A.L. Albee

Ages, Irradiation History, and Chemical Composition of Lunar Rocks from the Sea of Tranquillity

The 87Rb-87Sr internal isochrons for five rocks yield an age of 3.65 �0.05 x 109 years which presumably dates the formation of the Sea of Tranquillity. Potassium-argon ages are consistent with this result. The soil has a model age of 4.5 x109 years, which is best regarded as the time of initial differentiation of the lunar crust. A peculiar rock fragment from the soil gave a model age of 4.44 x 109 years. Relative abundances of alkalis do not suggest differential volatilization. The irradiation history of lunar rocks is inferred from isotopic measurements of gadolinium, vanadium, and cosmogenic rare gases. Spallation xenon spectra exhibit a high and variable 131Xe/126Xe ratio. No evidence for 129I was found. The isotopic composition of solar-wind xenon is distinct from that of the atmosphere and of the average for carbonaceous chondrites, but the krypton composition appears similar to average carbonaceous chondrite krypton.

Petrography of isotopically-dated clasts in the Kapoeta howardite and petrologic constraints on the evolution of its parent body

Detailed mineralogic and petrographic data are presented for four isotopically-dated basaltic rock fragments separated from the howardite Kapoeta. Clasts C and ρ have been dated at ~4.55 AE and ~ 4.60 AE respectively, and Clast ρ contains ^(244)Pu and ^(129)I decay products. These are both igneous rocks that preserve all the features of their original crystallization from a melt. They thus provide good evidence that the Kapoeta parent body produced basaltic magmas shortly after its formation (< 100 m.y.). Clast A has yielded a Rb-Sr age of ~ 3.89 AE and a similar ^(40)Ar/^(39)Ar age. This sample is extensively recrystallized, and we interpret the ages as a time of recrystallization, and not the time of original crystallization from a melt. Clast B has yielded a Rb-Sr age of ~ 3.63 AE, and an ^(40)Ar/^(39)Ar age of ⪆ 4.50 AE. This sample is moderately recrystallized, and the Rb-Sr age probably indicates a time of recrystallization, whereas the ^(40)Ar/^(39)Ar age more closely approaches the time of crystallization from a melt. Thus, there is no clearcut evidence for ‘young’ magmatism on the Kapoeta parent body. Kapoeta is a ‘regolith’ meteorite, and mineral-chemical and petrographic data were obtained for numerous other rock and mineral fragments in order to characterize the surface and near-surface materials on its parent body. Rock clasts can be grouped into two broad lithologic types on the basis of modal mineralogy—basaltic (pyroxene- and plagioclase-bearing) and pyroxenitic (pyroxenebearing). Variations in the compositions of pyroxenes in rock and mineral clasts are similar to those in terrestrial mafic plutons such as the Skaergaard, and indicate the existence of a continuous range in rock compositions from Mg-rich orthopyroxenites to very iron-rich basalts. The FeO and MnO contents of all pyroxenes in Kapoeta fall near a line with FeO/MnO ~ 35, suggesting that the source rocks are fundamentally related. We interpret these observations to indicate that the Kapoeta meteorite represents the comminuted remains of differentiated igneous complexes together with ‘primary’ undifferentiated basaltic rocks. The presently available isotopic data are compatible with the interpretation that this magmatism is related to primary differentiation of the Kapoeta parent body. In addition, our observations preclude the interpretation that the Kapoeta meteorite is a simple mixture of eucrites and diogenites. The FeO/MnO value in lunar pyroxenes (~60) is distinct from that of the pyroxenes in Kapoeta. Anorthositic rocks were not observed in Kapoeta, suggesting that plagioclase was not important in the evolution of the Kapoeta parent body, in contrast to the Moon. Both objects appear to have originated in chemically-distinct portions of the solar system, and to have undergone differentiation on different time scales involving differing materials.

PETROLOGIC AND MINERALOGIC INVESTIGATION OF SOME CRYSTALLINE ROCKS RETURNED BY THE APOLLO 14 MISSION.

Abstract Apollo 14 crystalline rocks (14053 and 14310) and crystalline rock fragments (14001,7,1; 14001,7,3; 14073; 14167,8,1 and 14321,191,X-1) on which Rb/Sr, 40 Ar- 39 Ar, or cosmic ray exposure ages have been determined by our colleagues were studied with the electron microprobe and the petrographic microscope. Rock samples 14053 and 14310 are mineralogically and petrologically distinct from each other. On the basis of mineralogic and petrologic characteristics all of the fragments, except 14001,7,1, are correlative with rock 14310. Sample 14073 is an orthopyroxene basalt with chemical and mineralogic affinities to ‘KREEP’, the ‘magic’ and ‘cryptic’ components. Fragment 14001,7,1 is very similar to Luny Rock I.

Constrained least-squares analysis of petrologic problems with an application to lunar sample 12040.

Many petrologic problems, which may be expressed as a set of linear equations, have been solved by least-squares analysis. In many cases insufficient attention has been paid to the physical conditions of the model resulting in incorrect application of the method. This paper presents a systematic treatment of the application of least-squares analysis to petrologic problems including the direct utilization of physical constraints and weighting factors in the problem, and the assessment of uncertainties in the solution. As an example, least-squares analysis is used to examine, in detail, the mass balance equations for lunar rock 12040 and to determine the consistency of the available analytical data.

Mineralogy, petrology, and chemistry of a Luna 16 basaltic fragment, sample B-1

Abstract Luna 16 sample B-1 was the largest fragment (62 mg) obtained in the sample exchange with the USSR. Petrologic, mineralogic, and chemical investigations have been made on this fragment in conjunction with Rb-Sr and40Ar/39Ar investigations by our colleagues. Sample B-1 is a fine-grained ophitic basalt but is distinguished from the Apollo samples by containing a single pyroxene, predominantly pigeonitic, an ilmenite content (7%) intermediate to that of the Apollo 11 and 12 samples, and subequal amounts of pyroxene (50%) and plagioclase (40%). Chemically it is distinguished by a high Sr content (437 ppm) and a high K/U value (4700). The K-content (1396 ppm) is higher than that of Luna 16 soil sample A-2.

Uranium-bearing minerals of lunar rock 12013

The U distribution in rock 12013 was studied by fission track and elemental mapping techniques. Major U-bearing phases are whitlockite, apatite, zircon, and phase β, which is a ZrTi mineral rich in Fe, Nb, Y, REE and containing up to 3.6% UO2, 4.7% ThO2, and 4.2% PbO. Calculated microprobe ages for phase β average 4.0 AE and are in reasonable agreement with RbSr and KAr ages. Phase β plays a significant role in the UThPb systematics of rock 12013 and may play a similar role in the model ages of lunar soil.

Comparative petrology of Apollo 16 sample 68415 and Apollo 14 samples 14276 and 14310

Petrographic and electron microprobe studies of Apollo 16 igneous rock 68415 and Apollo 14 rocks 14276 and 14310 show that all three samples differ from the mare basalts and are characterized by plagioclase as the first liquidus phase and by the abundance of plagioclase which is in part cumulate in origin. Major and minor element abundances and isotopic data prohibit the derivation of rocks like any of these samples from one another by magmatic fractionation during their crystallization. They could have originated by partial melting of an old, more Al-rich source material without isotopic equilibration with the residuum, by complete melting of three independent sources, or by contamination with old radiogenic material. The existence of such feldspathic basalts indicates that the generation of Al-rich magmas may have been an important and widespread lunar process.