Nebular or parent body alteration of chondritic material: Neither or both?

Abstract

Abstract-Discussions of meteorite properties often concern whether they are the result of 'hebular'' or "parent body" processes, but several decades of research have not resolved the issue. Part of the problem is that any gas-phase reaction involving meteoritic materials thought to have occurred is automatically assumed to be nebular, even though the most primitive solar system objects are water-and volatile-rich and could easily generate vapors. Reactions important in meteorite genesis are those involving (1) oxidation of Fe, (2) oxidation of other cations, (3) reduction, (4) olivine-pyroxene equilibria, and (5) hydration of silicates. The P-T-x conditions required are almost invariably incompatible with standard models for the early solar nebula, but clearly many of these reactions occurred prior to final agglomeration. Most of the reactions require high pressures atm) or, more importantly, major departures from cosmic composition, even though the final rocks are remarkably cosmic in elemental proportions. We suggest that most of these reactions occurred in a regolith rendered "dynamic" by the flow of copious volatiles. In such a scenario, liquid and gas phase reactions can occur at elevated temperatures and pressures relative to the nebula and with noncosmic gas phase compositions; but the system is "closed" to most components, so that cosmic proportions are essentially preserved.

Zoned chondrules in Semarkona: Evidence for high‐ and low‐temperature processing

“…(2000), silicate liquids calculated to form at 1000x dust enrichments and 10-3 bar only contained~10 ppm H20 at 1460 K. Water does not become an important alteration reactant in the solar nebula or on parent bodies unless the temperature is below 500 K (e.g., references cited in Bischoff, 1998;Sears and Akridge, 1998). This suggests that zoning of H and, by extension elements that strongly correlate with H, occurred at low temperatures, probably in the parent asteroid where aqueous processes are known to have occurred.…”

Thermal evolution of trans‐Neptunian objects, icy satellites, and minor icy planets in the early solar system

“…(Pisani 1864;Brearley 2006;Bland et al 2009). The magnetites were earlier thought to have condensed from the cooling solar nebula (Larimer and Anders 1967;Sears and Akridge 1998). It was also suggested that these water-bearing minerals are the byproducts of the aqueous alteration between water and anhydrous minerals in the cooling solar nebula (Grossman and Larimer 1974;Weisberg et al 1993).…”

I‐Xe measurements of CAIs and chondrules from the CV3 chondrites Mokoia and Vigarano

“…Nonetheless, the various components within the meteorites have been subject to one or more episodes of alteration either in the solar nebula or later within a parent body, or perhaps in both environments (Krot et al 1995;Bischoff 1998;Sears and Akridge 1998;Zolensky et al 1997). Radiometric dating of the alteration events may allow greater insight into the nature of the environment in which alteration occurred (e.g., very late ages would imply a parent body location rather than nebular).…”