Publications by authors named "Arnaud Agranier"

8 Publications

  • Page 1 of 1

The internal structure and geodynamics of Mars inferred from a 4.2-Gyr zircon record.

Proc Natl Acad Sci U S A 2020 12 16;117(49):30973-30979. Epub 2020 Nov 16.

Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, 1350 Copenhagen, Denmark;

Combining U-Pb ages with Lu-Hf data in zircon provides insights into the magmatic history of rocky planets. The Northwest Africa (NWA) 7034/7533 meteorites are samples of the southern highlands of Mars containing zircon with ages as old as 4476.3 ± 0.9 Ma, interpreted to reflect reworking of the primordial Martian crust by impacts. We extracted a statistically significant zircon population ( = 57) from NWA 7533 that defines a temporal record spanning 4.2 Gyr. Ancient zircons record ages from 4485.5 ± 2.2 Ma to 4331.0 ± 1.4 Ma, defining a bimodal distribution with groupings at 4474 ± 10 Ma and 4442 ± 17 Ma. We interpret these to represent intense bombardment episodes at the planet's surface, possibly triggered by the early migration of gas giant planets. The unradiogenic initial Hf-isotope composition of these zircons establishes that Mars's igneous activity prior to ∼4.3 Ga was limited to impact-related reworking of a chemically enriched, primordial crust. A group of younger detrital zircons record ages from 1548.0 ± 8.8 Ma to 299.5 ± 0.6 Ma. The only plausible sources for these grains are the temporally associated Elysium and Tharsis volcanic provinces that are the expressions of deep-seated mantle plumes. The chondritic-like Hf-isotope compositions of these zircons require the existence of a primitive and convecting mantle reservoir, indicating that Mars has been in a stagnant-lid tectonic regime for most of its history. Our results imply that zircon is ubiquitous on the Martian surface, providing a faithful record of the planet's magmatic history.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1073/pnas.2016326117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7733809PMC
December 2020

Early oxidation of the martian crust triggered by impacts.

Sci Adv 2020 Oct 30;6(44). Epub 2020 Oct 30.

Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France.

Despite the abundant geomorphological evidence for surface liquid water on Mars during the Noachian epoch (>3.7 billion years ago), attaining a warm climate to sustain liquid water on Mars at the period of the faint young Sun is a long-standing question. Here, we show that melts of ancient mafic clasts from a martian regolith meteorite, NWA 7533, experienced substantial Fe-Ti oxide fractionation. This implies early, impact-induced, oxidation events that increased by five to six orders of magnitude the oxygen fugacity of impact melts from remelting of the crust. Oxygen isotopic compositions of sequentially crystallized phases from the clasts show that progressive oxidation was due to interaction with an O-rich water reservoir. Such an early oxidation of the crust by impacts in the presence of water may have supplied greenhouse gas H that caused an increase in surface temperature in a CO-thick atmosphere.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1126/sciadv.abc4941DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7608801PMC
October 2020

The Jurassic magmatism of the Demerara Plateau (offshore French Guiana) as a remnant of the Sierra Leone hotspot during the Atlantic rifting.

Sci Rep 2020 05 4;10(1):7486. Epub 2020 May 4.

Anton de Kom University of Suriname, Paramaribo, SA, Suriname.

We report the discovery of 173.4 Ma hotspot-related magmatic rocks in the basement of the Demerara Plateau, offshore French Guiana and Suriname. According to plate reconstructions, a single hotspot may be responsible for the magmatic formation of (1) both the Demerara Plateau (between 180 and 170 Ma) and the Guinea Plateau (circa 165 Ma) during the end of the Jurassic rifting of the Central Atlantic; (2) both Sierra Leone and Ceara Rises (mainly from 76 to 68 Ma) during the upper Cretaceous oceanic spreading of the Equatorial Atlantic ocean; (3) the Bathymetrists seamount chain since the upper Cretaceous. The present-day location of the inferred Sierra Leone hotspot should be 100 km west of the Knipovich Seamount.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41598-020-64333-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7198611PMC
May 2020

A unifying model for the accretion of chondrules and matrix.

Proc Natl Acad Sci U S A 2019 09 4;116(38):18860-18866. Epub 2019 Sep 4.

Laboratoire Domaines Océaniques UMR/CNRS 6538, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Technopôle Brest-Iroise, 29280 Plouzané, France.

The so far unique role of our Solar System in the universe regarding its capacity for life raises fundamental questions about its formation history relative to exoplanetary systems. Central in this research is the accretion of asteroids and planets from a gas-rich circumstellar disk and the final distribution of their mass around the Sun. The key building blocks of the planets may be represented by chondrules, the main constituents of chondritic meteorites, which in turn are primitive fragments of planetary bodies. Chondrule formation mechanisms, as well as their subsequent storage and transport in the disk, are still poorly understood, and their origin and evolution can be probed through their link (i.e., complementary or noncomplementary) to fine-grained dust (matrix) that accreted together with chondrules. Here, we investigate the apparent chondrule-matrix complementarity by analyzing major, minor, and trace element compositions of chondrules and matrix in altered and relatively unaltered CV, CM, and CR (Vigarano-type, Mighei-type, and Renazzo-type) chondrites. We show that matrices of the most unaltered CM and CV chondrites are overall CI-like (Ivuna-type) (similar to solar composition) and do not reflect any volatile enrichment or elemental patterns complementary to chondrules, the exception being their Fe/Mg ratios. We propose to unify these contradictory data by invoking a chondrule formation model in which CI-like dust accreted to so-called armored chondrules, which are ubiquitous in many chondrites. Metal rims expelled during chondrule formation, but still attached to their host chondrule, interacted with the accreted matrix, thereby enriching the matrix in siderophile elements and generating an apparent complementarity.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1073/pnas.1907592116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6754599PMC
September 2019

Volatile element evolution of chondrules through time.

Proc Natl Acad Sci U S A 2018 08 6;115(34):8547-8552. Epub 2018 Aug 6.

Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7154, 75238 Paris Cedex 05, France.

Chondrites and their main components, chondrules, are our guides into the evolution of the Solar System. Investigating the history of chondrules, including their volatile element history and the prevailing conditions of their formation, has implications not only for the understanding of chondrule formation and evolution but for that of larger bodies such as the terrestrial planets. Here we have determined the bulk chemical composition-rare earth, refractory, main group, and volatile element contents-of a suite of chondrules previously dated using the Pb-Pb system. The volatile element contents of chondrules increase with time from ∼1 My after Solar System formation, likely the result of mixing with a volatile-enriched component during chondrule recycling. Variations in the Mn/Na ratios signify changes in redox conditions over time, suggestive of decoupled oxygen and volatile element fugacities, and indicating a decrease in oxygen fugacity and a relative increase in the fugacities of in-fluxing volatiles with time. Within the context of terrestrial planet formation via pebble accretion, these observations corroborate the early formation of Mars under relatively oxidizing conditions and the protracted growth of Earth under more reducing conditions, and further suggest that water and volatile elements in the inner Solar System may not have arrived pairwise.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1073/pnas.1807263115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6112700PMC
August 2018

Evidence for extremely rapid magma ocean crystallization and crust formation on Mars.

Nature 2018 06 27;558(7711):586-589. Epub 2018 Jun 27.

Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark.

The formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood. The mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium-to-lead (U-Pb) isotopic decay system and is resistant to subsequent alteration. Moreover, given the high concentration of hafnium in zircon, the lutetium-to-hafnium (Lu-Hf) isotopic decay system can be used to determine the nature and formation timescale of its source reservoir. Ancient igneous zircons with crystallization ages of around 4,430 million years (Myr) have been reported in Martian meteorites that are believed to represent regolith breccias from the southern highlands of Mars. These zircons are present in evolved lithologies interpreted to reflect re-melted primary Martian crust , thereby potentially providing insight into early crustal evolution on Mars. Here, we report concomitant high-precision U-Pb ages and Hf-isotope compositions of ancient zircons from the NWA 7034 Martian regolith breccia. Seven zircons with mostly concordant U-Pb ages define Pb/Pb dates ranging from 4,476.3 ± 0.9 Myr ago to 4,429.7 ± 1.0 Myr ago, including the oldest directly dated material from Mars. All zircons record unradiogenic initial Hf-isotope compositions inherited from an enriched, andesitic-like crust extracted from a primitive mantle no later than 4,547 Myr ago. Thus, a primordial crust existed on Mars by this time and survived for around 100 Myr before it was reworked, possibly by impacts, to produce magmas from which the zircons crystallized. Given that formation of a stable primordial crust is the end product of planetary differentiation, our data require that the accretion, core formation and magma ocean crystallization on Mars were completed less than 20 Myr after the formation of the Solar System. These timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle, which subsequently overturns, resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41586-018-0222-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6107064PMC
June 2018

A highly unradiogenic lead isotopic signature revealed by volcanic rocks from the East Pacific Rise.

Nat Commun 2014 Jul 16;5:4474. Epub 2014 Jul 16.

Laboratoire Domaines Océaniques, UMR6538, IUEM, 29280 Plouzané, France.

Radiogenic isotopes in oceanic basalts provide a window into the different geochemical components defining the composition of Earth's mantle. Here we report the discovery of a novel geochemical signature in volcanic glasses sampled at a sub-kilometre scale along the East Pacific Rise between 15°37'N and 15°47'N. The most striking aspect of this signature is its unradiogenic lead ((206)Pb/(204)Pb=17.49, (207)Pb/(204)Pb=15.46 and (208)Pb/(204)Pb=36.83). In conjunction with enriched Sr, Nd and Hf signatures, Pb isotopes depict mixing lines that trend away from any known mantle end-members. We suggest that this unradiogenic lead component sampled by magmatic melts corresponds to a novel upper mantle reservoir that should be considered in the Pb isotope budget of the bulk silicate Earth. Major, trace element and isotope compositions are suggestive of an ancient and lower continental origin for this unradiogenic lead component, possibly sulphide-bearing pyroxenites that were preserved even after prolonged stirring within the ambient upper mantle.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/ncomms5474DOI Listing
July 2014

Regulating continent growth and composition by chemical weathering.

Proc Natl Acad Sci U S A 2008 Apr 24;105(13):4981-6. Epub 2008 Mar 24.

Department of Earth Science, MS-126, Rice University, 6100 Main Street, Houston, TX 77005, USA.

Continents ride high above the ocean floor because they are underlain by thick, low-density, Si-rich, and Mg-poor crust. However, the parental magmas of continents were basaltic, which means they must have lost Mg relative to Si during their maturation into continents. Igneous differentiation followed by lower crustal delamination and chemical weathering followed by subduction recycling are possible solutions, but the relative magnitudes of each process have never been quantitatively constrained because of the lack of appropriate data. Here, we show that the relative contributions of these processes can be obtained by simultaneous examination of Mg and Li (an analog for Mg) on the regional and global scales in arcs, delaminated lower crust, and river waters. At least 20% of Mg is lost from continents by weathering, which translates into >20% of continental mass lost by weathering (40% by delamination). Chemical weathering leaves behind a more Si-rich and Mg-poor crust, which is less dense and hence decreases the probability of crustal recycling by subduction. Net continental growth is thus modulated by chemical weathering and likely influenced by secular changes in weathering mechanisms.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1073/pnas.0711143105DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2278177PMC
April 2008