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Content archived on 2024-05-29

Degassing systematics of Highly Siderophile Elements from magmas: New constrains on the convecting mantle, and the origins of global HSE anomalies in sedimentary rocks

Final Activity Report Summary - HSE DEGASSING SYSTEM (Degassing systematics ...: New constrains on the convecting mantle, and the origins of global HSE anomalies in sedimentary rocks)

A detailed assessment of the geochemistry of platinum group elements (PGEs), namely osmium (Os), iridium (Ir), ruthenium (Ru), platinum (Pt) and palladium (Pd) plus rhenium (Re) was made to understand
1. the distribution of these elements in the mantle and melts;
to provide an understanding of
2. the relative volatility of the PGEs;
3. the effects of degassing on PGE abundances in magmas;
4. how this might affect PGE anomalies found in sedimentary horizons, where there was a debate as to the origin of the elemental spikes, i.e. on whether they were of meteorite impact origin or large scale degassing of intense volcanic episodes.

The first detailed assessment of PGE abundances in sulphides within pyroxenites was made, in an effort to try to understand the factors controlling the relative abundances of Re, Pt and Os is basaltic rocks. This showed that it was possible for magmatic rocks with highly unusual Os isotopic signatures to be generated by melting of reacted zones between peridotites, i.e. the rock type formerly thought to control the chemistry of all mantle-derived melts, and pyroxenites. Although the melting of these reacted layers creates unusual isotopic signatures because of subtle fractionation of Re/Os and Pt/Os, the abundances of these elements in the resulting melts is ultimately controlled by their solubilities in silicate melts. Hence, even though it could be shown from this study that mantle source regions for magmatic rocks were much more diverse than previously recognised, there was an underlying limit on the amount of PGEs that could enter the silicate melts.

Establishing the limits and control on PGE abundances in mantle-derived melts then allowed a focus on the response to relative differences in eruption pressure on the PGE abundances of volcanic rocks. This part of the study demonstrated that Re abundances in lavas erupted under pressure, e.g. in substantial water depth or buried beneath glacial loads, contained significantly more Re than their lower pressure counterparts. However, for other PGEs, such as Pd, complications in the history of the mantle source region provided an over-riding control, as for example in the Philippine Sea Plate basalts, and degassing effects were minimal. No significant differences were so far observed for Pt, Ru, Os or Ir that could be attributed to differences in volatility during eruption.

The main implication of the results was that there was very little evidence that volatile emissions of any of the PGEs could be the main cause of elemental anomalies in sediment sequences, therefore so other mechanisms such as meteorite impacts appeared as more likely explanations.