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).
INAA | s.d. | XRF | s.d. | SSMS | s.d. | INAA | s.d. | SSMS | s.d. | ||
---|---|---|---|---|---|---|---|---|---|---|---|
wt.mg | 236 | 52.7 | 71 | wt.mg | 236 | 71 | |||||
% | ppm | ||||||||||
MgO | 24.50 | 1 | Zr | 5.89 | 3 | ||||||
Al2O3 | 1.25 | 3 | Nb | 0.42 | 7 | ||||||
SiO2 | 52.70 | 1 | Cs | 0.04 | 10 | ||||||
CaO | 1.7 | 15 | 1.82 | 1 | Ba | 4.03 | 3 | ||||
TiO2 | 0.20 | 1 | La | 0.13 | 15 | 0.15 | 5 | ||||
FeO | 17.6 | 3 | 17.60 | 1 | Ce | 0.43 | 5 | ||||
Na2O | 0.162 | 3 | Pr | 0.060 | 5 | ||||||
P2O5 | 0.014 | 15 | Nd | 0.265 | 3 | ||||||
S* | 0.011 | 10 | Sm | 0.10 | 4 | 0.104 | 3 | ||||
K2O | 0.022 | 5 | 0.026 | 5 | 0.024 | 5 | Eu | 0.036 | 15 | 0.032 | 3 |
Cr2O3 | 1.242 | 3 | Gd | 0.14 | 7 | ||||||
MnO | 0.443 | 3 | Tb | 0.04 | 15 | 0.030 | 7 | ||||
Dy | 0.27 | 15 | 0.24 | 5 | |||||||
ppm | Ho | 0.07 | 15 | 0.068 | 5 | ||||||
C* | 580 | 8 | Er | 0.21 | 5 | ||||||
Sc | 12.7 | 3 | Tm | 0.036 | 5 | ||||||
Co | 43 | 3 | Yb | 0.31 | 7 | 0.255 | 5 | ||||
Ni | <20 | Lu | 0.051 | 7 | 0.037 | 7 | |||||
Zn | 106 | 4 | Hf | 0.17 | 15 | 0.16 | 5 | ||||
Ga | 3.2 | 4 | Ta | <0.04 | |||||||
As | <0.08 | Ir | <0.002 | ||||||||
Se | <0.4 | Au | 0.001 | 25 | |||||||
Rb | 0.83 | 7 | Pb | 0.072 | 5 | ||||||
Sr | 4.46 | 3 | Th | <0.06 | 0.035 | 10 | |||||
Y | 1.63 | 5 | U | 0.012 | 20 | 0.010 | 10 |
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.