DISTRIBUTION OF TE, AS, BI, SB AND SE (TABS) IN MAFIC AND ULTRAMAFIC LAVAS: IMPLICATIONS FOR CRUSTAL ASSIMILATION, SULFIDE SEGREGATION AND MAGMA DEGASSING

The distributions of Te, As, Bi, Sb and Se (TABS) have been determined in ultramafic and mafic lavas from various geological contexts. The effects of: sulfide segregation, crustal assimilation and magma degassing are considered using a mantle-normalized plot, which combines the lithophile elements Th and Ta with the TABS, Cu, Pd and Pt. In the absence of sulfide phases all of these elements are incompatible. Niobium and Ta are highly incompatible lithophile elements and can be used to monitor the degree of fractional crystallization. Thorium, As, Sb and Bi are enriched in the continental crustal relative to mafic magmas and thus the combination of positive Th, As, Sb and Bi anomalies relative to Ta indicates that the magma has experienced continental crust assimilation. Due to the difference in volatility between the TABS and Cu, Pd and Pt effect of degassing can be observed using the Cu/TABS or Pd/TABS ratios. Comparison of these ratios in rocks formed sub-aqueously, such as MORB, with those formed sub-aerialy such as lava flows from Large Igneous Provinces (LIP) corroborate the idea that TABS have been lost during degassing. The effect is most readily observed for Te and Se which are depleted relative to Cu and Pd during degassing. The effect of degassing on Bi, As and Sb in some cases is less evident because these elements can be added to magma during crustal assimilation. The release of different gaseous species and metals in the atmosphere by degassing at low pressures upon the emplacement of LIP has been extensively considered over the past years. One of the main reasons is that mass extinctions may be linked to climate changes caused by these massive degassing events. Our results support the loss of TABS during the emplacement of LIP such as Emeishan, Karoo and the Siberian Traps. The quantification of the metal input into the atmosphere is a further step to be considered.

Keywords: TABS; sulfide segregation; crustal assimilation; degassing; mass extinctions