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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 (email@example.com); 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, 274–283, 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, 4915–4931, 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 176Lu–176Hf and 147Sm–143Nd 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,