Feldspathic clasts in Yamato-86032: Remnants of the lunar crust with implications for its formation and impact history

Low concentrations of Th and Fe in the Yamato (Y)-86032 bulk meteorite support earlier suggestions that Y-86032 comes from a region of the moon far distant from the Procellarum KREEP Terrain (PKT), probably from the lunar farside. ³⁹Ar–⁴⁰Ar, Rb–Sr, Sm–Nd, and Sm-isotopic studies characterize the chronology of Y-86032 and its precursors in the mega regolith. One of the rock types present in a light gray breccia lithology is an anorthosite characterized by plagioclase with An 93, i.e., more sodic than lunar FANs, but with very low ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr similar to those of FANs. (FAN stands for Ferroan Anorthosite). This “An93 anorthosite” has Nd-isotopic systematics similar to those of nearside norites. A FAN-like “An97 anorthosite” is present in a second light-colored feldspathic breccia clast and has a more negative ε_Nd value consistent with residence in a LREE-enriched environment as would be provided by an early plagioclase flotation crust on the Lunar Magma Ocean (LMO). This result contrasts with generally positive values of ε_Nd for Apollo 16 FANs suggesting the possibility of assymetric development of the LMO. Other possible explanations for the dichotomy in ε_Nd values are advanced in the text. The Y-86032 protolith formed at least 4.43 ± 0.03 Ga ago as determined from a Sm–Nd isochron for mineral fragments from the breccia clast composed predominantly of An93 anorthosite and a second clast of more varied composition. We interpret the mineral fragments as being predominatly from a cogenetic rock suite. An ³⁹Ar–⁴⁰Ar age of 4.36–4.41 ± 0.035 Ga for a third clast composed predominantly of An97 anorthosite supports an old age for the protolith. Initial ¹⁴³Nd/¹⁴⁴Nd in that clast was −0.64 ± 0.13 ε-units below ¹⁴³Nd/¹⁴⁴Nd in reservoirs having chondritic Sm/Nd ratios, consistent with prior fractionation of mafic cumulates from the LMO. A maximum in the ³⁹Ar–⁴⁰Ar age spectrum of 4.23 ± 0.03 Ga for a second sample of the same feldspathic breccia clast probably reflects some diffusive ⁴⁰Ar loss. Lack of solar wind and lunar atmosphere implanted Ar in the light gray breccia clast allows determination of an ³⁹Ar/⁴⁰Ar age of 4.10 ± 0.02 Ga, which is interpreted as the time of initial brecciation of this litholgy. After correction for implanted lunar atmosphere ⁴⁰Ar, impact melt and dark regolith clasts give Ar ages of 3.8 ± 0.1 Ga implying melt formation and final breccia assembly 3.8 Ga ago. Some breccia lithologies were exposed to thermal neutron fluences of 2 × 10¹⁵ n/cm², only about 1% of the fluence experienced by some other lunar highlands meteorites. Other lithologies experienced neutron fluences of 1 × 10¹⁵ n/cm². Thus, Y-86032 spent most of the time following final brecciation deeply buried in the megaregolith. The neutron fluence data are consistent with cosmogenic ³⁸Ar_cos cosmic ray exposure ages of 10 Ma. Variations among differing lithologies in the amount of several regolith exposure indicators, including cosmogenic noble gas abundances, neutron capture induced variations in Sm isotopic abundances, and Ir contents, are consistent with a period of early (>3.8 Ga ago) lunar regolith exposure, subsequent deep burial at >5 m depth, and ejection from the moon 7–10 Ma ago.     

Rb-Sr and K-Ar geochronologic and isotopic studies,Llano Uplift,central Texas

Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K₂O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses.Two foliated plutons in the southeast are 1,167±12m.y. (2) old, with distinctly different initial ⁸⁷Sr/⁸⁶Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial ⁸⁷Sr/ ⁸⁶Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial ⁸⁷Sr/⁸⁶Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial ⁸⁷Sr/⁸⁶Sr = 0.7061±0.0003. All of the intrusive rocks have initial ⁸⁷Sr/⁸⁶Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.     

Improved method for estimating the standard free energies of formation (ΔGf,298.150) of smectites

Two alternative empirical methods were developed for estimating the standard free energies of formation of layer silicate minerals with cation exchange capacity due to isomorphous substitutions. These methods involve the summation of either hydroxide or oxide components to form the desired smectite mineral similarly to a method developed by Nriagu, but differ in that (1) no ad hoc ‘calibrations’ are necessary for the $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values of alkali hydroxides and (2) a three-parameter equation is used for the correction factor, with the charge on the mineral due to isomorphous substitutions and the ionic radius and charge on the exchangeable cation taken as independent variables.A comparison between experimental and estimated $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values indicated that one of the methods (that involving the summation of hydroxide components) was significantly better than all other methods in predicting $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values of smectites while at the same time requiring no ad hoc thermochemical assumptions.     

Isotopic fractionation of noble gases by diffusion in liquid water: Molecular dynamics simulations and hydrologic applications

Despite their great importance in low-temperature geochemistry, diffusion coefficients of noble gas isotopes in liquid water (D) have been measured only for the major isotopes of helium, neon, krypton and xenon. Data on the diffusion coefficients of minor noble gas isotopes are essentially non-existent and so typically have been estimated by a kinetic-theory model in which D varies as the inverse square root of the isotopic mass (m): D ∝ m^−0.5. To examine the validity of the kinetic-theory model, we performed molecular dynamics (MD) simulations of the diffusion of noble gases in ambient liquid water. Our simulation results agree with available experimental data on the solvation structure and diffusion coefficients of the major noble gas isotopes and reveal for the first time that the isotopic mass-dependence of all noble gas self-diffusion coefficients has the power-law form D ∝ m^−β with 0 < β < 0.2. Thus our results call into serious question the widespread assumption that the ‘square-root’ model can be applied to estimate the kinetic fractionation of noble gas isotopes caused by diffusion in ambient liquid water. To illustrate the importance of this finding, we used the diffusion coefficients determined in our MD simulations to reconsider the geochemical modeling of ²⁰Ne/²²Ne and ³⁶Ar/⁴⁰Ar isotopic ratios in three representative hydrologic studies. Our new modeling results indicate that kinetic isotopic fractionation by diffusion may play a significant role in noble gas transport processes in groundwater.     

Metabasalts and metagabbros from the Llano Uplift,Texas: Petrologic and geochemical characterization with emphasis on tectonic setting

The 1.0-1.2 b.y. old rocks of the southeastern Llano Uplift, Texas include a 7 km thick sequence of amphibolite-grade, stratified, mafic metasedimentary rock (Packsaddle Schist) which is intruded by a varied suite of syntectonic and late-kinematic intrusions. The metasediments contain large blocks of serpentinized peridotite (Coal Creek serpentinite) and coarse hornblendite and metagabbro. Prior to the end of maximum deformation, the sequence was intruded by low to medium-K₂O tholeiitic basalts (0.40-0.72% K₂O). Late-kinematic low-K₂O tholeiitic basalts (0.38-0.40% K₂O) were intruded as dikes into the folded rocks. The Coal Creek serpentinite contains both syntectonic and late-kinematic low-K₂O tholeiitic metabasalts (0.13–0.36% K₂O). The Llano metabasalts and metagabbros are characterized by low Cr (67–378 ppm) and Ni (36–170 ppm), variable Rb (1.5–14.7 ppm), Sr (140–1229 ppm), TiO₂ (0.40–2.20%), P (568–2707 ppm), and Zr (18–230 ppm), and Y (16–45 ppm), Co (40–57 ppm), and Sc (36–49 ppm) similar to modern MORB. The metabasalts have La abundances from 7 to 39 times chondritic and exhibit light REE enrichment with ¦La/Sm¦_N from 1.13 to 1.45 and ¦La/Yb¦_N from 1.12 to 2.99. The metabasalts show a strong correlation of increasing Zr, Ti, and Y and decreasing Eu/Eu^* (1.56–1.00), CaO/TiO₂, Al₂O₃/TiO₂, and MgO/ MgO+FeO^* with increasing REE enrichment (La_N). The Llano metabasalts and metagabbros have initial ⁸⁷Sr/⁸⁶Sr=0.7029±0.0001. A likely petrogenetic model for these metabasalts is an island arc in which events from early arc development to final late-kinematic intrusion were dominated by tholeiitic volcanism and intrusion. The chemical systematics of the Llano mafic metaigneous rocks suggest they are products of fractionation of olivine, clinopyroxene, and plagioclase from more primitive basaltic magmas generated beneath the island arc complex.     

Channel systems and lobe construction in the Mississippi Fan

Morphological features on the Mississippi Fan in the eastern Gulf of Mexico were mapped using GLORIA II, a long-range side-scan sonar system. Prominent is a sinuous channel flanked by well-developed levees and occasional crevasse splays. The channel follows the axis and thickest part of the youngest fan lobe; seismic-reflection profiles offer evidence that its course has remained essentially constant throughout lobe development. Local modification and possible erosion of levees by currents indicates a present state of inactivity. Superficial sliding has affected part of the fan lobe, but does not appear to have been a factor in lobe construction.     

Oceanic thunderstorms

Summary The oceanic thunderstorm has long been a difficult phenomenon to investigate. The diurnal variation of these storms together with that of land based storms is essential in providing the statistical correlation between global thunderstorm activity and oceanic potential gradients upon which the classical explanation of the maintenance of the earth's electric charge is based. A data base of over seven million synoptic observations by ships throughout the world ocean has been analyzed to give a fresh approach and insight to the universal time variation of ocean areas affected by thunderstorms. According to these data, the annual average of this ocean area has a mean of 202×10⁴ km² with a diurnal peak of 240×10⁴ km² in the interval of 1800 to 2400 GMT. A diurnal minimum is observed to be 167×10⁴ km² in the interval of 0600 to 1200 GMT.

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Zusammenfassung Es war lange Zeit schwierig, die ozeanischen Gewitter zu untersuchen. Die klassische Erklärung für die Aufrechterhaltung der elektrischen Erdladung gründet sich auf den Zusammenhang zwischen dem luftelektrischen Potentialgefälle auf See und der weltweiten Gewittertätigkeit, wofür die Kenntnis der Tagesgänge der Gewitter über Land und über den Ozean eine wichtige Voraussetzung bilden. Beobachtungsmaterial mit mehr als sieben Millionen synoptischen Beobachtungen — gewonen von Schiffen auf allen Ozeanen — wurde ausgewertet um auf eine neue Weise die von Gewittern bedeckten ozeanischen Flächengrösse in ihrer nach Weltzeit verlaufenden Variation zu untersuchen. Es stellt sich heraus dass das Jahresmittel dieser Flächengrösse bei 202·10⁴ km² liegt, mit einem tageszeitlichen Höchstwert von 240·10⁴ km² in der Zeit zwischen 1800 und 2400 GMT. Ein tageszeitlicher Mindestwert wurde zu 167·10⁴ km² gefunden, welcher zwischen 0600 und 1200 GMT auftritt.

     

Pore pressure loads on gravity structures

It is common practice to compute wave-induced loads on the immersed surface of gravity structures exposed to the wave motion and disregard the pore-water pressure variation on the foundation surface. However, when the soil is porous, wave-induced pressures propagate within the soil under the structure and result in a rather significant contribution to overall loads. This paper describes a practical method for numerical modeling of the pore pressure under a gravity platform foundation for compressible water and a rigid, but porous soil. The porous soil may be bounded by an impermeable horizontal layer at some arbitrary depth.

The paper outlines the basic boundary element procedure for pore pressure analysis and presents numerical results for a typical gravity structure as well as results for comparison with an existing analytical solution for a vertical circular cylinder.     

The SEASWAB experiment

Abstract

SEASWAB is one element of the Delta Project of the U.S. Geological Survey, a cooperative effort with several universities and other governmental agencies to investigate the processes that cause marine‐sediment instability. The basic purpose of the SEASWAB experiment was to obtain field measurements of sediment motion and pore‐pressure variations in soft sediment affected by wave‐pressure perturbations. This article serves as an introduction to the six papers that follow and that together make up a report on the results of SEASWAB.