Gannoun A., Burton K. W., Barfod D. N., Schiano Pierre, Vlastelic I., Halliday A. N. (2015). Resolving mantle and magmatic processes in basalts from the Cameroon volcanic line using the Re-Os isotope system. Lithos, 224, p. 1-12. ISSN 0024-4937.
Titre du document
Resolving mantle and magmatic processes in basalts from the Cameroon volcanic line using the Re-Os isotope system
Gannoun A., Burton K. W., Barfod D. N., Schiano Pierre, Vlastelic I., Halliday A. N.
Source
Lithos, 2015,
224, p. 1-12 ISSN 0024-4937
This study presents major-, trace element and Re-Os isotope and elemental data for young alkaline basalts (<10 Ma) from oceanic (Annobon, S. Tome, Principe), continental (Manengouba) and continent-oceanic boundary (COB, Mt. Cameroon) sectors of the Cameroon volcanic line (CVL). The CVL is a chain of Tertiary to recent, transitional to strongly alkaline intraplate volcanoes extending from the south Atlantic island of Annobon to the continental interior of West Africa (Biu Plateau). The basalts from the oceanic sector display a range of initial Os-187/Os-188 ratios between 0.128 and 0.190 and those from the COB and continental sector range between 0.142 and 0.560. The samples with high (206)pb/(204)pb (e.g. ratios >20) possess Os-187/Os-188 isotope compositions between 0.14 and 0.18 (e.g., basalts from Mt Cameroon and Sao Tome) which reflect the chemical characteristics that are more likely to be primary features of CVL, and are close to the value of 0.153 attributed to the HIMU end-member (Tubuai-Mangaia). However, most of the lavas from the continental sector show highly radiogenic initial Os-187/Os-188 ratios (0.36 to 0.56) that are outside the range previously observed for ocean island basalts, with shifts to radiogenic Os isotope compositions accompanied by less radiogenic (206)pb/Pb-204 and increasing SiO2 contents. The increase in Os-187/Os-188 is also associated with the decrease of Os, Ni, MgO and phenocryst abundances. These data can be explained by fractional crystallisation and assimilation of continental crust by the ascending magma. The systematic shift to unradiogenic lead isotope compositions from the COB into the oceanic sector is positively correlated with variations in (OS)-O-187/(OS)-O-188 isotope composition (from 0.140 to 0.128). At first sight this covariation might be attributed to the mixing of HIMU material with the ambient upper mantle (DMM). However, there is a clear covariation of the Os isotope and elemental composition, best explained with contamination of the oceanic basalts by the physical entrainment of xenoliths and xenocrysts of mantle origin. Overall, these results indicate that Os in CVL basalts is highly susceptible to contamination from both oceanic and continental lithospheres, under these circumstances covariations with other isotopes and elements must be interpreted with caution.
Plan de classement
Géologie et formations superficielles [064]
;
Géophysique interne [066]
