Constraining deep mantle contributions to Canary Island lavas

Abstract

Much debate surrounds the original interpretation of coupled 186Os-187Os enrichments in ocean island basalts (OIB) as deriving from core-mantle interaction. Alternate models proposed to explain 186Os-187Os signatures include melting of pyroxenite-peridotite mixtures, or base metal sulphides, with long-term (Re+Pt)/Os enrichment. Canary Island lavas offer a means to test these models because they are characterised by radiogenic 187Os/188Os and Pb isotopes, and MORB-like 3He/4He – characteristics which can be interpreted as originating from a predominantly upper mantle source that includes recycled crust/lithosphere, rather than a ‘deep plume’ component. We present new coupled high-precision 186Os- 187Os isotope and highly siderophile element (HSE) abundance data on picrite and ankaramite lavas from the islands of El Hierro, La Palma, La Gomera, Tenerife, Lanzarote, and on harzburgite xenoliths from Lanzarote. Lavas analysed in this study have 0.15 to 1.2 ppb Os, and 187Os/188Os ranging from 0.137 to 0.166. These ratios are at the extreme radiogenic range of OIB. The Os concentrations are similar to rocks with comparable MgO worldwide, so there is no evidence of derivation from an HSE-depleted source. The harzburgite xenoliths are characterised by >1 ppb Os, chondritic to sub-chondritic 187Os/188Os and Re depletion ages consistent with their origin as lithospheric refractory melt residues. 186Os/188Os data for three aliquots of a Lanzarote harzburgite yield a ratio of 0.1198361±9; indistinguishable from estimates for the convecting upper mantle. Analysed Canary Island lavas have 186Os/188Os ratios that are also within the upper mantle range. The observation that 186Os/188Os of Canary Island lavas are typical of the upper mantle points to a lack of long-term Pt/Os enrichment, but the extreme range in 187Os/188Os indicates a source with long-term Re/Os enrichment. Combined, these characteristics are consistent with a mantle source containing variable proportions of recycled material within a peridotitic, depleted-MORB mantle. Such pyroxeniteperidotite, ‘layer-caked’ sources have been suggested as the cause of coupled 186Os-187Os enrichments in other OIB mantle sources. The new data obtained on Canary Island lavas do not appear to support this view.