The Rare Earth Elements (Lanthanum, Cerium, Prasedymium, Neodymium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium and Yttrium) are of vital importance in many modern high-tec industries. Unfortunately, as their name suggests they are extremely rare; present at extremely low levels in many soils and rock formations around the world, but seldom at concentrations high enough to make extraction viable. At the moment around 85% of the world’s Rare Earth Elements are produced from two areas in southern China. Several other recently deposits have been found to also contain high levels of uranium and thorium, which creates severe environmental problems to be overcome and some theorists have suggested that Rare Earth Elements could be extracted from the sea floor, where Rare Earth Elements eroded from deposits on land should in theory become concentrated over time, but as yet the technology to extract them from such an environment does not exist, and sites looked at have not yielded particularly high levels anyway.
In a paper published in the journal Gondwana Research on 5 November 2014, Poul Emsbo of the United States Geological Survey, Patrick McLaughlin of the Wisconsin Geological and Natural History Survey at the University of Wisconsin-Extension and George Breit, Edward du Bray and Alan Koenig, also of the United States Geological Survey, examine the presence of Rare Earth Elements in phosphorites (phosphate deposits) at a number of sites around the United States.
Phosphorites are formed in shallow marine settings by the concentration of phosphates from land washed out to sea over time, exactly the same scenario that has been suggested for the concentration of Rare Earth Elements. Thus if it is in theory possible to extract Rare Earth Elements from the sea floor, then it also ought to be possible to extract them from these ancient marine sediments, without the bother of finding ways to mine on the sea floor.
Emsbo et al. looked at 23 phosphorite deposits from locations across the US, ranging from Ordovician to Pliocene in age. All of the deposits are laterally extensive in range (i.e. found over a wide area) and already being commercially targeted for their phosphates.
Localities and stratigraphic age of studied phosphate deposits. Ages and colour coding in the stratigraphic chart are adapted from the International Commission on Stratigraphy. (a) Temporal distribution of major (i.e.,mined) and minor phosphorites in the United States. (b) Spatial distribution of phosphorites sampled by this study. (c) Distribution of major phosphorite occurrences across the United States. Emsbo et al. (2015).
The levels of Rare Earth Elements, and the proportion of each element within the sample, proved to be roughly similar in phosphorites of similar ages from different areas, suggesting that the level of Rare Earth Elements incorporated in sediments depended on the water chemistry at the time the sediment was laid down (the oceans are interconnected and their chemistry remains essentially the same across the globe, but is known to have varied over geological time). Pliocene and other more recent deposits were found to have similar levels of Rare Earth Elements to modern marine deposits, but the levels rose sharply in Miocene deposits, reaching levels similar to those found in commercially worked deposits in China, and remained high in Mesozoic and later Palaeozoic deposits. Deposits from the Late Ordovician and Mississippian were further highly enriched in Heavy Rare Earth Elements (Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium and Yttrium), which are particularly sought after in industry.
The discovery of high levels of Rare Earth Elements in American phosphorite deposits has important economic implications, particularly in the Miocene deposits, which are already heavily worked (around 80% of all phosphorus extracted in the US is recovered from Miocene-Pliocene sediments). Rare Earth Elements are generally recovered from sediments by acid leaching (i.e. soaking the ore in acid for long periods to dissolve the desired elements, then recovering these from the resultant sludge). This is an environmentally challenging procedure, but importantly, is also the method used to recover phosphorus.
This means that many plants in the US are currently already extracting Rare Earth Elements from phosphorites, but not then recovering it from the sludge (the easier and less destructive part of the process). Emsbo et al. show that for many Miocene deposits the value of Rare Earth Elements extracted could be similar to that of the phosphorous, doubling the value of the strata.
The raised levels of Heavy Rare Earth elements seen in some deposits are also interesting, not just because of the value of these elements, but because very high levels coincide with extinction events in the biological record. Emsbo et al. suggest that these are not so much evidence of extra elements entering the oceans at the time (as with the iridium anomaly at the end of the Cretaceous, which has been used as evidence of an impact by an iridium-rich meteorite), but rather of prevailing anoxic conditions in the oceans at these times, leading in turn to a drop in oyhyhydroxides in the ocean – chemicals which form bonds with Rare Earth Elements and keep them in the water column, leading to large amounts of Rare Earth Elements settling out of the oceans (with heavier elements settling faster, so that more were deposited before the oceans recovered).
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