CHEMICAL AND MINERAL COMPOSITION OF ALH 84001:  A MARTIAN ORTHOPYROXENITE.  G. Dreibus, A. Burghele, K. P. Jochum, B. Spettel, F. Wlotzka, and H. Wänke, Max-Planck-Institut für Chemie, Saarstraße 23, 55122 Mainz, Germany.

The orthopyroxenite ALH 84001 was recently reclassified by Mittlefehldt [1] as a martian meteorite. We analyzed several aliquots from a total of 1.7 g of split 40 for major and trace elements (Table 1). Mineral compositions were determined by SEM-EDS analysis on thin section ALH 84001,89, which does not come from the same split as our bulk sample. Nevertheless, the compositions found are practically identical to those reported by [1], showing that ALH 84001 is a rather homogeneous rock with regard to the composition of the minerals, although their relative proportions may vary (see below).

TABLE 1.  Chemical composition of ALH 84001,40.

  INAAs.d.XRFs.d.SSMSs.d.  INAAs.d.SSMSs.d.
wt.mg236  52.7  71  wt.mg236  71 
%            ppm       
MgO    24.501    Zr    5.893
Al2O3    1.253    Nb    0.427
SiO2    52.701    Cs    0.0410
CaO1.7151.821    Ba    4.033
TiO2    0.201    La0.13150.155
FeO17.6317.601    Ce    0.435
Na2O0.1623        Pr    0.0605
P2O5        0.01415Nd    0.2653
S*0.01110        Sm0.1040.1043
Cr2O31.2423        Gd    0.147
MnO0.4433        Tb0.04150.0307
ppm            Ho0.07150.0685
C*5808        Er    0.215
Sc12.73        Tm    0.0365
Co433        Yb0.3170.2555
Ni<20          Lu0.05170.0377
Zn1064        Hf0.17150.165
Ga3.24        Ta<0.04     
As<0.08          Ir<0.002     
Se<0.4          Au0.00125   
Rb        0.837Pb    0.0725
Sr        4.463Th<0.06  0.03510
Y        1.635U0.012200.01010

          *  S and C analyzed by Carbon-Sulfur Analyzer 2002 (Leybold Heraeus).
            INAA:  Instrumental Neutron Activations Analysis; XRF:  X-Ray Fluorescence Spectrometry;
            SSMS:  Spark Source Mass Spectrometry; s.d.:    standard deviation in %.
            All analyses were performed on aliquots from a 1.7-g sample.

The chemical composition confirms ALH 84001 as a martian meteorite and not as a diogenite. A typical feature for the HED meteorites is their low abundances in volatile, moderately volatile, and siderophile elements compared to terrestrial and martian rocks. An exception is S: HEDs contain between 1000 and 5000 ppm S. Compared to the 3900 ppm S in the diogenite Johnstown, the 108 ppm S in ALH 84001,40 is very low, but matches the S values for the nakhlites (300 ppm) and Chassigny (120 ppm). The exposure age of 14 ± 2 m.y. [2] indicates also a relationship to nakhlites and Chassigny. Furthermore, the high abundance of Zn, which is enriched by a factor of 160 relative to Johnstown, is typical for the martian rocks. In contrast to ordinary chondrites, not all the Zn is found in the chromite; its content of 0.1% ZnO accounts for only 20% of the Zn; the rest must be contained in the silicates. Cobalt is enriched by a factor of 4 compared to Johnstown and fits well into the correlation of Co vs. MgO + FeO found for the other SNCs. Compared to ALH 84001,20 analyzed by [1] our sample ,40 has a higher Na and K content, indicating about twice the amount of maskelynite. The Na/K ratio of 6.6 in our sample is close to that of 6.4 found in maskelynite. Our REE pattern is similar to that of split 20 [1], but with a lower negative Eu anomaly (Fig. 1), in agreement with the higher maskelynite content. Using the mineral composition in ALH 84001,89 and our bulk Na, Cr, C, P, and S values, we obtain for the bulk sample 2.0% maskelynite, 1.9% chromite, 0.46% carbonate, 0.031% phosphate, and 0.021% pyrite (the rest being opx with traces of cpx). The phosphate crystallizing from a residual liquid could be responsible for the enrichment of the LREE compared to the OPX separate of Johnstown [3]; see Fig. 1.

References:  [1] Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221. [2] Miura Y. N. et al. (1994) LPS XXV, 919-920. [3] Floran R. J. et al. (1981) GCA, 45, 2385-2391.