Characterization of the hafnium-tungsten systematics ((182)Hf decaying to (182)W and emitting two electrons with a half-life of 8.9 million years) of the lunar mantle will enable better constraints ...on the timescale and processes involved in the currently accepted giant-impact theory for the formation and evolution of the Moon, and for testing the late-accretion hypothesis. Uniform, terrestrial-mantle-like W isotopic compositions have been reported among crystallization products of the lunar magma ocean. These observations were interpreted to reflect formation of the Moon and crystallization of the lunar magma ocean after (182)Hf was no longer extant-that is, more than about 60 million years after the Solar System formed. Here we present W isotope data for three lunar samples that are more precise by a factor of ≥4 than those previously reported. The new data reveal that the lunar mantle has a well-resolved (182)W excess of 20.6 ± 5.1 parts per million (±2 standard deviations), relative to the modern terrestrial mantle. The offset between the mantles of the Moon and the modern Earth is best explained by assuming that the W isotopic compositions of the two bodies were identical immediately following formation of the Moon, and that they then diverged as a result of disproportional late accretion to the Earth and Moon. One implication of this model is that metal from the core of the Moon-forming impactor must have efficiently stripped the Earth's mantle of highly siderophile elements on its way to merge with the terrestrial core, requiring a substantial, but still poorly defined, level of metal-silicate equilibration.
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New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts ...per million lower. The tungsten isotopic data negatively correlate with ³He/⁴He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low ³He/⁴He.
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► We present a new high precision
182W/
184W measurement protocol by N-TIMS. ► Instrumental mass fractionation requires correction using a double normalization procedure. ► This ...procedure allows correction of isotope fractionation for both, tungsten and oxygen. ► This method permits an external reproducibility of <5
ppm for natural samples.
We describe a new technique for measuring the isotopic abundance of
182W with improved precision in natural silicate samples. After chemical purification of W through a four-step ion exchange chromatographic separation, the W isotopic composition is measured as WO
3
− by negative thermal ionization mass spectrometry using a
Thermo-Fisher Triton instrument. Amplifier biases are cancelled by using amplifier rotation, and Faraday cup biases are monitored by using a cup configuration that allows two-line data acquisition. Data are initially corrected for oxide interferences, assuming a predefined O isotope composition, and for mass fractionation, by normalization to
186W/
184W or
186W/
183W, using an exponential law. Despite these corrections, isotopic ratios exhibit small but strongly correlated variations. This second-order effect may reflect a mass dependent change of O isotope composition in the measured W (and Re) oxides, and is corrected by normalization to
183W/
184W using a linear law. Repeated analysis of an
Alfa Aesar W standard (
n
=
39), and of three dissolutions of a La Palma (Canary Islands) basalt, applying the double normalization procedure, demonstrate external reproducibility of
182W/
184W within ±4.5
ppm (2
σ SD). Repeated measurement of a gravimetrically prepared mixture of a natural W standard and a
182W enriched spike shows that differences in
182W/
184W of ∼10
ppm can be well resolved using this method. The external reproducibility of ±4.5
ppm is ∼5 times more precise than conventional W isotope measurements by MC-ICP-MS. The new technique constitutes an ideal tool for investigating the W isotope composition of terrestrial rocks for potential contributions from the core, and late accreted extraterrestrial materials.
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The
182Hf–
182W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the ...following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3
±
0.7
Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than ∼1
Myr. Chondrules from H chondrites formed 1.7
±
0.7
Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their
26Al–
26Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as ∼3
Myr after CAI formation and may have continued until ∼10
Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of
26Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf–W ages for eucrite metals postdate CAI formation by ∼20
Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf–W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained.
The strong fractionation of lithophile Hf from siderophile W during core formation makes the Hf–W system an ideal chronometer for this major differentiation event. However, for larger planets such as the terrestrial planets the calculated Hf–W ages are particularly sensitive to the occurrence of large impacts, the degree to which impactor cores re-equilibrated with the target mantle during large collisions, and changes in the metal-silicate partition coefficients of W due to changing
fO
2 in differentiating planetary bodies. Calculated core formation ages for Mars range from 0 to 20
Myr after CAI formation and currently cannot distinguish between scenarios where Mars formed by runaway growth and where its formation was more protracted. Tungsten model ages for core formation in Earth range from ∼30
Myr to >100
Myr after CAIs and hence do not provide a unique age for the formation of Earth. However, the identical
182W/
184W ratios of the lunar and terrestrial mantles provide powerful evidence that the Moon-forming giant impact and the final stage of Earth’s core formation occurred after extinction of
182Hf (i.e., more than ∼50
Myr after CAIs), unless the Hf/W ratios of the bulk silicate Moon and Earth are identical to within less than ∼10%. Furthermore, the identical
182W/
184W of the lunar and terrestrial mantles is difficult to explain unless either the Moon consists predominantly of terrestrial material or the W in the proto-lunar magma disk isotopically equilibrated with the Earth’s mantle.
Hafnium–tungsten chronometry also provides constraints on the duration of magma ocean solidification in terrestrial planets. Variations in the
182W/
184W ratios of martian meteorites reflect an early differentiation of the martian mantle during the effective lifetime of
182Hf. In contrast, no
182W variations exist in the lunar mantle, demonstrating magma ocean solidification later than ∼60
Myr, in agreement with
147Sm–
143Nd ages for ferroan anorthosites. The Moon-forming giant impact most likely erased any evidence of a prior differentiation of Earth’s mantle, consistent with a
146Sm–
142Nd age of 50–200
Myr for the earliest differentiation of Earth’s mantle. However, the Hf–W chronology of the formation of Earth’s core and the Moon-forming impact is difficult to reconcile with the preservation of
146Sm–
142Nd evidence for an early (<30
Myr after CAIs) differentiation of a chondritic Earth’s mantle. Instead, the combined
182W–
142Nd evidence suggests that bulk Earth may have superchondritic Sm/Nd and Hf/W ratios, in which case formation of its core must have terminated more than ∼42
Myr after formation of CAIs, consistent with the Hf–W age for the formation of the Moon.
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Genetic contributions to the final stages of planetary growth, including materials associated with the giant Moon-forming impact, late accretion, and late heavy bombardment are examined using ...siderophile elements. Isotopic similarities between the Earth and Moon for both lithophile and siderophile elements collectively lead to the suggestion that the genetics of the building blocks for Earth, and the impactor involved in the Moon-forming event were broadly similar, and shared some strong genetic affinities with enstatite chondrites. The bulk genetic fingerprint of materials subsequently added to Earth by late accretion, defined as the addition of ~0.5wt.% of Earth's mass to the mantle, following cessation of core formation, was characterized by 187Os/188Os and Pd/Ir ratios that were also similar to those in some enstatite chondrites. However, the integrated fingerprint of late accreted matter differs from enstatite chondrites in terms of the relative abundances of certain other HSE, most notably Ru/Ir. The final ≤0.05wt.% addition of material to the Earth and Moon, believed by some to be part of a late heavy bombardment, included a component with much more fractionated relative HSE abundances than evidenced in the average late accretionary component.
Heterogeneous 182W/184W isotopic compositions of some ancient terrestrial rocks suggest that some very early-formed mantle domains remained chemically distinct for long periods of time following primary planetary accretion. This evidence for sluggish mixing of the early mantle suggests that if late accretionary contributions to the mantle were genetically diverse, it may be possible to isotopically identify the disparate primordial components in the terrestrial rock record using the siderophile element tracers Ru and Mo.
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We obtained Hf–W metal-silicate isochrons for several H chondrites of petrologic types 4, 5, and 6 to constrain the accretion and high-temperature thermal history of the H chondrite parent body. The ...silicate fractions have 180Hf/184W ratios up to ∼51 and 182W/184W ratios up to ∼33 ε units higher than the whole-rock. These high 180Hf/184W and radiogenic W isotope ratios result in highly precise Hf–W ages. The Hf–W ages of the H chondrites become younger with increasing metamorphic grade and range from ΔtCAI=1.7±0.7 Ma for the H4 chondrite Ste. Marguerite to ΔtCAI=9.6±1.0 Ma for the H6 chondrites Kernouvé and Estacado. Closure temperatures for the Hf–W system in H chondrites were estimated from numerical simulations of W diffusion in high-Ca pyroxene, the major host of radiogenic 182W in H chondrites, and range from 800±50 °C for H4 chondrites to 875±75 °C for H6 chondrites. Owing to these high closure temperatures, the Hf–W system closed early and dates processes associated with the earliest evolution of the H chondrite parent body. Consequently, the high-temperature interval of ∼8 Ma as defined by the Hf–W ages is much shorter than intervals obtained from Rb–Sr and Pb–Pb dating. For H4 chondrites, heating on the parent body probably was insufficient to cause W diffusion in high-Ca pyroxene, such that the Hf–W age of ΔtCAI=1.7±0.7 Ma for Ste. Marguerite was not reset and most likely dates chondrule formation. This is consistent with Al–Mg ages of ∼2 Ma for L and LL chondrules and indicates that chondrules from all ordinary chondrites formed contemporaneously. The Hf–W ages for H5 and H6 chondrites of ΔtCAI=5.9±0.9 Ma and ΔtCAI=9.6±1.0 Ma correspond closely to the time of the thermal peak within the H chondrite parent body. Combined with previously published chronological data the Hf–W ages reveal an inverse correlation of cooling rate and metamorphic grade: shortly after their thermal peak H6 chondrites cooled at ∼10 °C/Ma, H5 chondrites at ∼30 °C/Ma and H4 chondrites at ∼55 °C/Ma. These Hf–W age constraints are most consistent with an onion-shell structure of the H chondrite parent body that was heated internally by energy released from 26Al decay. Parent body accretion started after chondrule formation at 1.7±0.7 Ma and probably ended before 5.9±0.9 Ma, when parts of the H chondrite parent body already had cooled from their thermal peak. The well-preserved cooling curves for the H chondrites studied here indicate that these samples derive from a part of the H chondrite parent body that remained largely unaffected by impact disruption and reassembly but such processes might have been important in other areas. The H chondrite parent body has a 180Hf/184W ratio of 0.63±0.20, distinctly lower than the 180Hf/184W=1.21±0.06 of carbonaceous chondrite parent bodies. This difference reflects Hf–W fractionation within the first ∼2 Ma of the solar system, presumably related to processes in the solar nebula.
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Early differentiation of the Earth and the Moon Bourdon, Bernard; Touboul, Mathieu; Caro, Guillaume ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2008, Volume:
366, Issue:
1883
Journal Article
Peer reviewed
We examine the implications of new 182W and 142Nd data for Mars and the Moon for the early evolution of the Earth. The similarity of 182W in the terrestrial and lunar mantles and their apparently ...differing Hf/W ratios indicate that the Moon-forming giant impact most probably took place more than 60 Ma after the formation of calcium-aluminium-rich inclusions (4.568 Gyr). This is not inconsistent with the apparent U-Pb age of the Earth. The new 142Nd data for Martian meteorites show that Mars probably has a super-chondritic Sm/Nd that could coincide with that of the Earth and the Moon. If this is interpreted by an early mantle differentiation event, this requires a buried enriched reservoir for the three objects. This is highly unlikely. For the Earth, we show, based on new mass-balance calculations for Nd isotopes, that the presence of a hidden reservoir is difficult to reconcile with the combined 142Nd-143Nd systematics of the Earth's mantle. We argue that a likely possibility is that the missing component was lost during or prior to accretion. Furthermore, the 142Nd data for the Moon that were used to argue for the solidification of the magma ocean at ca 200 Myr are reinterpreted. Cumulate overturn, magma mixing and melting following lunar magma ocean crystallization at 50-100 Myr could have yielded the 200 Myr model age.
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New Sm‐Nd, Lu‐Hf, Hf‐W, and Re‐Os isotope data, in combination with highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) and W abundances, are reported for the 3.55 Ga Schapenburg ...komatiites, South Africa. The Schapenburg komatiites define a Re‐Os isochron with an age of 3550 ± 87 Ma and initial γ187Os = +3.7 ± 0.2 (2SD). The absolute HSE abundances in the mantle source of the Schapenburg komatiite system are estimated to be only 29 ± 5% of those in the present‐day bulk silicate Earth (BSE). The komatiites were derived from mantle enriched in the decay products of the long‐lived 147Sm and 176Lu nuclides (initial ɛ143Nd = +2.4 ± 0.1, ɛ176Hf = +5.7 ± 0.3, 2SD). By contrast, the komatiites are depleted, relative to the modern mantle, in 142Nd and 182W (μ182W = −8.4 ± 4.5, μ142Nd = −4.9 ± 2.8, 2SD). These results constitute the first observation in terrestrial rocks of coupled depletions in 142Nd and 182W. Such isotopic depletions require derivation of the komatiites from a mantle domain that formed within the first ∼30 Ma of Solar System history and was initially geochemically enriched in highly incompatible trace elements as a result of crystal‐liquid fractionation in an early magma ocean. This mantle domain further must have experienced subsequent melt depletion, after 182Hf had gone extinct, to account for the observed initial excesses in 143Nd and 176Hf. The survival of early‐formed 182W and 142Nd anomalies in the mantle until at least 3.55 Ga indicates that the products of early planetary differentiation survived both later planetary accretion and convective mantle mixing during the Hadean. This work moreover renders unlikely that variable late accretion, by itself, can account for all of the observed W isotope variations in Archean rocks.
Key Points
Komatiites from the 3.55 Ga Schapenburg Greenstone Remnant in South Africa show coupled depletions in 142Nd and 182W
The komatiites were derived from a mantle domain that formed within the first 30 Ma of Solar System history
The products of early terrestrial differentiation survived planetary accretion and mantle mixing for 1.0 Ga
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Late accretion, early mantle differentiation, and core-mantle interaction are processes that could have created subtle (182)W isotopic heterogeneities within Earth's mantle. Tungsten isotopic data ...for Kostomuksha komatiites dated at 2.8 billion years ago show a well-resolved (182)W excess relative to modern terrestrial samples, whereas data for Komati komatiites dated at 3.5 billion years ago show no such excess. Combined (182)W, (186,187)Os, and (142,143)Nd isotopic data indicate that the mantle source of the Kostomuksha komatiites included material from a primordial reservoir that represents either a deep mantle region that underwent metal-silicate equilibration or a product of large-scale magmatic differentiation of the mantle. The preservation, until at least 2.8 billion years ago, of this reservoir-which likely formed within the first 30 million years of solar system history-indicates that the mantle may have never been well mixed.
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Acapulcoites and lodranites are highly metamorphosed to partially molten meteorites with mineral and bulk compositions similar to those of ordinary chondrites. These properties place the acapulcoites ...and lodranites between the unmelted chondrites and the differentiated meteorites and as such acapulcoites–lodranites are of special interest for understanding the initial stages of asteroid differentiation as well as the role of
26Al heating in the thermal history of asteroids. To constrain the accretion timescale and thermal history of the acapulcoite–lodranite parent body, and to compare these results to the thermal histories of other meteorite parent bodies, the Hf–W system was applied to several acapulcoites and lodranites. Acapulcoites Dhofar 125 and NWA 2775 and lodranite NWA 2627 have indistinguishable Hf–W ages of Δ
t
CAI
=
5.2
±
0.9 Ma and Δ
t
CAI
=
5.7
±
1.0 Ma, corresponding to absolute ages of 4563.1
±
0.8 Ma and 4562.6
±
0.9 Ma. Closure temperatures for the Hf–W system for acapulcoites and lodranites, estimated from numerical simulations of W diffusion in high-Ca pyroxene, are 975
±
50 °C and 1025
±
50 °C, respectively. Owing to these high closure temperatures, the Hf–W ages provide information on the earliest high-temperature evolution, and combined with thermal modeling indicate that the acapulcoite–lodranite parent body accreted ~
1.5–2 Ma after CAI formation, was internally heated by
26Al decay, and reached its thermal peak ~
3 Ma after CAI formation. Cooling rates for acapulcoites decreased from ~
120 °C/Ma just below the thermal peak to ~
50 °C/Ma at ~
600 °C. Over the same temperature interval the cooling rate for lodranites decreased from ~
100 °C/Ma to ~
40 °C/Ma. These thermal histories may reflect cooling in the uppermost ~
10 km of a parent body with a radius of ~
35–100 km. Acapulcoites and lodranites evolved with a
180Hf/
184W ratio of ~
0.64, which is indistinguishable from that of H chondrites but significantly lower than
180Hf/
184W~
1.23 for carbonaceous chondrites. The low
180Hf/
184W ratios of acapulcoites–lodranites were established before ~
2 Ma and, hence, prior to partial melting in the parent body at ~
3 Ma. Thus, they must reflect Hf–W fractionation of the precursor material by processes in the solar nebula. Combined with Hf–W ages of Δ
t
CAI
<
1 Ma for differentiation of the parent bodies of magmatic iron meteorites and an Hf–W age of Δ
t
CAI
~
2.5 Ma for the accretion of the H chondrite parent body, the Hf–W results for acapulcoites and lodranites reveal an inverse correlation between accretion age of asteroids and peak temperature in their interiors. The different thermal histories of most meteorite parent bodies, therefore, primarily reflect variations in their initial
26Al abundance, which is determined by their accretion time.
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