Water content of ignimbrites: Lunar implications

Abstract

The most important lunar resource may be ignimbrite because of its water content. The variation of water in tuff as a function of degree of welding, oxidation and flow thickness is of direct significance to lunar basing in volcanic terrains on the moon. The fact that many calderas on earth have ignimbritic « aprons » extending from them is correlatable with the apron-type ray patterns around certain post-mare and highland craters on the moon. If lunar craters can be positively identified with associated ignimbritic flows then such areas are potentially water richer than impacted environments. Electrolytic hygrometry and static dehydration analysis of the Tumulo Creek, Wineglass, Bishop, Pelée and Gharvegh tuffs indicate a wide span in water content from 0.2 to 4.1 % even within one flow unit. Devitrification of the mesostasis of these tuffs expels water. Water content apparently varies as a function of flow geometry and possibly thickness. The lower portions of the Tumulo Creek tuff are more hydrous than the upper portions. A slight increase in water content is observed with increase in oxidation.

Most of the water in all welded tuffs analysed with the hygrometer is driven off at around 300°C suggesting polymeric hydroxyl bonding with dissociation energies ranging from 2 to 8 kilocalories per mole. However, other types of bonding — Van der Waal, coordinated hydroxyl, free monomeric hydroxyl, etc. — are probably also present. No single, simple bond type controls water retention in rocks.

Lunar exploration techniques for hydrous portions of ignimbrite flows include the lateral and vertical application of the epithermal log.