SAMPLE RETURN AND CLIMATE: IGNEOUS ROCKS AND IMPACT BRECCIAS

Laurie A. Leshin
Department of Earth and Space Sciences
University of California
Los Angeles, CA 90095-1567, USA.

The major outstanding issue in studies of martian climate is whether or not Mars was warmer and/or wetter in its earliest history [1], i.e., during the Noachian, the time period when most of the valley networks were formed [2]. In order to address the scientific questions associated with evolution from the early martian climate to that of today, we must proceed with two types of studies. First, it is critical to understand the distribution and composition (both molecular and isotopic) of the current martian volatile inventory. Specifically, detailed chemical and isotopic analysis of the current atmosphere (discussed at this workshop from the perspective of sample return by B. Jakosky and T. Owen) and mineralogical, petrological and chemical characterization of young martian rocks (such as, but not limited to, most of the SNC meteorites), as well as mapping of the current distribution of groundwater/ice (not accomplished by sample return missions) will lead to an understanding of the current product of Mars' early climate/volatile evolution. Second, and even more critical, are studies of ancient (Noachian) rocks that preserve a record of this ancient martian climate. Ancient sediments are obviously an immensely interesting target for a sample return effort, but Noachian igneous rocks and impact breccias would also give important insights into the earliest volatile history of Mars, and they are the focus of this discussion.

In general, the minerals and glasses that comprise igneous rocks are unstable at the surface of a terrestrial planet. Given enough time and the proper physical and chemical conditions this disequilibrium leads to formation of secondary alteration products that record information about the environment in which they were formed. By studying the products of the interaction of igneous rocks with their environment, it is possible to reconstruct the environmental conditions under which the alteration products formed. In addition, magmas that come into contact with volatile-rich regions upon emplacement ( e. g., groundwater, oceans) have distinctive textures that are indicative of their eruptive setting. Therefore, textural studies of ancient igneous rocks can provide information of the presence of near-surface volatile reservoirs.

Specifically, studies that would be performed on returned ancient igneous rocks that will provide unique information on martian volatile history would include textural/petrologic investigations of the primary igneous minerals (relates to emplacement setting, and volatile history of magmatic source regions), mineralogical characterization of any secondary alteration products (relates to the environment and physical conditions under which the alteration took place, e.g., whether secondary minerals are "palagonites" or well ordered clay minerals), and detailed chemical and isotopic characterization of the alteration products, the source rock, and any volatiles derived from the primary or secondary phases (relates to the chemistry and history of the volatile reservoirs involved in the alteration, as well as the volatile history of the magmatic source region). These kinds of studies would involve the extensive use of optical microscopy, scanning electron microscopy (SEM), electron microprobe, transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), and gas source mass spectrometry, to name a few. Also, insight into the ancient magnetic field of Mars, which has important implications for early atmospheric loss by sputtering, could be gained from these samples. Numerous investigations could be performed on samples of on the order 10 grams in size, and (for example) multiple 10-20 gram samples are preferable to a single 200 gram sample. As a case in point, I note the large amount of data collected on the 12-gram SNC meteorite QUE94201 (see LPSC XXVII abstracts, 1996). Studies of relatively unshocked samples are preferable, but this is not a strict requirement as it may be difficult to achieve in the ancient cratered highlands.

Impact process supply samples of materials to planetary surfaces that might otherwise be unavailable at the surface due to burial or deep emplacement. Since deeper crustal rocks will likely show different effects of interaction with crustal volatiles than ancient igneous rocks that were emplaced at or very near the surface, studies of impact breccias are important. For example, deep crustal rocks may preserve evidence of Noachian hydrothermal activity which will give insight into ancient volatile cycling on Mars. Additionally, early, now buried sediments may be preserved in these samples. The types of measurements and questions addressed by the studies of ancient impact breccias are very similar to those outlined above for ancient igneous rocks. If breccias are to be collected, it is desirable to collect samples with dimensions larger than the size of the individual clasts, or if sampling of individual clasts is to be performed, documentation of the relationship of the samples to the "hand sample"-sized source rock is necessary.

Shocked rocks may also preserve direct samples of past martian atmospheres since the shock process implants atmospheric constituents into rock samples. The classic example of this is the shock-melted pockets in shergottite EETA79001, which reproduce with astonishing accuracy the composition of the current martian atmosphere (see figure in presentation materials, last page) [e.g., 3]. Laboratory experiments have shown that this implantation can occur even when extensive shock melting does not [3], therefore any samples of shocked rocks have some possibility of revealing information about martian atmospheric chemistry in past epochs.

A final word about sample preservation requirements. At least some of the alteration minerals that will provide the most information on ancient climates begin to lose volatiles at temperatures below 100°C and to completely breakdown at temperatures below 250°C. Therefore, if using volatile-bearing alteration products to constrain the early climate history of Mars is among the primary goals of a sample return mission, care should be taken to maintain the sample temperature at a low value (>50°C, if possible).

REFERENCES:

[1] Pepin R.O. and Carr M.H. (1992) in Mars, The University of Arizona Press: Tucson, 120-143.
[2] Baker V.R. et al. (1992) in Mars, The University of Arizona Press: Tucson, 493-522.
[3] Wiens R.C. and Pepin R.O. (1988) Geochim. Cosmochim. Acta, 52, 295-307.


Vugraphs:

VG 1: Title: Sample Return and Climate: Igneous Rocks and Impact Breccias

VG 2: Martian Climate - Reasonable Goal

VG 3: Ancient Igneous Rocks

VG 4: Ancient Igneous Rocks (cont.)

VG 5: Ancient Impact Breccias

VG 6: Igneous Rocks/Impact Breccias

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