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.

Komatiites and sedimentary rocks sampled during the International Continental Drilling Program (BARB1-2-3-4-5) in the Barberton greenstone belt, South Africa, were analyzed for Sm-146-Nd-142 systematics. Resolved negative mu Nd-142 values (down to -7.7 +/- 2.8) were identified in komatiites from the 3.48 Ga Komati Formation and this signature correlates with low Hf/Sm ratios measured in these samples. The negative mu Nd-142 point to a source with subchondritic Sm/Nd ratio which formed during the Hadean. No analytically resolvable Nd-142 anomalies were measured in crustal detritus-rich, Si-rich, Ca-Fe-rich sediments and cherts from the Buck Reef (3.42 Ga) and the Fig Tree Group (3.23-3.28 Ga). Our new measurements are incorporated into a larger set of Sm-147-Nd-143 and Lu-176-Hf-176 data to better understand the Nd-142,Nd-143-Hf-176 isotope signatures in the mantle source at the time of komatiite crystallization. Our calculations show that the Nd-142,Nd-143-Hf-176 isotope signatures and Hf/Sm ratios cannot be produced by recycling into the komatiite source of detrital sediments like those sampled in the Barberton area. Only cherts have the required trace element characteristics - low Hf/Sm, radiogenic epsilon Hf-176 -but the trace element concentrations in the cherts are so low that unrealistic amounts of chert would need to be added.

Globally distributed kimberlites with broadly chondritic initial Nd-143-Hf-176 isotopic systematics may be derived from a chemically homogenous, relatively primitive mantle source that remained isolated from the convecting mantle for much of the Earth's history. To assess whether this putative reservoir may have preserved remnants of an early Earth process, we report W-182/W-184 and Nd-142/Nd-144 data for "primitive" kimberlites from 10 localities worldwide, ranging in age from 1,153 to 89 Ma. Most are characterized by homogeneous mu W-182 and mu Nd-142 values averaging -5.9 +/- 3.6 ppm (2SD, n = 13) and +2.7 +/- 2.9 ppm (2SD, n = 6), respectively. The remarkably uniform yet mod-estly negative mu W-182 values, coupled with chondritic to slightly suprachondritic initial Nd-143/Nd-144 and Hf-176/Hf-177 ratios over a span of nearly 1,000 Mya, provides permissive evidence that these kim-berlites were derived from one or more long-lived, early formed mantle reservoirs. Possible causes for negative mu W-182 values among these kimberlites include the transfer of W with low mu W-182 from the core to the mantle source reservoir(s), creation of the source reser-voir(s) as a result of early silicate fractionation, or an overabundance of late-accreted materials in the source reservoir(s). By contrast, two younger kimberlites emplaced at 72 and 52 Ma and characterized by distinctly subchondritic initial Hf-176/Hf-177 and Nd-143/Nd-144 have mu W-182 values consistent with the modern upper mantle. These isotopic compositions may reflect contamination of the ancient kimberlite source by recycled crustal components with mu W-182 >= 0.