Thicknesses of some lunar mare basalt flows and ejecta blankets based on chemical kinetic data

Geocbimica

et Cosmochimxa

Acta,

1975, Vol. 39. pp. I135 to 1141. Pergamon

Press.

Printed

in Great

Britain

Thicknesses of some lunar mare basalt flows and ejecta blankets based on chemical kinetic data ROBIN BRETT* NASA Johnson Space Center, Houston, Texas 77058, U.S.A. (Received 7 June 1974; accepted in

revisedform 17 December 1974)

Abstract-Data on the kinetics of some chemical reactions occurring in lunar samples are used to estimate cooling rates over limited temperature ranges. These results, together with data on thermal conductivity and specific heats of lunar samples, have been applied to estimate the thicknesses of the cooling units. Estimates for mare basalts indicate that they were derived from cooling units no thicker than 10 m. Breccias are derived from cooling units up to a few meters in thickness. This suggests that most lunar breccias are the product of a series of relatively local events rather than large basin-forming events. If this conclusion is correct, the significance of the radiometric ages of lunar breccias needs to be reevaluated, as does the assignment of breccias to individual basin-forming events on the basis of their trace element contents. If the breccias are a product of the major basin-forming events, it is unlikely that so many would show an age of about 4.0 b.y. It is likely that a large proportion of materials sampled would have escaped resetting by these major events, and a wider spread of ages would be observed. Attempts to link a given breccia to a specific basin-forming event on the basis of its crystallization age therefore appear to be unfounded.

INTRODUCIION

OWING to the lack of outcrop at Apollo landing sites there have been few estimates of the thicknesses of mare basalt flows or ejecta blankets at these sites. Such information is important when considering the mechanism of mare filling and the ultimate source of returned breccia samples. The only observations of the thicknesses of mare basalt flows at Apollo sites are those of HOWARD et al. (1972) who state that most flow units in Hadley Rille are a few meters thick. Data on the thicknesses of cooling units of breccias are even sparser; HEIKEN et al. (1973) show possible stratigraphic units about 5 m thick in Station 6 boulders at the Apollo 17 site. The present paper attempts to place limits on the thicknesses of the units from which some lunar samples were derived. The method uses chemical kinetic data from mineral equilibria in selected rocks to establish cooling times over certain temperature intervals. These cooling times are then applied to thermal diffusivity equations in order to determine minimum thicknesses for the units from which the respective samples were derived. If it can be established that most returned breccias were derived from thick units, then the information * Present Center,

address:

Reston,

U.S. Geological

Virginia,

22092, U.S.A.

Survey

National

lends more credence to the hypothesis that most breccias were derived from major basin-forming events (e.g. SCHAEFER and HUSAIN, 1974). If on the other hand, breccias were derived from thin units, then it would seem more likely that they are the product of local impacts, e.g. OBERBECKet al. (1973). KINETIC

DATA

AND

THERMAL

CAL.CULATIONS

A literature survey allows compilation of a list of chemical kinetic data (Table 1). The list is short compared to a potential list that could be obtained after more chemical analyses, but it represents nearly all of the kinetic data that are available. The kinetic data listed in Table 1 are of six different types: equilibration of a minor element between two phases, stability of a phase, grain size of a phase, the diffusion profile across a phase, devitrification rate of glass, and synthesis of the texture of a rock by cooling experiments. Sufficient data exist on the physical properties of lunar rocks to allow calculations of the minimum thicknesses of the cooling units from which a number of lunar rocks were derived. The assumptions involved in the calculations are: (1) Cooling units were deposited instantaneously and isothermally as infinite sheets with the upper and lower surfaces initially at 0°C. The units were deposited on material of similar thermal properties. (2) Rocks for which calculations are made were derived from the center of the flow or ejecta blanket. Since most samples do not originate in the center of a cooling unit, calculated thicknesses of cooling units therefore represent a minimum.

1135

1136

Table

I. Kinetic

data

References: I. GK~EN ct al. (1971). 2. L. A. Taylor (oral commumcation 1974). 3. TAYL~H c’t ul. (1973). 4. GREEN and RINGW~~II ( 1972). 5. HUMPHRIB ~‘t al. (1972). 6. TAYLOR and WILLIAMS (1974). 7. MCCALLIS~IEK and TAYLOR (1972). 8. ROED~ER and WEIBLEN (1970). 9. RINGWOOD and ESSENE (1970). 10. LINDSLEY cr al. (1972). 1 I. USSELMAN and PIERCI- (1974). 12. Gcam et al. (1973). 13. LOFGREN clt al. (1974). 14. AXON and GOLDSTEIN (1972). 15. GRIFN ct ul. (1972). 16. G~LDSTI.IN and AXON (1973).

1137

Lunar basalt flows and ejecta blankets (3) The effects of latent heat of ~ys~ll~tion, convection, radiative cooling variations in surface temperature, and heat loss due to volatile loss and vesiculation were ignored in the calculations. The equation used to estimate minimum thickness of cooling units is from JAEGER(1968) for cooling of a flow with the upper surface at 0°C. f

0

= &6 r) - &2 - 5. rb

fH

where

I

(P(Rr) = - erfs 21

- er/‘&]

(2)

5 = x/a, T = temperatureatany point (“C), r, = initial temperature (“C), K = thermal conductivity of cooling unit and basement (Cal/cm set OK), t = time after deposition (set), a = half thickness of cooling unit (cm), p = density of cooling unit and basement (g/cm3), c = specific heat of cooling unit and basement (Cal/g “K), x = distance from center of cooling unit-zero in the present case. Since c = 0, t can be calculated if T/T, is known. The thickness of the cooling unit can then be calculated from equation (3) since

and an estimate of K, r, p and c may be obtained from the literature. Note that even if the values chosen are incorrect by a considerable amount, the square root term in the equation tends to diminish the error in calculated thickness. Some of the kinetic dam (Table 1) are concerned with cooling over a limited temperature range, well below the temperature of emplacement. In this case, simultaneous equations can be solved using the two values of T/T, to obtain both cooling time after emplacement and the thickness of the cooling unit. PROVOSTand E~JITINGA (1972) employed a more sophisticated method than the above for estimation of rates of ~lid~~tion of Apollo ii mare basalts. Their results are concerned with cooling in the solidus region; most of the present calculations for mare basalts involve cooling through both the solidus and subsolidus regions. Calculations on cooling unit thicknesses of breccias are based largely on cooling within the subsolidus region. Provost and Bottinga consider the effects of latent heat of crystallization, radiation of thermal energy at the upper surface of the cooling unit, and convection within the liquid cooling unit. They do not consider the effects of diurnal variations in lunar surface temperature, heat loss during outgassing, or increased porosity caused by vesiculation. All of the above effects, although theoretically significant, tend to cancel one another out, and none was considered in the present approach. The present approach was employed for ease of calculation, and Provost and Bottinga’s results lie within the error estimates of the present method. For example, Provost and Bottinga calculate that rocks containing pyroxferroite crystallized at a depth of less than 50cm in the lava flow unit; the present results indicate a depth of less than 2 m. They also estimate that 10020

crystallized at a depth of iOcm below the surface; the present results suggest a depth of 20-40cm. Provost and Bottinga’s thickness estimates are less than the present ones for a given rock, so that the major conclusions of this study still stand if their method is adapted to the present data. The range of each variable was determined from the literature, and a reasonable value for each was selected. Maximum errors were determined by computing the effect of using the most extreme values of each variable. Values chosen are as follows: 7; T, were taken from Table 1. Errors in temperatures estimation are assumed to be negligible. Thermal conductivity, K, varies markedly with temperature. No thermal conductivity data have been measured on lunar samples above about 6OO”K,which is below the temperature range of interest. MURA~E and MCBIRNEV (1973) have measured thermal conductivity on a simulated Apollo I1 basalt at temperatures up to 1500°C. Between 1300 and 5OO”C,the temperature range of interest, conductivities range between about 1.5 x 1Om3and 4 x low3 cal/ cm/scc/“K. A literature survey shows that variation in measured thermal conductivities of lunar rocks at 400°K (the highest temperature at which data are plentiful) is no greater than a factor of 10 regardless of composition, except for an anomalous sample measured by HORAI and WINKLER(1974). Thermal conductivities used in this study are assumed to lie between 5 x 10m3 and 5 x 10m4 with a preferred value of 2 x IO-” cal/cm/sec/“K for mare basalts since Murase and McBirney show this value for their simulated mare basalt at about 1000°C. The preferred value of 9 x 10m4caI/cm/sccpK has been chosen for nonmare rocks, since measured values are lower than those of mare basalts by about 3 x 10S3 cal/cm/sec/“K at 400°C. Time after deposition, t, was taken directly from the data of Table 1. In calculating errors, a factor of 3 was assumed, conservatively, to be the maximum error in time extimation. This factor is purely an estimate that allows for experimental error and differences, if any, between the kinetics of natural versus synthetic assemblages. The density, p, of the cooling unit and underlying basement was chosen as 3.2-3,4g/cm3 for mare basalts with a preferred value of 3.3 g/cm’ and 2.8-3.0g/cm3 for nonmare rocks with a preferred value of 2.9g/cm3. The specific heat, c, increases with temperature and at least over the temperature range 9ft350”K appears to be unrelated to composition or texture of the lunar material (e.g. HEMINGWAY et al., 1973). After extrapolating the work of a number of authors to higher temperatures, c is assumed to lie between 0.2 and 06 cal/g”K with a preferred value of about 0,4cal/g”K over the temperature range of interest. Calculations show that the range of estimated minimum thicknesses of any cooling unit is not greater than a factor of four even if the most extreme values are selected. RESULTS

Calculated minimum thicknesses are listed in Table 2. From the preceding discussion, most of these results are probably correct to within a factor of 4. Calculations of minimum thicknesses of cooling units of glassy matrix breccias, Apollo 16 and mesostasis-rich breceias, give totally unrealistic results 2 cm and 1Oem ‘thickness, respectively. These rocks

R. BRETT

1138 Table

2. Estimated

thickness

DeSCZiptiO”

of cooling

Thickness

units

ccmme”~s

Mare basalts

HOWARD et al. (1972) discuss thicknesses of units in Hadley Rille. the only place on the Moon where outcrops of mare basalts have been observed. Most units are only a few meters in thickness although a single massive unit 1S-20 m thick occurs. WARNER (1971) on the basis of the distribution and petrography of Apollo 12 mare basalts states that the stratigraphy is built up from a series of ‘thin’ units. Such conclusions are consistent with the present results. The above conclusion merely limits thickness of surnpled mare basalt flows. There is considerable evidence that thicker, but unsampled units exist, for example, G. Schaber (written communication, 1974) reports flow lobes in S.W. lmbrium up to 60m in thickness.

The thicknesses calculated for mare basalts are all no greater than 2 m, suggesting that the mare basalts sampled were derived from relatively thin flows unless all samples, by chance, were ejected from near the tops or bottoms of flows. The mare basalts listed in Table 2 cover almost the entire range in grain size of mare basalts, from coarse grained, almost gabbroic 15475 to the vitrophyre 15499. Applying the error range listed earlier (4 x ), it seems that all mare basalts sampled on the Moon were derived from Rows no thicker than about 8 m.

The present results on non-mare samples indicate that all breccias for which kinetic data are available are derived from cooling units with minimum thicknesses of a few centimeters to several meters. Although the present method calculates only minimum thicknesses, the fact that all thicknesses are less than 5 m suggests that this figure may be close to an upper limit for thickness of cooling units. Of all the returned lunar breccias, none have textures or

have therefore not been included in Tables 1 and 2. The reason for such results is that the rocks were not deposited under conditions even close to isothermal, and isothermal deposition is one of the initial assumptions. These rocks were deposited as a heterogeneous sample of melt and lithic, mineral, and glass clasts. It is noteworthy that the calculated thickness of both 12052 and 14310 are similar for the two types of kinetic data used for each rock.

DISCUSSION

Lunar basalt flows and ejecta blankets mineralogies differing greatly from those listed in Table 2 that would indicate significantly longer cooling times. It is noteworthy that the Station 6 boulders at the Apollo 17 site show possible stratigraphic units about 5 m thick (HEIKEN et al., 1973). One might argue that the samples are from high temperature portions of thick ejecta blankets primarily composed of cool, hence, unconsolidated material: however, field evidence at the Apollo 15, 16 and 17 sites does not suggest the presence of such unconsolidated layers. Calculations show that the samples could not have been deposited beneath thicknesses greater than about 6 m of sintered, glassy-fragment-matrix breccias because these were deposited at temperatures above 7OO’%Z, e.g. WILLU\MS(1972) and cooling rates therefore would have been slower than those-observed. It is noteworthy that the slowest cooled breccia (64567) is a poikilitic rock. WARNERet al. (1973) suggest that these are the slowest cooled breccias at the Apollo 16 site. There is therefore evidence that the majority of lunar breccias that have been sampled were derived from ejecta blankets no thicker than about 4 times the maximum thickness calculated, i.e. 4 x 5m = 20m. In addition to the breccias derived from relatively thin cooling units, some metal particles and the granulitic troctolite 76535 appear to have been derived originally from cooling units ranging in thickness from 40 m to 1@30 km. Since these are derived from deeper in the lunar crust than the breccias, it would appear that they originate from a different type of deposit. The metal particles derived from depths of 80 m-3 km were presumably formed by the indigenous processes of crustal formation for the deeper ones, and mare basin-forming events for the shallower ones. Rock 76535 is clearly of deeper crustal origin (&XlLEY et al., 1974) and is presumably a product of the Apollonian metamorphism discussed by STEWART(1973). Based on the limited data presented in this paper, it appears that breccias are derived from thin cooling units. This casts doubt upon their being unreworked products of the major basin-forming events, as suggested by a number of authors. MCGETCHIN et al. (1974) have calculated the thicknesses of ejecta from the various basin-forming events at the Apollo sites; thicknesses for Imbrian ejecta range from 50m (Apollo 16) to 812 m (Apollo 15). Their discussion does not take into account reworking by later smaller impacts. It would appear unlikely that many breccias were heated above 1000°C (e.g. WARNERet al., 1973) at distances as far from Imbrium and Orientale as the Apollo 16 site. It therefore appears possible that

1139

the vast majority of breccias represent products of ejecta blankets from local smaller impacts. This would readily explain the multi-stage brecciation histories observed in breccias from a number of Apollo sites. GRIEVEet al. (1974) point out that rocks from terrestrial impact structures the size of Clearwater Lake, Quebec (ca. 40 km diameter) display textures indicating a similar degree of impact metamorphism and annealing relative to the diabasic and poikilitic breccias seen in the Apollo 16 samples. Water aided crystallization of the terrestrial breccias so the comparison is not totally valid. Huge impact structures are therefore not required to produce such textures. Judging from their contents of siderophile elements and metal, non-mare breccias contain l-3 times as much meteoritic material as do mare soils (MORGAN et al., 1974a). This is difficult to explain if the breccias are the product of one major impact event and is more consistent with the breccias being the final product of repeated smaller events. A number of authors have suggested that the Cayley formation of Apollo 16 is largely of local to regional origin. For example, H~~Rzet al. (1974) suggest this from a consideration of cratering mechanics. They also suggest that the Fra Mauro formation may consist of locally derived materials to a much larger extent than was previously thought. OBERBECKet al. (1973) suggest that the Cayley formation may have been emplaced mainly by the ejecta from secondary craters rather than by direct transport of basin ejecta for hundreds of kilometers. HEAD (1974) concludes that the contributions from Imbrium at the Apollo 16 site are minor and those from Orientale are negligible. He states that a history dominated by local cratering is consistent with the local geology, stratigraphy, and petrography. The relatively high degree of metamorphism of the Apollo 16 breccias is more consistent with a local origin from craters tens of km in diameter, as suggested by Head, than derivation from a distant mare basin. Head points out that the presence of partially melted breccias at the Apollo 16 site “argues strongly for their association with local deposits of large local impact events”. SNIFFLER et al. (1974), on the basis of terrestrial analogs, suggest that the Fra Mauro formation ejected from the Imbrium basin was hardly sampled by Cone Crater. They suggest that the bulk of material sampled at the Apollo 14 site has been reworked, thus partly explaining the complexity of the observed textures. They also advocate a local origin for the Cayley formation. SCHONFELD and MEYER(1973) point out that Cone Crater stratigraphy is consistent with a series of ejecta

K. Brltrl

1140

blankets from relatively local impacts. They postulate that the bulk of the Apollo 15 breccias are derived from a post-Imbrium blanket. The Apollo 17 breccias do not appear representative of a significantly higher grade of metamorphism or greater annealing times than the Apollo 16 breccias, so that it appears likely that they too are derived from relatively thin (and local) ejecta blankets. Most breccias from all Apollo sites may therefore be of local origin, and not a product of the major basinforming events. Clasts formed by major basin-forming events are likely to be present within the young breccias. In the unlikely event that breccias represent clasts in thick unconsolidated blankets from basin-forming events, then it is clear that the crystallization age measured for the breccia is not the age of the event. If the breccias were formed by local impacts, then the whole question of the significance of the crystallization ages of lunar breccias needs to be re-examined. Also, the conclusion that a given breccia can be correlated with a specific basin-forming event on the basis of its meteoritic component (e.g. MORGAN rt ~1., 1974b) needs to be re-evaluated. The range in ages of several hundred million years for breccias from all landing sites appears to be consistent with the present model. The fact that an age of 3.95 + 0.1 b.y. predominates for all Apollo sites (TERA et al., 1974) suggests that the heavy bombardment which dominated the first 600 m.y. of lunar history ceased about that time, as suggested by SCHONFELD and MEYER (1973). The fact that ages older than 4 b.y. are rare is consistent with the present model for breccias-namely a continuous high flux of objects forming craters of all sizes and setting and resetting the radiometric clocks. This flux decreased markedly at about 4 b.y. ago so that most breccias retain this age. If the breccias are a product of the major basin-forming events, it is unlikely that so many would show an age of about 4.0 b.y. It is likely that a large proportion of material sampled would have escaped resetting by these major events, even if the material was excavated by such an event. A wider spread of ages than has been observed, therefore, would be observed if most breccias were derived from a few basin-forming events. Attempts by some authors (e.g. SYHAEFFERand HUSAIN, 1974) to link a given breccia to a specific major basin-forming event on the basis of its crystallization age therefore appear to be lacking foundation. -I thank an anonymous reviewer for an fierce, thorough, and helpful review that conimproved the paper.

.~cktlowledyetnrrlr

especially siderably

REFERENCES AXO\ H. J. and GOLI)S’~L.IUJ. 1. (1972) Temperature--time relationships from lunar two phase metallic particles (14310. 14163, 14003). Enrfh Planer. Sci. Lett. 16, 439

447. GOLDSTEIN J. I. and AXON H. J. (1973) Composition.

structure and thermal history of metallic particles from 3 Apollo 16 soils, 65701. 6X501. and 63501. Plot. Fourrll Lurlar Ser. Cor$, Geochim. Cosmoc/~irn. Acta Suppl. 4, pp. 751-775. Pergamon Press. GOOLEY R. C., BRETT R. and WARN~K J. L. (lY73) Crystalhration history of metal particles in Apollo 16 rake samples. Proc. Fourtll Lwrur 2;. Cot!/:. Gochim. Co,~mochim. Acfa Suppl. 4, pp. 799. 810. Pergamon Press. Goor~v R. C., BRETT R.. WARNER J. and SMYTH J. R. (1074) A lunar rock of deep crustal origin: sample 76535. Guoc,him. Cosmochim. Acta 38, I329 1340. GREEN D. H. and RINGW~~C) A. E. (1972) Signiticance 01 Apollo 15 mare basalts and primitive green glasses in lunar petrogenesis. The Apollo 15 Lunar Samples, (editors J. W. Chamberlain and C. Watkins), pp. X2-X3. Lunar Science Institute. GREEN D. H.. RINGW(X)L) A. E., WAKE N. G. Hmet:asoh W. 0.. MAJOR A. and KISS E. (1971) Experimental petrology and petrogcnesis of Apollo 12 basalts. Proc. Second Lunar

Sci. Coil&.

Geochim.

Cosmochim.

Actrr

Suppl.

2,

pp. 601-~615. M.I.T. Press. GREEN D. H., RINGWWU

A. E.. WAIU. N. G. and HIBBEKSON W. 0. (1972) Experimental petrology and petrogenesis of Apollo 14 hasalts. Proc. Third Lunur %i. Coil/.. Geochim. Cosmoc~/fim .4crtr SuppI, 3, pp. 197 706 M.I.T. Press. GRIEVE R. A. F., PLAUr A. G. and DENU M. R. (iY74) Characteristics of impact melts in the lunar highlands. Lwlar Scir~c, V. pp. 290-292. Lunar Science Institute. HEAD J. W. (1974) Stratigraphy of the Descartes region (Apollo 16): implications for the origin of samples. Tht, Moor? in press. HEIKEN G. H.. BCTLI:R P.. JR., SIMONI>S C. H., P~III\;NI.\ W. C.. WARYFR J., SCI~MIT.I.H. H.. E%OGARII D. D. and PEARCE W. G. (1973) Preliminary data on boulders at Station 6. Apollo 17 landing site. NASA Tech. Memo. X-581 16. S6 pp. HEMINGWAY B. S., ROHI~ K. A. and WILSON W. H. (1973) Specific heats of lunar soils, basalt and breccias from the Apollo 14, 15 and I6 landing sites, between 90 and 350°K. Proc. Fourth Lurlar Sci. Co,!/:, Geoch~m. Cosmochim. Acra Suppl. 4, pp. 24X1- 24X7. Pergamon Press. HORAI K. and WINKI.~:R J. (1974) Thermal diffusivity of lunar rock sample 12OU2.X5. Lunur Science V. pp. 353 356. Lunar Science Institute. HBRZ F.. ORI-.RBE(.K V. R. and MORRISON R. H. (1971) Remote sensing of the Cayley Plains and Imbrium Basm deposits. Lunar Scirncc V. pp. 3.57 -3.59. Lunar Science Institute. HOWARD K. A., HEAL) J. W. and SWANN G. A. (lY72) Gology of Hadley Rille. Proc. Third Lumzr Sci. Couf.. Geechim. Cosmochim. Acftr Suppl. 3, pp. t-14. M.I.T. Press. HUMPIIRIES 1~. J., BIGGAK G. M. and O’HARA M. J. (lY72) Phase equilibria and origin of Apollo 15 basal@ etc. The Apollo 15 Lunar Sump/es. (editors J. W. Chamberlain and C. Watkins), pp. 103 107. Lunar Science Institute. JAEGER M. C (196X) Cooling and solidification of igneous rocks m basalts. In The Poldcruuart Trratisr OH Rock.~ of’ Basultic Composition, (editors H. H. Hess and A. Pol-

dervaart).

pp. 503 536. Intersciencc.

Lunar basalt flows and ejecta blankets LINDSLEYD. H., PAPIKEJ. J. and BENCEA. E. (1972) Pyroxferroite: breakdown at low pressure and high temperature. Lunar Science III, (editor C. Watkins), pp. 483485. Lunar Science Institute. LOFGRENG. D., LMNALD~~N C. H., WILLIAMSR. J. and MULLINS0. (1974) Experimentally reproduced textures and mineral chemistry of A-15 quartz basalts. Lunar Science V, pp. 458-46t!l Lunar Science Institute. MCCALLISTERR. H. and TAYLORL. A. (1972) The kinetics of ulviispinel reduction: synthetic study and applications to lunar rocks. Earth Planet. Sci. Let; 17, 357-364. MCGETCHINT. R.. SETTLEM. and HEAD J. W. (1974) Radial thickness variation in impact crater deposits. Earth Planet. Sci. Lett. in press. MORGANJ. W., GANAPATHYR., HIGUCHI H. and ANDERS E. (1974a) Meteoritic material on the moon. U.S.U.S.S.R. Conference on Cosmochemistry of the Moon and Planets, Moscow. June 48, 1974. MORGANJ. W., GANAPATHY R., HIGUCHI H., KRKHENBDHL U. and ANDERS E. (1974b) Lunar basins: tentative characterization of projectiles, from meteoritic elements in Apollo 17 boulders. Lunar Science V, pp. 526528. Lunar Science Institute. MURASET. and MCBIRNEYA. R. (1973) Properties of some common igneous rocks and their melts at high temperatures. Bull. Geol. Sot. Amer. 84, 3563-3592. OBERBECKV. R., HBRZ F., MORRISONR. H. and QUAIDE W. L. (1973) Emplacement of the Cayley Formation. NASA Tech. Mem. X-62, 302 pp. PROVOSTA. and B~TTINGAY. (1972) Rates of solidification of Apollo 11 basalt and Hawaiian tholeiite. Earth Planet. Sci. Left. 15, 325-337.

RINGW~C~D A. E. and ESSENEE. (1970) Petrogenesis of Apollo 11 basalts, internal constitution and origin of the moon. Proc. Apollo 11 Lunar Sci. Conf, Geochim. Cosmochim. Acta Suppl. 1, pp. 769-799. Pergamon Press. ROEDDERE. and WEIBLENP. W. (1970) Lunar petrology of silicate melt inclusions, Apollo 11 rocks. Proc. Apollo 11 Lunar Sci. Con& Geochim. Cosmochim. Acta Suppl. 1, pp. 801-837. Pergamon Press. SCHAEFFER0. A. and HUSAIN L. (1974) Chronology of lunar basin formation and ages of lunar anorthositic rocks. Lunar Science V, pp. 663-665. Lunar Science Institute. SCHONFELD E. and MEVERC., JR. (1973) The Old Imbrium hypothesis. Proc. Fourth Lunar Sci. Co& Geochim. Cosmochim. Acta Suppl. 4, pp. 125138. Pergamon Press. SIMONDSC. H., PHINNEYW. C., WARNERJ. L. and HEIKEN G. H. (1975) Thermal regimes in crater debris as deduced from the petrology of Apollo 17 Station 6 boulder and

G.C.A. 39/x--t

1141

rake samples. Lunar Science VI, pp. 747-749. Lunar Science Institute. STEWARTD. B. (1975) Apollonian metamorphic rocks-the products of prolonged subsolidus equilibration. Lunar Science VI, pp. 774-776. Lunar Science Institute. ST~FFLERD.. DENCEM. R.. ABADIANM. and GRAUP G. (1974) Ejecta formations and pre-impact stratigraphy of lunar and terrestrial craters: possible implications for the ancient lunar crust. Lunar Science V, pp. 746748. TAVLORL. A., MCCALLISTERR. H. and SARDI 0. (1973) Cooling histories of lunar rocks based on opaque mineral geothermometers. Proc. Fourth Lunar Sci. Co$, Geochim. Cosmochim. Acta Suppl. 4, pp. 819-828. Pergamon Press. TAYLORL. A. and WILLIAMSK. L. (1974) Formation history of lunar rocks: applications of experimental geochemistry of the opaque minerals. Proc. Fifth Lunar Science Conf, Geochim. Cosmochim. Acta Suppl. 5, pp. 585-596. TERA F., PAPANASTASSIOU D. A. and WASSERBURG G. J. (1974) The lunar time scale and a summary of isotopic evidence for a thermal lunar cataclysm. Lunar Science V, pp. 7922794. Lunar Science Institute. USSELMAN T. M. and PEARCEG. W. (1974) Grain growth of iron: implications for the thermal conditions in an ejecta blanket. Lunar Science V, pp. 809811. Lunar Science Institute. WARNERJ. L. (1971) Lunar crystalline rocks: petrology and geology. Proc. Second Lunar Sci. Conf, Geochim. Cosmochim. Acta Suppl. 2, pp. 4699480. M.I.T. Press. WARNERJ. L., SIMONDSC. H. and PHINNEYW. C. (1973) Apollo 16 rocks: classification and petrogenetic model. Proc. Fourth Lunar Sci. ConfY, Geochim. Cosmochim. Acta Suppl. 4, pp. 481-504. ”

WILLIAMSR. J. (1972) The lithitication and metamorohism of lunar breccias. ‘Earth Planet. Sci. Lett. 16. 25b256. SIMONDSet al. (1975) point out that clast-laden lunar melts chill rapidly to solidus temperatures. This effect was not taken into account in the present calculations due to lack of quantitative data. The effect should be slight below solidus temperatures and thus should not affect the present results to any great degree. For example, Simonds et al. describe a boulder (Apollo 17, Station 6) that represents a section about 15 m thick. Textures range from poikilitic to ophitic to fine grained sub-ophitic breccias. Simonds et al. believe that the section represents an almost complete cooling unit. This conclusion is consistent with the present results.