Joel A. Baker

Rapid Timescales for Accretion and Melting of Differentiated Planetesimals Inferred from 26Al-26Mg Chronometry

Constraining the timescales for the assembly and differentiation of planetary bodies in our young solar system is essential for a complete understanding of planet-forming processes. This is best achieved through the study of the daughter products of extinct radionuclides with short half-lives, as they provide unsurpassed time resolution as compared to long-lived chronometers. Here we report high-precision Mg isotope measurements of bulk samples of basalt, gabbro, and pyroxenite meteorites obtained by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). All samples from the eucrite and mesosiderite parent bodies (EPB and MPB) with suprachondritic Al/Mg ratios have resolvable 26Mg excesses compared to matrix-matched samples from the Earth, the Moon, Mars, and chondrites. Basaltic magmatism on the EPB and MPB thus occurred during the life span of the now-extinct 26Al nuclide. Initial 26Al/27Al values range from (1.26 ± 0.37) × 10-6 to (5.12 ± 0.81) × 10-6 at the time of magmatism on the EPB and MPB, and are among the highest 26Al abundances reported for igneous meteorites. These results indicate that widespread silicate melting and differentiation of rocky bodies occurred within 3 million years of solar system formation, when 26Al and 60Fe were extant enough to induce planetesimal melting. Finally, thermal modeling constrains the accretion of these differentiated asteroids to within 1 million years of solar system formation, that is, prior to the accretion of chondrite parent bodies.

New Lu-Hf and Pb-Pb Age Constraints on the Earliest Animal Fossils

Abstract The Neoproterozoic Doushantuo Formation, South China, preserves a unique assemblage of early multicellular fossils and overlies rocks, which are thought to have formed during an ice age of global extent. The age of this formation is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. Until now, direct dating of sedimentary formations such as the Doushantuo has been difficult and associated with large uncertainties. Here, we show that dating of Doushantuo phosphorites by a novel Lu–Hf dating method and conventional Pb–Pb geochronometry independently yield ages of 584±26 Ma and 599.3±4.2 Ma, respectively. These ages are in agreement with bio- and chemostratigraphical observations and show that the Doushantuo animal remains predate diverse Ediacaran fossil assemblages, making them the oldest unambiguous remains of metazoans currently known. Furthermore, the Pb–Pb age for the post-glacial Doushantuo rocks suggests that the Neoproterozoic glaciation in China might predate glacial rocks in Eastern North America commonly associated with the younger (Marinoan) of two major Neoproterozoic glaciations. The combination of Lu–Hf and Pb–Pb dating shows considerable potential for dating other phosphorite successions and future application of these methods could therefore provide further constraints on Proterozoic biological and environmental history.

Matching conjugate volcanic rifted margins: 40Ar/39Ar chrono-stratigraphy of pre- and syn-rift bimodal flood volcanism in Ethiopia and Yemen

Abstract 40 Ar/ 39 Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from the pre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemen conjugate rifted volcanic margins. Sixteen new 40 Ar/ 39 Ar dates confirm that basaltic flood volcanism in Ethiopia was contemporaneous with flood volcanism on the conjugate margin in Yemen. The new data also establish that flood volcanism initiated prior to 30.9 Ma in Ethiopia and may predate initiation of similar magmatic activity in Yemen by ∼0.2–2.0 Myr. Rhyolitic volcanism in Ethiopia commenced at 30.2 Ma, contemporaneous with the first rhyolitic ignimbrite unit in Yemen at ∼30 Ma. Accurate and precise 40 Ar/ 39 Ar dates on initial rhyolitic ignimbrite eruptions suggest that silicic flood volcanism in Afro-Arabia post-dates the Oligocene Oi2 global cooling event, ruling out a causative link between these explosive silicic eruptions (with individual volumes ≥200 km 3 ) and climatic cooling which produced the first major expansion of the Antarctic ice sheets. Ethiopian volcanism shows a progressive and systematic younging from north to south along the escarpment and parallel to the rifted margin, from pre-rift flood volcanics in the north to syn-rift northern Main Ethiopian Rift volcanism in the south. A dramatic decrease in volcanic activity in Ethiopia between 25 and 20 Ma correlates with a prominent break-up unconformity in Yemen (26–19 Ma), both of which mark the transition from pre- to syn-rift volcanism (∼25–26 Ma) triggered by the separation of Africa and Arabia. The architecture of the Ethiopian margin is characterized by accumulation and preservation of syn-rift volcanism, while the Yemen margin was shaped by denudational unloading and magmatic starvation as the Arabian plate rifted away from the Afar plume. A second magmatic hiatus and angular unconformity in the northern Main Ethiopian Rift is evident at 10.6–3.2 Ma, and is also observed throughout the Arabian plate in Jordanian, Saudi Arabian and Yemeni intraplate volcanic fields and is possibly linked to tectonic re-organization and initiation of sea floor spreading in the Gulf of Aden and the Red Sea at 10 and 5 Ma, respectively.

Tropical sea temperatures in the high-latitude South Pacific during the Eocene

Sea-surface temperature (SST) estimates of ~30 °C from planktic foraminifera and archaeal membrane lipids in bathyal sediments in the Canterbury Basin, New Zealand, support paleontological evidence for a warm subtropical to tropical climate in the early Eocene high-latitude (55°S) southwest Pacific. Such warm SSTs call into question previous estimates based on oxygen isotopes and present a major challenge to climate modelers. Even under hypergreenhouse conditions (2240 ppm CO2), modeled summer SSTs for the New Zealand region do not exceed 20 °C.

Characteristics of volcanic rifted margins

Volcanic rifted margins evolve by a combination of extrusive flood volcanism, intrusive magmatism, extension, uplift, and erosion. The temporal and spatial relationships between these processes are influenced by the plate tectonic regime; the preexisting lithosphere (thickness, composition, geothermal gradient); the upper mantle (temperature and character); the magma production rate; and the prevailing climatic system. Of the Atlantic rifted margins, 75% are believed to be volcanic, the cumulative expression of thermotectonic processes over 200 m.y. Volcanic rifted margins also characterize Ethiopia-Yemen, India-Australia, and Africa-Madagascar. The transition from continental flood volcanism (or formation of a large igneous province) to ocean ridge processes (mid-ocean ridge basalt) is marked by a prerift to synrift transition with formation of a subaerial and/or submarine seaward-dipping reflector series and a significant thickness (to 15 km) of juvenile, high-velocity lower crust seaboard of the continental rifted margin. Herein we outline the similarities and differences between volcanic rifted margins worldwide and list some of their diagnostic features.

Extremely Brief Formation Interval for Refractory Inclusions and Uniform Distribution of 26Al in the Early Solar System

Calcium-aluminum-rich inclusions (CAIs) are millimeter-sized refractory objects commonly found in chondritic meteorites and are the oldest solids formed in our solar system. Primary CAI formation may have occurred through condensation and/or evaporation processes near the proto-Sun or, alternatively, during localized events in the asteroid belt. As such, these objects provide us with a unique window into the earliest development of the Sun and into the evolution of the protoplanetary disk. Here we report a 26Al-26Mg isochron for bulk CAIs from four CV carbonaceous chondrites, which yields an initial 26Al/27Al of (5.85 ± 0.05) × 10-5, suggesting that primary formation of the CV CAIs may have occurred within an interval as brief as 20,000 years. This timescale is inconsistent with the secular evolution of T Tauri stars but may be consistent with CAI formation during the infall stage of the protostellar evolution of the Sun. High-precision Mg isotope measurements of samples from the Earth, Moon, Mars, and bulk chondrite meteorites show that these have identically elevated 26Mg abundances compared to the initial 26Mg abundance (δ26Mg* = -0.0317‰ ± 0.0038‰) defined by the CAI isochron. This observation unequivocally demonstrates the homogeneous distribution of 26Al within the accretion region of the terrestrial planets. However, the initial 26Mg abundance of CAIs implies a brief history of elevated Al/Mg in CAI precursor material, which may represent primary condensation of refractory silicates and oxides from the solar nebula.

Osmium–oxygen isotopic evidence for a recycled and strongly depleted component in the Iceland mantle plume

Abstract Highly magnesian lavas characterised by strong light rare earth element depletion are a feature of Theistareykir and the Reykjanes Peninsula of Iceland, which are marginal to the proposed axis of the mantle plume. These lavas define positive covariations between whole rock osmium and olivine oxygen isotope ratios ( 187 Os/ 188 Os=0.1269–0.1369; δ 18 O olivine =4.2–5.7‰) that extend the array defined by Hawaiian samples to more unradiogenic Os isotope ratios and lower δ 18 O. The Os–O variation is difficult to explain in terms of high level crustal assimilation of Icelandic crust, with the possible exception of a subset of large volume lava flows from Theistareykir. The strong coupling of Os and O isotopic compositions of the lavas in addition to large excesses in large ion lithophile elements (Rb, Ba, Sr), positive Eu anomalies, and deficiencies in Hf and Zr relative to the rare earth elements clearly distinguishes these recent picrites from mid-ocean ridge basalts. The Reykjanes and Theistareykir lavas appear to represent melting of a very ancient (Archaean) mantle source which has isotopic and elemental characteristics suggestive of recycled oceanic lithosphere. We suggest that tapping of the refractory and depleted part of such a mantle plume (i.e. low 187 Os/ 188 Os and δ 18 O) is only possible due to the fortuitous location of the Iceland plume beneath a spreading ridge, which permits more extensive melting than would occur in an intraplate setting (e.g. Hawaii).

Precise and accurate in situ Pb-Pb dating of apatite, monazite, and sphene by laser ablation multiple-collector ICP-MS

To evaluate in situ Pb dating by laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), we analysed apatite, sphene, and monazite from Paleoproterozoic metamorphic rocks from West Greenland. Pb isotope ratios were also determined in the National Institute of Standards and Technology (NIST) 610 glass standard and were corrected for mass fractionation by reference to the measured thallium isotope ratio. The NIST 610 glass was used to monitor Pb isotope mass fractionation in the low Tl/Pb accessory minerals. Replicate analyses of the glass (1 to 2 min) yielded ratios with an external reproducibility comparable to conventional analyses of standard reference material 981 by thermal ionisation mass spectrometry (TIMS). Mineral grains were generally analysed with a 100-μm laser beam, although some monazite crystals were analysed at smaller spot sizes (10 and 25 μm). The common Pb isotope ratios required for age calculations were either measured on coexisting plagioclase by LA-MC-ICP-MS or could be ignored, as individual crystals exhibit sufficient Pb isotopic heterogeneity to perform isochron calculations on replicate analyses of single crystals. Mean mineral ages with the 204Pb ion beam measured in the multiplier were as follows: apatite, 1715 ± 23 m.y.; sphene, 1789 ± 11 m.y.; and monazite, 1783 to 1888 m.y., with relative uncertainties on individual monazite ages of <0.2% but highly reproducible age determinations on single monazite crystals (≪1%). Isochron ages calculated from several mineral analyses without assumption of common Pb also yield precise age determinations. Apatite and monazite Pb ages determined by in situ Pb isotope analysis are identical to those determined by conventional TIMS analysis of bulk mineral separates, and the analytical uncertainties of these short laser analyses with no prior mechanical or chemical separation are comparable to those obtained by TIMS. Detailed examination of the sphene in situ age data does, however, show a small discrepancy between the LA-MC-ICP-MS and TIMS ages (∼1% younger). High-resolution mass scans of the sphene during ablation clearly showed several small and as yet unidentified isobaric interferences that overlap with the 207Pb peak at the resolution conditions for measurement of isotope ratios. These might account for the age discrepancy between the LA-MC-ICP-MS and TIMS sphene ages. LA-MC-ICP-MS is a rapid, accurate, and precise method for in situ determination of Pb isotope ratios that can be used for geochronological studies in a manner similar to an ion microprobe, albeit currently at a somewhat degraded spatial resolution. Further modifications to the LA-MC-ICP-MS system, such as improved sensitivity, ion transmission, and LA methodology, may lead to this type of instrument becoming the method of choice for many types of in situ Pb isotope dating.

Rb isotope dilution analyses by MC-ICPMS using Zr to correct for mass fractionation: towards improved Rb–Sr geochronology?

A new technique is presented where mass fractionation during Rb isotope dilution analyses by multi-collector inductively coupled plasma mass spectrometry is corrected for by measuring the amount of fractionation on admixed Zr. Replicate analyses of natural Rb interspersed with analyses of 87Rb tracer enriched samples yield a mean 87Rb/85Rb=0.38540±19 (0.05%, 2 s.d.), assuming a natural 90Zr/91Zr of 4.588. Each Rb analysis takes 1 min, consumes 20 ng of Rb and has an internal precision of ∼0.02% (2 s.e.). Washouts between samples take 5 min. Persistent but small stable Rb backgrounds are overcome by an on-peak-zeroes (OPZ) measurement prior to data acquisition. Close examination of measured 87Rb/85Rb and 90Zr/91Zr ratios indicate small changes in relative fractionation of Rb and Zr during plasma ionisation occur when different sample introduction techniques are used (e.g., ‘wet’ vs. ‘dry’ nebulisation), although the differences are insignificant compared to the level of precision required for isotope dilution measurements. Replicate analyses of whole rock samples suggest a reproducibility for Rb concentration measurements of ≤0.5% and 87Rb/86Sr measurements of 0.2% when interfering Sr is reduced to satisfactory levels. However, it is difficult to ascertain to what extent this reproducibility reflects the limit of the technique or powder heterogeneity. Much of the error involved in the Rb isotope dilution and Sr isotope ratio measurements by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) is derived from uncertainties as to which 87Sr/86Sr (and 87Rb/85Rb) ratios to use when correcting for isobaric interferences due to the presence of spike Sr and Rb at mass 87. If isobaric interferences are minimised by efficient separation of Rb from Sr during cation exchange chemistry, the use of natural ratios for isobaric interference corrections yields the most reproducible data, indicating that the interferences are derived from environmental blank. Larger isobaric interferences at mass 87 are indicative of inefficient chemical separations, and the measured ratio from the complementary analysis provide more reproducible data. Burning off of Rb during conventional thermal ionisation mass spectrometry (TIMS) Sr isotope analysis nullifies this isobaric interference, and therefore, TIMS remains the method of choice for reliable and precise 87Sr/86Sr determinations on spiked samples. Application of our technique to minerals separated from Tertiary to Palaeozoic plutons yields age data consistent with previous determinations. Where different two-point isochron ages can be calculated for individual plutons, the ages reproduce to ≤±0.3%. The method represents an initial improvement in Rb isotope dilution measurements over TIMS by allowing a quantifiable correction to be made for mass fractionation, confirmed by duplicate analyses of standards and samples by both TIMS and MC-ICPMS. Mass fractionation corrected Rb isotope dilution analyses should result in: (1) improved Rb–Sr geochronology in examples where the Rb–Sr ratio provides the largest source of error; (2) application of this improved method to Rb–Sr geochronology on smaller samples such as single mica-flakes and micro-drill samples and; (3) by comparison with other geochronological techniques, more detailed cooling and crystallisation histories of igneous and metamorphic rocks. Taking advantage of these improvements requires a reevaluation of the Rb decay constant, which this technique should also permit.

RAPID TIMESCALES FOR MAGMA OCEAN CRYSTALLIZATION ON THE HOWARDITE-EUCRITE-DIOGENITE PARENT BODY

Asteroid 4 Vesta has long been postulated as the source for the howardite-eucrite-diogenite (HED) achondrite meteorites. Here we show that Al-free diogenite meteorites record variability in the mass-independent abundance of 26Mg (26Mg*) that is correlated with their mineral chemistry. This suggests that these meteorites captured the Mg-isotopic evolution of a large-scale differentiating magma body with increasing 27Al/24Mg during the lifespan of the short-lived 26Al nuclide (t 1/2 ~ 730,000 yr). Thus, diogenites and eucrites represent crystallization products of a large-scale magma ocean associated with the differentiation and magmatic evolution of the HED parent body. The 26Mg* composition of the most primitive diogenites requires onset of the magma ocean crystallization within 0.6–0.4 + 0.5 Myr of solar system formation. Moreover, 26Mg* variations among diogenites and eucrites imply that near complete solidification of the HED parent body occurred within the following 2-3 Myr. Thermal models predict that such rapid cooling and magma ocean crystallization could only occur on small asteroids (<100 km), implying that 4 Vesta is not the source of the HED meteorites.