Petrology of fine-grained rock fragments and petrologic implications of single crystals from the Luna 20 soil

Only fine-grained rocks are present in the Luna 20 samples, and coarser grained rocks are represented by fragments of single crystals. A petrologic study has been made of 47 fine-grained crystalline rocks, microbreccias, and glassy aggregates. In addition, a total of 33 single crystals of pyroxene, plagioclase, olivine and spinel, in the size range 125 to 500 μ, have been examined using electron microprobe and single crystal X-ray diffraction techniques.The most abundant fine-grained crystalline rocks in the samples we have examined are recrystallized anorthositic norite and anorthositic troctolite. Gabbroic rocks, anorthosite, and KREEP basalt are present but not common. Most of the single crystals of pyroxene and plagioclase could have been derived from coarser grained noritic, troctolitic and anorthositic rocks. However, three of the 14 pyroxene crystals, and 2 of the 5 olivine crystals have $\text{Fe}\text{(Fe + Mg)}$ contents greater than 0.45 and are believed to have been derived from mare basalts or related rocks. Two relatively sodic crystals of plagioclase were found. One is a crystal zoned at least over the range An₈₅ to An₆₃, and the second is a homogeneous crystal of albite (~An₃).     

Gravitational waves from scattering of stellar-mass black holes in galactic nuclei

Stellar-mass black holes (BHs) are expected to segregate and form a steep density cusp around supermassive black holes (SMBHs) in galactic nuclei. We follow the evolution of a multimass system of BHs and stars by numerically integrating the Fokker–Planck energy diffusion equations for a variety of BH mass distributions. We find that the BHs 'self-segregate', and that the rarest, most massive BHs dominate the scattering rate closest to the SMBH  (≲10⁻¹ pc)  . BH–BH binaries form out of gravitational wave emission during BH encounters. We find that the expected rate of BH coalescence events detectable by Advanced LIGO is  ∼1–10² yr⁻¹  , depending on the initial mass function of stars in galactic nuclei and the mass of the most massive BHs. We find that the actual merger rate is likely ∼10 times larger than this due to the intrinsic scatter of stellar densities in many different galaxies. The BH binaries that form this way in galactic nuclei have significant eccentricities as they enter the LIGO band (90 per cent with   e > 0.9  ), and are therefore distinguishable from other binaries, which circularize before becoming detectable. We also show that eccentric mergers can be detected to larger distances and greater BH masses than circular mergers, up to  ∼700 M_⊙  . Future ground-based gravitational wave observatories will be able to constrain both the mass function of BHs and stars in galactic nuclei.     

Regional seismic responses of shallow basins incorporating site‐city interaction analyses on high‐rise building clusters

This study assesses the 3D amplification effects in shallow basins and quantifies the effects of site‐city interaction (SCI) on high‐rise buildings. A regional‐scale 3D spectral element simulation is conducted on the Tuen Mun‐Yuen Long basin, which contains multiple subbasins with heterogeneous and nonlinear soil profiles, while 3D city models with various building layouts are fully integrated into the basin model for our SCI study. We found a good correlation between spectral amplification factors and soil depths. Site response is significantly amplified at basin edges and centers due to surface waves generated at basin edges and the focusing effects stemming from 3D basin geometry. Transfer functions of 3D basins can be up to fourfold at fundamental frequencies as compared to 1D response, and further amplifications occur at high frequencies due to surface waves. In the SCI simulations, we observe wave trapping in the open space amid buildings resulting in energy concentration and up to twofold PGA amplifications. The wave trapping effect diminishes as the space between buildings increase beyond their range of influence (～100 m). The SCI analyses show that destructive kinetic energy in superstructures increases 28% in one horizontal direction but decreases 22% in the other. Our study concluded that, 1D site response analysis can significantly underestimate the seismic demand in shallow basins. Site‐city interaction of high‐rise buildings increases the short‐period spectra of ground motions, leading to an increase in their story accelerations by up to 50% and to a substantial decrease in the seismic safety of short structures in their vicinity.     

IsoSpike: Improved Double‐Spike Inversion Software

The double‐spike approach for correction of instrumental mass bias in mass spectrometry data is well established. However, there is very little consistency within the scientific community in terms of double‐spike data reduction. Double‐spike solutions require computer calculation, using either geometric or algebraic approaches, and are often performed using spreadsheet calculations that vary from group to group and between isotope systems. Here, we present IsoSpike, a generalised computer procedure for the processing of double‐spike mass spectrometry data, built as an add‐on for the Iolite data‐reduction package ( www.iolite.org.au ). Use of this software permits visualisation of mass spectrometry data in a time window, and rigorous treatment and screening of data. Additionally, IsoSpike uses an integration‐by‐integration approach where the double‐spike calculations are performed on every integration within an analysis, providing straightforward quantification of uncertainties on double‐spike‐corrected isotope ratios. The advantages of this approach over traditional methods are discussed here. Platinum stable isotope data are presented as an example data set, although the procedure is applicable to any double‐spike system. IsoSpike is freely available from www.isospike.org .     

Petrogenesis of basalts from the FAMOUS area: Mid-Atlantic Ridge

Fresh basalt glasses most of which have Mg/(Mg + Fe²⁺) of 0.66–0.72 from outcrops within 3 km of one other in the rift valley at the Project FAMOUS locality have been analyzed for major, minor and trace elements in order to determine their petrogenesis.Transition metal abundances of the FAMOUS samples are similar to a wide variety of continental and oceanic basalts with high MgO and Ni, all of which show remarkably little variation, with the exception of Cu, Zn and Ti, on a chondrite-normalized plot. Modelling of these data suggests that the mantle beneath both continents and oceans is systematically fractionated relative to chondrites. This fractionation provides a constraint for models of earth formation and subsequent evolution.The abundances of the rare earth and the incompatible elements, Ba, La, Th, U, and Nb, vary by more than a factor of three and the La/Yb and La/Sm ratios vary by factors of 3.1 and 1.6, respectively, in samples with similar, high Mg/(Mg + Fe²⁺). There is no correlation between the degree of light-REE enrichment and the heavy-REE abundance. Furthermore, the trace element variations do not appear correlated with respect to location in the rift valley or to time of eruption. These trace element features demonstrate that successive eruptions in one small area of the rift valley can show wide variations in trace element chemistry over a short span of time; they preclude the derivation of these basalt glasses from a single magma chamber.Despite the heterogeneities in REE and the variable trace element abundances, a homogeneous mantle source is suggested by the similarities among the samples in the incompatible element ratios of La/Ce, Ba/Th, Zr/Nb and K/Ba and the small range in⁸⁷Sr/⁸⁶Sr isotope ratios observed in other samples from the FAMOUS region (White and Bryan, 1977). Thus, trace element heterogeneities appear to be generated by processes in the mantle during melting. However, processes such as batch partial melting, fractional fusion, fractional crystallization, zone refining, or mixing of magmas or sources acting alone are incapable of explaining the lack of correlation between the light and heavy REE.It is suggested that the observed variations are a consequence of dynamic partial melting of a homogeneous mantle source region. This process includes varying degrees of partial melting of an uprising mantle source with continuous but incomplete removal of melt as melting proceeds, varying extents of batch partial melting, and zone refining. Dynamic melting can produce different melts from a homogeneous source which have different degrees of light-REE enrichment and crossing REE patterns. The variable trace element abundances which may be produced through dynamic melting may be the cause of the apparent decoupling of major and trace elements (Bryan et al., 1976) which previously has been suggested for the FAMOUS region (Bryan and Moore, 1977).     

40Ar/39Ar spectrum ages for biotite,hornblende and muscovite in a contact metamorphic zone

Biotite, hornblende and muscovite from 2700 m.y. old rocks in northeastern Minnesota near the contact of the 1150 m.y. Duluth Complex have been analyzed by ⁴⁰Ar/³⁹Ar technique to determine whether spectrum ages can be used to distinguish partial loss of radiogenic argon due to a reheating event. Biotite and hornblende give plateau ages comparable to the ordinary K-Ar ages for all samples including those with intermediate ages. Muscovite gives plateau ages for the samples with less than 11% argon loss. An intermediate muscovite with a conventional K-Ar age of 1850 m.y. gives progressively older ⁴⁰Ar/³⁹Ar ages for higher temperature fractions.Microprobe analysis reveals no systematic correlation between biotite chemistry and loss of argon in the contact zone. This suggests that the rate-controlling process for the loss of argon from biotite in the contact zone may be volume diffusion or recrystallization without a measurable change in major element composition. Biotites with intermediate ages give plateaus because the rate-controlling processes in the vacuum furnace are related to dehydroxylation and delamination and are unrelated to the process causing loss of argon in the contact zone.The data for the muscovites are not easily interpreted, in part because of the limited number of samples. The hornblende data show a correlation between argon loss and change in major element composition suggesting that recrystallization may be a rate-controlling process for the loss of argon from hornblende in the contact zone. The small number of samples precludes a definitive statement.     

Towards a Method for Quantitative LA‐ICP‐MS Imaging of Multi‐Phase Assemblages: Mineral Identification and Analysis Correction Procedures

Here, we present an approach to laser ablation ICP‐MS mapping of multi‐phase assemblages that permits the use of different internal standard elements, concentration values and reference materials for each mineral. In this way, we obtain not only broad pictures of elemental distributions within samples but can also extract high accuracy concentration data for any user‐selected region. This is accomplished by assigning regions of an image to corresponding mineral phases on a pixel‐by‐pixel basis. In this way, accurate trace element concentrations can be determined for each mineral phase, despite potential variations in their ablation characteristics. We present an example where elemental maps are constructed from ablation of a gabbroic sample that includes the phases apatite, amphibole and plagioclase. This work represents an important first step towards development of a method to produce highly accurate LA‐ICP‐MS elemental maps of multi‐phase samples.     

New Reference Materials,Analytical Procedures and Data Reduction Strategies for Sr Isotope Measurements in Geological Materials by LA-MC-ICP-MS

Laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA-MC-ICP-MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr-baseline subtraction, Rb-peak-stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE²⁺ correction for rock glasses, yields the most accurate and precise ⁸⁷Sr/⁸⁶Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve < 100 ppm 2SE repeatability ("internal") precision for ⁸⁷Sr/⁸⁶Sr measurements for materials with < 1000 μg g^-1 Sr.