WATER CONTENT OF TEKTITES

Written By Aubrey Whymark 2013-2017
Tektites are extremely dry, with water contents of 0.002 to 0.030 wt% (Beran and Koeberl, 1997). This compares with terrestrial volcanic obsidian values being around 0.2 wt% (Gilchrist, Thorpe and Senfle, 1969). Water content values are summarised in the table and figure exhibited below.

Older analyses by Friedman (1958) found the water content of glasses to be in the range of 0.002 to 0.007 wt%, ranging up to 0.014 wt%. Later analyses, which were more accurate, placed the water values for tektites slightly higher at around 0.012 wt% (±0.004wt%) (Gilchrist, Thorpe and Senfle, 1969) and 0.002 to 0.030 wt% (Beran and Koeberl, 1997)

With reference to Beran and Koeberl (1997), the precise water content of tektites does vary between the different strewn fields. Ivory Coast tektites are the driest. All tektites are, however, extremely dry in comparison to other natural glasses.

Within the Australasian strewn field, water content increases with proximity to the impact location (see Figure 5.1). The distal, first formed, australites are the driest and proximal, last formed, Muong Nong-type layered impact glasses, whilst still extremely dry, have the highest H2O weight percentage. On a broader scale, analyses of impact glasses and impactites show much higher water contents. For Zhamanshin, Aouelloul, Rio Cuarto craters and Libyan Desert glass the H2O percentage ranges from 0.008 to 0.166 wt%, with an average of ~0.11% (Beran and Koeberl, 1997). Atomic bomb glass (Freidman, 1958) falls within the range of impactites. So, as a general comment, tektites are ten times drier than impactites and impactites are twice as dry as terrestrial volcanic obsidians.

O’Keefe (1964) saw the low water content as good evidence that tektites originated from the Moon. The loss of volatiles during the impact is, however, much better understood now. Melosh and Artemieva (2004) explain how the water and other volatiles can be removed from the tektite during the short time interval available for their formation in a large terrestrial impact event. The elimination of bubbles (containing the volatiles) from a tektite melt is discussed elsewhere on this site.
ABOVE: Selected analyses of water content of tektites and other glasses.
ABOVE: The water content of tektites and selected glasses, plotted on a logarithmic scale. Note the general trend of decreasing water content with distance from the impact (more data needed for indochinites). It also appears that one piece of terrestrial obsidian was muddled up with the philippinites, which would not be unexpected. Data sources: Terrestrial Obsidian; Peru (Friedman, 1958), Atom Bomb Glass, New Mexico (Friedman, 1958); Zhamanshin Impactite (unrelated to Australasian tektites, which have no known source crater impactite) (Beran & Koeberl, 1997); Layered Muong Nong-type impact glasses from Indochina (Beran & Koeberl, 1997 and Gilchrist, Thorpe & Senfle, 1969); Splashform indochinites (Beran & Koeberl, 1997, Gilchrist, Thorpe & Senfle, 1969 and Kadik, Lukanin, Zharkova & Feldman, 2003); Philippinites (Beran & Koeberl, 1997 and Gilchrist, Thorpe & Senfle, 1969); Javaites (Gilchrist, Thorpe & Senfle, 1969); Australites (Beran & Koeberl, 1997 and Gilchrist, Thorpe & Senfle, 1969).

Test for Water

From Wikipedia: 'Loss on ignition is a test used in inorganic analytical chemistry, particularly in the analysis of minerals. It consists of strongly heating ("igniting") a sample of the material at a specified temperature, allowing volatile substances to escape, until its mass ceases to change. This may be done in air, or in some other reactive or inert atmosphere. The simple test typically consists of placing a few grams of the material in a tared, pre-ignited crucible and determining its mass, placing it in a temperature-controlled furnace for a set time, cooling it in a controlled (e.g. water-free, CO2-free) atmosphere, and redetermining the mass. The process may be repeated to show that mass-change is complete. A variant of the test in which mass-change is continually monitored as temperature is changed, is thermogravimetry.

The loss on ignition is reported as part of an elemental or oxide analysis of a mineral. The volatile materials lost usually consist of "combined water" (hydrates and labile hydroxy-compounds) and carbon dioxide from carbonates.'

H2Otot and H2O-  are measured, with H2O+ being the difference. H2O minus is the moisture water and H2O plus is the bound water (crystal water if you like). When H2O minus and H2O plus are combined you have the H2O total.
A far cruder test for volatiles in a sample is to heat a specimen with a blow torch (taking the usual safety precautions including full eye and face protection). If the sample bubbles and froths then it clearly contains a higher volatile content. If it remains smooth and simply melts it contains far less volatile material. This is obviously a crude test and non-conclusive, but if frothing is observed then it is highly improbable that the sample is a tektite. If there is no frothing then maybe it is a tektite (but not proof the sample is a tektite).

Wet Tektites?

All proven tektites are very dry (even when found in wet tropical settings). It is extremely reasonable to assume that material formed in the same manner will also be extremely dry, regardless of depositional environment. Erickson et al. (2012) found that Healdsburg Glass contained 0.18 % to 0.53 % total water (0.08 % to 0.45% H2O plus or bound / crystal water).  This amount of water is an order of magnitude (or two) higher than that found in known tektites. At this point the logical conclusion is that Healdsburg Glass is obsidian (a notion supported by morphology and sculpture). Incredibly (and erroneously), despite the strongly contradictory evidence, it was concluded that Healdsburg Glass was a type of tektite!
 

Removing Volatiles from Tektites

Tektites have an extremely low volatile content (including water), yet they were only very briefly heated. Stoke's Law which deals with the way a bubble rises in liquid is frequently quoted as to why tektite glass cannot be terrestrial. In man-made glass, it may sit for 20 hours at about 1,750°C to remove bubbles. In contrast, tektites rapidly became fairly bubble-free (less so for proximal forms) in a matter of seconds. Tektites were melted by shock compression, then rapid decompression and ejected into a near vacuum. Bubbles of volatiles would literally boil out of the specimen exceedingly rapidly (think of a Champagne bottle going from 5 atmospheres to 1 atmosphere, now think of a tektite going from 1 atmosphere to 1/100,000th atmosphere).
 

References