G. A. McKay

Mineralogy and petrology of Apollo 12 sample no. 12013 - A progress report

Abstract A mineralogical-petrological survey of Apollo 12 sample 12013 has been made with petrographic microscope and electron microprobe. The rock is extremely heterogeneous and consists of at least two separate fragmental units (light and dark) permeated by a once-fluid granitic component. The fragmental material includes a wide variety of lithic and crystal fragments some of which have not yet been reported from other lunar samples. The granitic component is essentially bimineralic, with dominant potassic feldspar plus silica.

Lunar polymict breccia 14321 - A petrographic study

Abstract Seven petrographic thin sections of lunar rock sample 14321, ‘Big Bertha’, have been examined. It is a complex rock incorporating diverse lithic and single crystal fragments and represents a sampling of the heterogeneous Fra Mauro formation, considered by the writers to be lithified debris from the Imbrium impact event. Electron probe microanalysis and microscopic study of textures reveal the assembly history of this breccia which in turn allows some interpretation of the nature of the pre-Imbrium crust and the effect of the Imbrium impact and the subsequent transportation to the Apollo 14 site. The present-day polymict breccia 14321 is composed of basaltic clasts originating from the fragmentation of a single or closely related set of lava cooling units, a set of fragmental clasts designated as microbreccia 3 (themselves polymict microbreccias), and a light colored matrix which formed rock 14321 by cementing the two major groups of clasts. The light colored matrix material is derived from the fragmentation and mutual abrasion of the basalt and microbreccia 3. On the basis of consistent textural relations two older sets of microbreccias have been identified within microbreccia 3. Microbreccia 1 clasts are well-rounded, relatively light colored, and noritic. They are always completely enclosed within microbreccia 3, most often forming the central cores of rounded accretionary lapilli structures which we have designated as microbreccia 2. Microbreccias 1, 2, 3, and macrobreccia 14321 represent a chronological series of fragmentation and lithification events. Each of these events involved some thermal and/or shock metamorphism as evidenced by mineralogical and textural criteria, and the chronological order of formation of the breccias also corresponds to a decreasing intensity of associated thermal effects. The petrology and mineralogy of 14321 are described in detail in this paper. A more general interpretation of the combined petrographic and chemical data is given in Duncan et al. (1975a).

Microprobe studies of three Luna 16 basalt fragments

Abstract Three small fragments of Luna 16 basalt (NASA sample designation G38) have been analyzed with an electron microprobe. The mineralogy is generally similar to that found in Apollo 11 and 12 igneous rocks: major amounts of clinopyroxene, plagioclase, ilmenite and olivine; accessory spinel, troilite, Fe metal and residual phases. Unlike some Apollo 12 basalts, there is continuous zoning with no composition gap in the G38 spinels. Plagioclase analyses follow the same non-stoichiometric trends previously found in Apollo 11 and 12. The samples indicate a magmatic history (rapid crystallization from reduced melts) similar to that inferred from studies of Apollo 11 and 12 basalts.

Breccias from the lunar highlands: preliminary petrographic report on apollo 16 samples 60017 and 63335.

Lunar samples 60017,4 and 63335,14 are composed of microbreccias and devitrified glass. These components are predominantly anorthositic, with the exception of a cryptocrystalline clast found in the microbreccia portion of 63335,14 which contains 2.7 percent potassium oxide and 66.7 percent silicon dioxide. The samples have been subjected to extreme shock and thermal metamorphism. The parent materials of the microbreccias include both a coarse-grained anorthosite and a fine-grained subophitic anorthositic gabbro.

Ignimbrites of the Seven Springs Formation, Barrilla Mountains, Western Texas

The Seven Springs Formation of the Barrilla Mountains, western Texas, consists of 4 tuff units, 3 ignimbrite units, and 1 basalt unit. The ignimbrites consist of anorthoclase (Or 35–40 ) phenocrysts in a matrix of glass, alkali feldspar, and quartz. The percentage of anorthoclase phenocrysts, the degree of welding of the matrix, and the grain size of the matrix all increase from the oldest to the youngest ignimbrites. The ignimbrites can be genetically related to the underlying Star Mountain Rhyolite flow in terms of sequential eruption of all units from a single magma chamber in which anorthoclase phenocrysts were crystallizing, settling, and being partially resorbed. All ignimbrites underwent posteruptive loss of Na 2 O.