Abstract
Variations in the niobium/thorium ratio in basaltic rocks are thought to be related to the long-term extraction of continental crust from the mantle and the extent of sediment recycling and mixing in the mantle. However, basalts erupted between 13 and 3 Ma from a single volcanic center in the Cantal alkali massif (France) have Nb/Th of 10.5-18.7, a range encompassing nearly the entire basalt record. Cantal basalts are isotopically homogeneous, ruling out variable sediment contamination of their mantle sources. Instead, the new data indicate a mineralogical control on Nb/Th in a veined mantle source. We postulate that the large Nb/Th variations are the result of a metasomatic process, called percolative fractional crystallization, that produced veins containing pyroxene and Nb-rich oxide within the upper mantle. Short-term metasomatic-induced variations in mantle Nb/Th may have occurred throughout the geologic record, and provide an alternative explanation to sediment recycling for Nb/Th heterogeneity in the upper mantle.
Original language | English |
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Pages (from-to) | 113-116 |
Number of pages | 4 |
Journal | Geology |
Volume | 32 |
Issue number | 2 |
DOIs | |
State | Published - Feb 2004 |
Keywords
- Cantal massif
- Geochemistry
- Heterogeneity
- Metasomatism
- Oceanic-island basalt
- Recycling
- Upper mantle