Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules

Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules ALAN E. RUBIN, KLA~JS KEIL, G. JEFFREYTAYLOR Department

of Geology

and Institute

of Meteoritics, University NM 87131, U.S.A.

of Neu

Mexico.

Albuquerque.

and

M.-S. MA. R. A. SCHMITT Radiation

Center,

Oregon

State University.

Corvallis.

OR 97331. I1.S.A.

and D. D. BOGARD Geochemistry

Branch.

NASA

-Johnson

Space Center.

Houston.

TX 77058. L!.S.A.

Abstract--The Bovedy L-group chondrite contains a light-colored poikllitic lithic fragment with oli\me. low-Ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites. Its texture, compositional similarities to porphyritic chondrules, and low Na,O. KZO and P205 content indicate that the fragment represents a solidified. slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules. The fragment is hcterogeneous. with a progressive increase in the bulk MgO!FeO ratio and in MgO content of olivines and low-Ca pyroxenes across its length. -“Ar;4”Ar analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating. The approximate age of O&O.94 Byr dates the reheating event and not the formation of the lithic fragment or the Bovedy breccia. This reheating event renders the fragment’s and host’s metallographic cooling rate of -5 C/Myr (through 500 C) imprecise. However. the absence of martensite and the presence of kamacite. zoned taemte and tetrataenite m the fragment and host are consistent with such slow coolmg through 500 C. This cooling rate must have resulted from burial of the fragment-host assemblage beneath insulating material on the Bovedy parent body. If the thermal diffusivity (K) of this overburden was approximately comparable to that of the lunar regolith (1O-4 cm’jsec), then the fragment was buried at a depth 26.5 km: if K = IO * cm’,‘sec (similar to chondritic material). then the fragment was buried at a depth ~65 km

1. INTRODUCTION

have been identified in a number of ordinary chondrites. These fragments are either of carbonaceous chondrite composition (e.g. VAN SCHMLS, 1967; FREDRIKSSON et d., 1969; KURAT, 1970; WILKENING and CLAYTON, 1974; F~DOR and KEIL, 1976a; FODOR et crl., 1976, 1977; LEITCH and GROSSMAN,1977: WILKENIN(;. 1977; KEIL and FODOR. 1980) or of ordinary chondrite composition (e.g. FODOK et d., 1972, 1976; FODOR and KEIL, 1973, 1975, 1976a. b, 1978; KEII. and FODOR, 1973, 1980; PRINZ et al.. 1977; KEIL et (I[., 1980). Many of the latter fragments are light-colored and depleted in metallic NipFe and troilite compared to average ordinary chondrites. They display a variety of igneous (nonchondritic) textures. In most cases the light-colored lithic fragments have bulk compositions similar to those of the silicate portions of the ordinary chondrite hosts in which they occur. On this basis, these fragments have been interpreted as having formed from ordinary chondrite material by impact-melting and fractionation of metallic NipFe and troilite in the regoliths of their parent bodies (FODOR rt cd.. 1972, 1976; KFII and FOIX)R. 1973. 1980; F~DOK and KI.IL.. LITHIC fragments

1973, 1975, 1976a, b, 1978; WILKENIN~;, 1978: KEIL (‘I (II., 1980). In the case of a light-colored lithic fragment in the Plainview H5 chondrite, this process took place 3.63 + 0.07 Byr ago (KEIL et t/l.. 1980). Studies of lithic fragments can thus provide valuable information about the formation of brecciated meteorites and meteorite parent body regoliths. In a few cases the lithic fragments of ordinary chondrite parentage have bulk compositions different from that of their host: the Paragould LL chondrite has a fragment of L chondrite composition (fragment 4. FODOK and KEIL. 1978); the St Mesmin LL chondrite has two fragments of bulk H chondrite composition (DODD, 1974): and the Ngawi LL chondrite has a fragment of bulk H chondrite composition (FOUOR and KEIL, 1975). However, the rarity of such occurrences suggests that H-, L- and LL-group chondrice parent bodies were separate entities. In this paper we report the results of petrographic. electron microprobe, instrumental neutron activation analysis, oxygen isotope analysis and 3’Ar,,J”Ar agedating studies of a light-colored lithic fragment in the Bovedy meteorite that has a bulk composition different from that of the L-group chondrite host. We con-

2213

,A E. Rt’BIN PI tr!

2214

elude that the fragment most likely formed by impactmelting of a target rich in porphyritic chondrules and that the melt experienced partial volatilization of Na*O, K20 and P205 and heterogeneously crystallized olivine and low-Ca pyroxenc. The Bovedy meteorite fell on April 25. 1969 in Northern Ireland and three days iatar two stones were recovered. one weighing 513 p (found near Sprucefield), and the other weighing 4.94 kg. found near Bovedy, about 60 km away. (&AtiAh$ cf (I/. (1976) described the meteorite ah an 1.3 chondrite with homogeneous olivines. heterogeneous pyroxenes and Ca-rich glass inclusions. Our modal analysis indicates that the Bovedy host is composed of approximate11 50”,,, porphyritic chondrules. IO”,, non-porphyritlc chondrules and 4O”,, other components. 2. ASALYTIC‘AL PROCEDURES Polished thin sections of the lithlc Iragmenr. host and the fragment--host boundary (from British Museum specimen BM 1975, M.12 of the Sprucefield stone) were studied microscopically in transmitted and reflected light and electron microprobe mineral analyses were made using crystal spectrometers, following the procedures of FODOR and KEIL (1976a). Metallic Ni-Fe grains in the fragment and host were etched by a dilute solution of mtric acid and alcohol to bring out structural details, and metallic Ni-Fe analyses were corrected by ZAF procedures described by KEIL (1967). Major, minor and trace element contents of the fragment were determined by INAA usmp standard procedures (WAKITA er ul.. 1970; LA~IL. and SCHMITT 1973, 1974). The accuracy of the method was determined by the procedures described by KEEL ('Iul.(1980). Broad-beam electron microprobe analyses were made following the procedures of Lrix et trl. (1980). A 0.099 g chip of arca D of the fragment (F’lg. I ). which was _ 1 cm from the fusion crust. was used for “Ar 40Ar analyses. The sample and NL-75-2 hornblende fluencr tnonitors were irradiated wtth a nommal fast neutron fluence of 2 x 10’“. The fragment material was subsequently heated for 45 minutes at each of a series of increaaing temperature steps. and the isotopic composition of the extracted argon was measured on a mabs spectrometer Temperature was monitored with a thermocouple embedded in the crucible. Argon from the hornblende monitors gave 39Ar/40Ar ratios of 0.0312 and 0.0303 and were used to calculate ages for the fragment. Further details of the irradiation and gas extraction techniques, the hornblende monitor and the age calculations are given by HL’SAIN (1974) and BOGARD rt ctl. (1976). Oxygen isotopic analyses were carried out by ‘1‘ K. Mayeda and R. N. Clayton (University of Chicago) on a 1,I mg chip of the lithic fragment. following previously described procedures (Ct A~'TONt'r (I/.. 1')76l. 3. RESULTS

The lithtc fragment measure\ ahout 2 x I cm and IS light in color relative to the host chondrite (Fig. 1). Because it borders the fusion crust, it is likely that some of the original fragment was destroyed by ablation. The contact between fragment and host is sharp and well-defined.

* The percent mean deviation of olivlncs in the Bovedy host is _ 1. characteristic of chondrites of petrologic type > 4.

Many areas of the fragment h,t\c pronounced poikthtlc textures (Figs Za, b), consisting of subhrdral to I-oundrd chadacrysts of olivine 0.01-0.12 mm in diameter ttqptcally 0.04 mm) enclosed by oikocryats of orthopyroxenc 0.3~ 1 mm in diameter. These textures are vcr! ~lrnllal to those of lithic fragments from the Plam\rc\\ H chondrltc (Kent. er NI., 1980). the Abbott H chondrite (F~)~x)K ~‘1J.. 1976), several LL-group chondrttel (K~II and I~‘o~)c)K. 1973; FODOR and KEII., 1975) and to the Itght-cillorcd (impact-melted) portions of the Shaw I- chondrite (7 ‘41I.OK cjl ul., 1979). The olivine chadacrysts near the bounda:tc\ ol the enclosing orthopyroxene oikocrqs!s tend to hc larger and sltghtly more ldtomorphtc than oli\tne\ I~?~,:!~~:the centers of the pyroxenes. The fragment also contams a feu larser. mhedrsi oll\lnc grains up 10 0.4 mm in diameter Area .A iI.tg. ! : .:clntal;l~ some elongated olivines up to 0.25 >- 0.04 nlm ill ~IJL‘ (Fig, 2~) that do not differ in cotnpo’\ttlon from iicighb~~rmg olivme chadacrysts. At one end of the frayrncnt (:~Ic‘! F). subhedral to rounded oltvines 0.03 -0.16 mm dia it!,pitally 0.07 mm) form an equtgranul.ir tc‘xturc (Fig ?& Although the fragment is significantly depleted to met,& Itc Ni--Fe and troilite relative to the host chantit-itc. sotnc small. angular metallic Ni~Fe grains (0.006 0.14 mm didi were observed. Microprobe analysts and etching indlcatch that the metallic Ni -Fe in the fragment consis:l\ of three phases: kamacite, zoned taemte and tetl.ataL’nlte :I thorough search was made for martensite (10 15rWt, Nil. but none was identified. Whereas kamacite tn many area\ of the host is rimmed by taenite. the kamacite ,lnd taenitc in most areas of the fragment occur as separate grams Metallic Ni- Fe grains in the fragment are typically \mailc:1than those in the host. Many of the kamacite and taentie grains in the fragment and host are polycrystallitlc A Fe\+ isolated grains of tetrataenite (48 57 wt”,, Ni. Cr AKKt- and SCOTT. 1980). approximately 10 itrn dia. occur in the htj\t and area E of the fragment. In area E. some troilite gratns (5 40 /cm ~CI’C)S:~I ~~c‘u~~ associated with metallic Ni Fc. Tl-oihtr in the othet JI-~‘I\ of the fragment is submicron-st;red and too
Area D was removed for age-dating beiorc dny pctrographic or mineralogical studies were made. Electron mic. roprobe analysis shows that the remaining porttons of the fragment have distinct olivine and low-Ca pyloucne con,positions (Figs 3.4). These areas are labelled A. B C. E and F in Fig. lb. Areas G and H arc similar in composition ti> areas B and C, respectively. Olivines in each area of the fl-agmrnt arc iaui) irt~rnogeneous. varying by only a fea percent from the mean FIX content of the area in which they occur. The I;I!I, of the olivine compositional distributions overlap from oue area of the fragment to the next (Fig. 3). Olivines in the host arc homogeneous. averaging Foih in this study as ~~11 ‘1’ 113 that of GRAHAM rt trl. (1976) and are t>plcal for L-group chondrites (Fo,, 4 to Fo-’ .%.Kbtl and FK~:IXI~~SSOX.1964: FODIX ef trl., 1976).* Mean Fo content of olivlnc Increases across the fragment and is Fo,- (area A), Fo,,~ [area 1st. FosO (area C), Fast (area E) and Fo,, (area F). Thus. olivine in area A is very similar in composition t&j ,)li\lne in the host chondrite. Composition of olivinc ID &trca B i\ intermediate between that typical of H and L chondriteh. whereas compositions of olivine in areas C. E and F are tn the range of olivine composition> !n H chondl http\ (t;o~, /

Fig. I. Photograph (a) and schematic outline (b) of the lithic fragment (top: light-colored) m the Bovcdy chondrite. A-H designate areas studied. Boundary between A and B is defined compositionally; all other boundaries are along fractures formed when material was removed from the fragment for study. Fusion crust is shown in (b) by heavier line at the left and top. Each division of scale is 1 mm.

2215

b

F1.g. 2. Photomicrugrapha of the lithlc fragment. 1.1) Polkilitic texture m area A conatstmg 01 i:u,g~ orthopyroxene oikocrysts enclosing small olivine chadacrysts (schematic outline m b). Olivme chad~ trysts tend to be larger and shghtly more idiomorphic toward the margins of the orthopyroxenc olk(i trysts. (c) Elongated olivines m area A are compositwnally similar to neighboring ohvine chadacryit(d) Area F has equigranular texture consisting of subhedral to rounded olivines and contams ;I dark branching glassy shock vein. Transmitted light. crossed polarizers: scale bar equals 200 ktm (RI1 phlrtcjmicrographs to same scale.)

Derivation

of a heterogeneous

L

lithic fragment

I

2317

H

I

1

% Fo Fig. 3. Histograms of the compositions of 29 olivines from the Bovedy host chondrite and of 435 &vines from different areas of the lithic fragment shown in Fig. lb (in molt,<; forsterite, Fo, MgzSiO,). There is a progressive increase in Fo in olivines across the fragment from area A to F. For comparison. ranges of average compositions of o&vine in equilibrated H- and L-group chondrites are given (after KEIL and FREDRIKSSON. 1964, as revised by FODOR et crl., 1976).

Host ilR ““”

5’

L ,n

0=

AmaA

5-

1.

0, AmuB

5’ h P 0 2 I=

----

o-

-

5. 6,

AMOC

5’

Area E

&_L.-p_.__L__

L.-l

0 Area F

5’ O-

-

L

.

I..

5. 0 73

1 I 74

. . * . . 1 .= ,=.a 75

76

77

70

7 X En

Fig. 4. Histograms of the compositions of 21 low-Ca pyroxines from the Bovedy host chondrite and of 74 grains from the different areas of the lithic fragment shown in Fig. lb (in mol?;, enstatite. En. MgSiO,). There is a progressive increase in En in pyroxenes across the fragment from area A to F. but the trend is not as clearly defined as for Fo content of olivine (see Fig. 3). For comparison, ranges of average compositions of low-Ca pyroxene in equilibrated H-, L- and LL-group chondrites are given (after KEIL and FREDRIKSSON, 1964, as revised by FOLXIR et ul., 1976).

to Fo,,.,.

KEIL and FREDRIKSSON. 1964; I.‘OIX)R et cl/.. 1976). By inference, we suggest that olivine in area D (although not analyzed because the material was entirely used up for .“Ari4”Ar age measurements) has a composition intermediate between that of areas C and E, The average olivine composition of all five areas of the fragment (IX. areas A, B, C, E and F). statistically weighting each area equally, is Faso. but if the different ctreas of the fragment are statistically weighted according to theu sizes (areas E and F are the largest), the overall average olivine composttton IS FosI. Both of these average values lie in the compositional range of olivines in H-group chondrites (Fo,~,~_~~.~. KEIL and FREDRIKSSO~. 1964, as modified by FODOR er trl., 1976) and are also simtlar to the average composition of olivines in porphyritic chondrules from unequilibrated ordinary chondrites (Fogq. GOODING. 1979). Compositions of low-Ca pyroxene in the host chondrite are highly variable (Graham 6’1trl.. 1976; this study. Fig. 4). as they are in the fragment (Fig. 4). The rneun composition of low-Ca pyroxene in the host is En,,, typical of L-group chondrites (En,,., to En*,,,. KEIL and FKEURIKSSON, 1964. as modified by F~WR L’Iul., 1976). Although there IS considerable overlap in pyroxene compositions in the different areas of the fragment. there is a systematic increase tn mean En content from area A to F: EnR, in A. Ens1 tn B, En,, in C. En,, in E and En,, in F (Fig. 4). However. this increase is not as clear-cut as it is for Fo in olivinc (Fig. 3). The average composition of low-Ca pyroxene of the entire fragment. calculated by weighting the areas by size. is En,,. This average is within the range of pyroxene compositions of H-group chondrites (Ens,.s to Ensr.j, KEII. and FREDRIKSSON, 1964, as modified bv FOLIOR et (II.. 1976) and similar to the average cornposit& of pyrnxene in porphyritic chondrulea from unequilibrated ordinary chondritcs (En”,,. GOOlXMi. 1979). Small grains of plagioclase (or glass of plagtoclase composition) ranging from An,, to AnsJ (average An-,) show no detectable progressive change in composition across the fragment: average compositions are An,, in A. An,, in B. AnT4 in C. An74 in E and AnTj in F The average host plagioclase composition is An,- (thts study). although GRAHAM ct ul. (1976) reported An,,. This discrepancy could be due to volatilization of Na in plagioclase during microprobe analysis by GRABAM er ul. (1976). Plagioclasr in the host and fragment is thus very An-rich when compared to plagioclase from equilibrated (petrologic type 6) ordinary chondrites (An,~ ,i. VAN S~XMU and RIBBI. 1968). A literature search indicates that less equilibrated chondrites also normally contain plagtoclase (or glass) of oligoclase composition. Electron microprobe (crystal spectrometer) analyses of kamacite and taenite indicate minor Si in solid solution (Table 1).Taenite appears to contain more Si than coexisting kamacite, consistent with the preference of Si for taenite in the anomalous iron meteorites Tucson (WAI and WASSON. 1969) and Horse Creek (A. E. Rr~nu. unpublished). Silicon contents are relatively high for kamacite and taenite in the host and in area A, whereas in the other areas of the fragment, Si contents are near the detection limit (about O.Ol”,, Si) (Table I). The detection of St tn kamacite and taenite of the Bovedy host and lithic fragment confirms the presence of Si in metallic Ni- Fe of some ordinary chondrites (RAMRALIII and WASSON. 1980: RAMHALDI et d., 1980: TAYLOR et al.. 1981 : Wtwxw t’t d.. 1981).

The bulk composition of two combined pteces of the lithic fragment (total weight 48.3 mg) was determined by INAA (Table 2). Because the fragment is heterogeneous, its major-element bulk composition is probably better represented by the broad beam electron probe analysis obtained by combining data from sections of all areas of

Derivation Table 2. Bulk compositions

of the Bovedy

(wt”,,)

020, FeO* MgG MnO CaO Na?O KZO P205 H&I.

SC

host, lithic fragment,

Lithic fragmentt

Host? SiOz TiOz A103

of a heterogeneous

47.1 0.1 1 2.47 0.43 15.4 29.8 0.32 1.93 0.99 (I.10 0.39 0.36

13.8 33.9 0.31 2.72 0.67 0.05 0.14

11.8 88 220 5220 0.59 0.34 0.15 0.6 0.30 0.03 0.3 89 183 100

(ppm)

.-

(svb) (wt’:,)

f 00.00

100.00

chondrules

Porphyriti~ chondrulesl

(42.6) 0.12 f 0.08 3.54 ) 0.03 0.651 +_ 0.002 18.5 * 0.2 31.8 f 0.7 0.265 & 0.001 2.0 * 0.2 0.519 * 0.009 0.020 f 0.010

V CO Ni La Sm Eu Dy Yb Lu Hf .AU lr Total

porphyritic

Lithic fragmenth

43.5 0.14 4.1 0.67

221’)

lithic fragment and H- and L-group

H group

L group

(49.3) 0.14 2.5 0.66 12.7 31.0 0.38 2.2 1.01 0.13

(46.8) 0.13 2.4 0.64 17.1 2Y.3 0.3: 21 1.02 0. I2

(48.5) 3.0 0.619 15.7 28.3 0.368 2.3 1.20

+ 0.1 i 3 * 2 + 80 &- 0.04 & 0.01 * 0.02 f 0.4 * 0.07 * 0.02 f 0.1 + 4 * 5

12.1 90 180 4360 0.44 0.30 0.12

Y.5 75 550 11500 0.4 I 0.25 0.095 0.40 0.2: 0.044 0.20 160 460 100

9.9 80 834 16900 0.42 0.26 0.101 0.43 0.26 0.046 0.24 210 730 100

0.30 0.054 0.22 58 254 100

chondrites

*Total iron as FeO. +Normalized bulk composition of the Bovedy (Sprucefield) host (GRAHAM cr d.. 1976). :Normalized bulk composition of the lithic fragment (d.etermined by broad beam electron probe analysis), $Lithic fragment, INAA (SiO, by difference). 7Average for 84 porphyritic chondrules from unequil&:ated ordinary chondrites, INAA (after GOOIIIW, 1979). (SiOZ by difference). ! Averages compiled from EVENSEN or ul. (1978). FELTHE and HERMANN (1970). MASON (1965, 1971, 1979). MAS~WA c’t tri. (1973). NAKAMURA (1974) SCHMITT rt NI. (1972). and SHIMA (1979). The values for Co. NI, Au and lr are for the bulk chondrites: all other values are for the silicate portions of the chondrites. (INPlA), (SiOZ difference).

Lithlc fragment Porphyrltk chondrubs H chondrite silicate fraction L chondritesilicate fraction

-_-

1 T

La

Ce

R

Nd

.

I

Pm Sm

-

Eu

b

Gd

t

Tb

T

11

Dy

Ho

Fig. 5. REE patterns of the Bovedy lithic tragment. porphyritic chondrules H and L chondrites, normalized to Type I carbonaceous

I

Er

Tm

.

Yb

.

I

Lu

and the silicate chondrites.

fractions

of

A. E. RCRIN et ul.

2220

1.8 39Ar/40Ar

,

AGE-Gy

,4

aoo"c

r

0

0.2

0.6

0.4

CUMULATIVE FRACTION

0.8

1.0

39 Ar

39Ari4uAr ages and K. C‘a ratios of the light-colored lithic fragment from the BoL~J~ Fig. 6. Calculated meteorite as a function of fraction of j9Ar released, from stepwise temperature extractions

the fragment (Table 2). This broad beam analysis IS used here for all major-element comparisons. Compared to the major-element bulk composition of the L chondrite host (GRAHAM etul., 1976), the fragment has more Ti02. AIIOJ. Cr,O,, MgO and CaO, and less SiOz. FeO, NazO, KzO and PzOS. In addition, the fragment is significantly depleted in metallic NiiFe and troilite: CIPW norms indicate that the host contains lO.O”, Ni-Fe and 5.8”,, troilite (GRAHAM et(I/.. 1976), whereas the lithic fragment contains 1.X”,, Ni ~Fe and O.lq,, troiiite. ‘The siderophile elements Ni, Co. Au and Ir are depleted in the fragment relative to bulk average H and L chondrites (Table 2), consistent with the fragment’s relative depletion of metallic Ni-Fe. However, the abundances of Ni. Co, Au and Ir in the fragment are similar to those of porphyritic chondrules (Table 2). The REE pattern (+ la errors) of the fragment. normalized to Type I carbonaceous chondrites, appears somewhat depleted in the heavier REE (Fig. 5); however, within the _+1~ error of Yb and f2u error of Lu, the fragment’s REE pattern is flat at 1.8 times Cl chondrites. The fragment’s REE abundances most closely approximate those of porphyritic chondrules (Fig. 5). The average ratio for the measured REE in the fragment versus the bulk compo-

1400

6

9k =A,

600

0~

200 3gAr/36Ar

isochron plot for the Fig. 7. ‘“Ari3(‘Ar versus “Ar/3hAr first five temperature extractions of the Bovedy lithic fragment, The strongly linear relationship gives a 4oAr.““Ai intercept value essentially identical with the atmospheric ratio and a slope of 40Ar/39Ar = 6.59, which corresponds to an age of 0.94 Byr. Extraction temperatures in hundreds of degrees C are shown.

sition of porphyritic chondrules is 1.05. wherea, tlie I atlct\ of the fragment‘s REE versus the silicate fractions of H aim1 L chondrites are 1.23 and 1.X respectivei! Ihe ahundances of REE as well as many other trace elements lNi> Co. Ir. Au. SC. V. Hf) of the fragment arc moic ~l<~\el! matched by those of the porphyritic chondrules from unequilibrated ordinary chondrites than by those UCfhe hulh and silicate fractions of H or 1. c,hondrites ifahlc li

Argon results for the lithic fragment arc presented r/r Table 3. and the calculated “9Ar “‘Ar age and K Ca rati<) for each extraction temperature are plotted versuc the fractional release of ‘“Ar in Fig. 6. The Ca and K contents are 1.73 t 0.17”,, and 0.030 f 0.003”,,, respectively, in good agreement with the bulk analysis obtained hy hrond beam electron probe analysis (Table 2). The fragment has been extensively degassed ol it, radio. genie “OAr relative to an age of 1.5 Byr. 7‘hc trend for3‘iAr,‘4”Ar ages to increase and the K Ca ratio 1,) snioothly decrease by a factor of _ 30. with increasing <\traction temperature. indicates that at least two nuner,tl phases with quite different KCa ratios are contributing to the calculated ages. However, a plot of ” Ar ahundanic, \crsu> the extraction temperature does not resolve these two K-hearmg phases. in contrast to the case for nl& whole rock chondrite samples (e.g. BWARI) and HIKS ft. 1980) The extractions through 600 C show atmospheric ,\I. ‘1s i\ evjident from the appreciably higher “‘Ar ‘-\r ratic)\ 01 these extractions compared to higher temperatiiie extractions (see BOGARD etd., 1976. for a discussion of ihe u?c i?f the ‘hAr,!J7Ar ratio as an indicator of atmospheric Ail Because the fragment contains a relatively small amount <>t radiogenic ‘“Ar ( . 1 x IO-’ cm3 in the 500 C‘ c\rr,tctionl. the presence of atmospheric Ar I . 5 x IO ” LITI’ in the 500 C analysis) causes the calculated ‘“Ar ‘“4r- :igr> 10 hc too great for the extractions through 600 C‘. A “‘Ar -“Ar versus J9Ar;‘6Ar isochron plot gives a very lineat trend for the 300’.400., 500 , 600 and 700 C data (43”,, (if the total “Ar; Fig. 7) and defines a 40Ar -‘“Ar intercept identical to the atmospheric value of 296 and a “4r “‘Ar \/ape which yields an age of 0 94 Byr. 4. DISCI>SSIO% 4. I lyneous The

lithic

origin

fragment

ogical similarities

bears

to materials

textural

anti

mmerd-.

for \vbich igtwu

i)rl-

Derivation

of a heterogeneous

lithic

fragment

?,I, -__

gins have been previously ascribed. The fragment’s poikilitic texture is very similar to those of fragments in Plainview (KEIL et trl., 1980) as well as to the Shaw chondrite (TAYLOR et trl., 1979). RYDER and BOWER (1976) concluded that porkilitic textures with plagioclase chadacrysts and pyroxene oikocrysts in Apollo 14 white rocks (friable breccias) resulted from heterogeneous nucleation of plagmclasc caused by abundant tiny seed nuclei in an rmpactgenerated melt. This conclusion was supported by NARELEKat cd. (1978) who found that poikilitic textures readily form in cooling melts containing numerous non-faceted crystals or unfused particles that acted as seeds for heterogeneous nucleation of chadacrysts. It is possible that numerous unfused olivine crystals remained in the Bovedy lithic fragment’s parent impact-melt which subsequently acted as seeds for the abundant heterogeneous nucleation of olrv~ne chadacrysts. MCBIRNEY and NOYES (1979) descrtbed potkilitic textures (clinopyroxene oikocrysts enclosing olivme and plagioclase chadacrysts) in Lower Zone A of the Skaergaard Intrusion in eastern Greenland. They found that the olivine and plagioclase chadacrysts tend to increase in size toward the margins of the enclosing pyroxenes. This indicated to them that all of the crystals nucleated irr situ. growing from adjacent liquids and that the growth of the chadacrysts was arrested when they were enveloped by the pyroxcnc oikocrysts. In the Bovedy lithic fragment. the ohvine chadacrysts tend to be larger and slightly more idiomorphic toward the margins of the enclosmg orthopyroxene oikocrysts (Figs Za, b), indicating that semilar processes may have been operating in hoth instances.

FOUOR et trl. (1976) suggested that Itght-colored igneous-textured and metallic NipFe- and trorlitedepleted lithic fragments formed by impact-melting in their parent body regoliths. The lithic fragment in Plainview. for example. was derived from tmpactmelted host chondrite material (KEIL (21 trl.. 19X01. whereas bulk and mineralogical data indicate that the source of the Bovedy fragment was probably impactmelted porphyritic chondrules. The bulk composition of the Bovedy fragment (Table 2) is depleted in Na>O, KLO and PLOS relative to the host chondrtte. probably due to volatilizatron during impact-melting. The average plagioclase composition of the fragment is Ani4, compared to AnT- in the host and An, r5 in most ordinary chondrrtes. Because the occurrence of calcic plagioclasc in the Bovedy host is unusual (GRAHAM cr trl. 1976). the presence of a phase of similar composrtron in the fragment suggests a genetic relationship between fragment and host. However. the average composlttons of OIIvine and low-Ca pyroxene in the fragment (T--o”,: tlil 83) are different from those in the host (Fo-,,: En-,,) the L-group chondrite range and lie outside

2222

A. E. RUBIN et cd

(Fo,~.~~Fo,~.~; En77,4-En81.3; KEIL and FREDRIKSSON, 1964, as modified by FOD~R et u1., 1976). Porphyritic chondrules* from unequilibrated ordinary chondrites have olivines (Fo& and low-Ca pyroxenes (En& (GOODING, 1979), similar to those of the fragment. Although the compositions of these phases in the fragment are also within the ranges of these minerals in H-group chondrites (Fo,~,~ to Foej,,, Ene1.9 to Ens4.3; KEIL and FREDRIKSSQN,1964, as modified by FODOR et al., 1976), G~DING (1979) and Huss et (11.(1981) showed that individual chordrules in H-, Land LL-group chondrites have mineral compositions overlapping those in H chondrites. Trace element contents of the Bovedy fragment also favor a source more like porphyritic chondrules than H or L chondrites. The fragment’s REE abundances (Table 2) and, more significantly, trace siderophile element contents, strongly favor a porphyritic chondrule source. Because Ir and Au are both strongly siderophile, the Ir/Au ratio of meteoritic materials should remain fairly constant (or increase slightly because Au is more volatile than Ir) through episodes of melting and fractionation. Thus, the Ir/Au ratio of the lithic fragment should be a sensitive indicator of the fragment’s relationship to other meteoritic materials. For instance, the Ir/Au ratio of the Plainview fragment studied by KEIL et ui. (1980) is 3.7, typical of bulk H chondrites (RAMBALDIet cd., 1979), indicating that the fragment was indeed derived from bulk H chondrite material. However, the Ir/Au ratio of the Bovedy _______ * Porphyritic chondrules (as defined by GOODING, 1979) comprise the following chondrule types: porphyritic olivine,

porphyritic

pyroxene.

and barred olivine.

porphyritic

olivine-pyroxene

lithic fragment (2.1) is closer to the median ratio of 2.7 for 88 porphyritic chondrules (Fig. 8) from unequilibrated ordinary chondrites (GOODINC~.1979) than to the mean ratios for bulk H-. L- and LL-group chondrites (3.7, 3.4, 3.0 respectively: RAMBALUIel (ii.. 19791. Because the fragment’s h/Au ratio is /ower than that of bulk chondrites. metamorphism or meltmg of ;1 chondrite source cannot account for the fragment’s ratio. A porphyritic chondrule source for the fragment seems more plausible, especially in view of the large Ir/Au range observed in porphyritic chondrules. AFIATTALAB and WASSON (1980) found that the Co contents of kamacite grains form distinct no~~r/trpping ranges for the three groups of ordinary chondrites: H chondrite kamacite contains 0.33 to 0.48 wtT,ACo, L chondrite kamacite 0.67 to 0.X2 wt”,, Co, and LL chondrite kamacite I.5 to I 1.0 ut”,, CU. The Bovedy L chondrite host has kamacite with (‘
a

H-group

30-

Fig. 8. Histogram of the Ir/Au ratios of 88 porphyritic chondrules from unequilibrated ordinary chon. drites (after GOODING. 1979) and the lr/Au ratios of 22 ordinary chondrites (after RAMBALDI Ed t/1.. 19791 Chondrites with ‘+’ pattern are H chondrites, those with dotted pattern are L chondrites and those MII~ striped pattern are LL chondrites. The lr/Au ratio of the Bovedy lithic fragment (2.1) lies in the porphyritic (P) chondrule range and is closer to the median porphyritic chondrule value (2.71 than to the median chondrite value (3.2).

Derivation of a heterogeneous lithic fragment fragment and still falls between the non-overlapping ordinary chondrite ranges. Oxygen isotopic analysis of the lithic fragment yields 6180 and ijl’O values of +4.3”,,,, and +2.6”,,,,, respectively. The Bovedy fragment thus lies near the ordinary chondrite mixing line (e.g. Fig. 3 of CLAYTON md MAYEDA. 1978). near the whole-rock values of equihbrated H chondrites and at one extreme of the cluster of chondrules from unequilibrated ordinary chondrites (COODING er trl.. 1980a; R. N. CLAYTON. personal communication. 1981). The fragment’s oxygen isotopic composition is consistent with the fragment’s derivation from H-group chondrites or from porphyritic chondrules with very low 6”O and (5”O values. In summary. bulk chemical and mineralogical compositions indicate that the Bovedy lithic fragment is more closely akin to porphyritic chondrules than to bulk H or L chondrites, although the oxygen isotopic composition of the fragment is nearer the mean of equilibrated H chondrites than the mean of chondrules (R. N. CLAYTOY. personal communication, 1981). It appears. therefore. that the fragment may have formed by impact-melting of a target composed predominantly (possibly entirely) of porphyritic chondrules, although the possibility remains open that the fragment represents an unusual H chondrite projectile that melted upon impacting the Bovedy parent body regolith. However, it is certain that the fragment was not derived from an L-group chondrite. A concentration of porphyritic chondrules is not hard to envisage: GWDIW (1979) found that about W’,, of the chondrules in unequilibrated ordinary chondrites are porphyritic. Perhaps areas exceptionally rich in porphyritic chondrules existed in the unconsolidated regoliths of meteorite parent bodies or on the surfaces of accreting parent bodies. Rocks made from these areas might be too friable to survive impacts in space or entry into the earth’s atmosphere. thereby accountmg for the apparent absence of meteorites composed almost entirely of porphyritic chondrules. Another possibility is that the fragment formed before the Bovedy parent body agglomerated. GO~IXX VI trl. (I 980b) concluded from a study of the compositions and physical properties of chondrules that the porphyritic and non-porphyritic varieties formed in separate environments by melting of preexisting materials. It is possible that an impact made the Bovedy fragment from a concentration of porphyritic chondrules prior to their mixing with nonporphyritic chondrule types. Alternatively. the fragment could have formed by impact-melting of the same parent materials from which the porphyritic chondrules were derived. In any case, the data suggest that the Bovedy fragment formed by impact-melting of a target very similar in composition to porphyritic chondrules. 4.3

Origin of he/c3w+wcilies

The observed

progressive

increase in the MgO con-

22’3

tent of olivines and low-Ca pyroxenes and in the bulk MgOiFeO ratio across the lithic fragment (from areas A to F) is one of the fragment’s most distinctive characteristics. Such heterogeneities could result from (a) quenching of a heterogeneous melt, (b) cooling of different areas of the fragment at different rates. or (c) heterogeneous nucleation of MgO-rich phases in area F caused by an abundance of nuclei, resulting in progressive Fe0 enrichment in the residual melt away from area F. The five areas of the fragment contain similar amounts of SiO,. TiOz, CaO. Na20. K20 and P,05. indicating that the melt was initially homogeneous and lost volatiles at a similar rate. Thus. model (a) is rejected. If the melt which formed the fragment was initially part of the margin of a larger melt pool. a progressive decrease in cooling rate from the very margin toward the interior of the solidifying melt would be expected. The melt nearest the surface (i.e. area A of the fragment) would cool faster. supercool deeper and. if no pre-existing nuclei were present, olivines rich in Fa (relative to olivines formed by equilibrium crystallIzation) would crystallize first (DONALDSOS er (I/.. 1975: BIANCO and TAYLOR, 1977). Areas increasingly distant from the surface would experience lower cooling rates and lesser degrees of supercooling and. hence. crystallize progressively more Fo-rich olivines (toward area F of the fragment). However, the lack of textures typical of supercoohng and the high degree of supercooling required to generate such Fa-rich olivines (BIANCO and TAYLOR. 1977) suggest that model (b) should also be rejected. Small relict olivines in the fragment may have contributed an abundance of nucleation sites in area F. helping to form the equigranular texture. causing crystallization and Fo-rich olivine and enriching the residual melt in FeO. At larger distances from area F. the number of heterogeneous nuclei may have been lower. causing crystallization of olivine (including elongated olivine in area A) from an FeO-enriched and more supercooled liquid. Crystallization of pyroxene would follow that of olivine and show a similar progressive FeO-enrichment across the fragment. Alternatively, the nucleation sites may have been heterogeneously distributed submicron-sized bubbles (of NazO, KzO and PzOs vapor) in the impact melt. [PLASNER (1979) found that minute bubbles can serve as heterogeneous nucleation sites for olivines in melts of bulk H chondrite composition.] In either case. we suggest that model (c) best accounts for the observed characteristics of the fragment. After crystallization of the fragment and incorporation into the Bovedy breccia, some sub-solidus equllibration between the host and the fragment appears to have taken place. Evidence for this equilibration is only apparent from area A within 1 mm of the host border. but may have been present around the entire circumference of the fragment. Unfortunately. except for the A-host boundary, the other portions of the

2224

A. E.

RUBIN

lithic fragment along the host-fragment boundary are not preserved. At the boundary of area A with the host, there is almost no compositional difference between host and A. This similarity decreases with increasing distance from the boundary (out to about I mm), as area A becomes more magnesian. In addition, there are similarities in mineral composition between area A and the host: olivines in area A have an average composttton of Fo,,. whereas those in the host are Fo,~; !ow-Ca pyroxenes in area A are of Ens, composition, whereas those in the host are En,,: the Si content of metallic Ni--Fe grains in area A is much higher than in the rest of the fragment and is similar to that of the host (Table 1); and the average Co content of kamacites in area A is much closer to that of the host kamacites than to those in the remaining areas of the fragment (Table 1). 4.3 Implications

of young

“‘Ar--““Ar

uyr

Ar data (Fig. 6) demonstrate that the Bovedy lithic fragment has been partly degassed of its radiogenic Ar. An isochron plot (Fig. 7) suggests that the outgassing event took place ~0.94 Byr ago. According to G. TURNER (personal communication, 1980), the Bovedy host has also been partly outgassed and the heating event could have taken place as recently as -0.5 Byr ago. These data suggest that the young age of the fragment reflects a degassing event that affected the entire Bovedy chondrite (including the lithic fragment). The Ar age, therefore, places no constraints on the time when the lithic fragment formed or when it was incorporated into the Bovedy host. However. we infer from the highest apparent age recorded in the step-wise heating experiment that the fragment formed prior to -2.2 Byr ago (Fig. 6); it could have formed 4.5 Byr ago. This situation is quite different from the case of a light-colored lithic fragment in Plainview which clearly formed 3.63 Byr ago (KEIL rr ~1.. 1980). The event that reheated the Bovedy assemblage ~0.94 Byr ago was probably caused by shock. This conclusion is supported by the evidence for moderate shock in the Bovedy fragment and host and the evidence in other chondrites for colhsional reheating in this time range (BOGARD ef al., 1976). If the pre-shock K-Ar age of the fragment was > 4 Byr and the impact occurred 0.940.5 Byr ago, the total fractional loss of 40Ar would have been ~95’,,,. 3.5 Thermal

er ul

the absence of nuclei (DONALDSON. 1976; I,oK;RE~~ and SMITH, 1980). SCOTT (1981) used Figs 7 and X of 1 AYI.OR and HEYMANN(1971a) to empirically deduce an equation relating (with order-of-magnitude accuracy) the radius R (in m) of a spherical melt to the cooling ra)c during solidification C (in C,sec): log R = -0.5

log (’

I .X

If the impact melt that produced the Bovvedl hthic fragment was approximately the same silt :rs the present lithic fragment ( - I cm in radius). then a cooling rate of 2.5 C/set or 9000 C,,hr is indicated by the equation. It is thus possible that the fragment cooled from the hquidus at a rate of one or two degrees per second, but the presence of nuclei in the melt caused the olivines to assume morphologies far less skeletai than such a rapid cooling rate might have otherwise caused. Similarly rapid cooling rates (I 300 (.*:secl were determined by SCOTT (1981) for clasts III the chondrites MezBMadaras, Weston. San Emigdio. Dimmitt, Pulsora, Tysnes Island and Tell, based on the dimensions of metallic Ni---Fe dendrites. The composition of the metallic Ni Fe phases arc related to the rate of cooling of the metallic Ni Fc (and, presumably. the entire fragment) through the two-phase field of the FeeNi phase diagram at temperatures near 5OOC (the temperature at which the taenite diffusion gradients were established. Woon. 1967). Slow cooling of the lithic fragment ar this temperature is indicated by the presence of kamacite, zoned taenite and tetrataenite and the absence OI martensite. In Fig. 9, we have plotted the central Ki content of the fragment’s (and host’s) taenite grains against the distance from the microprobe analysis point to the nearest grain boundary. Much of the scatter in this diagram is due to analytical error: grain

I I’>/

--.7_~T_~.---

r---

a lithlc frogent o Bovedy host

historjs

D~NALD%JN (1976) estimated that a mafic or ultramafic melt must cool at a rate of 2.5’C/hr or faster in order for skeletal ohvine morphologies to form. Because there are no skeletal olivines in the fragment, it appears that it cooled from the liquidus at a rate somewhat slower than this, i.e. at < -2’C/hr. However, this rate must be considered a lower limit: the inferred presence of nuclei in the melt would cause crystallization of olivines with less skeletal morphologies than might result from ohvine crystallization in

20 -

Apparent distance to nsarest grain bcuncky (microns) Fig. 9. Cooling rate of taenite grams m the Bovedy host (open circles) and lithic fragment (filled circles). ‘Theoretical cooling curves are from Wool (1967). Data for the raenite grains indicate that the fragment and host both cooled at d rate of - S’C/Myr, typical for ordinary chondrites.

1

Derivation of a heterogeneous lithic fragment boundaries of the polycrystalline taenites are difficult to locate accurately and many of the taenites are only a few microns across. Because this size is just slightly larger than the electron beam, the centers of the taenite grains (where the minimum Ni content occurs in the normal M-shaped profile) cannot be accurately analyzed. Moreover, the reheating event (indicated by the extensive degassing of Ar experienced by the meteorite) must have annealed the metallic Ni-Fe to some degree, rendering the metallographic cooling rate less precise. Nevertheless, the data suggest that the metallic Ni-Fe grains of the fragment and host cooled at a rate of _ 5 C/Myr (at temperatures near 5t)O”C), very similar to the cooling rates of ordinary chondrites (WOOD, 1967; TAYLOR and HEYMANN. 1971b). WOOD (1979) indicated that metallographic cooling rates may be systematically too slow by a factor of -6. If so, then the fragment and host cooled through 5oO‘C at - 30’C:‘Myr. It thus appears that the fragment cooled fast at high temperatures and very slowly at lower temperatures. These two cooling rates represent separate events: one records the rapid cooling of the Bovedy fragment as a small body of impact melt; the other records the much slower cooling at sub-solidus temperatures within the Bovedy parent body. The similarity in metallographic cooling rate and timing of ““Ar-degassing of the fragment and host, as well as evidence of equilibration of the outer 1 mm of the fragment with the host indicates that the fragment and host cooled together at low temperatures as one assemblage. The argon temperature release data can also be used to place constraints on the thermal history of Bovedy (e.g. BGGARD and HIRSCH, 1980). Argon data for many chondrites indicate the presence of two distinct mineral phases with quite different characteristics for Ar diffusion. Argon data from the Bovedy fragment seem to behave similarly. although the two mineral components are not well resolved. For an arbitrary reheating temperature of 5OO”C, the Ar diffusion coefficient. D:‘a’. for these two phases in many chondrites is approximately lo-’ and lo-‘” cm2/sec. The low temperature phase of the Bovedy fragment has apparently lost most of its 40Ar. but the high temperature phase appears to have retained approximately half of its 4”Ar. In order not to lose all of its “Ar during reheating to 5oo’C, the Hugh-temperature phase would have to cool at a rate of approximately O.Ol’-Ciyr. if D;‘a’ = lo-” (BOCARD and HIRSCH, 1980). (Higher values of D/a’ would require even faster cooling rates.) This cooling rate is orders of magnitude faster than those derived from the metallic Ni-Fe. We conclude. therefore, that the metallic Ni-Fe compositions were established during an earlier epoch in the meteorite’s history when slow cooling occurred compared to the more recent shockreheating event which caused partial loss of **Ar. The shock reheating would not be expected to appreciably affect the metallic NiLFe, although it could have slightly affected taenite compositions, giving rise to

?215

some of the scatter on the composition dimension plot (Fig. 9). The shock-reheating temperature is not well-defined by either the metallic Ni Fe compositions or the argon data. For example, reheating to 6OO’C or 400’ C would require cooling rates of -0.1’ C;yr and _ 10. a Cyr. respectively. to produce the observed argon loss in the high temperature phase of the Bovedy fragment.

The slow cooling rate of the Bovedy- meteorite at lower temperatures must have resulted from burial beneath insulating material on the Bovedy parent body. The metallographic cooling rates can be used to estimate the post-impact burial depth of the Bovedy fragment host assemblage by employing eqn (19) of JAEGER (1968) and making a number of assumptions about the shape of the Bovedy parent body. its temperature and temperature distribution, thermal conductivity and abundance of heat sources. We assumed that the Bovedy assemblage was located in the center of an infinite extrusive slab (with no internal heat sources and a uniform temperature of 1000 C) that cooled by conduction on the bottom and had a top surface temperature of 0 C. If we assume that the slab had an extremely loa thermal diffusivity (K) of lo-“ cm%ec. approximately comparable to that of the lunar regolith (~.AKCiSFTH t’r trl.. 1976). then a depth of about 6.5 km is calculated for a cooling rate of 5 C’Myr. For a cooling rate of 30 C Myr. a burial depth of 2.6 km is indicated. This is the shallowest possible burial depth for the Bovedq assemblage (granting the above assumptions). If h- = 0.01 cm’:sec is assumed. typical of mafc rocks (JAtifii:R. 196X) and similar to chondritic material (Woor~ l%h7). then a burial depth of 65 km is indicated for a cooling rate of 5 C,‘Myr and 26 km for 30 C’Myr. From F‘ig. 26 of WOOD (1967), cooling rates of 5 C Myr and 30 C Myr correspond to respective burial depths of 27 56 km and 13-38 km. It thus seems likely that by the time the Bovedy assemblage had cooled to 500 C. it was buried at depths of at least several kilometers. Because the lithic fragment cooled very fast from its liquidus, this requires continued accretion after the formation of the Bovedy assemblage. SCOTT and RAJ~N (1981) have silggested that maximum metamorphic temperat~lres {from an lhAl heat source) occurred in meteorite parent bodies only . 10 km in radius which subsequently accreted into larger objects before their residual heat had been lost. This planetesimal-conglomerate model allows chondrites to have cooled rapidly at high temperatures and more slowly at lower temperatures. The Bovedy fragment may then have formed in the unconsohdated regolith of such a planetesimal (by impact-melling of an area rich in porphyritic chondrules) and cooled rapidly from the liquidus. but this requires that the fragment formed very early in the history of the solar “Ai was still available. system. when sufficient (Although there are apparent difficLiities in retaining

2226

A. E. RUBIN

an impact melt on a body with a very low escape velocity, this melt may have been driven into the crater floor.) Alternatively, the fragment may have formed from a concentration of porphyritic chondrules off the Bovedy parent body and been subsequently accreted to the planetesimal. The planetesima1 may have then accreted (along with other L chondrite parent objects) into a much larger body wherein the Bovedy fragment--host assemblage resided at least several kilometers from the surface and cooled at a rate of approximately 5 30”CiMyr. The cooling rates after shock-reheating of Bovedy were much faster than the metallographic cooling rate. Consequently, the post-shock cooling must have occurred at shallower depths than that discussed above, after the fragment-host assemblage had been brought to the near-surface by excavation of overlying material. The argon-loss cooling rate of -O.Ol”C/yr suggested by shock-reheating to an arbitrary temperature of 500°C requires a burial depth _ 1OOOm of rock or -3Om of lunar-like beneath regolith. Higher reheating temperatures and faster cooling rates would require even less burial.

5. HISTORY OF THE LITHIC FRAGMENT

(I) Accretion of the Bovedy parent body of L-group chondrite composition. (2) Impact-melting of a target rich in porphyritic chondrules with consequent partial volatilization of sodium, potassium and phosphorus. (3) Rapid cooling of this melt from the liquidus and preferential nucleation of Fo-rich olivines in area F. resulting in compositional heterogeneities in the fragment. (4) Incorporation of the quenched melt into the Bovedy parent body regolith. (5) Burial of the fragment-host assemblage to depths of at least several kilometers, possibly the result of incorporation of the Bovedy parent body into a much larger object, and cooling of the Bovedy fragment-host assemblage through 500°C at a rate of approximately 5’.-30”C/Myr. During this period, the outer 1 mm of the fragment experienced sub-solidus equilibration with the Bovedy host. This most likely happened >4 Byr ago. (6) Impact-induced shock deformation of the assemblage, major degassing of radiogenic 4”Ar, probably 0.94-0.5 Byr ago, and transport to near-surface areas of the parent object. (7) Ejection of the fragment-host assemblage from its parent object, initiation of cosmic ray exposure. and perturbation into an earth-crossing orbit. (8) Collision with Northern Ireland on Friday evening, April 25, 1969. Acknowledgements--We thank R. HUTCHIWN (British Museum, Natural History) for calling our attention to the Bovedy lithic fragment and supplying us with fragment material and thin sections of &vedy. L. HIJSAIN-(N~~ York State Department of Health) arranged for the neu-

et ul

tron irradiation for K-Ar dating of the hthic fragment. WC also thank G. TURNER for providing us with his unpublished age-dating analysis of the Bovcdy hoat and R. h CLAYTON for his unpublished oxygen isotope analysis 01 the Bovedy lithic fragment. We gratefully ~+cknowledgc E. R. D. SCOTT for his helpful discussions and \uggestions and S. 0. AORELI., J. W. MOR(;AN, F. J. OI.S~N and M. J DRAKE for reviews and comments. This \ro,rk \\;I> supported in part by National Aeronautics and Space Admmistration grants NGL 32-004-064 CK KEII 1 .SII~ NGI. 32-002-039(R. A. SCHMITT).

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Left.

23, 206 ~219

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A. R. and NOYES R. M. (1979) and layering of the Skaergaard Intrusion.

Crystalh/atlon .1. I’ctrr~l.20.

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2228

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