ISOTOPE CHEMISTRY INTRODUCTION

Written By Aubrey Whymark 2013-2017
In the Shaw and Wasserburg, 1982, paper published in Earth and Planetary Science Letters entitled ‘Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematic’ the authors use the address ‘Lunatic Asylum, Division of Geological and Planetary Sciences, California Institute of Technology’. I think this serves as a good introduction and warning for those venturing into isotope geology! Evidently the editor failed to get beyond the fourth line of this paper, so I am hoping the reader manages to get a little further on this web page! The isotopic chemistry of tektites is a hard subject to approach; however, it offers a wealth of information and is extremely important to the understanding of tektites.

Many elements have naturally occurring isotopes. An element has a fixed number of protons, but the number of neutrons may vary and this number defines the isotope. Some of these isotopes are stable and others are unstable and radioactive. The radioactive isotopes will decay to non-radioactive isotopes at a constant rate known as a half life (see below).

At this point, the mention of radioactivity to the lay person may cause concern or be suggestive that tektites are rather special, even so far as suggesting they may be the product of an ancient nuclear war. Radioactivity is all around us and tektites are no more radioactive than any normal rock on Earth, any window glass, any other human being or household object. Radioactive materials are naturally everywhere, but for the purpose of energy and weapon production naturally high concentrations of radioactive material (ores) have been sought after and mined and then extensively refined and concentrated. Only in these high concentrations will the material pose a health risk.

Isotopes resolve, help to resolve or remove doubt over a large number of questions such as the age of the tektite; the type of source rock; the age of the source rock; the origin of the source rock (terrestrial or otherwise); the age of the precursor rock to the source rock; the time the tektite has spent at or near the surface; the approximate location of the source crater relative to the tektites; a clear ‘signature’ link to the identified source crater; the amount of extra-terrestrial material mixed in with the tektite; the amount of time (if any) the tektite has spent in space; the strewn field to which a tektite belongs to, the age of the sedimentary rock into which the tektite is reworked and incorporated into and so on.

Isotopic chemistry simply measures and compares the different abundances and ratios of isotopes. The isotopes being compared may be of a different or the same element.  The relationship of these two or more isotopes must be understood. Isotopes may be related by a decay series or a common method of formation, such as by interaction with cosmic rays or origin in a star. Isotopic ratios may be compared with isotopic ratios in other systems or with elemental abundances/ratios in the samples to see if they match the expected isotopic ratios observed. This builds up a clearer picture of what the data is revealing.

In studying isotopes, one must understand the systems involved. For instance, is the system open or closed? Many results will rely on a system being closed, as in none of the parent or daughter isotopes are lost over time. One must understand the nature of the different elements/isotopes such as are they stable or what is their half-life, how they form, how they may become concentrated or how they may be lost relative to another element/isotope.

In tektites, which were once molten (due to impact melting), it is important to know how elements/isotopes behave. Some calculations hinge on the fact that isotopic ratios are not significantly altered by impact processes. Other calculations, the K-Ar system for example, rely on the retention of one isotope (parent 40K) and loss of the other isotope (daughter 40Ar), which resets the radiometric clock. From this understanding one can establish if one is calculating the age of the solar system, age of the ultimate parent igneous rock, age of the source rock or age of the tektite.

Much of the isotopic work is like a jigsaw or crossword puzzle. By cross referencing different isotopic results a clearer picture is built up. These assumptions, such as if the source rock is established as a recent terrestrial sedimentary argillaceous rock, can then be fed back into the system. As events/histories of parent rocks/tektites are constrained the messages from the isotopes become less ambiguous, where ambiguity existed.

Unstable isotopes that decay can be used to date rocks. Different radioactive isotopes will have different half lives. After one half life, only 50% of the radioactive parent isotope remains, after two half lives only 25% remains, after 3 half lives only 12.5% remains and so on (see below).
ABOVE: The radioactive clock. Parent isotope is black, daughter isotope is white.
In order to establish the age of tektites one must know the half life and select a suitable radioactive isotope where the half life is neither too long nor too short for accurate measurements to be taken. For instance, Carbon-14, with a half life of 5,730 years, is of no use in dating tektites. Carbon-14 is only valid up to around 60,000 years, after which the low percentage of Carbon-14 remaining results in significant age uncertainties.

As already alluded to in this section, for each isotope system one must understand the event(s) that resets the clock, i.e. the event it is measuring (see below for examples). One must also assess how closed the system is, with regards loss/gain of parent/daughter isotope.
LEFT: Different isotope systems date different events in the history of the tektite. One might be dating the original rock or crustal formation (A).
One might be dating the age that the original crustal rock was weathered and re-deposited as a sedimentary rock (B).
One might be dating the age the tektite formed by impact melting of the sedimentary rock (C).
One might be dating the age of the sedimentary rock into which the tektite has been incorporated (D).
One might also be dating the age the sedimentary rock was metamorphosed (if applicable), time the tektite spent in space, the time the tektite has lain on or close to the surface. One needs to choose the correct isotope system and understand what resets the system (or partially resets the system) to interpret the event one is dating.

Dating a Sample

Many people think that the half life dates the age of the sample. This is true, but what age are we really dating? Are we dating the time the sample differentiated from the mantle? The time the sample solidified as an igneous rock? The time the sample was transported and deposited as a sedimentary rock? The time the sedimentary rock was metamorphosed? The time the sample was last melted and formed a tektite? To understand the event being dated we must understand the isotope system and what resets the clock. In the K-Ar system the clock is reset by melting which results in loss of the daughter isotope (Argon, which is a gas). So, in the K-Ar system we are dating the time the sample was last fully melted (i.e. the formation of the tektite, asssuming it hasn't been melted since! 

Half-Lives

A half-life is the time taken for, on average, 50% of the parent isotope to undergo radioactive decay to the daughter isotope. After zero half-lives the remaining parent isotope is 100%, 1 half-life = 50% remaining, 2 half-lives = 25% remaining, 3 half-lives = 12.5% remaining, 4 half-lives = 6.25% remaining, 5 half -lives = 3.125% remaining and so on. Some isotopes decay too rapidly and others too slowly, so the correct isotope must be chosen such that the amount of decay can be measured and remains statistically viable, within margins of error.

References