To constrain the deformation, thermal evolution, and seismic properties of the mantle lithosphere beneath the Hangay Dome, we have analyzed the microstructures, crystal preferred orientations (CPO), and hydrogen concentrations of olivine and pyroxenes of 50 mantle xenoliths carried up by Cenozoic basalts from Zala, Haer, and Shavaryn-Tsaram from Tariat, Mongolia. Most xenoliths are medium- to coarse-grained spinel-lherzolites, but four contain garnet + spinel. Coarse granular, highly annealed microstructures predominate. The microstructures are associated with well-developed CPO, typical of deformation under high temperature, moderate pressure, and dry conditions. The hydrogen concentrations in olivine, orthopyroxene, and clinopyroxene are low and range around 5, 75, and 147 ppm H2O wt, respectively. Together, microstructures and CPO indicate that ductile deformation was followed by static recrystallization, which has annealed the microstructures but preserved the CPO and, hence, the anisotropy of physical properties. Lack of correlation between annealing and equilibrium temperatures suggest that the annealing is due to a long quiescence episode since the last deformation episode. Here, there is not evidence that the formation of Hangay Dome is associated with recent deformation in the lithospheric mantle. Calculated seismic properties show moderate seismic anisotropy, with fast propagation of P waves and polarization of S waves parallel to the flow direction and low birefringence for S waves propagating obliquely to the flow plane. The results are consistent with weak P wave anomalies but not with the strong low S wave velocity anomalies predicted by some tomographic models or with the high conductivity inferred from magnetotelluric data for the lithospheric mantle beneath the Hangay Dome.

The Mn-53-Cr-53 short-lived radionuclide decay system is a powerful tool to investigate the timescales of early solar system processes. A complication arises, however, from the fact that spallation and thermal/epithermal neutron capture processes induced by cosmic rays can significantly alter Cr-53/Cr-52 ratios in solar system objects that have long exposure ages and high Fe/Cr ratios. Quantifying these cosmogenic effects helps constrain the cosmic ray exposure history of extraterrestrial samples. The isotopic shifts produced by cosmic ray irradiation also need to be corrected before the Cr isotope systematics can be used as a dating tool and as a tracer of nucleosynthetic provenance. To investigate the impact of cosmogenic production on Cr, the Cr isotopic compositions of 25 samples from 16 iron meteorites belonging to nine different chemical groups were measured. The measurements show that exposure to cosmic rays can cause large coupled excesses in epsilon Cr-53 (up to +268.29 +/- 0.14; 2SE) and epsilon Cr-54 (up to +1053.78 +/- 0.72; 2SE) with a best fit line of epsilon Cr-54 = (3.90 +/- 0.03) x epsilon Cr-53. The magnitude of Cr isotope production is controlled by various factors including the exposure age, the chemical composition (i.e., Cr concentration and Ni/Fe ratio) and shielding conditions. Nevertheless, the correlation of epsilon Cr-53 and epsilon Cr-54 is independent of these factors, which provides an effective method to evaluate the cosmogenic contribution to Cr-53 by monitoring the cosmogenic variations in epsilon Cr-54 in meteoritic irons. The results are compared with modeling results that yield a slightly shallower slope of 3.6 +/- 0.2. Modeling results for the olivine in stony meteorites yield a higher slope (similar to 5.4). However, the previous estimated results for lunar samples (stony targets for comic ray irradiation) exhibit an observably shallower slope (similar to 2.62). The reason for the different slopes is that the production rates of different cosmogenic Cr isotopes in iron meteorites and lunar samples are in different proportions. The differences may not be completely controlled by the higher thermal and epithermal neutron fluencies in lunar samples than in iron meteorites, but instead may largely reflect different radiation geometry between the two. More studies are needed to solve this open question. (C) 2019 Published by Elsevier Ltd.

Recycling of crust into the mantle has left only small remnants at Earth's surface of crust produced within a billion years of Earth formation. Few, if any, of these ancient crustal rocks represent the first crust that existed on Earth. Understanding the nature of the source materials of these ancient rocks and the mechanism of their formation has been the target of decades of geological and geochemical study. This traditional approach has been expanded recently through the ability to simultaneously obtain U-Pb age and initial Hf isotope data for zircons from many of these ancient, generally polymetamorphic, rocks. The addition of information from the short-lived radiometric systems Sm-146-Nd-142 and Hf-182-W-182 allows resolution of some of the ambiguities that have clouded the conclusions derived from the long-lived systems. The most apparent of these is clear documentation that Earth experienced major chemical differentiation events within the first tens to hundreds of millions of years of its formation, and that Earth's most ancient crustal rocks were derived from these differentiated sources, not from primitive undifferentiated mantle. Eoarchean rocks from the North Atlantic Craton and the Anshan Complex of the North China Craton have sources in an incompatible-element-depleted mantle that dates to 4.4-4.5 Ga. Hadean/Eoarchean rocks from two localities in Canada show the importance of remelting of Hadean mafic crust to produce Eoarchean felsic crust. The mafic supracrustal rocks of the Nuvvuagittuq Greenstone Belt are a possible example of the Hadean mafic basement that is often called upon to serve as the source for the high-silica rocks that define continental crust. Many, but not all, ancient terranes show a shift in the nature of the sources for crustal rocks, and possibly the physical mechanism of crust production, between 3.0-3.6 Ga. This transition may reflect the initiation of modern plate tectonics. Eoarchean/Hadean rocks from some terranes, however, also display compositional characteristics expected for convergent margin volcanism suggesting that at least some convergent margin related magmatism began in the Hadean. The persistence of isotopic variability in Nd-142/Nd-144 into the mid-Archean, and the eventual reduction in that variability by the end of the Archean, provides new information on the efficiency by which mantle convection recombined the products of Hadean silicate-Earth differentiation. The rate of crust production and recycling in the Hadean/Archean, however, is not resolved by these data beyond the observation that extreme isotopic compositions, such as expected for Hadean evolved, continent-like, crust are not observed in the preserved Eoarchean rock record. The lack of correlation between Nd-142/Nd-144 and W-182/W-184 variation in Archean rocks suggests that these two systems track different processes; the Sm-Nd system mantle-crust differentiation while Hf-W is dominated by core formation. The major silicate differentiation controlling Sm/Nd fractionation occurred at similar to 4.4 Ga, possibly as a result of the Moon-forming impact, after the extinction of Hf-182.

A new collection of spinel-peridotite xenoliths from Quaternary basaltic centers in the Tariat region of Mongolia provide a sample of the shallow lithospheric mantle beneath this area. Ninety-seven of these xenoliths were analyzed for whole rock and mineral major element composition. Both orthopyroxenes and clinopyroxenes separated from the samples were analyzed for trace element content. For a selection of samples, whole rocks were analyzed for trace element contents and clinopyroxenes for the isotopic composition of Sr, Nd, Hf, and Pb. A number of the whole rocks also were analyzed for their Re-Os systematics.

One requirement for isotope ratio measurement results with small measurement uncertainties is that the element of interest is effectively separated from the sample matrix. Efficient chemical separation of W from matrix components, especially Ti, can be challenging, particularly for large test portion masses (>1g). We present a new W separation procedure that takes advantage of the distinct complexation behaviour of Ti and W with citrate ligand in a moderately low pH, oxidising solution. This preparation procedure can reduce the Ti/W ratio of large (4-10g) basaltic (i.e., high-matrix) test portions by a factor of 10(5), relative to their original compositions, in a two-step separation procedure. The procedure additionally provides a separate, well-purified Mo fraction. We show that optimal separation requires precise selection of reagent concentrations and sample load. The procedure was employed to determine the W-182 composition of BHVO-2 as -6.7 +/- 4.2 (2 standard deviation, 2s). The principles derived from this method may prove useful for chemical separation of other elements used for geochemical and cosmochemical applications given an appropriate selection of organic acid. Future successful applications of this method may reveal that the use of organic acids as procedural reagents is a currently under-utilised tool for efficient chemical separation protocols.

The stable isotope compositions of chromium (Cr) are fractionated during magmatic differentiation of lunar mare basalts, which might be attributed to redox-related mineral crystallization. It has yet to be demonstrated whether magmatic differentiation fractionates Cr isotope composition of terrestrial samples. Here, we present high-precision stable Cr isotope measurements, reported as delta Cr-53 relative to NIST SRM 979, of well-characterized Hawaiian tholeiitic basalts from Koolau, Mauna Kea and Kilauea. The studied Makapuu-stage Koolau lavas have MgO ranging from 6.58 to 21.54 wt.%, and they have homogeneous delta Cr-53 ranging from -0.21 parts per thousand to -0.17 parts per thousand. Similarly, studied Mauna Kea lavas have MgO ranging from 9.11 to 17.90 wt.%, and they also have homogeneous delta Cr-53 ranging from -0.17 to -0.13 parts per thousand. Some Makapuu-stage Koolau and Mauna Kea lavas experienced subaerial or submarine alteration. The homogenous delta Cr-53 within each sample suites implies that the post-magmatic alterations have not significantly changed the Cr isotope compositions of these samples. Conversely, nine Kilauea Iki basalts have MgO ranging from 7.77 to 26.87 wt.% reflecting varying degrees of magmatic differentiation, and they show resolvable Cr isotope variations with delta Cr-53 ranging from -0.18 parts per thousand to 0.00 parts per thousand. The delta Cr-53 values of the Kilauea Iki samples are positively correlated with indicators of magmatic differentiation such as Cr and MgO contents, and Mg# values. The most evolved samples have the lightest isotope compositions, whereas the olivine-spinel cumulates display complementary heavy isotope compositions. This fractionation is most likely generated by the crystallization and accumulation of spinel, which is dominated by Cr3+ and, hence, enriched in heavier Cr isotopes relative to the residual melt. At a given MgO content, Kilauea and Mauna Kea lavas, both Kea-trend volcanoes, have higher delta Cr-53 than Makapuu-stage Koolau lavas, a Loa-trend volcano. This difference might reflect recycling of altered oceanic crusts or redox differences of their magmatic sources, with the mantle source of Makapuu-stage lavas being more reducing.

Compositional signatures of subducted crust in the deep-mantle sources of ocean island volcanoes in the Atlantic Ocean but not the Pacific reveal that plate motions on Earth's surface influence the characteristics of Earth's deepest interior.

Continental flood basalts are more prone to compositional modification from passage through thicker and (or) more felsic crust in comparison to their oceanic counterparts. The Steens Basalt in southeast Oregon (similar to 17 Ma) is among the oldest and most mafic members of the Columbia River Basalt Group and provides a record of the early stages of flood basalt volcanism. We evaluate the balance of mantle sources in time during the onset of Columbia River Basalt Group magmatism and assess the effect of crustal passage using stratigraphically controlled Sr, Nd, Pb, Hf, Os, and O isotopic compositions, as well as whole rock major and trace element data.

The principles governing ionization techniques used in thermal ionization mass spectrometers are relatively well understood and have remained largely unchanged for many decades. Though significant advances have been made in ion signal quantification for isotope ratio measurements, particularly for analyses of small samples by using multiple detector systems and low-noise amplifiers, the fundamental approach to sample ionization has received little focus. Modern TIMS techniques attempting to achieve parts-per-million level isotope ratios precisions are realizing limits imposed by the physics of the ionization source. A type of high-ionization efficiency thermal source employed in nuclear physics communities for decades is the so-called cavity thermal ionization source. Here, we provide a proof-of-concept study that shows cavity sources may provide a path forward to achieve a new level of precision in isotope ratio measurements from solid samples. We document our new, simple, cavity ion source design, show preliminary results from Nd isotope measurements, and discuss these new data in the context of current precision limits imposed during traditional thermal ionization methods. We show that, within the limits of our testbed mass spectrometer, mass fractionation within the cavity ion source appears similar to that from filament ion sources. We also demonstrate that oxide-versus-metal ion production plays a significant role in cavity ionization processes for Nd. Cavity ion sources may provide a viable path forward to achieving isotope ratios precisions at the sub-ppm precision level.

Detection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long-term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short-lived Sm-146-Nd-142 and Hf-182-W-182 systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between Nd-142/Nd-144 and W-182/W-184 ratios among Reunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of Hf-182. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long-lived Nd-143-Hf-176 isotopic systematics mirror classical models invoking mixing of recycled trace-element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources.