Lunar and Planetary Science XXXVIII (2007) 1649.pdf

ALGONQUIN CLASS ROCKS OF COLUMBIA HILLS IN THE GUSEV CRATER, MARS, AND THEIR RELATIONSHIP TO SNC METEORITES. G. Dreibus1, J. Brückner1, R. Gellert2, E. Jagoutz1, G. Klingel- höfer3, M. E. Schmidt4 and Athena Science Team5, 1Max-Planck-Institut f. Chemie, J.-J.-Becher-Weg 27, D-55128 Mainz, Germany ([email protected]), 2Dep. of Physics, Univ. of Guelph, Guelph, On, Canada, 3Inst. Anorg. Analyt. Chemie, Joh. Gutenberg-Univ., Mainz, Germany, 4Dep. Mineral Sci., National Museum of Natural History, Smithsonian Institution, Washington, D.C., 5Cornell Univ., Ithaca, NY.

Introduction: The Mars Exploration Rover Chemical and mineralogical composition: At (MER) Spirit discovered the first ultramafic rocks on Husband Hill, a sequence of outcrop rocks was en- the surface of Mars as it descended the south flank of countered and putatively called “Algonquin Class”: ‘Husband Hill’ (Columbia Hills). Data provided by the ‘Larry’s Bench’, ‘Seminole’, ‘Algonquin’ and ‘Co- Alpha Particle X-Ray Spectrometer (APXS) showed manche’. The APXS and the Mössbauer (MB) spectra the highest concentrations of Mg, Fe and the lowest of these rocks indicate that the samples are relatively contents of Al, K, Ca and Ti (Fig.1) of almost all rocks unaltered igneous rocks with high olivine content (also encountered so far [1] for two samples from the Al- observed by Mini-TES [3]). Possibly, they could be gonquin Class, ‘Algonquin Iroquet’ and ‘Comanche derived from a primary magma as an olivine cumulate. Palomino’. Among the SNC meteorites no such rock A magmatic sequence of these four rocks becoming type is found. However, there is a close similarity of increasingly olivine-rich was postulated [4]. Among these two rocks (both brushed surfaces) to the Martian the SNCs the two chassignites contain mostly olivine meteorites with respect to elemental compositions. and are thus classified as dunites. Evidently, the Ni concentrations of the Algonquin Class rocks are higher than in SNCs. MER data dis-

cussed are for brushed (and abraded) surfaces, only. NWA2737 1 ALHA84001 Larry's Bench Thrasher Algonquin Iroquet ALHA77005 Normalized to Humphrey RAT 2 Seminole Osceola DaG Mars Mantle Algonquin Iroquet Brushed Adirondack Chassigny 10 Comanche Palomino Seminole Earth Mantle Zagami CI Pot of Gold Lafayette Nakhla Comanche Palomino Shergotty

1 0.1 Larry's Bench

Cr [wt.%] QUE 94001 Clovis Backstay Relative Concentration [1] 0.1 Na Mg Al Si P S Cl K Ca Ti Cr Mn Fe Ni Zn Br SNCs Figure 1 Relative concentrations of four related rocks (all Gusev brushed) at Husband Hill (see text). Values are normalized to Gusev plains rock ‘Humphrey RAT2’ [1]. 0.01 Los Angeles

110100 Classification: The composition of ‘Algonquin’ Mg [wt. %] and ‘Comanche’ with high Mg, low Na, Al, K, and Ca Figure 2 Cr versus Mg concentrations for SNC meteorites, contents resemble to terrestrial picrites or komatiites. Gusev rocks, Earth mantle, and carbonaceous chondrite CI. There are considerable differences between Mars and Earth. The Martian mantle has more than twice the Figure 2 shows the similarity of the brushed ul- Fe content compared to the Earth’s mantle. Therefore, tramafic Gusev rocks with the most unfractionated the two ultramafic rocks encountered by the rover Martian meteorites, like Chassigny or the lherzolitic Spirit are with 22.3 and 24.8 wt % MgO not so mag- shergottites. Whereas the primitive basalt ‘Adiron- nesian as terrestrial samples, but richer in iron. For the dack’ from the Gusev plains [1] does not fall on the Earth, the classification of high-magnesium volcanic chondritic (CI) Mg-Cr line, the Algonquin Class sam- rocks leads to MgO > 18 wt % for komatiites and > 12 ples have a close to chondritic Mg-Cr ratio as postu- wt % for picrites [2]. The total alkali content (Na2O + lated for the Martian mantle [5]. Furthermore, the ratio K2O) of the komatiites and picrites is between 1 and 2 of the refractory elements Ca and Al of these rocks wt % and between 2 and 3 wt %, respectively. The matches closely that of CI. This is a strong hint for (at alkali content of Algonquin and Comanche is 1.7 and least) a local, primitive magma source for potential 1.1 wt %, respectively. volcanism beneath Gusev crater, similarly postulated Lunar and Planetary Science XXXVIII (2007) 1649.pdf

for alkaline volcanic rocks in Husband Hill by [6]. The The Ni contents of the Algonquin Class rocks, ex- 3+ Fe /Fetotal ratios measured by MB on brushed surfaces cept for Larry’s Bench, are also much higher than the of these rocks are 0.20 (Larry’s Bench) 0.24 (Semi- primitive Gusev plains basalts [1]. A Ni enrichment of nole), 0.11 (Algonquin), and 0.22 (Comanche), and the Gusev soils relative to the plains basalts was ex- illustrate that these outcrops are only weakly altered. plained by a meteoritic component that was admixed However, compared to the SNCs with similar Fe con- by impacts [1,7]. Most olivine-rich shergottites, Chas- tents their Mn contents are lower compared to SNCs signy, and the Gusev plains basalts have an Mg/Ni of resulting in an overall higher Fe-Mn ratio of 53 (Fig. 400, except for NWA2737 with 250. All soils and the 3). A similar trend can be observed for the olivine cu- Columbia Hills rocks have even lower ratios (Fig. 4). mulate Chassigny. The Ni/Mg scattering of all Gusev rocks is rather large and approaches the lower ratios of terrestrial rocks. 0.5 Fe/Mn = 40 While Ni/Mg in most SNCs and terrestrial ultramafics SNCs Fe/Mn = 60 Gusev are strongly correlated, Ni in rocks at Gusev Crater 0.4 Chassigny might be mobilized by the acidic Martian environment. Nickel is strongly partitioned into olivine. A weak 0.17 Fe*, Adirondack Comanche Palo., 0.22 Fe* correlation of Ni with the (calculated) olivine content 0.3 NWA2737 Algonquin, 0.11 Fe* 0.24 Fe*, Seminole Abiaka Larry's Bench, 0.20 Fe* of all Martian rocks exists (Fig. 5). Chassigny and the 0.36 Fe*, Irvine Mn [wt %] [wt Mn plains basalts plot along the SNC correlation line.

Clovis, 0.84 Fe* However, most of the Algonquin Class rocks seem to Fe*= Fe3+/Fe total follow a different correlation. Assuming olivine is the 0.2 0.23 Fe*, Backstay Pot of Gold, 0.63 Fe* host mineral of Ni, the source region of the igneous 46810 3050 Algonquin Class rocks must contain considerably Fe [wt %] higher Ni concentrations than derived for the Martian Figure 3 Fe-Mn ratios for SNCs and Gusev rocks. mantle based on the SNC compositions [5]. A frac-

tional crystallization of the olivine cumulates could Olivine-rich SNCs have changed their Ni content considerably, which Basalts in Gusev plains 2000 Gusev soils would explain the apparent heterogeneity of the Mar- Earth: Komatiites+Lherzolite tian mantle. Meridiani soils Rocks in Columbia Hills Earth 1200 1500 Montmorillonites 1000 Comanche Palomino Assemblee Algonquin Iroquet NWA2737 Gruyere 800 Mars ElDorado Scuff 1000 Shadow Comanche Palomino 600 Seminole Abiaka Pot of Gold NWA2737

Ni [ppm] Chassigny Ni [ppm] Ni Algonquin Iroquet Comanche 400 Clovis Horseback Irvine Inde- SNCs Adirondack RAT pendence Penn2 Larry's Bench Peace 200 500 Seminole Chassigny Abiaka Humphrey RAT2 Mg/Ni = 400 0 Irvine 0 20406080100 Backstay Olivine [wt %] Scuppers Larry's Bench 0 Figure 5 Nickel versus olivine for Gusev rocks, Chassigny, 0 5 10 15 20 25 and NWA2737. The Gusev olivine content is based on the Mg [wt %] APXS data adjusted to 0.3 % S and 0 % Cl and the Fe3+/Fe2+ Figure 4 Nickel [ppm] versus Mg [weight %] of olivine-rich ratio obtained by the Mössbauer data [6] and this work. SNCs, NWA2737, Gusev rocks and soils, Meridiani soils, and terrestrial komatiites and lherzolites. References: [1] Gellert R. et al. (2006) JGR, 111, E02S05. [2] LeBas H.J., (2000) J. Petr., 41, 1467. [3] Ruff S.W. et al. The Ni matter: Compared with the olivine-rich (2006) LPSC 37, #1989. [4] Mittlefehldt D.W. et al. (2006) SNCs most of the Algonquin Class rocks have an ex- LPSC 37, #1505. [5] Wänke H. a. Dreibus G. (1988) Phil. tremely high Ni content. Only, a recently discovered Trans. R. Soc. Lond. A 325, 545. [6] McSween H.Y. et al. chassignite, NWA2737, has with 875 ppm a similar Ni (2006) JGR, 111, E09S91. [7] Yen A.S. et al. (2006) JGR, concentration. 111, E12S11.