An unexpected source of Earth's water
The abundances of Earth's chemical elements and their isotopic ratios can indicate which materials formed Earth. Enstatite chondrite (EC) meteorites provide a ...good isotopic match for many elements but are expected to contain no water because they formed in the hot inner Solar System. This would require Earth's water to be from a different source, such as comets. Piani
et al.
measured hydrogen contents and deuterium/hydrogen ratios (D/H) in 13 EC meteorites (see the Perspective by Peslier). They found far more hydrogen than is commonly assumed, with D/H close to that of Earth's mantle. Combining these data with cosmochemical models, they show that most of Earth's water could have formed from hydrogen delivered by EC meteorites.
Science
, this issue p.
1110
; see also p.
1058
Enstatite chondrite meteorites contain more hydrogen than expected and could have been the source of Earth’s water.
The origin of Earth’s water remains unknown. Enstatite chondrite (EC) meteorites have similar isotopic composition to terrestrial rocks and thus may be representative of the material that formed Earth. ECs are presumed to be devoid of water because they formed in the inner Solar System. Earth’s water is therefore generally attributed to the late addition of a small fraction of hydrated materials, such as carbonaceous chondrite meteorites, which originated in the outer Solar System where water was more abundant. We show that EC meteorites contain sufficient hydrogen to have delivered to Earth at least three times the mass of water in its oceans. EC hydrogen and nitrogen isotopic compositions match those of Earth’s mantle, so EC-like asteroids might have contributed these volatile elements to Earth’s crust and mantle.
FeO-poor (type I) porphyritic chondrules formed by incomplete melting of solid dust precursors via a yet-elusive mechanism. Two settings are generally considered for their formation: (i) a nebular ...setting where primordial solids were melted, e.g. by shock waves propagating through the gas and (ii) a collisional planetary setting. Here we report a method combining high-current electron microprobe X-ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors. We find that these olivine crystals are Ca–Al–Ti-poor relative to host olivine crystals. Their variable Δ17O, even in individual chondrule, is inconsistent with derivation from planetary interiors as previously argued from 120 ° triple junctions also exhibited by the chondrules studied herein. This indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions.
We propose that porphyritic chondrules formed during gas-assisted melting of nebular condensates comprising relict olivine grains with varying Δ17O values and Ca–Al–Ti-rich minerals such as those observed within amoeboid olivine aggregates. Incomplete melting of chondrule precursors produced Ca–Al–Ti-rich melts (CAT-melts), allowing subsequent crystallization of Ca–Al–Ti-rich host olivine crystals via epitaxial growth on relict olivine grains. Incoming MgO and SiO from the gas phase induced (i) the dilution of CAT-melts, as attested by the positive Al–Ti correlation observed in chondrule olivine crystals, and (ii) buffering of the O-isotope compositions of chondrules, as recorded by the constant Δ17O values of host olivine grains. The O-isotopic compositions of host olivine grains are chondrule-specific, suggesting that chondrules formed in an array of environments of the protoplanetary disk with different Δ17O values, possibly due to variable solid/gas mixing ratios.
•High-resolution X-ray maps reveal different populations of olivine in chondrules.•High-current microprobe analyses allow relict olivine grains to be characterized.•Relict olivine crystals in chondrules are Ca–Al–Ti-poor but show varying Δ17O.•Chondrules result from nebular condensate melting and gas–melt interactions.•Host olivine grains formed via gas-assisted epitaxial growth on relict olivine.
We report microscopic, cathodoluminescence, chemical, and O isotopic measurements of FeO‐poor isolated olivine grains (IOG) in the carbonaceous chondrites Allende (CV3), Northwest Africa 5958 ...(C2‐ung), Northwest Africa 11086 (CM2‐an), and Allan Hills 77307 (CO3.0). The general petrographic, chemical, and isotopic similarity with bona fide type I chondrules confirms that the IOG derived from them. The concentric CL zoning, reflecting a decrease in refractory elements toward the margins, and frequent rimming by enstatite are taken as evidence of interaction of the IOG with the gas as stand‐alone objects. This indicates that they were splashed out of chondrules when these were still partially molten. CaO‐rich refractory forsterites, which are restricted to ∆17O <−4‰ likely escaped equilibration at lower temperatures because of their large size and possibly quicker quenching. The IOG thus bear witness to frequent collisions in the chondrule‐forming regions.
Among the different groups of carbonaceous chondrites, variable concentrations of hydrous minerals and organic matter are observed that might be related to the time and/or place of formation of their ...asteroidal parent bodies. However, the precise distribution of these volatile-bearing components between chondrite groups and their chemical and isotopic compositions remain fairly unknown. In this study, we used a novel secondary ion mass spectrometry analytical protocol to determine the hydrogen isotopic composition of water-bearing minerals in CV-type carbonaceous chondrites. This protocol allows for the first time the D/H ratio of CV chondrite hydrous minerals to be determined without hindrance by hydrogen contributions from adjacent organic material. We found that water in the altered CV chondrites Kaba, Bali, and Grosnaja has an average D/H ratio of D/HCV-water = 144−21+8 × 10−6 (or δDCV-water = −77−131+54‰, 2σ), significantly higher than water in most CM-type carbonaceous chondrites (D/HCM-water = 101 ± 6 × 10−6 or δDCM-water = −350 ± 40‰, 2σ). We show that because organic matter in CV chondrites is depleted in deuterium compared to that in CM chondrites, such differences could result from isotopic exchange between water and organics. Another possibility is that the CM and CV parent bodies sampled different reservoirs of water ice and organics characterized by variable isotopic compositions due to their different time and/or place of accretion.
•In situ measurements by ion probe reveals the D/H ratio of water in chondrite.•CV-type carbonaceous chondrites contain water with a D/H = 144 ± 8/21 × 10−6.•Water is enriched in D in CV- relative to CM-type carbonaceous chondrites.•Isotope exchange with organic matter could be a cause for the water D-enrichment.•CM and CV parent bodies could sample different reservoirs of water ice and organics.
The origin and abundance of water accreted by carbonaceous asteroids remains underconstrained, but would provide important information on the dynamic of the protoplanetary disk. Here we report the in ...situ oxygen isotopic compositions of aqueously formed fayalite grains in the Kaba and Mokoia CV chondrites. CV chondrite bulk, matrix and fayalite O-isotopic compositions define the mass-independent continuous trend (δ17O = 0.84 ± 0.03 × δ18O − 4.25 ± 0.1), which shows that the main process controlling the O-isotopic composition of the CV chondrite parent body is related to isotopic exchange between 16O-rich anhydrous silicates and 17O- and 18O-rich fluid. Similar isotopic behaviors observed in CM, CR and CO chondrites demonstrate the ubiquitous nature of O-isotopic exchange as the main physical process in establishing the O-isotopic features of carbonaceous chondrites, regardless of their alteration degree. Based on these results, we developed a new approach to estimate the abundance of water accreted by carbonaceous chondrites (quantified by the water/rock ratio) with CM (0.3–0.4) ≥ CR (0.1–0.4) ≥ CV (0.1–0.2) > CO (0.01–0.10). The low water/rock ratios and the O-isotopic characteristics of secondary minerals in carbonaceous chondrites indicate they (i) formed in the main asteroid belt and (ii) accreted a locally derived (inner Solar System) water formed near the snowline by condensation from the gas phase. Such results imply low influx of D- and 17O- and 18O-rich water ice grains from the outer part of the Solar System. The latter is likely due to the presence of a Jupiter-induced gap in the protoplanetary disk that limited the inward drift of outer Solar System material at the exception of particles with size lower than 150 μm such as presolar grains. Among carbonaceous chondrites, CV chondrites show O-isotopic features suggesting potential contribution of 17–18O-rich water that may be related to their older accretion relative to other hydrated carbonaceous chondrites.
•Bulk, matrix and fayalite grains of CV chondrites define a continuous trend.•O-isotopic compositions of C-rich asteroids are established by isotopic exchange.•We estimated the abundance of water accreted by carbonaceous chondrites.•Carbonaceous asteroids formed in the asteroid belt and accreted local water.•Our results support low influx of 17O- and 18O-rich water ice from the outer Solar System.
•Chondrules are characterized by large, in situ mass-dependent silicon isotopic variations.•These variations are inherited from chondrules precursors and kinetic effects during their formation.•These ...variations imply that chondrules formed in open-system in interaction with a SiO-rich gas.
We determined the silicon isotopic compositions of silicates (olivine and low-Ca pyroxene) in type I and type II chondrules of the carbonaceous chondrites Allende, Kaba, NWA (Northwest Africa) 5958, and MIL (Miller Range) 07342. Type I chondrule silicates show large, mass-dependent Si isotopic fractionations, with δ30Si values ranging from −7‰ to +2.6‰, whereas the δ30Si values of type II chondrule silicates are close to zero and vary by less than 2‰. When present, Mg-rich relict olivine grains in type II chondrules show larger Si variations than their FeO-rich counterparts. In type I chondrules, low-Ca pyroxenes yield systematically lighter δ30Si values than Mg-rich olivines. Our results show that type I chondrules are complex objects whose Si isotopic compositions derived from their precursors and SiO-rich gas-melt interactions. This corroborates that type I chondrules are nebular products that formed under open-system conditions. Our data also suggest that at least some type II chondrules derived from their type I counterparts. Overall, this demonstrates that recycling was common during the evolution of the protoplanetary disk.
Chondrules are sub-millimetric spheroids that are ubiquitous in chondrites and whose formation mechanism remains elusive. Textural and oxygen isotopic characteristics of chondrules in carbonaceous ...chondrites (CCs) suggest that they result from the recycling of isotopically heterogeneous early-condensed precursors via gas–melt interactions. Here, we report high-resolution X-ray elemental maps and in situ O isotopic analyses of FeO-poor, olivine-rich chondrules from ordinary chondrites (OCs) to compare the conditions of chondrule formation in these two main classes of chondrites. OC chondrules show minor element (e.g., Ti, Al) zonings at both the chondrule and individual olivine grain scales. Considering the entire isotopic data set, our data define a mass-independent correlation, with olivine grains showing O isotopic variations spanning more than 40‰. Though 16O-rich relict olivine grains were identified in OC chondrules, they are much less abundant than in CC chondrules. They appear as two types: (i) those with low minor element abundances and Δ17O < −15‰ and (ii) those with varying minor element abundances and less negative Δ17O values averaging −5.5‰. The host olivine grains exhibit mass-dependent O isotopic variations within individual chondrules.
Our results reveal that similar processes (precursor recycling and interactions between chondrule melts and a SiO- and Mg-rich gas) established the observed features of OC and CC chondrules. The mass-dependent isotopic variations recorded by host olivine grains result from kinetic effects induced by complex evaporation/recondensation processes during the gas–melt interactions. This suggests that OC chondrules formed through enhanced recycling processes, in good agreement with the lower abundances of relict olivine grains in OC chondrules compared to CC chondrules. We use the Δ18O = δ18O − δ17O parameter to demonstrate that there is no genetic relationship between CC and OC chondrules, suggesting limited radial transport in the protoplanetary disk. Finally, to the first order, the Δ18O–Δ17O diagram may allow the non-carbonaceous vs. carbonaceous origin of a given chondrule to be deciphered.
Chondrites are rocky fragments of asteroids that formed at different times and heliocentric distances in the early solar system. Most chondrite groups contain water-bearing minerals, attesting that ...both water-ice and dust were accreted on their parent asteroids. Nonetheless, the hydrogen isotopic composition (D/H) of water in the different chondrite groups remains poorly constrained, due to the intimate mixture of hydrated minerals and organic compounds, the other main H-bearing phase in chondrites. Building on our recent works using in situ secondary ion mass spectrometry analyses, we determined the H isotopic composition of water in a large set of chondritic samples (CI, CM, CO, CR, and C-ungrouped carbonaceous chondrites) and report that water in each group shows a distinct and unique D/H signature. Based on a comparison with literature data on bulk chondrites and their water and organics, our data do not support a preponderant role of parent-body processes in controlling the D/H variations among chondrites. Instead, we propose that the water and organic D/H signatures were mostly shaped by interactions between the protoplanetary disk and the molecular cloud that episodically fed the disk over several million years. Because the preservation of D-rich interstellar water and/or organics in chondritic materials is only possible below their respective sublimation temperatures (160 and 350–450 K), the H isotopic signatures of chondritic materials depend on both the timing and location at which their parent body formed.
•The water D/H ratios are distinct and unique among the different chondrite groups.•Parent-body processes play secondary roles in controlling the D/H variations.•Water and organics record sublimation and isotopic exchanges occurring in the disk.•Interaction with the parent molecular cloud can have shaped the D/H variations.•The D/H of water and organics might be related to the timing of asteroid accretion.
Rapid condensation of the first Solar System solids Marrocchi, Yves; Villeneuve, Johan; Jacquet, Emmanuel ...
Proceedings of the National Academy of Sciences - PNAS,
11/2019, Volume:
116, Issue:
47
Journal Article
Peer reviewed
Open access
Chondritic meteorites are composed of primitive components formed during the evolution of the Solar protoplanetary disk. The oldest of these components formed by condensation, yet little is known ...about their formation mechanism because of secondary heating processes that erased their primordial signature. Amoeboid Olivine Aggregates (AOAs) have never been melted and underwent minimal thermal annealing, implying they might have retained the conditions under which they condensed. We performed a multiisotope (O, Si, Mg) characterization of AOAs to constrain the conditions under which they condensed and the information they bear on the structure and evolution of the Solar protoplanetary disk. High-precision silicon isotopic measurements of 7 AOAs from weakly metamorphosed carbonaceous chondrites show large, mass-dependent, light Si isotope enrichments (–9‰ < δ30Si < –1‰). Based on physical modeling of condensation within the protoplanetary disk, we attribute these isotopic compositions to the rapid condensation of AOAs over timescales of days to weeks. The same AOAs show slightly positive δ25Mg that suggest that Mg isotopic homogenization occurred during thermal annealing without affecting Si isotopes. Such short condensation times for AOAs are inconsistent with disk transport timescales, indicating that AOAs, and likely other high-temperature condensates, formed during brief localized high-temperature events.
•First combined Cr, Ti, O isotope data for individual chondrules.•Isotopic compositions of NC chondrules distinct from CC chondrules.•Isotopic variability not due to chondrule transport, but ...precursor heterogeneity.•Incomplete mixing of NC-like and CAI-like nebular components in the disk at all scales.
Isotopic anomalies in chondrules hold important clues about the dynamics of mixing and transport processes in the solar accretion disk. The meaning of these anomalies is debated and they have been interpreted to indicate either disk-wide transport of chondrules or local heterogeneities of chondrule precursors. However, all previous studies relied on isotopic data for a single element (either Cr, Ti, or O), which does not allow distinguishing between source and precursor signatures as the cause of the chondrules' isotope anomalies. To overcome this problem, we obtained the first combined O, Ti, and Cr isotope data for individual chondrules from enstatite, ordinary, and carbonaceous chondrites. We find that chondrules from non-carbonaceous (NC) chondrites have relatively homogeneous Δ17O, ε50Ti, and ε54Cr, which are similar to the compositions of their host chondrites. By contrast, chondrules from carbonaceous chondrites (CC) have more variable compositions, some of which differ from the host chondrite compositions. Although the compositions of the analyzed CC and NC chondrules may overlap for either ε50Ti, ε54Cr, or Δ17O, in multi-isotope space, none of the CC chondrules plot in the compositional field of NC chondrites, and no NC chondrule plots within the field of CC chondrites. As such, our data reveal a fundamental isotopic difference between NC and CC chondrules, which is inconsistent with a disk-wide transport of chondrules across and between the NC and CC reservoirs. Instead, the isotopic variations among CC chondrules reflect local precursor heterogeneities, which most likely result from mixing between NC-like dust and a chemically diverse dust component that was isotopically similar to CAIs and AOAs. The same mixing processes, but on a larger, disk-wide scale, were likely responsible for establishing the distinct isotopic compositions of the NC and CC reservoirs, which represent in inner and outer disk, respectively.
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