Discover Space

Wood, John A.

doi: 10.1007/bf00569666pmid: N/A

Wolfe, E. W.

doi: 10.1007/bf00569667pmid: N/A

Boulder 1 at Station 2 is one of three boulders sampled by Apollo 17 at the base of the South Massif, which rises 2.3 km above the floor of a linear valley interpreted as a graben formed by deformation related to the southern Serenitatis impact. The boulders probably rolled from the upper part of the massif after emplacement of the light mantle. Orbital gravity data and photogeologic reinterpretation suggest that the Apollo 17 area is located approximately on the third ring of the southern Serenitatis basin, approximately 1.25 times larger than the analogous but fresher Orientale basin structure. The massif exposures are interpreted to represent the upper part of thick ejecta deposited by the southern Serenitatis impact near the rim of the transient cavity. Basin ring structure and the radial grabens that give the massifs definition were imposed on this ejecta at a slightly later stage in the basin-forming process. There is no clear-cut compositional, textural, or photogeologic evidence that Imbrium ejecta was collected at the Apollo 17 site.

Marvin, Ursula B.

doi: 10.1007/bf00569668pmid: N/A

The external morphologies of Boulder 1, Station 2, and of the four samples taken from it by the Apollo 17 crew, are briefly described. The boulder is a polymict breccia, containing the following principal materials as clasts: gray competent breccias (GCBx), black competent breccias (BCBx), anorthositic breccias (AnBx), pigeonite basalt (PB), coarse norite (CN). All are enclosed in a matrix of light-colored friable breccia (LFBx).

Ryder, Graham; Stoeser, Douglas B.; Marvin, Ursula B.; Bower, Janice F.; Wood, John A.

doi: 10.1007/bf00569669pmid: N/A

Boulder 1, Station 2, Apollo 17 is a stratified boulder containing dark clasts and dark-rimmed light clasts set in a light-gray friable matrix. The gray to black clasts (GCBx and BCBx) are multigenerational, competent, high-grade metamorphic, and partially melted breccias. They contain a diverse suite of lithic clasts which are mainly ANT varieties, but include granites, basaltic-textured olivine basalts, troctolitic and spinel troctolitic basalts, and unusual lithologies such as KREEP norite, ilmenite (KREEP) microgabbro, and the Civet Cat norite, which is believed to be a plutonic differentiate. The GCBxs and BCBxs are variable in composition, averaging a moderately KREEPy olivine norite. The matrix consists of mineral fragments derived from the observed lithologies plus variable amounts of a component, unobserved as a clast-type, that approximates a KREEP basalt in composition, as well as mineral fragments of unknown derivation. The high-temperature GCBxs cooled substantially before their incorporation into the friable matrix of Boulder 1.The light friable matrix (LFBx) is texturally distinct from the competent breccia clasts and, apart from the abundant ANT clasts, contains clasts of a KREEPy basalt that is not observed in the competent breccias. The LFBx lacks such lithologies as the granites and the Civet Cat norite observed in the competent breccias and in detail is a distinct chemical as well as textural entity. We interpret the LFBx matrix as Serenitatis ejecta deposited in the South Massif, and the GCBx clasts as remnants of an ejecta blanket produced by an earlier impact. The source terrain for the Serenitatis impact consisted of the competent breccias, crustal ANT lithologies, and the KREEPy basalts, attesting to substantial lunar activity prior to the impact. The age of the older breccias suggests that the Serenitatis event is younger than 4.01±0.03 b.y.

Blanchard, Douglas P.; Haskin, Larry A.; Jacobs, Jeffrey W.; Brannon, Joyce C.; Korotev, Randy L.

doi: 10.1007/bf00569670pmid: N/A

Twenty-seven samples of matrix and clast materials from Boulder 1 at Station 2, Apollo 17 have been analyzed for major and trace elements as part of the study of this boulder by Consortium Indomitabile. Both unusual and common types of material have been characterized.Gray and black competent breccia (GCBx and BCBx) and anorthositic breccia (AnBx) have compositions which are common at the Apollo 17 site and were common at the site of boulder formation. Light friable breccias (LFBx) have compositions which are not found at the Apollo 17 site other than in the boulder. Pigeonite basalt is a new type of lunar rock and has characteristics that would be expected of a highland volcanic rock. It is associated with LFBx material, and like LFBx material it is exotic to the Apollo 17 site. Coarse norite is an old primitive rock which is no longer (if ever) found as millimeter fragments at the Apollo 17 site. It was, however, present as millimeter fragments associated with GCBx and BCBx materials at the site and time of boulder formation. Therefore the boulder-forming process combined materials from at least two different localities or vertical strata; at least one of these (LFBx) has not been previously sampled and analyzed.

Morgan, John W.; Higuchi, H.; Anders, Edward

doi: 10.1007/bf00569671pmid: N/A

Sixteen samples of Boulder 1 from Station 2 at the Apollo 17 site were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, Ir, Ni, Rb, Re, Sb, Te, Tl, U, and Zn. Two clast samples contam no meteoritic material and appear to consist of relatively pristine igneous rocks: an unusual, KREEP-rich pigeonite basalt of very high Ge content, and an alkali-poor coarse norite. Nine grey or black breccia samples contain a unique, Group 3 meteoritic component of Ir/Au ratio 0.65–0.82, which appears to separate into subgroups 3H and 3L on the basis of Ni, Ge, and Re content. It is quite distinct from the Group 2 component (Ir/Au - 0.46–0.54) that dominates at the Apollo 17 site.The unique black-rimmed clasts from this boulder show striking compositional zoning. The cores of anorthositic breccia are very low in Rb, Cs, and U, and have a distinctive 5L meteoritic component (Ir/Au≈1.1). The black rinds are 5- to 10-fold richer in Rb, Cs, and U and have a Group 3 meteoritic component. The cores may represent breccias formed in an earlier impact that became coated with alkali-rich ejecta during the event that produced the boulder.Because of the rarity of the Group 3 meteoritic component at the Apollo 17 site, this boulder cannot represent ordinary Serenitatis ejecta, with their characteristic admixture of the Group 2 Serenitatis projectile. It may represent pre-Serenitatis material excavated from the fringes of the crater during late stages of the Serenitatis impact, but only lightly shocked and hence uncontaminated by the Serenitatis projectile.

Jovanovic, Stanka; Reed, George W.

doi: 10.1007/bf00569672pmid: N/A

Correlations among the trace and minor element pairs Cl and Br, Cl and P2O5, and Ru and Os, present in parent igneous rocks, generally survived the processes of boulder breccia formation.Fractions of the Cl, Br, and Hg that are mobilized by water leaching and/or volatilization at moderate temperatures (⩽450°C) place constraints on the thermal history of Boulder 1 and its component breccias. Since, and possibly during, consolidation, the boulder has probably not been subjected to temperatures of ⩾450°C.The parent rocks of the Apollo 17 boulder and breccia samples studied could have been derived from two initial magmas. Boulder 1, Station 2 gray competent breccias 72255 and 72275 Clast #2 appear to be genetically unrelated to gray competent breccia and anorthositic material 72215, or to light friable breccia 72275; they do appear to be related to samples 72395 (Boulder 2) and 76315 (Station 6 boulder).Vapor clouds from apparently external sources permeated the source regions of the boulders.

Goswami, J. N.; Hutcheon, I. D.

doi: 10.1007/bf00569673pmid: N/A

Fossil track analyses of a ∼ 3 cm section of boulder fragment 72255, collected at the base of the South Massif, yield a surface exposure age for this boulder in its present location of ∼ 40 m.y. This age is in good agreement with the81Kr-Kr exposure age (Leichet al., 1975), suggesting that the boulder was either never exposed to cosmic radiation prior to its emplacement at the foot of the South Massif or that it was heavily shielded during any previous irradiation. High-voltage electron microscope observations reveal no evidence of solar flare irradiation prior to breccia compaction, indicating that the breccia components were never part of a pre-Serenitatis near-surface regolith. The fission track record of a whitlockite crystal from 72255 yields a fission track age of 3.96−0.07+0.04 g.y. Comparison with the40Ar-39 Ar age of 4.00±0.03 g.y. suggests that this age represents the compaction age of the parent boulder.

Leich, D. A.; Kahl, S. B.; Kirschbaum, A. R.; Niemeyer, S.; Phinney, D.

doi: 10.1007/bf00569674pmid: N/A

Rare gas isotopic analyses have been performed on both pile-irradiated and unirradiated samples from Boulder 1, Station 2. Two samples from rock 72255, the Civet Cat clast and a sample of adjacent breccia, have concordant40Ar-39 Ar ages of 3.99±0.03 b.y. and 4.01±0.03 b.y., respectively. Several samples from rock 72275 have complex thermal release patterns with no datable features, but an intermediate-temperature plateau from the dark rim material of the Marble Cake clast yields an age of 3.99±0.03 b.y. - indistinguishable from the age of rock 72255. We regard these ages as upper limits on the time of the Serenitatis basin-forming event.The absence of fossil solar-wind trapped gases in the breccia samples implies that a prior existence for the boulder as near-surface regolith material can be regarded as extremely unlikely. Instead, the small trapped rare-gas components have isotopic and elemental compositions diagnostic of the terrestrial-type trapped component which has previously been identified in several Apollo 16 breccias and in rock 14321. Excess fission Xe is found in all Boulder 1 samples in approximately 1:1 proportions with Xe from spontaneous fission of238U. This excess fission Xe is attributed to spontaneous fission of244Puin situ.Cosmic-ray exposure ages for samples from rocks 72215 and 72255 are concordant, with mean81Kr-Kr exposure ages of 41.4±1.4 m.y. and 44.1±3.3 m.y., respectively. However a distinctly different81Kr-Kr exposure age of 52.5±1.4 m.y. is obtained for samples from rock 72275. A two-stage exposure model is developed to account for this discordance and for the remaining cosmogenic rare-gas data. The first stage was initiated at least 55 m.y. ago, probably as a result of the excavation of the boulder source-crop. A discrete change in shielding depths ∼ 35 m.y. ago probably corresponds to the dislodgement of Boulder 1 from the South Massif and emplacement in its present position.

Compston, W.; Foster, J. J.; Gray, C. M.

doi: 10.1007/bf00569675pmid: N/A

Rb, Sr and87Sr/86Sr have been determined for fragments of matrix and clasts from three of the hand-specimens of Boulder 1, 72275, 72255, and 72215. Total-rock and certain plagioclase samples from a crushed norite clast (Civet Cat) define an age of 4.17±0.05AE (2σ) for the pre-Serenitatis igneous differentiation of the norite. Pyroxene and other mineral separates were affected by a later event at about 3.9±0.1AE. An unshocked clast of pigeonite basalt has a well-fitted mineral isochron of 4.01±0.04AE. Samples of the competent breccia matrix comparatively rich in small clasts of highly radiogenic microgranite define a mixing line equivalent to 4.03±0.03AE, which denotes the age of the microgranite. Other samples of the matrix dominated by small anorthosite clasts define a 4.4AE mixing-line and demonstrate that Sr isotope equilibration between plagioclase and matrix did not occur during the high-temperature event that indurated the matrix.