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Asuka 12325: A new depleted shergottite



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Asuka 12325: A new depleted shergottite
Vinciane Debaille1, Geneviève Hublet1, Jérôme Roland1,2, Hamed Pourkhorsandi1, Steven Goderis2
1Laboratoire G-Time, Université Libre de Bruxelles, Brussels, Belgium (; 2AMGC, Vrije Universiteit
Brussel, Brussels, Belgium
Introduction: Besides their mineralogical classification, shergottites, a subgroup of martian meteorites, have also been divided
in 3 categories according to their content in rare earth elements (REE) (e.g.
Debaille et al., 2007). As such, shergottites that are depleted in the most
incompatible REE, the light REE (LREE), are called the depleted
shergottites, with a progressive enrichment in LREE, to obtain the
intermediate shergottites and then the enriched shergottites. Interestingly,
the mineralogical and chemical classifications do not match, as the
geochemical range encompasses all mineralogical categories, as well as
several ejection events (e.g. Lapen et al., 2017). The chemical variation
between depleted and enriched shergottites has been attributed to
contamination of the magma by the martian crust (e.g. Humayun et al.,
2013) or contamination by enriched cumulates resulting from the
solidification of the martian magma ocean (Debaille et al., 2007;
Armytage et al., 2017). In any case, while the enriched shergottites clearly
represent a contamination by an enriched endmember, the depleted shergottites are thought to be representative of the depleted
martian mantle (Borg et al., 1997; Debaille et al., 2007; 2008), hence bringing important information about the geological
evolution of Mars and the martian interior. As such, they are particularly important within the shergottite compositional
continuum. During the Belgian-Japanese field expedition on the Nansen Blue Ice field in 2012-2013, a greenish stone was
collected and later identified as a shergottite (NIPR-RBINS Meteorite Newsletter April 26 2018). The stone weights 28 g and is
devoid of fusion crust (Fig. 1). Geochemical investigation has been performed at the Laboratoire G-Time (Université Libre de
Bruxelles) in order to understand the geochemical affinities of Asuka 12325 and bring new light of the geological evolution of
Method: Around 50 mg of samples has been dissolved by alkaline fusion for major and trace elements content. Major elements
were measured on the iCAP ICP-AES at ULB using Y as internal standard. Overall, the total reproducibility estimated using
USGS rock standards is better than 2%. Trace elements were measured on the Agilent 7700 ICP-MS at ULB, using In as internal
standard. The total reproducibility estimated using USGS rock standards is better than 10%. For isotope analyzes, around 0.8 g
of sample was gently crushed and sieved, and minerals separation was performed using heavy liquids and Frantz magnetic
separator. After dissolution using 3:1 mixture of HF:HNO3, and then HCl, a small aliquot was removed for Lu-Hf and Sm-Nd
spiking. Hafnium and rare earth elements (REE) were purified first using a cationic resin and 2N HCl and 6N HCl respectively.
Hafnium was subsequently purified first on anionic column to eliminate Fe
and then on a HDEHP column, where Ti was removed and Hf eluted with 4N
HF. On the other hand, REE were purified on HDEHP resin. The Nd cut of
the whole rock was then further purified for removing the Ce because of the
isobaric interference of the mass 142 with Nd, using again an HDEHP column
(Armytage et al., 2017) and the Na introduced for that purification was finally
removed using a small cationic column. All spiked and unspiked cuts for Lu-
Hf and Sm-Nd have been measured on the Nu II HR-MC-ICP-MS at ULB
using an Aridus 2. The bulk rock Nd fraction will be measured on the
Thermoscientific Triton plus at ULB.
Figure 1: Picture of Asuka 12325, scale is 1 cm. © NIPR
Figure 2: REE pattern normalized to CI chondrites,
of Asuka 12325, compared to literature data for
Results and discussion: According to the REE pattern (Fig. 2), Asuka 12325 is a depleted shergottite, showing a clear depletion
in LREE. The Mg# is 0.42, similar to mafic shergottites, according to the classification introduced by Irving et al. (2010).
However, the CaO content is unusually low for mafic shergottites, of 4.1 %wt, resembling the permafic or even the ultramafic
shergottites. Because of its very low trace element contents, the Lu-Hf and Sm-Nd isochrons are inconclusive at the present time,
giving an age of 394 ± 61 Ma for the Sm-Nd systematics and 272 ± 39 Ma for
the Lu-Hf systematics. The two ages are not concordant within error.
However, the Sm-Nd age is not definitive as the bulk fraction has currently
not yet been measured. On the other hand, the Hf content of the different
fractions were very low and not all fractions could actually be measured. As
such, the Lu-Hf age is potentially erroneous and more investigations are
needed. The initial -values obtained on the isochrons are i143Nd of +6.6 ±
3.1 and i176Hf of +29.6 ± 0.3. Those values do not correspond to the depleted
shergottites range (i143Nd from ~+36 to +39; and i176Hf from ~+46 to +50).
However, they are preliminary and will be tested with new data. In any case,
from major elements concentration already, Asuka 12325 might be a new
flavor in the shergottite continuum.
Figure 3: Sm-Nd and Lu-Hf isochrons for Asuka 12325.
Armytage, R. M. G., et al., A complex history of silicate differentiation of Mars from Nd and Hf isotopes in crustal breccia NWA
7034. Earth and Planetary Science Letters 502, 274283, 2018.
Borg, L. E., et al., Constraints on Martian differentiation processes from Rb-Sr and Sm-Nd isotopic analyses of the basaltic
shergottite QUE 94201. Geochimica et Cosmochimica Acta 61, 49154931, 1997.
Debaille, V., et al., Coupled 142Nd-143Nd evidence for a protracted magma ocean in Mars. Nature 450, 2007.
Debaille, V., et al., Martian mantle mineralogy investigated by the 176Lu176Hf and 147Sm143Nd systematics of shergottites. Earth
and Planetary Science Letters, 269, 2008.
Humayun, M. et al. Origin and age of the earliest Martian crust from meteorite NWA 7533. Nature 503, 2013.
Irving, A. J. et al. Petrologic, Elemental and Multi-Isotopic Characterization of Permafic Olivine-Phyric Shergottite Northwest
Africa 5789: A Primitive Magma Derived from Depleted Martian Mantle. 41st Lunar and Planetary Science Conference, 2010.
Lapen, T. J., et al., Two billion years of magmatism recorded from a single Mars meteorite ejection site. Science advances 3,
e1600922e1600922, 2017.
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Resolving the possible mantle and crustal sources for shergottite meteorites is crucial for understanding the formation and early differentiation of Mars. Orbiter and rover characterization of the martian surface reveal that the major element composition of most of its surface does not match the shergottites (McSween et al., 2009) leaving the relationship between them poorly understood. The identification of the meteorite NWA 7034 and its pairs as a Mars surface rock (Cartwright et al., 2014) provides access to a representative sample of Mars' crust (Agee et al., 2013; Humayun et al., 2013). Utilizing the short-lived ¹⁴⁶Sm–¹⁴²Nd, and long-lived ¹⁴⁷Sm–¹⁴³Nd and ¹⁷⁶Lu–¹⁷⁶Hf chronometers, which are sensitive to silicate differentiation, we analyzed three fragments of NWA 7034. The very negative mean isotopic compositions for this breccia, μ¹⁴²NdJNdi-1=−45±5 (2SD), ε¹⁴³NdCHUR=−16.7±0.4 (2SD) and ε¹⁷⁶HfCHUR=−61±9 (2SD) point to an ancient origin for this martian crust. However, modeling of the data shows that the crust sampled by NWA 7034 possesses a Hf/Nd ratio and coupled ε¹⁴³Nd–μ¹⁴²Nd model age that are incompatible with this crustal reservoir being an end-member that generated the shergottite source mixing array. In addition, this crust is not juvenile, despite its rare earth element profile, but has had a multistage formation history. Therefore, early crustal extraction alone was not responsible for the creation of the reservoirs that produced the shergottites. Instead mantle reservoirs formed via other early differentiation processes such as in a Mars magma ocean must be responsible for the trace element and isotopic signatures present in shergottites.
We report on an extensive petrological-chemical study of a very primitive depleted shergottite, which is similar in many (but not all) respects to Yamato 980459/98497.
Chemical heterogeneities in the Martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history. Shergottite meteorites from Mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths. The coupled 176Lu–176Hf and 147Sm–143Nd isotope systematics in 9 shergottites are used here to investigate these issues. Three compositional groups in the shergottites display distinct isotope systematics. One group, commonly termed as depleted, is characterized by positive ɛ176Hfi from + 46.2 to + 50.4 and ɛ143Ndi from + 36.2 to + 39.1. Another, termed as enriched, has negative ɛ176Hfi = − 16.5 to − 13.2 and ɛ143Ndi = − 7.0 to − 6.5. The third group is intermediate between the depleted and enriched groups with positive ɛ176Hfi = + 30.0 to + 33.4 and ɛ143Ndi = + 16.9. Together, they describe mixing curves between 176Hf/177Hf, 143Nd/144Nd, Lu/Hf, and Sm/Nd, implying that they sample two distinct sources in the Martian mantle. All shergottites are characterized by (Sm/Nd)source < (Sm/Nd)sample, but (Lu/Hf)source > (Lu/Hf)sample. This decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the Martian upper mantle. The genesis of shergottites can be modeled using non-modal equilibrium partial melting in a source initially composed of 60% olivine, 21% clinopyroxene, 9% orthopyroxene, and 10% garnet, with degrees of partial melting of 8.8% and 3.9%, respectively, for the two successive events. The enriched end-member of the shergottite mixing curve is best modeled by late-stage quenched residual melt resulting from the crystallization of a magma ocean. The depleted shergottite source may be modeled as a mixture of cumulates and residual melt, as convection in the Martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers. Consequently, equilibrium crystallization is preferred to model the crystallization of the Martian magma ocean. The models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in Mars.
Isotopic analyses of mineral, leachate, and whole rock fractions from the Martian shergottite meteorite QUE 94201 yield RbSr and SmNd crystallization ages of 327 ± 12 and 327 ± 19 Ma, respectively. These ages are concordant, although the isochrons are defined by different fractions within the meteorite. Comparison of isotope dilution Sm and Nd data for the various QUE 94201 fractions with in situ ion microprobe data for QUE 94201 minerals from the literature demonstrate the presence of a leachable crustal component in the meteorite. This component is likely to have been added to QUE 94201 by secondary alteration processes on Mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractions. Initial 87Sr/86Sr of 0.701298 ± 14, ϵNd143 of +47.6 ± 1.7, and whole rock ϵNd142 of +0.92 ± 0.11 indicate that QUE 94201 was derived from a source that was strongly depleted in 87Rb/86Sr and enriched in 147Sm/144Nd early in its history. Modeling demonstrates that the SmNd isotopic compositions of QUE 94201 can be produced by either four episodes of melting at 327 Ma of cumulates crystallized from a magma ocean at 4.525 Ga or five episodes of melting of an initially solid Mars at 4.525 Ga and 327 Ma. The neodymium isotopic systematics of QUE 94201 are not consistent with significant melting between 4.525 Ga and 327 Ma. The estimated timing of these events is based on initial neodymium isotopic ratios and is independent of differentiation of the QUE 94201 parental magma. Rb-Sr-based partial melting models are unable to reproduce the composition of QUE 94201 using the same model parameters employed in the SmNd-based models, implying a decoupling of RbSr and SmNd isotopic systems. The initial decoupling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fractionate Rb from Sr compared to Sm from Nd. The fact that all Martian meteorites analyzed so far define a RbSr whole rock isochron age of 4.5 Ga suggests that virtually all Rb was partitioned out of their mantle source regions and into either fractionated residual liquids trapped in the cumulate pile or into the crust at that time. Thus, the Martian mantle cumulates and restites are not expected to evolve past of 0.700 and could not have been significantly enriched in incompatible elements by crustal recycling processes. All Martian meteorites have initial values that are higher than ∼0.700 and are, therefore, likely to be produced by mixing between evolved crustal-like and depleted mantle reservoirs. The absence of crustal recycling processes on Mars may preserve the geochemical evidence for decoupling of the RbSr and SmNd isotopic systems, underscoring one of the fundamental differences between geologic processes on Mars and the Earth.