The Isotopic Composition of the Subcontinental Mantle Lithosphere in Yukon, Canada, as Infered by Ultramafic Nodules and Alkaline Lavas

J. Carignan GEOTOP, Université du Québec à Montréal, C.P. 8888, succ. Centre-Ville, Montréal, Canada, H3C 3P8

J. N. Ludden CRPG-CNRS, 12 rue N.D. des Pauvres, Vandoeuvre-lès-Nancy, 54501 France

D. Francis Department of Earth and Planetary Sciences, McGill University, Montréal, Canada, H3A 2A7

Quaternary alkaline magmatism ranging in composition from olivine nephelinite (OL-NEPH) to hypersthene-normative olivine basalts (HY-NORM AOB) have erupted through the various Cordillera terranes accreted to ancestral North America. Many of these lavas bear spinel lherzolite xenoliths. Major and trace element modelling indicates that the major source for OL-NEPH magmas had amphibole as a residual phase and that the progressive melting of the mantle lithosphere resulted in the formation of the less Si-undersaturated basanite and HY-NORM AOB magmas (Francis and Ludden, 1995). OL-NEPH lavas from all the volcanic centres display very homogeneous Pb, Sr and Nd isotopic compositions, similar to those of kaersutite in nodules from the Nunivak Island, Alaska (Ben Othman et al., 1990). The OL-NEPH and kaersutite isotopic compositions lie within the field of Aleutian Island volcanic rocks (Morris et al., 1983; Miller et al., 1994), similar to the least radiogenic 206Pb/204Pb (18.77) and 87Sr/86Sr (0.703) ratios of the island arc magmas. HY-NORM AOB lavas yield more heterogeneous isotopic compositions than OL-NEPH lavas. However, a coherent distribution of the data is observed for individual centres. Lavas from Hirschfeld, Llangorse and Mt.Edziza have radiogenic 206Pb/204Pb (up to 19.4), retain relatively low 87Sr/86Sr (0.7033 - 0.7028) and high 143Nd/144Nd (0.5129 - 0.5131), and form trends towards the HIMU mantle component in isotope diagrams. HY-NORM AOB lavas from Fort Selkirk have positively correlated Pb and Sr isotopic ratios (206Pb/204Pb from 19.0 to 19.3 and 87Sr/86Sr from 0.7033 to 0.7036) while those from Alligator Lake have homogeneous and high Sr ratios (87Sr/86Sr=0.7039) but heterogeneous Pb and Nd isotopic compositions. This may reflect local isotopic heterogeneities within the subcontinental mantle lithosphere, which might be related to the various Cordillera terranes. A spinel lherzolite xenolith CPX that is depleted in light rare earth elements (LREE) relative to chondrite has less radiogenic Sr (87Sr/86Sr=0.7025) and Pb (206Pb/204Pb=18.50) isotopic ratios than OL-NEPH lavas, and falls in the compositional field for northeastern Pacific MORB (White et al., 1987). Other xenoliths, with variably LREE enriched patterns, have 206Pb/204Pb ratios comparable to those of their host lavas, but have significantly more radiogenic 207Pb/204Pb (up to 15.75) and 87Sr/86Sr (up to 0.705). At one site (Alligator Lake), the Pb isotopic composition of mineral seperate from spinel lherzolite xenoliths indicates that the minerals are out of isotopic equilibrium, with the CPX being more radiogenic than the co-existing olivine and orthopyroxene. Olivine and orthopyroxene may have 206Pb/204Pb as low as 18.75, similar to or less radiogenic than that of OL-NEPH. The distribution of the xenolith data in a 206Pb/204Pb vs. 238U/204Pb diagram suggests that the spinel lherzolites were affected by a recent (less than 100 Ma?) metasomatism involving metasomatic agents with an isotopic composition similar to that of marine sediments in the northeastern Pacific. Three whole rock xenoliths have nearly chondritic gOs values (-0.3 to +3.0). 187Os/188Os and 187Re/188Os are positively correlated with indices of fertility such as Ca and Al contents, and therefore, may record partial melting of the subcontinental lithosphere in the area. If the Re-Os system was closed since then, the melting event may have occured ca. 650 Ma ago, an age much older than that infered for the metasomatism which lead to the last incompatible element enrichment.

Whereas CPX from the ultramafic nodules as well as HY-NORM AOB lavas record isotopic heterogeneities, amphibole and OL-NEPH rather suggest a homogeneous reservoir within the lithosphere. Apparently, metasomatic magmas/ fluids responsible for the formation of amphibole within the mantle lithosphere had an isotopic composition reflecting mixing between juvenile (slab?) and more differentiated material (sediments?). On the other hand, metasomatic magmas/fluids in the spinel lherzolite field of the subcontinental mantle lithosphere had a more radiogenic isotopic composition, and exchanged progressively with lherzolites which had an isotopic composition close to that of northeastern Pacific MORB.


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