Qeochimica et Cosmochimica Acta, 19iS, Vol. 39. pp. 265 to 273. Pergamon Preee. Printed in Northern Ireland
The life and times of Bii Bertha: lunar breccia 14321 A. R. DUNCAN,* R. A. F. GRIEVEDand D. F. WEILL Center for Volcanology, University of Oregon, Eugene, Oregon 97403, U.S.A. (Receiveb 9 July 19’73; accepted in revised form 12 April 1974) Abstr&--The lithic units of polymiot breccia 14321 (Big Bertha) have been grouped according to composition, texture, degree of metamorphism, and additional criteria based on a systematic study of the interrelationships of all clast-matrix pairs. From this information it has been possible to reconstruct the assembly and metamorphic history of this breccia. The earliest formed fragmental component of 14321 (microbreccia-1) is dominated by KREEP-rich norite, extruded and subsequently brecciated and lithified in an ejects blanket at approximately 1OOO’Cin the general region of Mare Imbrium after the Serenitatis impact but prior to the Imbrium impact. This early microbreccia component and lesser amounts of mare-type basalt, microgranite, rhyolite glass, anorthosite and olivine microbreccia were assembled at the Apollo 14 site as part of the Fra Mauro ejecta blanket from the Imbrium impact. The resulting microbreccia-3 incorporates all the lithic types above and accretionary lapilli structures (microbreccia-2) in a dark matrix annealed at approximately 700%. A later impact on the Fra Mauro excavated and mutually abraded microbreccia-3 and a local, 14321-type, basalt which were assembled into polymict breccia 14321. Final placement of 14321 at its sampling location was accomplished during the minor Cone Crater impact event. INTRODUCTION
THE PETROORAPHIC character, minerology and chemical composition of 14321 have been discussed by GRIEVEet d. (1976), W~NER and HEXEN (submitted for publication) and MORGANet al. (1976). What remains to be considered here is the evidence bearing on the assembly order of the components, the nature of the source rocks from which the components were derived, the assembly processes, and their possible relation to larger-scale events in lunar history. A summary of the assembly sequence for 14321,184 is shown in Fig. 1. The ‘flow chart’ illustrates the relative ages of the components and indicates the immediate source material for the elastic lithic units. The relative ages have been assigned to the various components according to the simplest of criteria, viz. an older component is systematically incorporated in a younger one. It should be stressed that the relative ages discussed here and depicted diagrammatically in Fig. 1 are only relative assembly or lithification ages of the fragmental lithic units as opposed to the other possible meanings of ‘age’ when applied to geological systems. The resolution of this simple dating technique ia inherently poor. For example, we cannot say anything about the relative assembly ages of microbreccia-1 and the olivine microbreccia (cf. Fig. 1) because one is not observed incorporated in the other. On the other hand, they are both systematically incorporated within microbreocia-3 and they are clearly older than the latter. In spite of its obvious shortcomings this simple breccia ‘stratigraphy’ is a very valuable aid in unraveling the complex history of lunar breccias. * Present address: Department of Geochemistry, University of Cape Town, Rondebosch, C. P., S. Africa. t Present address: Gravity Division, Earth Physics Branch, Department of Energy, Mines and Resources, Ottawa KIA-OEC, Ontario, Canada. 265
A. R. DUNCAN, R. A. F. GRIEVE and D. F. WEILL
Fig. 1. Flow chart illustrating the various components of breccia 14321, their relative ages, and the assembly history of the breccia. Reasons for grouping the components arc discussed in the text. ASSEMBLY HISTORY OF 14321 Group 1 components
The oldest recognizable fragmental lithic unit in 14321 is the noritic microbreccia-1 described by GRIEVE et ~2. (1975). Crystalline norite fragments are also associated with microbreccia-1. The mineral assemblages of microbreccia-1 and the crystalline norite fragments are identical, and it is likely that they represent different degrees of comminution of an older noritic (KREEP-rich) basalt. These noritic clasts are always well rounded and are always found incorporated in microbreccia-2 and microbreccia-3, along with lesser amounts of microgranite, mare-basalt, rhyolite glass, anorthosite, and olivine-rich microbreccia. These group 1 lithic fragments plus a volumetrically important fraction of unresolved fine-grained dark matrix and individual crystal fragments have been assembled into microbreccia-2 and microbreccia-3. Although we cannot differentiate these components chronologically, it is possible to divide them into two sub-groups according to the degree of thermal recrystallization. Microbreccia-1, the norite, and the microgranite all show granoblastic or poikiloblastic textures typical of relatively high grade thermal metamorphism (sub-group 1A). The other components (sub-group 1B) have largely preserved their original igneous or fragmental textures with comparatively minor crystallization. Group 1 materials are dominantly KREEP-rich in composition and noritic in mineralogy. Group 2 components Microbreccia-2 and microbreccia-3 both contain group 1 lithic clasts and a variety of mineral fragments set in very fine-grained dark matrix. The dark matrix major element compositions of the two microbreccias are very similar and their suites of crystal and lithic clasts are identical (GRIEVE et al., 1975). Macroscopically
The life and times of Big Bertha: lunar breccia 14321
(Fig. 1 in DUNCANet al., 1976) microbreccia-2 is seen as dark, rounded clasts incorporated within microbreccia-3. The microbreccia-2 clasts are seen in thin section to be outlined by trails’of crystal fragments disposed concentrically to a central core of a group 1 clast or a large crystal clast (Fig. If in GRIEVEet al., 1975). We consider it most likely that microbreccia-2 represents accretionary lapilli structures formed during the same impact event that gave rise to microbreccia-3. The fine materials thrown out as impact ejecta accreted around large fragments during transport in a hot cloud to form the lapilli structures, while the entire ejects blanket formed microbreccia-3 during post-depositional lithification. Although microbreccia-2 is older than microbreccia-3 in terms of the textural ‘systematic incorporation’ criterion, the assembly ages of the two are the same on the geologic time scale. Microbreccia-3 is KREEP-rich. According to compositional mixing models, average microbreccia-3 can be closely approximated by a mix of 86 per cent KREEP-rich component, 7 per cent mare basalt, 4 per cent anorthosite and 3 per cent chondritic materials (see DUNCANet al., 1975,.for definition of components and details of calculations). As can be seen from Fig. 1 this compositional mix, with the exception of the meteoritic component, can be derived from the group 1 lithic materials. The addition of meteoritic material could have been brought about either cumulatively by a series of small impacts on (i.e. gardening of) a regolith predominantly composed of group 1 material, or by a single, large impact without a regolith stage. Some of the large single crystal fragments found in microbreccias 2 and 3 are compositionally similar to their counterparts in the group 1 lithic fragments but must have been derived from rocks which were much coarser grained than any we have observed. The presence of pleonaste spine1fragments of distinctly different compositions than that found in any of the group 1 lithic fragments suggests that a small fraction of microbreccia-3 was derived from material not represented in group 1. The group 2 basalt (designated as 14321~type basalt in DUNCANet al., 1976) is a compositionally distinct basalt type which comprises a significant proportion of 14321,184, including one particularly large clast of 20g. The textural variation of the 14321-type basalt clasts (Fig. la-d in GRIEVE et al., 1975) and their relatively constant composition suggest a related set of thin lava flows or shallow sills and dikes as a source for this component. Mixing model calculations suggest that the group 1 basalts are compositionally distinct and contain lower concentrations of the KREEP characteristic elements than the 14321-type basalts. The anorthosite component of group 2 was not observed in thin section but was recognized as small clasts in the bulk portion of 14321. Also, photographs of 14321 indicate that large (up t,o 10 cm) clasts of anorthosite are present in other portions of the rock. It is evident from the microprobe study (GRIEVEet al., 1976), and compositional mixing models (DUNCAKet al., 1975) that the light matrix which now holds 14321 together was produced by mutual abrasion of the group 2 clasts (14321-type basalt, microbreccia-3, and minor anorthosite) during the final assembly process, with litt,le else added. If they resided in the lunar regolith prior to the final assembly of 14321, the group 2 components were not significantly diluted as a result. The major compositional evolution of 14321 during its assembly history appears to have involved successive dilution of an early KREEP-rich noritic component.
A. R. DUNCAN, R. A. F. GRIEVEand D. F.
THERMAL HISTORY OF 14321
There is considerable evidence that some of the lithic components of 14321 have been subjected to high temperatures. The degree of metamorphism as judged by visible textural or recrystallization criteria is consistent for each type of component, and it decreases with decreasing assembly age, i.e. the effect of metamorphic overprinting appears to be minimal. This circumstance allows us to attempt a reconstruction of the thermal history of 14321. The thermal history we speak of presumably took place while the material was part of an impact-produced debris cloud or an ejecta blanket deposited from such a cloud. It is in this context that we somewhat loosely speak of the thermal metamorphism of lunar breccias. Perhaps it would be more accurate petrologically to speak of the autometamorphism of lunar ‘pyroclastic’ rocks. The degree of autometamorphism will be a function of time as well as temperature and consequently will depend on the total thickness cf the ejecta blanket and position within the deposit. In what follows it is not possible to consider the probable temperature-time curves for the fragmental components of 14321 except in a qualitative sense. First thermal metamorphic event We observe two distinct episodes of thermal metamorphism in 14321. The first affected only the group IA components. In addition to the obvious recrystallization textures seen in these components, the pyroxenes show evidence of subsolidus re-equilibration. Orthopyroxenes and Ca-pyroxenes have uniform compositions in individual clasts of microbreccia-1 and the related crystalline norite, although both pyroxenes have a wide range of composition from clast to clast. This suggests equilibration within clasts, and we have used the Fe-Mg distribution to estimate the temperature range of metamorphism for the group 1A components. Figure 2a shows coexisting pyroxene pairs from nine microbreccia-1 and norite clasts. The frequency distribution of In K[K = (Fe/Mg)“p”/(Fe/Mg)cpx; mole ratio] for these pairs are shown in Fig. 3a. An average temperature of 1050% is arrived at using the calibration of MCCALLUM(1968). This may be compared to an estimate of 850-1OOO’C for the metamorphism of noritic clasts in 14321 and other Apollo 14 breccias by ANDERSON et al. (1972) using the Fe-Mg distribution between ilmenite and pyroxene. This early, high temperature metamorphic event affected only the dominantly noritic, KREEP-rich components, and predates the assembly of microbreccias 2 and 3. Second thermal metamorphic event The mineral clasts, dark matrix and 1B lithic components of microbreccias 2 and 3 lack the well-developed recrystallization textures of the 1A components but contain distinct evidence of a less severe thermal event that either coincided with or postdated the assembly of microbreccia-3. Textural evidence for this metamorphism includes reaction rims between the dark matrix and plagioclase and pyroxene clasts, exsolution lamellae in pyroxene clasts, a well sintered, non-porous dark matrix and devitrification textures in the rhyolitic and plagioclase glass fragments (see Fig. lk,m in GRIEVE et al., 1975). Representative host and lamellae pyroxene compositions are plotted in Fig. 2b, and the frequency distribution of
The life and times of Big Bertha: lunar broccia 14321
Mol. % Fig. 2 (a). Composition of coexisting pyroxene pairs in microbreccia-I and micronorite clasts. Dashed line is trace of miscibility gap defined by exsolved pyroxenes shown in b. (b) Composition of representative exsolved pyroxene crystal clasts in microbreccia-2 and -3. e = Host, A = exsolved lamellae, n = approximate bulk composition of host-lamellae pair.
Fig. 3. Fe-Mg distribution between orthopyroxenes and high-Ca pyroxenes [ILD = \Fe/J1g)oPX/ (Fe/Mg)CD’; mole ratio]. (a) Individual pyroxene grains in microbreocia-1 and micronorite. (b) Exsolution lamellae and host pyroxene clast in microkeccia-2 and -3. See text for definition of Ko and explanation of temperatures.
A. R. DUNCAN, R. A. F. GRIEVEand D. F.
In K for 19 pairs is shown in Fig. 3b. The distribution is not as tightly grouped as for the coexisting pyroxene grains in the group 1A material. The scatter is partly due to analytical errors in obtaining the composition of the t#hinlamellae, but it is also an indication of incompIete equilibration. Nevertheless, the distribution clearly indicates an overall larger value of K and a lower average temperature of approximately 700°C. This is in agreement with the estimates of >66O”C by SIMONDS(1973) for the sintering temperature of Fra Mauro breccias. The pyroxene grains of the 1A components show no exsolution features (>l pm), and yet they were subjected to whatever temperature regime affected microbreccia-3 in its entirety. The 1A pyroxene compositions (Fig. 2a) lie close to the trend of the hos%lamellae pairs of Fig. 2b, implying a relatively small supersaturat,ion to drive the exsolution mechanism in the 1A pyroxenes during the second period of metamorphism. In contrast, the bulk compositions of the pyroxenes that developed exsolution lamellae during this lower temperature event are all located well within t.he inferred immiscibility field (Fig. 2b). Possible additional thermal events The 14321-type basalt and the light matrix do not show any textural or mineralogical evidence of metamorphism. The mineral phases of the basalt and light matrix show no sub-solidus equilibration features and have the unequilibrated compositions common to rapidly cooled lunar igneous rocks. The light matrix is partially sintered, giving 14321 its overall cohesiveness, but it still has appreciable pore space. It is possible that the assembled 14321 has undergone intense heating of relatively short duration which would not be recorded obviously texturally or mineralogically. Such short-lived temperature increases might explain the stable thermoremanent magnetization found to be directionally consistent in three distinct fragments of 14321 by PEARCEet al. (1972). The presence of this TRM direction implies that 14321 was heated close to the Curie point (770°C) after its assembly, but the necessary time is very short. IMPACTHISTORY OF 14321 According to the accumulated evidence on the origin of lunar fragmental rocks in general it is reasonable to assume that the history of 14321 is directly related t,o one or more impact events. We stress at the outset that this is a ‘model’ history which can only hope to express the minimum number of events and relate them stratigraphically in somewhat simplistic fashion, while it must be admitted that many minor and/or distant impacts may well have left no decipherable record in 14321. The first recognizable impact (no. 1) occurred on a terrain of dominantly KREEP-rich composition and resulted in the formation of microbreccia-1 in the high temperature portions of the resulting ejecta blanket. Less cornminuted norite and minor amounts of microgranite were also included and thermally metamorphosed in the same ejecta blanket. Clasts of olivine microbreccia suggest another possible early impact (no. 2). The olivine microbreccia is not thermally annealed to the extent of the 1A components, and the lack of any signs of mutual inclusion between it and microbreccia-1 also points to a separate impact event. The relative ages of
The life and times of Big Bertha: lunar breccia 14321
these two impacts are not known, but both are necessarily older than impact no. 3 which formed microbreccias 2 and 3. We envisage the latter impact as having occurred on terrain containing mare basalt and the other 1B components, overlain by the ejecta blanket from impact no. 1 and minor fragments from impact no. 2, probably in a regolith. Impact no. 3 resulted in the formation of microbreccias 2 and 3, with microbreccia 2 formed as accretionary lapilli. Breccia 14321 as we now see it was assembled after impact no. 4 which occurred in terrain rich in 14321type basalt (along with minor anorthosite) and probably overlain by the no. 3 impact ejecta blanket. The light matrix material was developed in the impact and during transport of the no. 4 impact ejecta blanket. Final annealing and sintering of 14321 into a cohesive rock was comparatively short-lived or took place at relatively low temperature. Which, if any, of the impacts discussed here excavated the Imbrium basin and deposited the Fra Mauro formation as its ejecta blanket? In the area of the Apollo 14 landing site the Fra Mauro formation has well preserved topographic alignments radial to the Imbrium basin which are considered to be depositional features of the Imbrium ejecta blanket (SWANNet al., 1971). The preservation of these features strongly suggests that no other major ejecta blanket overlies the Imbrium blanket at the Apollo 14 site. The thermally metamorphosed character of the breccias produced by impacts 1 and 3 makes it likely that their respective ejecta blankets would be large enough to be readily discernible if they had been deposited over the Imbrium ejecta. Consequently, we believe that the Imbrium impact must be impact no. 3 or younger. In view of the common occurrence of dark matrix microbreccias similar to microbreccia-3 at the Apollo 14 location (WILSHIRE and JACKSON, 1972; CHAOet al., 1972) it seems highly probable that they were all formed in a common, large event,. We therefore consider impact no. 3 to correspond to the Imbrium event. Although the formation of the light matrix and the final annealing and sintering of 14321 (impact no. 4) were relatively low temperature or short-lived processes, it is unlikely that the Cone Crater impact was large enough to account for it. Impact no. 4 must have occurred on the Fra Mauro formation, but prior to the excavation of 14321 (Cone Crater impact no. 5). COMPOSITIONAL CUACTER AND HISTORYOF TEE SOURCETERRAIN The overall composition of 14321 and its assembly history imply that it was assembled on a KREEP-rich terrain. The relatively KREEP-poor nature of Serenitatis and Crisium ejecta, indicated by orbital gamma-ray data (METZUER et al., 1972) and returned samples from Apollo 17 and Luna 20, does not reinforce the origin of Spollo 14 multistage breccias by re-brecciation of such material as suggested by PET (1971) and WILSHIREand JACKSON(1972). We consider it more probable that the Imbrium-Procellarum area was partially flooded with KREEPrich basalts after the Serenitatis impact. The relative dates of extrusion (as opposed to time of incorporation in 14321) of mare-type Apollo 14 basalts (14053,14072), 14321-type basalts and KREEP-rich basalts cannot be determined, but all three types were extruded prior to 3.88 AE (P~PANA~TASSIOU and WASSERBVRG,1971; YOR6 et al., 1972). Impact events after the extrusion of the basalts incorporated the various basalts and fragments of the surrounding and/or underlying highland-type terrain into breccias such as 14321.
A. R. DUNCAN, R. A. F. GRIEVE and D. F. WEILL
We suggest the following partial schematic history of this area of the Noon based on our interpretation of 14321: 1. Serenitatis impact ; deposition of ejecta blanket over Imbrium area. 2. Extrusion of KREEP-rich basalts, mainly in the south Imbrium-north Procellarum regions ; extrusion of lesser amounts of Apollo 14 type mare basalt. 3. Impacts on south Imbrium region to form early (group 1) breccia stages seen in 14321. 4. Imbrium impact forming Fra Mauro ejecta. Inhomogeneous distribution of KREEP-rich ejecta due to inhomogeneous distribution of KREEP-rich basalts in t,he impact area. Formation of microbreccias 2, 3. 5. Extrusion of 14321-type basalt and additional Apollo 14-type mare basalt on the Fra Mauro formation and adjacent areas. 6. Impact events, including no. 4, recorded in 14321, on Fra Nauro formation; final assembly of 14321. 7. Excavation of 14321 in Cone Crater (no. 5) impact. There are a number of similarities between our proposed history and the ‘Old Imbrium’ hypothesis of SCHONFELD and &~EYER (1973), but our study of 14321 does not support the concept that its formation is entirely post-Imbrium and due entirely to local impacts. The high metamorphic grades recorded in 14321 and other Apollo 14 breccias (WARNER, 1972) suggest large ejecta blankets which would have modified the topographical alignments radial to Imbrium had they been post-Imbrium. A single large event common to all Apollo 14 breccias accounts for the dark microbreccia clasts common to almost all the samples collected. Explanations involving a series of small, localized impacts are less direct. Analysis and interpretation of a complex rock like 14321 is rewarded with few categorical conclusions, but we believe that the elucidation of its compositional character and assembly history leads to a very probable evolutionary picture for this area of the Moon. Comparable work on additional breccias from the Apollo 14 and other sites is now needed to test the validity of our approach and conclusions. Acknowledgments-We thank our colleagues in the 14321 consortium. Without their careful work any attempt at synthesis would have been futile. We also thank A. REID, J. WARNER, and D. YORK for profitable discussions and incisive critical reviews. Our work was supported by NASA grants NGL 38-003-20, 38-003-22 and 38-003-24. REFERENCES ANDERSONA. T., BRAZIUNAST. F., JACOBYJ. and SMITHJ. V. (1972) Thermal and mechanical history of breccias 14306, 14063, 14270 and 14321. Proc. 3rd Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 3, Vol. 1, pp. 819-835. M.I.T. Press. CRAOE. C., MINEIN J. A. and BEST J. B. (1972) Apollo 14 breccias: general characteristicsand classification. Proo. 3rd Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 3, Vol. 1, pp. 645659. M.I.T. Press. DUNCANA. R., MCKAY S. M., STOESIZR J. W., LINDSTROM M. M, LINDSTROJX D. J., FRUCETER J. S. and GOLESG. G. (1975) Lunar polymict breccia 14321: a compositional study of its principal components. Geochim. Coemochim. Acta 39, 247-260. GRIEVER. A., MCKAY G. A., SMITE H. D. and WEILL D. F. (1975) Lunar polymict breccia 14321: a petrographic study. Geoohim. Cosmochim. Acta 39, 229-245. MCCALLUMI. S. (1968) Equilibrium relationships among co-existing minerals in the Stillwater Complex, Montana. Ph.D. thesis, University of Chicago.
The life and times of Big Bertha: lunar breccia 14321
METZGERA. E., TROMBKAJ. I., PETERSONL. E., REEDY R. C. and ARNOLDJ. R. (1972) A first look at the lunar orbital gamma-ray data. PTOC.3rd Lunar Sci. Conj., Geochim.Cosmochim. Acta Swppl. 3, Vol. 3, frontispiece. M.I.T. Press. MORUANJ. W., GANAPAT~ R., K.R&EENB~ V. and ANDERS E. (1975) Meteoritic trace elements in lunar rock 14321, 184. Geochim. Cosmochim. Acta 39, 261-264. PAPANASTASSIOU D. A. and WASSERBTJRU G. J. (1971) Rb-Sr ages of igneous rocks from the Apollo 14 mission and the age of the Fra Mauro formation. Earth Planet. Sci. Lett. 12, 36-48. PEARCEG. W., STRANGWAYD. W. and GOSE W. A. (1972) Romanent magnetization of the lunar surface. Proc. 3rd Lunar Sci. Con.., Geochim. Cosmochim.Acta Suppl. 3, Vol. 3, pp. 2449-2464. M.I.T. Press. PET (1971) Preliminary examination of lunar samples from Apollo 14. Science 179, 681-693. SCHONFELD E. and MEYER C., JR. (1973) The Old Imbrium hypothesis. Proc. 4th Lunar Sci. Conj., Geochim. Cosmochirn. Acta Suppl. 4, pp. 125-138. Pergamon Press. SIMONDSC. H. (1973) Sintering and hot pressing of Fra Mauro composition glass and the lithification of lunar breccias. Amer. J. Sci. 273,428-439. SWA~‘NG. A., BAILEY N. G., BATSONR. M., EGGLETONR. E., HAIT M. H., HOLTH. E., LARSON K. G., MCEWEN M. C., MITCHELLE. D., SCHABERG. G., SCHAFERJ. P., S~EPARDA. B., SUTTONR. L., TRASK N. J., ULRICHG. E., WILSHIREH. B. and WOLFE E. W. (1971) Preliminary geologic investigations of the Apollo 14 landing site. U.S. Geol. Surv. Ivateragency Report 29. WARNERJ. L. (1972) Metamorphism of Apollo 14 breccias. Proc. 3rd Lunar Sci. Conj., Geochim. Cosmochim.Acta Suppl. 9, Vol. 1, pp. 623-643. M.I.T. Press. WARNER J. L. and HEIKEN G. Metamorphism and surface mapping of lunar sample 14321. Submitted to Geochim. Cosmochim. Acta. WILSHIRE H. G. and JACKSONE. D. (1972) Petrology and stratigraphy of the Fra Mauro formation at the Apollo 14 site. U.S. Geel. Sure. Prof. Pap.-785. YORE D., KENYON W. J. and DOD R. J. (1972) 4oAr-aeArages of Apollo 14 and 15 samples. Proc. 3rd Lunar Sci. Conf., Geoohim. Cosmochim. Acta Suppl. 3, pp. 1613-1622. M.I.T. Press.