Randy L. Korotev

The 'North American shale composite' - Its compilation, major and trace element characteristics

Abstract The compilation and major element composition of the “North American shale composite” (NASC) are reported for the first time, along with redeterminations for the REE and selected other elements by modern, high precision analytical methods. The NASC is not strictly of North American origin; 5 of the constituent samples are from Africa and Antarctica, and 15 are from unspecified locations. The major element composition of the NASC compares quite closely with other average shale compositions. New analyses of the NASC document that significant portions of the REE and some other trace elements are contained in minor phases (zircon and possibly other minerals) and that their uneven distribution in the NASC powder appears to have resulted in heterogeneity among analyzed aliquants. The results of this study show that the REE distributions of detrital sediments can be dependent to some extent on their minor mineral assemblages and the sedimentological factors that control these assemblages. Consequently, caution should be exercised in the interpretation of the REE distributions of sediment samples as they may be variable and biased relative to average REE distribution of the crustal rocks supplying detritus. These effects appear to be largely averaged out in sediment composites, with the result that their REE distributions are more likely to be representative of their provenances.

Major lunar crustal terranes: Surface expressions and crust‐mantle origins

In light of global remotely sensed data, the igneous crust of the Moon can no longer be viewed as a simple, globally stratified cumulus structure, composed of a flotation upper crust of anorthosite underlain by progressively more mafic rocks and a residual-melt (KREEP) sandwich horizon near the base of the lower crust. Instead, global geochemical information derived from Clementine multispectral data and Lunar Prospector gamma-ray data reveals at least three distinct provinces whose geochemistry and petrologic history make them geologically unique: (1) the Procellarum KREEP Terrane (PKT), (2) the Feldspathic High-lands Terrane (FHT), and (3) the South Pole-Aitken Terrane (SPAT). The PKT is a mafic province, coincident with the largely resurfaced area in the Procellarum-Imbrium region whose petrogenesis relates to the early differentiation of the Moon. Here, some 40% of the Th in the Moons crust is concentrated into a region that constitutes only about 10% of the crustal volume. This concentration of Th (average ∼5 ppm), and by implication the other heat producing elements, U and K, led to a fundamentally different thermal and igneous evolution within this region compared to other parts of the lunar crust. Lower-crustal materials within the PKT likely interacted with underlying mantle materials to produce hybrid magmatism, leading to the magnesian suite of lunar rocks and possibly KREEP basalt. Although rare in the Apollo sample collection, widespread mare volcanic rocks having substantial Th enrichment are indicated by the remote data and may reflect further interaction between enriched crustal residues and mantle sources. The FHT is characterized by a central anorthositic region that constitutes the remnant of an anorthositic craton resulting from early lunar differentiation. Basin impacts into this region do not excavate significantly more mafic material, suggesting a thickness of tens of kilometers of anorthositic crust. The feldspathic lunar meteorites may represent samples from the anorthositic central region of the FHT. Ejecta from deep-penetrating basin impacts outside of the central anorthositic region, however, indicate an increasingly mafic composition with depth. The SPAT, a mafic anomaly of great magnitude, may include material of the upper mantle as well as lower crust; thus it is designated a separate terrane. Whether the SPA basin impact simply uncovered lower crust such as we infer for the FHT remains to be determined.

A well-tested procedure for instrumental neutron activation analysis of silicate rocks and minerals

Procedures for instrumental neutron activation analysis (INAA) have been developed and used on more than a thousand small samples of terrestrial and lunar silicate rocks and minerals for determination of Co, Cr, Fe, Hf, Na, Ni, Sc, Ta, Th, and the rare earths La, Ce, Sm, Eu, Tb, Yb, and Lu. Precision has been determined by repeated analysis of Knippa basalt and DTS-1 to be better than ±5 percent for all elements except Ni, Yb, Lu, and Hf. Mean values and estimates of accuracy are given for Knippa basalt and USGS standards AGV-1, G-2, GSP-1, and W-1. Important features of the method are its precision and ease of data reduction.

The great lunar hot spot and the composition and origin of the Apollo mafic (“LKFM”) impact‐melt breccias

Thorium-rich, mafic impact-melt breccias from the Apollo 14–17 missions, that is, those breccias identified with the composition known as “LKFM,” are regarded largely as products of basin-forming impacts that penetrated the feldspathic crust and sampled underlying mafic material and magma-ocean residuum carrying the compositional signature of KREEP (potassium, rare earth elements, phosphorous). Despite considerable compositional variation among such breccias, compositions of all of them correspond to mixtures of only four components: (1) a norite with composition generally similar to that of Apollo 15 basalt (mean abundance: 58%; range: ∼30–95%), (2) Fo∼90 dunite (mean: 13%, range: 1–27%), (3) feldspathic upper crust (mean: 29%, range: 4–50%), and FeNi metal (0.1–1.7%). Petrographic evidence has shown that much of the feldspathic component, but none of the KREEP component, is clastic. This observation and the high proportion of KREEP norite component in the breccias suggest that the melt zone of the impact or impacts forming the breccias contained little feldspathic material but consisted predominantly of material with the average composition of KREEP norite. The dunite component probably derives ultimately from the upper mantle. These conclusions support the hypothesis that the breccias were not formed in typical feldspathic crust but instead by one or more impacts into what is designated here “the great lunar hot spot,” that is, the anomalous Th-rich terrane in the Imbrium-Procellarum area identified by the Apollo and Lunar Prospector gamma-ray spectrometers. The LKFM composition is a special product of the great lunar hot spot and is not the average composition of the lower crust in typical feldspathic highlands. Similarly, Mg-suite and alkali-suite plutonic rocks of the Apollo collection are likely all differentiation products of the hot spot, not of plutons that might occur in typical feldspathic crust.

Raman spectroscopy for mineral identification and quantification for in situ planetary surface analysis: A point count method

Quantification of mineral proportions in rocks and soils by Raman spectroscopy on a planetary surface is best done by taking many narrow-beam spectra from different locations on the rock or soil, with each spectrum yielding peaks from only one or two minerals. The proportion of each mineral in the rock or soil can then be determined from the fraction of the spectra that contain its peaks, in analogy with the standard petrographic technique of point counting. The method can also be used for nondestructive laboratory characterization of rock samples. Although Raman peaks for different minerals seldom overlap each other, it is impractical to obtain proportions of constituent minerals by Raman spectroscopy through analysis of peak intensities in a spectrum obtained by broad-beam sensing of a representative area of the target material. That is because the Raman signal strength produced by a mineral in a rock or soil is not related in a simple way through the Raman scattering cross section of that mineral to its proportion in the rock, and the signal-to-noise ratio of a Raman spectrum is poor when a sample is stimulated by a low-power laser beam of broad diameter. Results obtained by the Raman point-count method are demonstrated for a lunar thin section (14161,7062) and a rock fragment (15273,7039). Major minerals (plagioclase and pyroxene), minor minerals (cristobalite and K-feldspar), and accessory minerals (whitlockite, apatite, and baddeleyite) were easily identified. Identification of the rock types, KREEP basalt or melt rock, from the 100-location spectra was straightforward.

Teabags: Computer programs for instrumental neutron activation analysis

Described is a series of INAA data reduction programs collectively known as TEABAGS (Trace Element Analysis By Automated Gamma-ray Spectrometry). The programs are written in FORTRAN and run on a Nuclear Data ND-6620 computer system, but should be adaptable to any medium-sized minicomputer. They are designed to monitor the status of all spectra obtained from samples and comparison standards irradiated together and to do all pending calculations without operator intervention. Major emphasis is placed on finding all peaks in the spectrum, properly identifying all nuclides present and all contributors to each peak, determining accurate estimates of the background continua under peaks, and producing realistic uncertainties on peak areas and final abundances.

Early Proterozoic oceanic crust and the evolution of subcontinental mantle: Eclogites and related rocks from southern Africa

Seven eclogitic nodules from kimberlites in southern Africa have been studied in detail for whole-rock, major- and trace-element geochemistry, petrography, and mineral chemistry by electron microprobe; high-purity mineral separates from six of these samples have been analyzed for trace elements and for the isotopic composition of Sr, Nd, and oxygen. Three eclogite groups are recognized: group A eclogites have very high Mg/Fe, low Na in pyroxene, moderate δ18O (+4.7 to +5.3), and low 87Sr/86Sr and 143/144Nd. Olivine and enstatite may also be present as accessory phases in this group. Group B eclogites have moderate to high Mg/Fe ratios, high Na in pyroxene, low δ18O (+3.0 to +3.4), high 87Sr/86Sr and 143Nd/144Nd ratios, and extremely LREE-depleted/ HREE-enriched garnets with no Eu anomalies. Group C eclogites have low Mg/Fe ratios, high Na in pyroxene, and variable Sr-, Nd-, and O-isotope compositions. Accessory feldspar is present in one sample that may be of crustal origin. Mineral separates of garnet and pyroxene have positive Eu anomalies. The group A eclogites are too refractory to represent magma compositions and must have formed as cumulate dike rocks in the upper mantle that contain a minor trapped liquid component. This is supported by the presence of accessory olivine and enstatite, and by their KSm and KNd, which are similar to empirical pyroxene/garnet partition coefficients. The Group B eclogites are extremely depleted in incompatible elements and have ϵNdvalues 10x to 20x MORB. The high 87Sr/86Sr of these eclogites is not consistent with their strongly depleted LREE and high ϵNd, and cannot be primary—it must have been imposed on the protolith. The low δ18O of these eclogites cannot form by mantle fractionation processes and must also be inherited from the protolith. Both the Sr- and oxygen-isotope data are consistent with high- temperature hydrothermal alteration of a basaltic protolith, followed by partial melting to form the refractory compositions observed now. The hydrothermal fluid may have been sea water, but secondary enrichment of the protolith in Rb is required to generate the observed high Sr ratios. The high Na content of the pyroxenes supports a spilitic alteration event. The major- and trace-element characteristics of the group C eclogites are consistent with recrystallization of a cumulate gabbro protolith. One group C rock is probably a garnet granulite derived from the lower crust. The other may represent the plutonic portion of oceanic crust. A reconstructed whole-rock isochron for the three Bellsbank eclogites yields an age of 2.1 ± 0.1 b.y. This implies that plate-tectonic processes involving the generation and subduction of oceanic crust have been active since the early Proterozoic. The early Proterozoic ocean crust consisted of two components: a plutonic section of gabbro cumulates (group C eclogites) and a volcanic section of basalt (group B eclogites). The volcanic component has undergone substantial hydro-thermal alteration and subsequent partial melting to form a refractory residue; the plutonic component is less modified.

Compositional Variation in Apollo 16 Impact-Melt Breccias and Inferences for the Geology and Bombardment History of the Central Highlands of the Moon

Abstract High-precision data for the concentrations of a number of lithophile and siderophile elements were obtained on multiple subsamples from 109 impact-melt rocks and breccias (mostly crystalline) from the Apollo 16 site. Compositions of nearly all Apollo 16 melt rocks fall on one of two trends of increasing Sm concentration with increasing Sc concentration. The Eastern trend (lower Sm Sc , Mg Fe , and Sm Yb ratios) consists of compositional groups 3 and 4 of previous classification schemes. These melt rocks are feldspathic, poor in incompatible and siderophile elements, and appear to have provenance in the Descartes formation to the east of the site. The Western trend (higher Sm Sc , Mg Fe , and Sm Yb ratios) consists of compositional groups 1 and 2. These relatively mafic, KREEP-bearing breccias are a major component (~35%) of the Cayley plains west of the site and are unusual, compared to otherwise similar melt breccias from other sites, in having high concentrations of Fe-Ni metal (1–2%). The metal is the carrier of the low-Ir/Au (~0.3 × chondritic) siderophile-element signature that is characteristic of the Apollo 16 site. Four compositionally distinct groups (1M, 1F, 2DB, and 2NR) of Western-trend melt breccias occur that are each represented by at least six samples. Compositional group 1 of previous classification schemes (the “poikilitic” or “LKFM” melt breccias) can be subdivided into two groups. Group 1M (represented by six samples, including 60315) is characterized by lower Al2O3 concentrations, higher MgO and alkali concentrations, and higher Mg Fe and Cr Sc ratios than group 1F (represented by fifteen samples, including 65015). Group 1M also has siderophile-element concentrations averaging about twice those of group 1F and Ir Au and Ir Ni ratios that are even lower than those of other Western-trend melt rocks ( Ir Au = 0.24 ± 0.03 , CI-normalized). At the mafic extreme of group 2 (“VHA” melt breccias), the melt lithology occurring as clasts in feldspathic fragmental breccias from North Ray crater (group 2NR) is compositionally distinct from the melt lithology of dimict breccias from the Cayley plains (group 2DB) in having higher concentrations of Sc, Cr, and heavy rare earth elements and lower concentrations of siderophile elements. The distinct siderophile-element signature (high absolute abundances, low Ir Au ratio) suggest that the four groups of mafic melt breccia are all somehow related. Ratios of some lithophile elements also suggest that they are more closely related to each other than they are to melt breccias from other Apollo sites. However, none of the breccia compositions can be related to any of the others by any simple process of igneous fractionation or mixing involving common lunar materials. Thus, the origin of the four groups of mafic melt breccia is enigmatic. If they were produced in only one or two impacts, then a mechanism exists for generating regimes of impact-melt breccia in a single impact that are substantially different from each other in composition. For various reasons, including the problem of delivering large volumes of four different types of melt to the Apollo 16 site, it is unlikely that any of these breccias were produced in basin-forming impacts. If they were produced in as many as four crater-forming impacts, then the unusual siderophile-element signature is difficult to explain. Possible explanations are 1. (1) the four groups of melt breccia all contain metal from a single, earlier impact, 2. (2) they were each formed by related metal-rich meteoroids, or 3. (3) some common postimpact process has resulted in metal of similar composition in each of four melt pools. Within a compositional group, most intrasample and intersample variation in lithophile element concentrations is caused by differences among samples in the proportion of a component of normative anorthosite or noritic anorthosite. In most cases, this compositional variation probably reflects variation in clast abundance. For group 2DB (and probably 2NR), differences in abundance of a component of ferroan anorthosite (estimated Al2O3 ≈ 32%) accounts for the compositional variation. For groups 1M and 1F, the anorthositic component is more mafic (estimated Al2O3 ≈ 26%). Some group-2 samples may be related by a troctolitic component of varying abundance.

Concentrations of radioactive elements in lunar materials

As an aid to interpreting data obtained remotely on the distribution of radioactive elements on the lunar surface, average concentrations of K, U, and Th as well as Al, Fe, and Ti in different types of lunar rocks and soils are tabulated. The U/Th ratio in representative samples of lunar rocks and regolith is constant at 0.27; K/Th ratios are more variable because K and Th are carried by different mineral phases. In nonmare regoliths at the Apollo sites, the main carriers of radioactive elements are mafic (i.e., 6–8% Fe) impact-melt breccias created at the time of basin formation and products derived therefrom.

The materials of the lunar Procellarum KREEP Terrane: A synthesis of data from geomorphological mapping, remote sensing, and sample analyses

Major features of the Moons Procellarum KREEP Terrane include subdued relief and extensive resurfacing with mare basalt, consistent with high concentrations of Th and other heat-producing elements at depth. We relate the chemistry of sampled materials to the geomorphology, Th surface concentrations determined by the Lunar Prospector (2° pixels), and FeO and TiO2 concentrations derived from Clementine ultraviolet-visible spectral data. On the basis of geologic maps, each pixel was classified as mare, terra, or mixed. Near the periphery of the terrane, terra pixel compositions are relatively feldspathic; in the interior they mainly represent Imbrium basin rim or ejecta deposits and are mainly incompatible trace element rich norites and presumably represent materials from a thick section (tens of kilometers) of the pre-Imbrium crust of the terrane excavated by the Imbrium event. (Although Imbrium ejecta are the principal source of surface terra materials, the Imbrium event did not create the Th-rich Procellarum KREEP Terrane.) Broad, continuous expanses of mare pixels are observed, with little interruption from protruding terra or terra-penetrating craters. The mare-basalt-dominated regoliths of these areas have a wide range of TiO2 concentrations ( 18%), leading to the conclusion that the high Th concentrations are in the mare basalts and are not present in the regoliths as terra-derived materials. Volcanic glasses and impact glasses of mare basalt composition collected from the Procellarum KREEP Terrane support this conclusion.