Journal of Petrology, Volume 40, Issue 2: February 1999.

Os Isotope Systematics in the Canary Islands and Madeira: Lithospheric Contamination and Mantle Plume Signatures

E. WIDOM¹, K. A. HOERNLE², S. B. SHIREY³ AND H.-U. SCHMINCKE²

¹DEPARTMENT OF GEOLOGY, MIAMI UNIVERSITY, OXFORD, OH 45056, USA
²DEPARTMENT OF VOLCANOLOGY AND PETROLOGY, GEOMAR, WISCHHOFSTRASSE 1-3, 24148 KIEL, GERMANY
³DEPARTMENT OF TERRESTRIAL MAGNETISM, CARNEGIE INSTITUTION OF WASHINGTON, 5241 BROAD BRANCH ROAD NW, WASHINGTON, DC 20015, USA

Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr-Nd-Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. ¹⁸⁷Os/¹⁸⁸Os ratios in the basalts range from 0·129 to 0·183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a `radiogenic' group with high <sup>187</sup>Os/<sup>188</sup>Os from 0·152 to 0·183; (2) an `unradiogenic' group with low ¹⁸⁷Os/¹⁸⁸Os from 0·129 to 0·138; (3) an `intermediate' group with ¹⁸⁷Os/¹⁸⁸Os between 0·139 and 0·151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of <= 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb but lower ¹⁴³Nd/¹⁴⁴Nd than samples with similar ²⁰⁶Pb/²⁰⁴Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower ²⁰⁶Pb/²⁰⁴Pb. The radiogenic Os and moderate ²⁰⁶Pb/²⁰⁴Pb signature of the Canary plume is consistent with a plume which contains 25-35% of relatively young (~1·2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in ²⁰⁶Pb/²⁰⁴Pb isotopic composition.

Keywords: Canary Islands;crustal contamination;lithospheric mantle; ocean island basalts; Os isotopes

Pages 279-296