Possible carbonaceous inclusion

Halite (vug is 1 mm)

Introduction:

Over the past three decades we have become increasingly aware of the fundamental importance of water, and aqueous alteration, on primitive solar system bodies [1].

Nevertheless, we are still lacking fundamental information such as the location and timing of the aqueous alteration, and the nature of the aqueous fluid itself. A major impediment to our understanding of aqueous alteration has been the complete absence of direct samples of aqueous fluids in meteorites. Here we describe aqueous fluid inclusions present in blue/purple halite and sylvite found within the matrix of two ordinary chondrite falls, Monahans (1998) (H5) and Zag (H3-6); both meteorites are gas-rich regolith breccias [2&3].

Fluid Inclusions:

Fluid inclusions are micro-samples of fluid that are trapped at the crystal/fluid interface during growth (primary inclusions) or some later time along a healed fracture in the mineral (secondary inclusions). Both primary and secondary fluid inclusions are found in Monahans (1998) and Zag halite; the latter predominate.

The presence of secondary inclusions in the halite indicates that aqueous fluids were locally present following halite deposition, suggesting that aqueous activity could have been episodic. We have completed preliminary study of the Monahans (1998) inclusions; these range up to 15 micrometers in longest dimension. At room temperature, approximately 25% of the inclusions contain bubbles that are in constant motion, proving that the inclusions contain a low viscosity liquid and "vapor". During cooling under the microscope, the inclusions solidified (froze) at -45 to -50êC. When the frozen inclusions are heated, first melting is observed at about -35 to -40êC, indicating that the inclusions likely contain divalent cations such as Fe 2+ , Ca 2+ or Mg 2+ , in addition to Na + and K + ; given the environment of formation, dissolved Fe and Ca are the most likely dissolved species.

The presence of water in the Monahans (1998) inclusions was confirmed by Raman microprobe analysis. Raman spectra of inclusions show a significant peak at approximately 3400 cm -1 , diagnostic of aqueous salt solutions. The rarity of vapor bubbles in fluid inclusions in Monahans (1998) and Zag halite suggests a low formation temperature (probably less than 50êC). The bubbles that are present are small, and probably resulted from freeze-stretching of the inclusions during space exposure. The vapor bubbles represent the water vapor in equilibrium with liquid water at room temperature and, as such, are essentially a poor vacuum with a pressure of about 0.03 bars. No gases, such as CO2, N2, or CH4, were detected during Raman analysis of the inclusions.

Implications:

It is possible that halite is commonly present in chondrites, but has been overlooked, resulting in considerable errors in bulk Cl determinations for chondrites. It is also possible that a considerable fraction of the ubiquitous sulfate/halide efflorescence noted on Antarctic meteorites is derived from dissolution and reprecipitation of indigenous halite (and sulfides), rather than from components introduced from the ice as is commonly assumed.

The halite may have been deposited from indigenous briny asteroidal water trickling through the regolith, or by a water-bearing meteoroid impacting the surface. Measurement of the oxygen and hydrogen isotopic composition of the fluid should answer this question, and shed new light on the source of water. Measurement of the fluid composition within the inclusions will permit more realistic modeling of asteroid alteration. Apparently, water was more common on asteroids than is generally realized, and thus chondrite metamorphism paths should be reconsidered. It should also be possible to date more precisely the halite/ sylvite by the 39 Ar/40 Ar laser ablation technique, increasing our understanding of the timing of asteroidal alteration