Examples of such islands include the Hawaiian and Galápagos chains. Prevailing scientific theory says that mantle plumes involve recycled basaltic ocean crust as a major component. However, a new paper by DCO scientists published in Nature presents new evidence that in some mantle plumes, old continental roots are key constituents .
The team of scientists, Yaakov Weiss, Cornelia Class, Steven L. Goldstein (of Lamont-Doherty Earth Observatory, Columbia University, USA) and Takeshi Hanyu (of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC, Japan), focused on lavas from the Cook-Austral islands in the south Pacific that have the highest lead isotopic compositions among oceanic lavas. Known as the “HIMU compositional endmember” (high-μ, the Greek letter geochemists use as shorthand for the ratio of uranium to lead), the sources of these lavas have been generally considered to represent ‘recycled’ subducted basaltic ocean crust. However, trace elements in olivine crystals show extremely high calcium-to-aluminum ratios, far outside the range previously measured in olivines from other islands, indicating involvement of a component high in Ca and low in Al, which is a fingerprint for carbonatite (magma/fluid composed of carbonate). An important role for carbonatite is further supported by comparison of the unique trace-element patterns of HIMU lavas and carbonatitic fluids encapsulated in diamonds.
“Diamonds provide unique ‘windows’ into the deep Earth because they uniquely preserve actual fluids from down there.” said lead author Yaakov Weiss, “The key linkage here is that carbon-rich fluids in diamonds that formed in continental roots at ~100 miles depth, and magmas that formed from mantle plumes rising from the lower mantle, have the same unique chemical signatures, indicating similar histories”.
Coauthor Cornelia Class said “This means that parts of carbonatite-altered old continental roots drop off and sink to the core-mantle boundary, to later re-emerge as part of an island-forming eruption”.
“The idea that the ‘subcontinental mantle’ contributes significantly to mantle plumes has been around for over 30 years, but never found general acceptance,” said coauthor Steven Goldstein. “While this is likely not the last piece to the HIMU puzzle, it signals a major shift in our view of deep Earth dynamics.”
The study was partly funded by the U.S. National Science Foundation.
Article adapted from source.
Image: Crystals of the mineral olivine, taken from Grand Comore Island in the Indian Ocean, have a calcium-to-aluminum ratio similar to that of fluids found in diamonds. This suggests that both materials have a common source. Credit: Yaakov Weiss/Lamont-Doherty Earth Observatory.