62nd Annual Meteoritical Society Meeting 5050.pdf

ASTEROIDAL WATER WITHIN FLUID-INCLUSION-BEARING HALITE IN ORDINARY CHONDRITES. M. E. Zolensky1, R. J. Bodnar2, and A. E. Rubin3, 1Mail Code SN2, NASA, Johnson Space Center, Houston, TX 77058 USA, 2Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061 USA; 3Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095 USA.

Introduction: Over the past three decades we have temperature and, as such, are essentially a poor vac- become increasingly aware of the fundamental im- uum with a pressure of about 0.03 bars. No gases, portance of water, and aqueous alteration, on primi- such as CO2, N2, or CH4, were detected during Raman tive solar-system bodies [1]. Nevertheless, we are still analysis of the inclusions. lacking fundamental information such as the location Implications of the Halite and Fluid Inclusions and timing of the aqueous alteration, and the nature of in Ordinary Chondrite Regolith Breccias: It is pos- the aqueous fluid itself. A major impediment to our sible that halite is commonly present in chondrites, understanding of aqueous alteration has been the but has been overlooked, resulting in considerable complete absence of direct samples of aqueous fluids errors in bulk Cl determinations for chondrites. It is in meteorites. Here we describe aqueous fluid inclu- also possible that a considerable fraction of the ubiq- sions present in blue/purple halite and sylvite found uitous sulfate/halide efflorescence noted on Antarctic within the matrix of two ordinary chondrite falls, meteorites is derived from dissolution and reprecipita- Monahans (1998) (H5) and Zag (H3-6); both meteor- tion of indigenous halite (and sulfides), rather than ites are gas-rich regolith breccias [2&3]. from components introduced from the ice as is com- Fluid Inclusions: Fluid inclusions are micro- monly assumed. samples of fluid that are trapped at the crystal/fluid Based upon the evidence, we can construct the interface during growth (primary inclusions) or some following basic chronology of events for both Mona- later time along a healed fracture in the mineral (sec- hans (1998) and Zag: (1) asteroid accretion, (2) differ- ondary inclusions). Both primary and secondary fluid ent degrees of fluid-assisted metamorphism of chon- inclusions are found in Monahans (1998) and Zag dritic materials within the asteroid, (3) impact gar- halite; the latter predominate. The presence of secon- dening to produce a regolith, and impact deposition of dary inclusions in the halite indicates that aqueous different materials into the regolith, (4) deposition of fluids were locally present following halite deposition, the halite into the regolith, (5) impact welding of re- suggesting that aqueous activity could have been epi- golith materials and chondritic fragments, (6) impact sodic. launching of the meteoroid from the surface. We have completed preliminary study of the The halite may have been deposited from indige- Monahans (1998) inclusions; these range up to 15 nous briny asteroidal water trickling through the re- micrometers in longest dimension. At room tem- golith, or by a water-bearing meteoroid impacting the perature, approximately 25% of the inclusions contain surface. Measurement of the oxygen- and hydrogen- bubbles that are in constant motion, proving that the isotopic composition of the fluid should answer this inclusions contain a low-viscosity liquid and “vapor”. question, and shed new light on the source of water. During cooling under the microscope, the inclusions Measurement of the fluid composition within the in- solidified (froze) at -45 to -50°C. When the frozen clusions will permit more realistic modeling of aster- inclusions are heated, first melting is observed at oid alteration. Apparently, water was more common about -35 to -40°C, indicating that the inclusions on asteroids than is generally realized, and thus chon- likely contain divalent cations such as Fe2+, Ca2+ or drite metamorphism paths should be reconsidered. It Mg2+, in addition to Na+ and K+; given the environ- should also be possible to date more precisely the hal- ment of formation, dissolved Fe and Ca are the most ite/sylvite by the 39Ar/40Ar laser ablation technique, likely dissolved species. The presence of water in the increasing our understanding of the timing of asteroi- Monahans (1998) inclusions was confirmed by Raman dal alteration. microprobe analysis. Raman spectra of inclusions Acknowledgement: We thank Edwin Thompson show a significant peak at approximately 3400 cm-1, for first noting and providing the Zag halite. diagnostic of aqueous salt solutions. References: [1] Zolensky and McSween (1988) In The rarity of vapor bubbles in fluid inclusions in Meteorites and the Early Solar System (J. Kerridge Monahans (1998) and Zag halite suggests a low for- and M. Matthews, Eds., U. A. Press, pp. 114-143;. mation temperature (probably less than 50°C). The [2] Zolensky et al. Science, submitted; [3] Meteoriti- bubbles that are present are small, and probably re- cal Bulletin 83, July 1999 (in press). sulted from freeze-stretching of the inclusions during space exposure. The vapor bubbles represent the wa- ter vapor in equilibrium with liquid water at room