A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites. However, the accessible Earth has a greater ...(142)Nd/(144)Nd ratio than do chondrites. Because (142)Nd is the decay product of the now-extinct (146)Sm (which has a half-life of 103 million years), this (142)Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation and implies the formation of a complementary (142)Nd-depleted reservoir that either is hidden in the deep Earth, or lost to space by impact erosion. Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate, and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution. Here we show that, compared with chondrites, Earth's precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher (142)Nd/(144)Nd ratios; after correction for this effect, the (142)Nd/(144)Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The (142)Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth's bulk chemical composition.
Comparisons between the predicted abundances of short-lived
r-nuclides (
107Pd,
129I,
182Hf, and
244Pu) in the interstellar medium (ISM) and the observed abundances in the early solar system (ESS) ...conclusively showed that these nuclides cannot simply be derived from galactic chemical evolution (GCE) if synthesized in a unique stellar environment. It was thus suggested that two different types of stars were responsible for the production of light and heavy
r-nuclides. Here, new constraints on the
244Pu
/
238U production ratio are used in an open nonlinear GCE model. It is shown that the two
r-process scenario cannot explain the low abundance of
244Pu in the ESS and that this requires either than actinides be produced at an additional site (A-events) or more likely, that
129I and
244Pu be inherited from GCE and
107Pd and
182Hf be injected in the ESS by the explosion of a nearby supernova.
Analyses of chemical sedimentary precipitates such as banded iron formation (BIF) provide a direct means to explore the nature and composition of the early hydrosphere. The recently discovered ...>3750Myr old Nuvvuagittuq Supracrustal Belt (NSB) in the Northeast Superior Province (Québec, Canada) hosts a suite of iron oxide-rich (±pyroxene and amphibole) units that are interpreted to be the metamorphosed equivalents of Fe oxide-facies BIF, and a collection of BIF-like Ca–Fe–Mg silicate rocks. The NSB rocks provide a rare glimpse of trace metal availability in Eoarchean (ca. 3800Ma) seawater. As they may be contemporaneous with the relatively well-studied Isua Supracrustal Belt of southern West Greenland, their comparison provides an opportunity to enhance our basic understanding of the Eoarchean oceans at a global scale. Work since the initial discovery of the NSB in 2001 has established the basic lithological, geochemical and petrographic characteristics of these BIF and BIF-like rocks. Here we review the current state of knowledge of NSB rocks of probable chemical sedimentary origin, including aspects of their geology, likely origin and age. We conclude by examining the implications of results thus far for our understanding of early seawater compositions, and for the emergence of life in the context of early metallo-enzyme evolution.
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► The >3.75Ga NSB has Fe-oxide & Mg–Fe silicate-rich iron formation and jaspilite. ► They have low Al, Ti and HFSE; heavy δ56Fe and seawater REE+Y profiles. ► The NSB provides new opportunities to study early hydro–geo–biosphere systematics.
The deuterium to protium (D/H) ratio of the deep mantle may be a remnant of the hydrogen isotopic composition of Earth forming planetesimals, which later evolved as a result of the late accretion of ...asteroids and comets. If so, the mass of asteroids and comets incident on Earth since the time of its accretion is estimated to be 4×10
20–2×10
22 kg. The combined use of water D/H ratios, the lunar cratering record, and terrestrial mantle siderophiles would favor a rather low mass fraction of comets among impacting bodies (⋦0.01). Asteroids, comets, and the early Earth contributed to 0–0.5, 0–0.1, and 0.5–0.9 of Earth's water inventory, respectively. A two stage model is advocated in which escape to space of terrestrial volatiles predated the late accretion of extraterrestrial gases. We wish to emphasize that our interpretations and conclusions might evolve in the future when additional data on asteroids, comets, and Earth's interior become available.
The data available for short-lived p-nuclides are used in an open nonlinear model of the chemical evolution of the Galaxy in order to discuss the origin of extinct radionuclides, the stellar sources ...of
p-nuclides, and the chronology of solar system formation. It is concluded that the observed abundances of
97Tc,
98Tc,
92Nb, and
146Sm in the early solar system are consistent with nucleosynthetis in type II supernovae during continuous chemical evolution of the Galaxy and a subsequent short isolation of the presolar molecular cloud from fresh nucleosynthetic inputs. However; further work on supernova models is needed before
p-radionuclides will comprise reliable cosmochronometers. Despite these limitations, we argue that niobium-92 can be used to test whether the
rp-process contributed to the synthesis of light p-nuclides in the Mo-Ru region.
High-temperature isotopic variations documented in natural materials span a narrow range and call for precise and accurate isotopic analyses. Precisions better than ~
0.03‰ (95% confidence interval) ...for
δ
56Fe have been obtained by High-Resolution Multi-Collector Inductively Coupled Plasma Mass Spectrometry (HR-MC-ICPMS). Whether these uncertainties encompass all sources of error has not been carefully evaluated. The current study examines all parameters that can affect accuracy and precision of Fe isotopic measurements by HR-MC-ICPMS. It is demonstrated that accurate
δ
56Fe measurements can be routinely achieved within the quoted uncertainties.
In the modern ocean, U reduction and incorporation into anoxic sediments imparts a large isotopic fractionation of approximately +0.6‰ that shifts the seawater δ238U value (238U/235U, expressed as ...δ238U per mil deviation relative to CRM-112a) relative to continental runoff. Given the long residence time of U in the modern oceans (∼400 kyr), the isotopic composition of carbonates (taken as a proxy for seawater) reflects the global balance between anoxic and other sinks. The U isotopic composition of open-marine carbonates has thus emerged as a proxy for reconstructing past changes in the redox state of the global ocean. A tenet of this approach is that the δ 238U values of seawater and anoxic sediments should always be fractionated by the same amount.
In order to test this hypothesis, we have measured the U concentrations and isotopic compositions of carbonates spanning ages from 3250 Ma to present. A first-order expectation for the Archean and possibly Proterozoic is that near-quantitative U removal to extensive anoxic sediments should have shifted the uranium isotopic composition of seawater and carbonates towards lower values. Instead, the measurements reveal that many Archean and Proterozoic carbonates have unfractionated δ238U values similar to those of continents and riverine runoff. These results are inconsistent with the view that the U isotopic composition of seawater simply reflects the areal extent of anoxic sediments in the past.
We consider two plausible explanations for why the U isotopic composition of Archean and Proterozoic carbonates is not fractionated from the crustal and riverine composition: (1) the residence time of U could have been much shorter in the Precambrian oceans when anoxic settings were much more extensive, and (2) the process of incorporation of U into anoxic sediments in the Precambrian imparted a smaller U isotopic fractionation than in the modern because of differences in the efficiency or mechanism of uranium removal. This study highlights the challenges inherent to applying knowledge of the modern marine U isotopic cycle to periods of Earth’s history when ocean-floor anoxia was much more extended, anoxic basins were ferruginous, and atmospheric oxygen content was significantly lower than present.
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