186 Os and 187 Os enrichments and high- 3 He/ 4 He sources in the Earth's mantle:evidence from Icelandic picrites.

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Brandon, A.D., Graham, D.W., Waight, Tod Earle, Gautason, B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2007
• Subjects: /dk/atira/pure/core/keywords/TheFacultyOfScience; Faculty of Science; Island; Iceland; He isotopes; Os isotopes; mantle plume
• Online Access: https://curis.ku.dk/portal/da/publications/186os-and-187os-enrichments-and-high3he4he-sources-in-the-earths-mantle(103b05b0-a4bd-11dc-bee9-02004c4f4f50).html; https://doi.org/10.1016/j.gca.2007.07.015

Picrites from the neovolcanic zones in Iceland display a range in 187Os/188Os from 0.1297 to 0.1381 (¿Os = + 2.1 to +8.7) and uniform 186 Os/ 188 Os of 0.1198375 ± 32 (2r). The value for 186 Os/ 188 Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186 Os/ 188 Os and 187 Os/ 188 Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high 3 He/ 4 He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not. A positive correlation between 187 Os/ 188 Os and 3 He/ 4 He from 9.6 to 19 Ra in Iceland picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1-1.5 Ga, which preserves the high 3 He/ 4 He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently mixes with the present-day convecting MORB mantle during ascent and melting. This multistage mixing scenario requires convective isolation in the deep mantle for hundreds of million years or more to maintain these compositionally distinct hybrid sources. The 3 He/ 4 He of lavas derived from the Iceland plume changed over time, from a maximum of 50 Ra at 60 Ma, to approximately 25-27 Ra at present. The changes are coupled with distinct compositional gaps between the different aged lavas when 3 He/ 4 He is plotted versus various geochemical parameters such as 143 Nd/ 144 Nd and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period. The positive correlation between 187 Os/ 188 Os and 3 He/ 4 He demonstrates that ...