Lithium isotope constraints on crust–mantle interactions and surface processes on Mars

We present a geochemical study of recently discovered Martian meteorite Northwest Africa (NWA) 5790 and use our results to constrain its origin and relationship with the other nakhlites. This nakhlite is a clinopyroxene cumulate composed of phenocrysts of augite, olivine, and rare oxides surrounded by a mesostasis composed of vitrophyric glass, feldspars, oxides, phosphates, and fine‐grained olivines and augite. Petrography, and major and trace element compositions of the phases present are consistent with derivation of NWA 5790 from a parental magma common to all the nakhlites. Olivine cores grew from a distinct, incompatible‐element enriched magma and are surrounded by rims containing augite inclusions that grew from the nakhlite parental liquid, supporting previous arguments for xenocrystic olivine cores in nakhlites. Rare earth element microdistributions suggest derivation of NWA 5790 augites from an evolved, relatively oxidized magma, produced by augite fractionation from the common nakhlite parental liquid. Augite grain shapes and CSD patterns are consistent with rapid cooling and derivation near the top of the nakhlite cumulate pile, but patterns are distinct from other nakhlites thought to have formed near the stratigraphic top. The high mesostasis abundance (~44 vol%) indicates solidification near the top of the nakhlite pile close to locations suggested for nakhlites NWA 817 and Miller Range (MIL) 03346. However, the geochemical and petrographic characteristics of these three samples do not permit their placement in a simple stratigraphic order as would occur in a single lava flow. This lack of simple ordering suggests that the nakhlite lava flow split into multiple sections as would occur during breakouts from a single lava flow. Finally we note that NWA 5790 is unique among currently available nakhlites in having phenocryst abundances low enough to allow it to flow.

Section: Introductionsupporting

confidence: 62%

“…Other studies have characterized the abundance and isotopic composition of Li in NWA 5790 (Magna et al. ), and suggested that it contains little Martian alteration compared with other nakhlites (Tomkinson et al. ).…”

Section: Introductionmentioning

confidence: 99%

Northwest Africa 5790: A previously unsampled portion of the upper part of the nakhlite pile

Balta

Sanborn

Mayne

et al. 2016

“…They have some of the highest δ 7 Li values of any Martian meteorite (Magna et al. ), have similar rare gas chemistry (Schwenzer et al. ) and even crystal size distributions (Lentze and McSween ).…”

Section: Discussionmentioning

confidence: 99%

The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars

Sharp

Williams

Shearer

et al. 2016

We determined the chlorine isotope composition of 16 Martian meteorites using gas source mass spectrometry on bulk samples and in situ secondary ion microprobe analysis on apatite grains. Measured d 37 Cl values range from À3.8 to +8.6&. The olivinephyric shergottites are the isotopically lightest samples, with d 37 Cl mostly ranging from À4 to À2&. Samples with evidence for a crustal component have positive d 37 Cl values, with an extreme value of 8.6&. Most of the basaltic shergottites have intermediate d 37 Cl values of À1 to 0&, except for Shergotty, which is similar to the olivine-phyric shergottites. We interpret these data as due to mixing of a two-component system. The first component is the mantle value of À4 to À3&. This most likely represents the original bulk Martian Cl isotope value. The other endmember is a 37 Cl-enriched crustal component. We speculate that preferential loss of 35 Cl to space has resulted in a high d 37 Cl value for the Martian surface, similar to what is seen in other volatile systems. The basaltic shergottites are a mixture of the other two endmembers. The low d 37 Cl value of primitive Mars is different from Earth and most chondrites, both of which are close to 0&. We are not aware of any parent-body process that could lower the d 37 Cl value of the Martian mantle to À4 to À3&. Instead, we propose that this low d 37 Cl value represents the primordial bulk composition of Mars inherited during accretion. The higher d 37 Cl values seen in many chondrites are explained by later incorporation of 37 Cl-enriched HCl-hydrate.

“…Their δ 7 Li values are thus considered representative of their respective parent bodies (Magna et al. , , ; Seitz et al. , ; Pogge von Strandmann et al.…”

Section: Discussionmentioning

confidence: 99%

“…It should be noted, that condensation temperatures listed by Lodders (2003) are those derived for low-pressure gas of the solar composition, conditions of which are quite different from those encountered by moldavites or in other postimpact environments (Canup et al 2015). A number of studies have provided evidence for uniform distribution of Li isotopes in large planetary and asteroidal bodies of the inner solar system, such as Earth, Mars, Moon, and Vesta (Seitz et al 2004(Seitz et al , 2006(Seitz et al , 2007Elliott et al 2006;Magna et al 2006Magna et al , 2008Magna et al , 2014Magna et al , 2015Jeffcoate et al 2007;Tomascak et al 2008;Gao et al 2011;Pogge von Strandmann et al 2011;Lai et al 2015;Day et al 2016), as well as in chondrites (Seitz et al 2007(Seitz et al , 2012Maruyama et al 2009;Sephton et al 2013;Day et al 2016). From these studies it appears that Li isotope composition is well constrained for mantles of planetary bodies with mean d 7 Li values broadly between 3.5 and 4.2&.…”

Section: Introductionmentioning

confidence: 99%

The fate of moderately volatile elements in impact events—Lithium connection between the Ries sediments and central European tektites

Rodovská

Magna

Žák

et al. 2016

Self Cite

Lithium abundances and isotope compositions are presented for a suite of sediments from the surroundings of the Ries Impact structure, paralleled by new Li data for central European tektites (moldavites) from several substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia), including a specimen from the newly discovered substrewn field in Poland. The data set was supplemented by three clay fractions isolated from sedimentary samples. Moldavites measured in this study show a very narrow range in δ7Li values (−0.6 to 0.3‰ relative to L‐SVEC) and Li contents (23.9–48.1 ppm). This contrasts with sediments from the Ries area which show remarkable range in Li isotope compositions (from −6.9 to 13.4‰) and Li contents (0.6–256 ppm). The OSM sediments which, based on chemical similarity, formed the major part of moldavites, show a range in δ7Li values from −2.0 to 7.9‰ and Li contents from 5.8 to 78.9 ppm. Therefore, the formation of moldavites was apparently accompanied by large‐scale mixing, paralleled by chemical and isotope homogenization of their parent matter. The proposed Li mixing model indicates that sands, clayey sediments, and low volumes of carbonates are the major components for tektite formation whereas residual paleokarst sediments could have been a minor but important component for a subset of moldavites. Striking homogenization of Li in tektites, combined with limited Li loss during impacts, may suggest that moderately volatile elements are not scavenged and isotopically fractionated during large‐scale collisions, which is consistent with recent models. In general, whether homogenization of bodies with distinct Li isotope systematics takes place, or collision of bodies with similar Li systematics operates cannot be resolved at present stage but Li isotope homogeneity of solar system planets and asteroidal bodies tentatively implies the latter.