ABSTRACT Mars exhibits ample evidence for an ancient surface hydrosphere. The oxygen isotope comp... more ABSTRACT Mars exhibits ample evidence for an ancient surface hydrosphere. The oxygen isotope compositions of carbonate minerals and alteration products in martian meteorites suggest that this ancient hydrosphere was not in isotopic equilibrium with the martian lithosphere(1-4). Martian meteorite NWA 7533 is composed of regolith breccia from the heavily cratered terrains of ancient Mars and contains zircon grains for which U-Pb ages have been reported(5). Here we report variations between the oxygen isotopic compositions of four zircon grains from NWA 7533. We propose that these variations can be explained if the mantle melts from which the zircon crystallized approximately 4.43 Gyr ago had assimiliated O-17-enriched regolith materials, and that some of the zircon grains, while in a metamict state, were later altered by low-temperature fluids near the surface less than 1.7 Gyr ago. Enrichment of the martian regolith in O-17 before the zircon crystallized, presumably through exchange with the O-17-enriched atmosphere or hydrosphere during surface alteration, suggests that the thick primary atmosphere of Mars was lost within the first 120 Myr after accretion. We conclude that the observed variation of O-17 anomalies in zircon from NWA 7533 points to prolonged interaction between the martian regolith, atmosphere and hydrosphere.
Martian meteorite NWA 7533 is a regolith breccia that compositionally resembles the Martian surfa... more Martian meteorite NWA 7533 is a regolith breccia that compositionally resembles the Martian surface measured by orbiters and landers. NWA 7533 contains monzonitic clasts that have zircon with U-Pb ages of 4.428 Ga. The Pb isotopic compositions of plagioclase and alkali feldspars, as well as U-Pb isotopic compositions of chlorapatite in the monzonitic clasts of NWA 7533 have been measured by Secondary Ion Mass Spectrometry (SIMS). The U-Pb isotopic compositions measured from the chlorapatite in NWA 7533 yield an age of 1.357 ± 81 Ga (2σ ). The least radiogenic Pb isotopic compositions measured in plagioclase and K-feldspar lie within error of the 4.428 Ga Geochron. These data indicate that the monzonitic clasts in NWA 7533 are a product of a differentiation history that includes residence in a reservoir that formed prior to 4.428 Ga with a μ-value ( 238 U/ 204 Pb) of at least 13.4 ± 1.7 (2σ ) and a κ-value ( 232 Th/ 238 U) of ∼4.3. This μ-value is more than three times higher than any other documented Martian reservoir. These results indicate either the Martian mantle is significantly more heterogeneous than previously thought (μ-value of 1-14 vs. 1-5) and/or the monzonitic clasts formed by the melting of Martian crust with a μ-value of at least 13.4. Therefore, NWA 7533 may contain the first isotopic evidence for an enriched, differentiated crust on Mars.
ABSTRACT The study of the paleomagnetism of carbonaceous chondrites can lead to an estimate of th... more ABSTRACT The study of the paleomagnetism of carbonaceous chondrites can lead to an estimate of the magnetic fields present in the early solar system. CM chondrites contain abundant magnetite formed during aqueous alteration on their parent body, and have not been heated after that, making them interesting targets for paleomagnetism. We performed a detailed and comparative magnetic study (paleomagnetism and rock magnetism) of three CM chondrites: Paris, Cold Bokkeveled and Murchison. These three meteorites cover a wide range of aqueous alteration, with increasing alteration from Paris [1] to Murchison to Cold Bokkeveld [2]. Paris is a unique CM chondrite significantly less aqueously altered than other CM chondrites. Our magnetic data show that in contrast with other CM, Paris meteorite contains abundant FeNi metal (of nebular origin) together with magnetite and pyrrhotite (of asteroidal origin). Paleomagnetic results of Paris show that unfortunately the meteorite has been exposed to a strong artificial magnetic field (magnet), precluding the study of the natural magnetization (of possible nebular origin) carried by FeNi. However, a high-coercivity magnetization carried by pyrrhotite is still preserved in the meteorite. It is homogeneous in direction and intensity at the scale of the meteorite. We interpret this high-coercivity magnetization as a pre-terrestrial chemical remanent magnetization acquired on the parent body in a field of a few µT. Our preliminary results on Murchison also evidenced an stable and homogeneous magnetization in the meteorite. Therefore a long-lasting stable magnetizing field seems necessary to account for the paleomagnetism of both meteorites. Because crystallization of pyrrhotite and magnetite occurred several Myr after the formation of the parent body [3] (i.e. after possible existence of strong solar and nebular magnetic field), the magnetizing field was most probably created on the parent body. In view of its intensity, the most plausible origin for the magnetizing field is an internally generated dynamo field. This would imply that the parent body of CM chondrites was partially differentiated with a convecting metallic core. Such process has recently been proposed for the parent body of CV chondrites [4, 5]. [1] Zanda et al., 2010. Meteoritics Planetary Sci., 45, 222-222. [2] Rubin et al., 2007. Geo. et Cosmo. Acta, 71, 2361-2382 [3] Krot et al., 2005. UCRL-BOOK-217207 [4] Carporzen et al., 2011. Proc. National Acad. Sci., 108, 6386-6389. [5] Elkins-Tanton et al., 2011. Earth Planet . Sci. Lett., 305, 1-10.
Paris: The slightly altered, slightly metamorphosed CM that bridges the gap between CMs and Cos C... more Paris: The slightly altered, slightly metamorphosed CM that bridges the gap between CMs and Cos Conference Item How to cite: Bourot-Denism, M.; Zanda, B.; Marrocchi, Y.; Greenwood, R. C.; Pont, S.; Hewins, R. H.; Franchi, I. A. and Cornen, G. (2010). Paris: The slightly altered, slightly metamorphosed CM that bridges the gap between CMs and Cos.
ABSTRACT Weathering of an olivine-bearing regolith to form Fe-oxides, followed by impact melting,... more ABSTRACT Weathering of an olivine-bearing regolith to form Fe-oxides, followed by impact melting, creates an opx and magnetite mineralogy in the early martian crust.
Introduction: Metal (Fe-Ni alloy) is ubiquitous in carbonaceous chondrites (CCs) [e.g. 1, 2]. How... more Introduction: Metal (Fe-Ni alloy) is ubiquitous in carbonaceous chondrites (CCs) [e.g. 1, 2]. However, the origin of metal remains debated, and different models of formation have been proposed such as direct condensation from the nebular gas [e.g. 3], condensation of vaporized Fe-Ni alloy [e.g. 4], reduction of iron contained in silicates [e.g. 4, 5], and destabilization of sulfides [e.g. 6]. The study of metal grains in CR chondrites, one of the most metal-rich groups of CCs [e.g. 7], can supply important information about their origin and history and their further solid-state thermal history during secondary processes. Large isolated CR metal grains appear to have originated as liquid droplets, but little evidence of igneous differentiation in these former droplets has been reported so far. In the course of a study of volatile element zonation in the CR2 Renazzo chondrite [8] we encountered a large metal grain with excellent igneous zoning preserved. To enhance our understanding of the origin of metal in CCs and the possible formation of metal from a liquid as argued by [9], we present here a detailed LA-ICP-MS analysis of this zoned metal grain.
ABSTRACT NWA 7533 is a pre-Noachian regolith breccia, containing siderophiles consistent with 5% ... more ABSTRACT NWA 7533 is a pre-Noachian regolith breccia, containing siderophiles consistent with 5% chondritic impactor, 4.44 Ga zircons, and LILE enrichments implying a crustal thickness of 50 km established in the first 100 Ma of martian history.
ABSTRACT This martian breccia contains feldspathic clasts with Ni-rich pyroxene and some Ni-poor ... more ABSTRACT This martian breccia contains feldspathic clasts with Ni-rich pyroxene and some Ni-poor orthopyroxene clasts, respectively interpreted as impact-related and pristine material.
ABSTRACT Oxygen-isotope compositions of zircon from martian meteorite NWA 7533 suggest existence ... more ABSTRACT Oxygen-isotope compositions of zircon from martian meteorite NWA 7533 suggest existence of different oxygen reservoirs during zircon formation and alteration.
ABSTRACT Implications of the matrix chemistry of the new martian impact breccia, NWA 7533, for th... more ABSTRACT Implications of the matrix chemistry of the new martian impact breccia, NWA 7533, for the origin of martian soils and crustal thickness will be presented.
ABSTRACT LA-ICP-MS analyses of unaltered matrix, chondrules, and bulk CM and CR chondrites show t... more ABSTRACT LA-ICP-MS analyses of unaltered matrix, chondrules, and bulk CM and CR chondrites show that the preaccretionary matrix had a CI composition. Chondrules could have formed in the inner disk and been transported to be embedded in matrix further out.
ABSTRACT Mars exhibits ample evidence for an ancient surface hydrosphere. The oxygen isotope comp... more ABSTRACT Mars exhibits ample evidence for an ancient surface hydrosphere. The oxygen isotope compositions of carbonate minerals and alteration products in martian meteorites suggest that this ancient hydrosphere was not in isotopic equilibrium with the martian lithosphere(1-4). Martian meteorite NWA 7533 is composed of regolith breccia from the heavily cratered terrains of ancient Mars and contains zircon grains for which U-Pb ages have been reported(5). Here we report variations between the oxygen isotopic compositions of four zircon grains from NWA 7533. We propose that these variations can be explained if the mantle melts from which the zircon crystallized approximately 4.43 Gyr ago had assimiliated O-17-enriched regolith materials, and that some of the zircon grains, while in a metamict state, were later altered by low-temperature fluids near the surface less than 1.7 Gyr ago. Enrichment of the martian regolith in O-17 before the zircon crystallized, presumably through exchange with the O-17-enriched atmosphere or hydrosphere during surface alteration, suggests that the thick primary atmosphere of Mars was lost within the first 120 Myr after accretion. We conclude that the observed variation of O-17 anomalies in zircon from NWA 7533 points to prolonged interaction between the martian regolith, atmosphere and hydrosphere.
Martian meteorite NWA 7533 is a regolith breccia that compositionally resembles the Martian surfa... more Martian meteorite NWA 7533 is a regolith breccia that compositionally resembles the Martian surface measured by orbiters and landers. NWA 7533 contains monzonitic clasts that have zircon with U-Pb ages of 4.428 Ga. The Pb isotopic compositions of plagioclase and alkali feldspars, as well as U-Pb isotopic compositions of chlorapatite in the monzonitic clasts of NWA 7533 have been measured by Secondary Ion Mass Spectrometry (SIMS). The U-Pb isotopic compositions measured from the chlorapatite in NWA 7533 yield an age of 1.357 ± 81 Ga (2σ ). The least radiogenic Pb isotopic compositions measured in plagioclase and K-feldspar lie within error of the 4.428 Ga Geochron. These data indicate that the monzonitic clasts in NWA 7533 are a product of a differentiation history that includes residence in a reservoir that formed prior to 4.428 Ga with a μ-value ( 238 U/ 204 Pb) of at least 13.4 ± 1.7 (2σ ) and a κ-value ( 232 Th/ 238 U) of ∼4.3. This μ-value is more than three times higher than any other documented Martian reservoir. These results indicate either the Martian mantle is significantly more heterogeneous than previously thought (μ-value of 1-14 vs. 1-5) and/or the monzonitic clasts formed by the melting of Martian crust with a μ-value of at least 13.4. Therefore, NWA 7533 may contain the first isotopic evidence for an enriched, differentiated crust on Mars.
ABSTRACT The study of the paleomagnetism of carbonaceous chondrites can lead to an estimate of th... more ABSTRACT The study of the paleomagnetism of carbonaceous chondrites can lead to an estimate of the magnetic fields present in the early solar system. CM chondrites contain abundant magnetite formed during aqueous alteration on their parent body, and have not been heated after that, making them interesting targets for paleomagnetism. We performed a detailed and comparative magnetic study (paleomagnetism and rock magnetism) of three CM chondrites: Paris, Cold Bokkeveled and Murchison. These three meteorites cover a wide range of aqueous alteration, with increasing alteration from Paris [1] to Murchison to Cold Bokkeveld [2]. Paris is a unique CM chondrite significantly less aqueously altered than other CM chondrites. Our magnetic data show that in contrast with other CM, Paris meteorite contains abundant FeNi metal (of nebular origin) together with magnetite and pyrrhotite (of asteroidal origin). Paleomagnetic results of Paris show that unfortunately the meteorite has been exposed to a strong artificial magnetic field (magnet), precluding the study of the natural magnetization (of possible nebular origin) carried by FeNi. However, a high-coercivity magnetization carried by pyrrhotite is still preserved in the meteorite. It is homogeneous in direction and intensity at the scale of the meteorite. We interpret this high-coercivity magnetization as a pre-terrestrial chemical remanent magnetization acquired on the parent body in a field of a few µT. Our preliminary results on Murchison also evidenced an stable and homogeneous magnetization in the meteorite. Therefore a long-lasting stable magnetizing field seems necessary to account for the paleomagnetism of both meteorites. Because crystallization of pyrrhotite and magnetite occurred several Myr after the formation of the parent body [3] (i.e. after possible existence of strong solar and nebular magnetic field), the magnetizing field was most probably created on the parent body. In view of its intensity, the most plausible origin for the magnetizing field is an internally generated dynamo field. This would imply that the parent body of CM chondrites was partially differentiated with a convecting metallic core. Such process has recently been proposed for the parent body of CV chondrites [4, 5]. [1] Zanda et al., 2010. Meteoritics Planetary Sci., 45, 222-222. [2] Rubin et al., 2007. Geo. et Cosmo. Acta, 71, 2361-2382 [3] Krot et al., 2005. UCRL-BOOK-217207 [4] Carporzen et al., 2011. Proc. National Acad. Sci., 108, 6386-6389. [5] Elkins-Tanton et al., 2011. Earth Planet . Sci. Lett., 305, 1-10.
Paris: The slightly altered, slightly metamorphosed CM that bridges the gap between CMs and Cos C... more Paris: The slightly altered, slightly metamorphosed CM that bridges the gap between CMs and Cos Conference Item How to cite: Bourot-Denism, M.; Zanda, B.; Marrocchi, Y.; Greenwood, R. C.; Pont, S.; Hewins, R. H.; Franchi, I. A. and Cornen, G. (2010). Paris: The slightly altered, slightly metamorphosed CM that bridges the gap between CMs and Cos.
ABSTRACT Weathering of an olivine-bearing regolith to form Fe-oxides, followed by impact melting,... more ABSTRACT Weathering of an olivine-bearing regolith to form Fe-oxides, followed by impact melting, creates an opx and magnetite mineralogy in the early martian crust.
Introduction: Metal (Fe-Ni alloy) is ubiquitous in carbonaceous chondrites (CCs) [e.g. 1, 2]. How... more Introduction: Metal (Fe-Ni alloy) is ubiquitous in carbonaceous chondrites (CCs) [e.g. 1, 2]. However, the origin of metal remains debated, and different models of formation have been proposed such as direct condensation from the nebular gas [e.g. 3], condensation of vaporized Fe-Ni alloy [e.g. 4], reduction of iron contained in silicates [e.g. 4, 5], and destabilization of sulfides [e.g. 6]. The study of metal grains in CR chondrites, one of the most metal-rich groups of CCs [e.g. 7], can supply important information about their origin and history and their further solid-state thermal history during secondary processes. Large isolated CR metal grains appear to have originated as liquid droplets, but little evidence of igneous differentiation in these former droplets has been reported so far. In the course of a study of volatile element zonation in the CR2 Renazzo chondrite [8] we encountered a large metal grain with excellent igneous zoning preserved. To enhance our understanding of the origin of metal in CCs and the possible formation of metal from a liquid as argued by [9], we present here a detailed LA-ICP-MS analysis of this zoned metal grain.
ABSTRACT NWA 7533 is a pre-Noachian regolith breccia, containing siderophiles consistent with 5% ... more ABSTRACT NWA 7533 is a pre-Noachian regolith breccia, containing siderophiles consistent with 5% chondritic impactor, 4.44 Ga zircons, and LILE enrichments implying a crustal thickness of 50 km established in the first 100 Ma of martian history.
ABSTRACT This martian breccia contains feldspathic clasts with Ni-rich pyroxene and some Ni-poor ... more ABSTRACT This martian breccia contains feldspathic clasts with Ni-rich pyroxene and some Ni-poor orthopyroxene clasts, respectively interpreted as impact-related and pristine material.
ABSTRACT Oxygen-isotope compositions of zircon from martian meteorite NWA 7533 suggest existence ... more ABSTRACT Oxygen-isotope compositions of zircon from martian meteorite NWA 7533 suggest existence of different oxygen reservoirs during zircon formation and alteration.
ABSTRACT Implications of the matrix chemistry of the new martian impact breccia, NWA 7533, for th... more ABSTRACT Implications of the matrix chemistry of the new martian impact breccia, NWA 7533, for the origin of martian soils and crustal thickness will be presented.
ABSTRACT LA-ICP-MS analyses of unaltered matrix, chondrules, and bulk CM and CR chondrites show t... more ABSTRACT LA-ICP-MS analyses of unaltered matrix, chondrules, and bulk CM and CR chondrites show that the preaccretionary matrix had a CI composition. Chondrules could have formed in the inner disk and been transported to be embedded in matrix further out.
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Papers by Brigitte Zanda