ALH 84001 A “SNC ORTHOPYROXENITE”:  INSIGHTS FROM SIMS ANALYSIS OF ORTHOPYROXENE AND COMPARISONS TO DIOGENITES. J. J. Papike, G. W. Fowler, G. D. Layne, M. N. Spilde, and C. K. Shearer; Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque NM 87131-1126, USA.

Published in Lunar and Planetary Science XXV, pp. 1043-1044, LPI, Houston.

Introduction:  Recently we were informed that thin sections, labeled ALH 84001,45 and ALH 84001,46, that we had been working on for the last two years were mislabeled and really are EETA 79002,154 and EETA 79002,155 respectively. We received “the real ALH 84001” on November 23, 1993, and discovered [through the receipt of Antarctic Meteorite Newsletter, 16(3)] that ALH 84001 is an SNC orthopyroxenite. Although we are not actively working on SNC meteorites, we thought we could make a contribution by running a set of trace elements by SIMS (Table 1) on orthopyroxene in 84001 using the techniques that we have been using on diogenites.

Table 1.  ALH 84001 orthopyroxene (ppm).

ALH 84001,5  SrYZrLaCeNdSmEuDyErYb
  Point 10.1481.854.180.0060.0440.0960.0790.0120.2180.1630.204
  Point 20.1101.411.150.0020.0170.0300.0430.0070.1500.1150.169
  Point 30.1101.801.900.0060.0250.0640.0440.0090.2070.1650.209
  Point 40.1371.833.980.0050.0470.0860.0860.0130.2100.1450.182
Average:ALH 84001,50.1261.722.800.0050.0330.0690.0630.0100.1960.1470.191

bd = below detection.

Results and Discussion:  Selected microprobe and SIMS data are presented in Table 1 and Figs. 1, 2, and 3. Table 1 presents data for 11 trace elements for ALH 84001 and for Roda. Roda is the most REE enriched diogenite we have studied [1]. Even though, as Table 1 illustrates, Roda is considerably more REE enriched than 84001, Eu is an exception. Figure 1 presents chondrite normalized [2] REE patterns for orthopyroxene from ALH 84001 and estimates of REE patterns for melts assumed to be parental to the new SNC meteorite. As usual, the choice of partition coefficients (Ds) make such calculations approximate. We used a combination of Ds used in [3, Table 3]. It is informative to compare the REE patterns for orthopyroxene from Roda and ALH 84001 (Fig. 3). Even though Roda orthopyroxene is significantly enriched in REE over ALH 84001, it has a larger negative Eu anomaly. The anomaly that we portray is a minimum and is approximate because the point we plot is the detection limit for Eu; the true anomaly could be much greater. We interpret the higher abundance of Eu in ALH 84001 orthopyroxene than in Roda as a direct result of a higher Eu3+/Eu2+ in the parental melt of the SNC orthopyroxenite. This, in turn, is likely the result of a higher fO2 on "Mars" than the HED parent body. Other than the behavior of Eu, most other trace element comparisons are unremarkable. The trace-element attributes such as position of ALH 84001 on a Y vs. Zr or a Y vs. Yb plot places ALH 84001 intermediate to high (excluding Roda) in the diogenite fractionation series somewhat like Johnstown and Aioun El Atrouss but less trace-element enriched than Ibbenburen, ALHA 77256, LEW 88008, or Roda [4].

Our mineral chemical data, including that for plagioclase (~60% Ab), is in complete agreement with that of Mittlefehldt [5]. We also analyzed several accessory carbonate veinlets and found that the carbonate ranged from (Mg0.75Ca0.07Fe0.18)CO 3 to (Mg0.48Ca0.19Mn0.02Fe0.31)CO3. For orthopyroxene, on a diagram of Mn (afu) vs. Fe (afu) 84001 plots about halfway along the positive trend but with a slightly lower Mn/Fe ratio than for the average diogenite (approximately 40 microprobe analyses of ALH 84001 compared with approximately 1000 microprobe analyses of a suite of 23 diogenites). Mittlefehldt noted that the albite content of ALH 84001 plagioclase is considerably higher than the albite content of plagioclase in diogenites. Figure 1 illustrates that the higher activity of Na in “martian” magmas also results in a slight, but probably significant, elevated Na content of ALH 84001 orthopyroxene compared to diogenite orthopyroxene.

Acknowledgments:  SIMS analyses were performed at the UNM/SNL Ion Microprobe Facility, a joint operation of the Institute of Meteoritics, UNM, and Sandia National Laboratories. This research was funded by NASA grant NAGW-3347 and the Institute of Meteoritics.

References:  [1] Papike J. J. et al. (1993) Eos AGU, 74, 380-381. [2] Anders E. and Grevesse N. (1989) GCA, 53, 197-214. [3] Lundberg L. L. et al. (1990) GCA, 54, 2535-2547. [4] Fowler G. W. et al. (1994) LPS XXV, this volume. [5] Mittlefehldt D. W. (1993) Antarctic Meteorite Newsletter, 16(3).