Engineering topics
Publications and source records attributed to Gast, P. W..
A chemically distinct group of lunar rocks with the trace element characteristics of basaltic lunar rocks is apparently ubiquitous on the lunar surface. Such rocks have been found at the Apollo 15, Apollo 16, and Luna 20 landing sites. They may be derived from the plains-forming material that has been designated Cayley Formation.
Some nonmare and highland rock types (14310 type KREEP and very high Al2O3 basalts) have the internal chemical variations expected for a plagioclase-liquid system. The observed Eu variations in these rock types suggest a D(Eu) 1/p of 0.6 to 0.7. The Sr variations suggest a D(Sr) 1/p of about 0.6, with values as low as 0.35 suggested for some materials from sample 14063. Common Apollo 14 KREEP and Apollo 15 KREEP do not show internal Sr, Eu, Al2O3 variations consistent with the D(Eu, Sr) 1/p values derived for 14310 type KREEP. Major element and experimental data indicate that olivine or pyroxene is a large, perhaps dominant, controller of chemical variations within common Apollo 14 KREEP. The application of these distribution coefficients to pure anorthosites like 15415 yields the model dependent conclusion that the silicate liquids with which such anorthosites may have been chemically equilibrated have not yet been analyzed and perhaps not directly sampled.
Concentrations of Rb and Sr and the Sr87/86 ratios are determined in a large number of Apollo 15 samples, including KREEP basalts, mare basalts, anorthosite breccia clasts, lithic fragments, a spinel-bearing clast, green glass samples, breccia matrix, and soils. Relative concentrations of Sm, Eu, Rb, and Sr are also examined in the samples, with particular attention to Rb/Sr systematics vs other trace element abundances and total chemical compositions of rocks. Analysis of crystalline KREEP samples rich in trace elements indicates that the differentiation of these rocks has extended to -4.25 AE age, a significantly later time than the presumed time of accretion of the moon.
The concentrations of the rare earth elements K, Rb, Sr, Ba, U, Zr, and Cr for the Luna 20 soil and four different Apollo 16 soils are reported. These trace element abundances imply: (1) that the lunar highlands consist of a mixture of rocks rich in large ion lithophile (LIL) elements and LIL-element improverished anorthosites; or (2) that the bulk of the aluminum-rich crust did not originate by upward segregation of plagioclase in a primitive liquid shell. The Luna 20 soil is distinguished from the Apollo 16 soil by lower aluminum and LIL element abundances.
It is attempted to derive the constraints that can be placed on the density of the lunar interior. The moment of inertia of the moon and its mean density are being considered in the investigation together with the mass and density of the lunar crust that have been inferred from the seismic refraction data recorded by the passive seismometer. The calculations presented show that the density of the lunar interior can easily approach values as high as 3.5 for a fraction of the lunar mass which lies in the range from 1/2 to 2/3.
New information is used to demonstrate that better models for the thermal history of the moon are required. As a first step, account is taken of (1) a nonuniform initial composition in terms of fraction of low melting to high melting phase present, and for variation in the uranium, potassium, and thorium contents as a function of depth, (2) partitioning of the radioactive elements between the melt and the solid phases, and (3) a cutoff value of melt which must be exceeded before magma can move to the surface. The results of several attempts to determine whether reasonable conditions, composition, and thermal properties can be expected to give rise to two separate periods of volcanism are discussed. Two models with somewhat different distributions of radioactive heat sources and different conductivities are examined.
It is assumed that most of the igneous rocks on the lunar surface are the product of partial melting in the lunar interior, followed by segregation and upward transport of an igneous liquid. An attempt is made to determine constraints on the composition of the lunar interior that derive from the chemical composition of the lunar igneous rocks. The salient chemical characteristics of igneous rocks from the lunar surface are summarized, and are compared to analogous characteristics of terrestrial volcanic rocks so that major similarities and differences between terrestrial and lunar basaltic rocks can be established.
Chemical characteristics of KREEP basalts from the Apollo 12 site are discussed. It is indicated that nonmare basalts are chemically distinct from mare basalts, primarily in FeO and Al2O3 concentrations and their ratios, and that the spectra of the former are closely related to the degree of partial melting. It is also noted that the chemical compositions of KREEP basalts from Apollo 12, 13, and 14 show very little chemical variation.
The Rb and Sr systematics were determined for eight lithic fragments from the coarse fines of Apollo 11, Apollo 12, and Apollo 14; for bulk soils from Luna 16, Apollo 14, and Apollo 15 and for 'whole rocks' from Apollo 14 and the Apollo 15 anorthosite 15415. The objective of the studies was to identify important lunar rock types by their major and trace element chemistry and utilize Rb and Sr systematics to obtain a chronology for events which establish or modify rock chemical composition.
Data obtained in a trace element analysis of some anorthosite and anorthositic fragments are presented. The low Mg concentrations of the Apollo 15 anorthosite and the KREEP anorthosite reflect the mineralogical purity of these samples. They are so pure that their composition can be taken as that of liquidus plagioclases. Some of the characteristics of the parent liquids of these plagioclases can be calculated from the liquid-plagioclase distribution coefficients.
Canary Islands volcanic rocks, investigating lead isotope compositions for upper mantle multistage history
Apollo 12 soils composition and derivation, finding feldspathic orthopyroxene rich rock and chemically comparable glass fragments
Nonmare basalts chemical composition and origin from Apollo 11 and 12 sites
Rare earth abundance in Apollo 12 basalts and soils and achondritic meteorites, using partial melting model
Apollo 12 lunar rock 12013 rare earth, alkaline and alkali metal and Sr 87/Sr 86 data, discussing light and dark component composition
Oceanic ridge volcanic rocks alkali metal, alkaline earth, rare earth, nickel and major element content, observing partial melting
Oceanic volcanic rocks, discussing dispersed element characteristics to describe origin of magma types
Apollo 11 lunar basalt chemical composition and petrogenesis, using stable isotope dilution method