TRACE ELEMENT CHEMISTRY

Written By Aubrey Whymark 2013-2017
Trace elements include, but are not limited to, Scandium (Sc), Chromium (Cr), Manganese (Mn), Cobalt (Co), Arsenic (As), Rubidium (Rb), Zirconium (Zr), Antimony (Sb), Cesium (Cs), Barium (Ba) and Hafnium (Hf).

Detailed trace and rare earth element chemistry is beyond the scope of this book, but analyses, particularly for the Australasian strewn field can be obtained in the scientific literature. We will briefly examine the main points in this book.

Trace element analyses can be used infer similarities and differences between tektites from a certain strewn field and source rocks (Koeberl, 1990). So, trace elements can be used to prove certain microtektites are related to certain macro-tektites. They can also be used to link geographically separated macro-tektites, such as georgiaites and bediasites to the same impact event and then to the Chesapeake Bay Crater itself.

The study of trace element ratios, such as Rubidium:Barium and Thorium:Samarium, just two examples of many, clearly demonstrate that tektites show a terrestrial upper crust ratio and do not show a lunar ratio (which has been well established by extensive study of lunar rock samples).

Another useful plot is the Thorium/Scandium plot. This plot allows the distinction of mafic and felsic sources. Mafic sources are lower crustal or oceanic crustal materials. Felsic sources are upper crustal or continental materials. Tektites clearly demonstrate a felsic, upper crustal, signature with a small or absent mafic component. The same is true of Lanthanum/Scandium plots (Koeberl, 1990).

Rare Earth Element Chemistry

Rare earth elements can be divided into light rare earth elements (LREE) and heavy rare earth elements (HREE). LREE are Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm) and Samarium (Sm). HREE are Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb) and Lutetium (Lu). During the impact process REE patterns are not significantly altered.

As with other trace elements, REE can be used to link different tektite groups and craters to a single impact event. They can also be used to divide up tektites within a single strewn field due to the heterogeneous nature of the target material.

Investigation of the Eu (Europium) anomaly also helps identify the tektite precursor material. Eu can occur as Eu3+ or Eu2+. Eu2+ and Ca2+ can readily exchange places in calcium bearing rocks (limestone is calcium carbonate for example). Fractional crystallisation in igneous magma chambers can also alter the Eu anomaly. Archaean rocks, including sediments, do not show a pronounced Eu anomaly, whereas this is a significant feature in post-Archaean sediments. This comes to the conclusion that only post-Archaean sediments (those formed after 2.5 billion years ago) have a REE signature that resembles that in tektites (Koeberl, 1990).

The LREE/HREE ratio is a good indicator of the precursor rock of tektites. The (La/Lu)CN ratio (CN = chondrite-normalised) of all tektites is typically close to 10. This compares with the LREE/HREE ratio in the mantle of between 1 and 2; oceanic crust being about 1.5; total continental crust being between 5 and 6 and upper crust, especially the sedimentary component, being close to 10 (Koeberl, 1990). REE patterns of tektites and impactites follow very strictly the post-Archaean (last 2,500 million years) average sediment pattern. REE point generally to a sedimentary parent material for tektites (Koeberl et al., 1985).

REE's

Rare Earth Elements point generally to a sedimentary parent material for tektites (Koeberl et al., 1985).
 

Platinum Group Element Chemistry

Platinum group elements (PGEs) comprise Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Osmium (Os), Iridium (Ir) and Platinum (Pt). PGEs can be used to identify the type of impactor. Only very limited PGE analyses has been carried out on tektites. PGE analyses, however, are far more suited to impactites within or in close proximity to the crater. Tektites typically are not mixed with the impactor and will contain, at best, just a trace of the impactor. Koeberl (1990) offers an excellent summary of PGEs.

PGE's

Platinum Group Elements include Iridium. Iridium has an affinity to iron and is exceedingly rare in the Earth's crust, but far more abundant in meteorites. In 1980, Luis Alvarez, Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel discovered the Cretaceous–Paleogene boundary which contains a concentration of iridium hundreds of times greater than normal. Source: Wikipedia.

References