~

"ul

0

,.,d d d d d ,-; d d d d

!

~o,-4

o

m .~

d d d d d d d ~ d d

oqd

o

~g <

~

s~ ...-, ~

OOoooooOo

u

242 ¢',10~ "~" O

~'

O

¢",1 ,,,,,~ ',~ O

O

O

~D O

O

O

¢'q O

'~1" O

~

~

+~ +~ ~

O

¢-.I o"~ ¢-,I tt~ ¢,e'~ O,-~ Cq O

+1

,,,-~ O

O

O

O

O

+~

~

~ + ~ + ~

O

t"',- O

O

O

O

O

O

O

O

O

O

O

~ O O

O

O

O

O

O

Ott)

'~

tt"a t"-- t"-,- ¢'-.I ',~- '.~ O.I r-~ ~:'~ ',,~ O

"~ O

O

O

¢'.1 ,/) ,-~ ',,~D ,,-~ rID

O

O

O

',~ e q ,,,,,~ ,,--~ O

O

O

O

O

O

O

O

O

O O

~

O

O

O

O

O

O

O

O

O

O

O ~ O O

O

~,~ O

ee', "~" e,'~ " ~

O

O

O

O ~

O

O

O

,,,,,~ q') ¢,'~ O

~

O

O

O

O

~

O

,~

¢',~ O

O O O

O

O

,

~

~

O

O

O

O

O

O

O

O

~

O

O

O O O

O

,

~

:

D

O

O

O

O

O

O

o

!

,"'~ O

•--~ O

O

O O O o'~

~8

O

O

O

O

O

O

O O

O

O

~ O

O O

~

O

,,,,,~ O

O O O O

O O

O O

O O O O O

z O

O

~'~ O

O

O

O

O

O

O

O

O

O

O

O

O

O

O O O

O

O

O

O

O

O

' ~ ' ~ O O O O O O O O O O O

,..~ ',,~

~.'O~.m.m.e';

.~

~,v ~

~.~.,-2. m . ~ . ~ , , , £

243 standard is not included but is probably <1% [171. The K and Ca abundances determined for the lightcolored fragment and the host chondrite are K = 0.135% and Ca = 1.34% for the fragment and K = 0.067% and Ca = 0.95% for the host (uncertainties are -+10%). These values differ slightly from those determined by other analytical techniques presented in Table 1, but are in reasonably good agreement considering error bars and sampling. A sample with the same K-At age for all K-bearing phases would theoretically give constant 4°Ar-39Ar ratios and, hence, constant age, throughout the gas release. Such constant 4°Ar-a9Ar ratios (ages) do not usually occur, however, and were not observed for either Plainview sample analyzed. Nevertheless, current interpretations of 4°Ar-a9Ar release profiles (Fig. 4) enable us to determine an age for the lightcolored lithic fragment. Somewhat lower ratios and ages for the low-temperature portion of the agAr release are commonly seen in a variety of sample types and are generally attributed to simple diffusion loss of 4°Ar from phases characterized by low Ar retention during the history of the rock. For the lithic fragment we attribute the lower ages associated with the first 21 of the 39Ar released compared to the maximum ages of ~3.6-3.7 b.y. to a diffusion loss of ~2% of the total 4°Ar measured in the sample. From 21% to 87% of the 39Ar released, five extractions give nearly constant 4°Ar-a9Ar ratios and yield a mean age of 3.63 + 0.07 (-+1o). The small amount of diffusion loss observed is unlikely to have appreciably affected this plateau age, and is evidenced by the fact that the 3.52-b.y. summed age is only slightly lower than the highest observed age of 3.67 b.y. We attribute the drop in 4°Ar-a9Ar ages for the last ~ l 3% of the a9Ar released in both the Plainview fragment and host chondrite to a9Ar recoil induced in the irradiation, a phenomenon not uncommon in 4°Ar-aeAr analyses. Recoil from the reaction 39K (n, p) a9Ar can eject 39Ar from near-surface layers of grains and implant it into surface layers of adjacent grains. Huneke and Smith [33] have demonstrated that this effect is to be expected for any sample with small grain sizes (<100/am) and with a lower-temperature phase containing most of the K and a highertemperature phase containing considerably less K. During stepwise heating, irradiated ordinary chondrites commonly release the first ~80% of their 39Ar

from a phase with a high K/Ca ratio, which is probably feldspar or associated trace minerals, and the last "~20% of their a9Ar from a phase with a low K/Ca ratio, which may be pyroxene [18]. The Plainview fragment shows such a rapid drop in K/Ca beginning at ~80% of the 39Ar released (Fig. 4). If this recoiled 39Ar originated from those phases releasing from 21 to 87% of the total a9Ar, the observed plateau age for the fragment would be too high by only ~ 1 - 2 % . Therefore, we assign a 4°Ar-39Ar age to lhe fragment of 3.63 -+ 0.07 b.y., where the uncertainty encompasses the total error spread in the plateau extractions and includes reasonable uncertainties due to diffusion loss of Ar and recoil effects. The 4°Ar-a9Ar release of the Plainview host chondrite sample is too complex to assign a reliable age. This sample may have contained materials of different ages. The lower age observed for the host (3.8 b.y.) is higher than the highest age observed for the lithic fragment, and two other extractions of the host gave ages of 4.4 b.y.. This indicates that the bulk of the host chondrite is considerably older than the lithic fragment and possibly contains material as old as 4.4 b.y.

4. Origin and implications of the lithic fragment The light-colored poikilitic lithic fragment studied here is one of several described from Plainview [1 ]. Those previously studied were interpreted to have formed by impact melting and fractionation (largely of metallic nickel-iron and troilite) of ordinary chondritic material in the regolith of their parent body [1-7]. That the fragment described here also had such an origin is strongly suggested by its compositional and textural similarities to previously described fragments [1'-7] and to the light-colored portions of the Shaw chondrite which consists of poikiliticallytextured material of chondritic composition [19]. Compelling evidence was presented for an igneous origin of Shaw: it has skeletal olivine of high CaO content (indicative of rapid crystallization), preferred orientation of olivine, and contains silica-alkali-rich interstitial material (residual igneous melt), vugs and vesicles filled with delicate crystals, low metallic nickel-iron and troilite contents, and metallic nickeliron of martensitic composition.

244 The Plainview lithic fragment studied here is similar to Shaw and previously described light-colored lithic fragments in ordinary chondrites in many ways; its texture is poikilitic which we suggest formed from a melt that initially crystallized olivine. That mineral later (i.e., at lower temperature) became surrounded by more slowly crystallizing, larger orthopyroxene. At still lower temperatures, a residual liquid of feldspar composition crystallized that contains minute interstitial areas of alkali-rich material approaching feldspar stoichiometry. We further suggest that the melt from which the fragment formed, cooled relatively rapidly. This suggestion is based on the presence of small amounts of martensite and olivine and orthopyroxene with relatively high CaO and Cr203 contents. Thus, on the basis of texture and mineral compositions, we conclude that the poikilitic fragment in Plainview has an igneous origin. However, the melt initially did not cool as rapidly as the Shaw melt (the former does not have skeletal olivine), but probably faster than most ordinary chondrites, as is indicated by the presence of martensite and olivine and orthopyroxene of relatively high CaO and Cr203 contents. As suggested previously for similar lithiC fragments and for the Shaw chondrite [1,6,7,19], we further contend that the igneous melt from which the fragment crystallized was produced by impact melting and fractionation of ordinary chondrite material in the regolith of the Plainview parent body. The source material for the lithic fragment was characterized by major, minor and trace element compositions very similar to those of the Plainview host silicate fraction and the average silicate fraction of H-group chondrites (Tables 1-3). The most compelling evidence for derivation of the fragment from ordinary chondrite parent material stems from the REE abundances which are identical to those in the silicate fraction of ordinary chondrites (Table 2). This is the first time that a non-carbonaceous, light-colored, chondrulefree lithic fragment in an ordinary chondrite has unambiguously been related to an origin from ordinary chondrite material. The compositions of minerals in the fragment are also compatible with parentage from ordinary H-group chondrite material, although there are some minor differences in the compositions of the same minerals in the Plainview host (Table 3). The slightly

higher FeO/MgO ratios in olivine and orthopyroxene of the fragment are, however, still well within the compositional ranges of those phases in H-group chondrites. The relatively high CaO and Cr203 contents in olivine and orthopyroxene in the fragment are attributed to trapping in the silicate structures during relatively rapid crystallization and lack of metamorphic annealing. It is not clear, however, why orthopyroxene and clinopyroxene in the fragment have somewhat higher A1203, although this may be related to the slightly higher bulk A1203 content of the fragment in comparison to H-group chondrite silicate fraction (Table 1). Relatively early (i.e., high temperature) crystallization from the melt may account for the higher refractory oxide contents (AlzO3, MgO) in chromite in the fragment in comparison to chromite in the host. Furthermore, slightly higher Na20 and K20 in oligoclase in the fragment in comparison to oligoclase in the host are probably the result of slightly higher bulk Na20 and K20 in the fragment. The mean 4°Ar-39Ar age for ordinary chondrites is 4.44 + 0.03 b.y. [34] and the age of the Plainview host is close to 4.4 b.y. Those portions of Plainview that are known to contain gases implanted by the solar wind and trapped within carbonaceous material [35] could not have been heated appreciably or else these gases would have been driven off. The 4°Ar39Ar age of the fragment, therefore, could not have been reset after it was incorporated into the host material, which coul have occurred no earlier than 3.63 b.y. ago. The fragment must, therefore, have formed independently of the host material, a conlusion consistent with the petrographic and compositional evidence that it formed from an impact-generated melt. Thus, regolith-forming processes, including impact melting, fractionation, mixing, irradiation by solar wind and implantation of solar wind gases must have taken place on the Plainview (H-group chondrite) parent body as recently as 3.63 b.y. ago, or even later (the 3.63-b.y. age of the fragment gives a maximum age for the process that formed the Plainview polymict breccia, i.e., the regolith formation process). This finding is in contrast to the suggestion by Pellas [36] that irradiation and formation of gas-rich chondrite breccias took places 4.0-4.3 b.y. ago, at a time when accretion was in its final stages in the asteroidal belt. As the cosmic ray exposure age of Plainview is

245 only 2.9 m.y. [35], the rock was probably buried at least several meters on its parent body by the event that formed the breccia. The rock remained shielded until ~<2.9 m.y. ago, when it was broken up in space into a, say, meter-sized chunk. The burial and later disinternment could not have been accompanied by strong heating because of the presence of solar-windimplanted gases and numerous unequilibrated and carbonaceous chondrite fragments in Plainview [ 1]. Thus, the process that made the regolith breccia into a tough, coherent rock could not have been prolonged annealing, but was possibly flash heating and short-term melting along grain boundaries and quick cooling in response to impact into the porous Plainview regolith, as suggested by Ashworth and Barber [42] and Keil and Fodor [8]. Additional evidence for the duration of regolith and impact breccia formation on meteorite parent bodies comes from age measurements of a limited number of other lithic fragments. Ages of lithic fragments in brecciated chondrites were determined by Schultz and Signer [37] who reported a conventional K-Ar age of 1.36 b.y. for an apparent H-group chondrite fragment in the St. Mesmin LL-group chondrite. They also reported mean K-At ages of 4.40 -+ 0.26 b.y. for apparently LL-group chondrite fragments in St. Mesmin. One fragment in St. Mesmin (having a high K-Ar age) and one in the Weston H-group chondrite [38] yielded high concentrations of cosmic ray produced gases which were interpreted as indicating pre-irradiation of these fragments for 1.5-4 m.y. on the surfaces of their parent bodies prior to incorporation into the breccias. Like Plainview, both St. Mesrain and Weston are rich in solar gases acquired during irradiation of dispersed surface material on their parent bodies. Ages of basaltic achondritic lithic fragments in the howardite Kapoeta by the Rb-Sr technique were reported by Papanastassiou et al. [39] and by the 4°Ar-39Ar technique by Huneke et al. and Rajan et al. [40]. The fragments range in age from 3.48 to 3.89 b.y., and others gave ages around 4.4 b.y. The howardite Kapoeta (and many other howardites as well) represents a regolith bjceccia that contains solar wind gases, particle tracks and micrometeorite craters, all of which apparently developed in the regolith on a parent body whose surface was of basaltichowarditic composition.

Since chondritic breccias most commonly only contain lithic fragments of the same composition as the host material (i.e., H-group fragments in H-group host; L-group fragments in [,-group host: LL-group fragments in LL-group hosts: [1 7]), it is likely that tt-, L-, and LL-group chondrites and their breccias formed on separate parent bodies. This suggestion is also supported by oxygen isotope data [41 ]. Furthermore, howardites clearly also come from a separate parent body. Hence, although only few lithic fragments and their host breccias have been dated so far, it is clear that regolith and breccia formation proceeded at least on four separate meteorite parent bodies over a long time span since the formation of the solar system and until relatively recent times. Specifically, on the Plainview (tt-groupl parent body, these processes took place until at least 3.63 b.y. ago and on the St. Mesmin LL-group parent body until at least 1.36 b.y. ago. Similarly, on the Kapoeta howardite parent body, regolith and breccia formation took place at least as recently as 3.48 b.y. ago.

5. Summary of the history of the Plainview breccia (l) Agglomeration of tire Plainview tf-group parent body of asteroidal size large enough to support formation of a regolith. (2) Equilibration of the Plainview chondrite material to petrologic type 5. (3) Repeated impact brecciation and regolith forination of the tt5 material at or near the surface of the Plainview parent body: impact melting, partial melting and fractionation of some of this material to form the light-colored poikilitic lithic fragments; and impact mixing of unequilibrated silicate grains and chondrules~carbonaceous and unequilibrated lithic fragments into the regolith. These impact melting and mixing events occurred at least as recently as 3.6 b.y. ago. (4) Formation of the dense, tough Plainview rock ~<3.6 b.y. ago, possibly by flash heating and localized melting along grain boundaries, caused by impact into the porous regolith. (5) Breakup of the Plainview parent object by major collisions, further breakup to a meter-sized object 2.9 m.y. ago, and perturbation into an earth-crossing orbit.

246 Acknowledgements We thank Mr. Glenn Huss, American Meteorite Laboratory, Denver, Colorado, for bringing this specimen of Plainview to our attention and for providing a photograph o f it (Fig. 1). We also thank Ms. R.L. Conrad for assistance in the INAA and RNAA work. This work is supported in part by the National Aeronautics and Space Administration, grant NGL 32-004-064 (K. Keil, Principal Investigator) and grant NGL 38-002-020 (R.A. Schmitt, Principal Investigator).

References 1 R.V. Fodor and K. Keil, Carbonaceous and non-carbonaceous inclusions in the Plainview, Texas, chondrite, Meteoritics 8 (1973) 33. R.V. Fodor and K. Keil, Carbonaceous and non-carbonaceous lithic fragments in the Plainview, Texas, chondrite: origin and history, Geochim. Cosmoehim. Acta 40 (1976) 177. 2 R.V. Fodor, K. Keil and E. Jarosewich, The Oro Grande, New Mexico, chondrite and its lithic inclusion, Meteoritics 7 (1972) 495. 3 R.V. Fodor and K. Keil, Composition and origin of lithic fragments in L- and H-group chondrites, Meteoritics 8 (1973) 366. 4 K. Keil and R.V. Fodor, Composition and origin of lithic fragments in LL-group chondrites, Meteoritics 8 (1973) 394. 5 R.V. Fodor and K. Keil, Implications of poikilitic textures in LL-group chondrites, Meteoritics 10 (1975) 325. 6 R.V. Fodor and K. Keil, A komatiite-like lithic fragment with spinffex texture in the Eva meteorite: origin from a supercooled impact melt of chondritic parentage, Earth Planet. Sci. Lett. 29 (1976) 1. 7 R.V. Fodor, K. Keil, L.L. Wilkening, D.D. Bogard and E.K. Gibson, Origin and history of a meteorite parent body regolith breccia: carbonaceous and non-carbonaceous lithie fragments in the Abbott, New Mexico, chondrite, N.M. Geol. Soc., Spee. Publ. 6 (1976) 206. 8 K. Keil and R.V. Fodor, Origin and history of the polymict-brecciated Tysnes Island ehondrite and its carbonaceous and non-carbonaceous lithic fragments, Chem. Erde 39 (1980) 1. 9 L.L. Wllkening, Tysnes Island: an unusual clast composed of sohdified immiscible, Fe-FeS and silicate melts, Meteoritics 13 (1978) 1. 10 H. Wakita, R.A. Schmitt and P. Rey, Elemental abundances of major, minor and trace elements in Apollo 11 lunar rocks, soil, and core samples, Proc. Apollo 11 Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 1, 2 (1970) 1685.

11 J.C. Laul and R.A. Schmitt, Chemical composition of Luna 20 rocks and soil and Apollo 16 soils, Geochim. Cosmochim. Acta 37 (1973) 927. 12 J.C. Laul and R.A. Schmitt, Chemical composition of Boulder-2 rocks and soils, Apollo 17, Station 2, Earth Planet. Sci. Lett. 23 (1974) 206. 13 D.N. Sunderman and C.W. Townley, The radiochemistry of barium, calcium and strontium, NAS-NS 3010 (1960). 14 R.O. Allen, Multi-element neutron activation analysis: development and application to trace element study of the Bruderheim chondrite, Ph.D. Dissertation, University of Wisconsin (1970) unpublished. 15 P. Rey, H. Wakita and R.A. Sehmitt, Radiochemical activation analysis of In, Ce, and the 14 rare-earth elements and Y in rocks, Anal. Chim. Acta 57 (1970) 163. 16 K. Sporek and A.F. Williams, The quantitative determination of K as the tetraphenyl boron salt, Analyst 80 (1955) 347. 17 L. Husain, 40Ar39Ar chronology and cosmic ray exposure ages of Apollo 15 samples, J. Geophys. Res. 79 (1974) 2588. 18 D.D. Bogard, L. Husain and R.J. Wright, 4°Ar-39Ar dating of collisional events in chondrite parent bodies, J. Geophys. Res. 81 (1976) 5664. 19 G.J. Taylor, K. Keil, J.L. Berkley, D.E. Lange, R,V. Fodor and R.M. Fruland, Shaw meteorite: history of a chondrite consisting of impact-melted and metamorphic lithologies, Geochim. Cosmochim. Acta 43 (1979) 323. 20 H.C. Urey and H. Craig, The composition of the stone meteorites and the origin of meteorites, Geochim. Cosmochim. Acta 4 (1953) 36. 21 R.A. Schmitt, G.G. Goles and R.M. Smith, Elemental abundances in stone meteorites, Meteoritics 7 (1972) 131 22 B. Mason, The chemical composition of olivine-bronzite and olivine-hypersthene chondrites, Am. Mus. Novitates 2222 (1965); B. MaSon, Handbook of Elemental Abundances in Meteorites (Gordon and Breach, 1971). 23 C.B. Moore, Cobalt, in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, 1971) 215. 24 C.B. Moore, Nickel, in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, 1971) 221. 25 P.A. Baedeker, Iridium, in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, 1971) 463. 26 W.D. Ehmann, Gold, in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, 1971) 479. 27 A. Masuda, N. Nakamura and T. Tanaka, Fine structures of mutually normalized rare earth patterns, Geochim. Cosmochim. Acta 37 (1973) 239. 28 R.A. Sehmitt, R.H. Smith, J.E. Lasch, A.W. Mosen, D.A. Olehy and J. Vasllenskis, Abundances of the fourteen rare-earth elements, scandium and yttrium in meteoritic and terrestrial matter, Geochim. Cosmochim. Acta 27 (1963) 577.

247 29 K. Keil, On the phase composition of meteorites, J. Geophys. Res. 67 (1962) 4055. 30 K. Keil and K. Fredriksson, the iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites. J. Geophys. Res. 69 (1964) 3481. 31 T.E. tlunch, K. Keil and K.G. Snetsinger, Chromite composition in relation to chemistry and texture of ordinary chondrites, Geochim. Cosmochim. Acta 31 (1967) 1369. 32 R.H. Steiger and E. J/iger, Subcommission on Geochronology: convention on the use of decay constants in geoand cosmochronology, Earth Planet. Sci. Lett. 36 (1977) 359. 33 J.C. Huneke and S.P. Smith, The realities of recoil out of small grains and anomalous age patterns in 39Ar-4°Ar dating, Proc. Lunar Sci. Conf. 7 (1976) 1987. 34 G. Turner, M.C. Enright and P.H. Cadogan, The Early history of chondrite parent bodies inferred from 4°Ar39Ar ages, Proc. Lunar Planet. Sci. Conf. 9 (1978) 989. 35 EL. Wilkening and R.N. Clayton, Foreign inclusions in stony meteorites, II. Rare gases and oxygen isotopes in a carbonaceous chondritic xenolith in the Plainview gasrich chondrite, Geochim. Cosmochim. Acta 38 (1974) 937. 36 P. Pellas, Irradiation history of grain aggregates in ordinary chondrites: possible clues to the advanced stages of accretion, in: From Plasma to Planet, A. Alvius, ed. (Wiley, New York, N.Y., 1973) 65.

37 L. Schultz and P. Signer, Noble gases in the St. Mesmin chondrite: implications to the irradiation history of a brecciated meteorite, Earth Planet. Sci. Lett. 36 (1977) 363. 38 L. Schultz, P. Signer, J.C. Latin and P. Pellas, Complex irradiation history of the Weston chondrite, Earth Planet. Sci. Lett. 15 (1972) 403. 39 D.A. Papanastassiou, R.S. Rajan, J.C. Huneke and G.J. Wasserburg, Rb-Sr ages and lunar analogs in a basaltic achondrite: implications for early solar system chronologies, in: Lunar Science V (The Lunar Science Institute, 1974) 583 (abstract). 40 J.C. Huneke, S.P. Smith, R.S. Rajan, D.A. Papanastassiou and G.J. Wasserburg, Comparison of the chromology of the Kapoeta parent planet and the Moon, in: Lunar Science VIII (The Lunar Science Institute, 1977) 484 (abstract); R.S. Rajan, J.C. Huneke, S.P. Smith and G.J. Wasserburg, Argon 40-Argon 39 chronology of lithic clasts from the Kapoeta howardite, Geochim. Cosmochim. Acta 43 (1979) 957. 41 R.N. Clayton, N. Onuma and T.K. Mayeda, A classification of meteorites based on oxygen isotopes, Earth Planet. Sci. Lett. 30 (1976) 10. 42 J.R. Ashworth and D.J. Barber, Lithification of gas-rich meteorites, Earth Planet. Sci. Lett. 30 (1976) 222.