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Rutile-bearing refractory eclogites: missing link between continents and depleted mantle
1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA.
Science (New York, N.Y.)
|January 15, 2000
Summary
Earth
Area of Science:
- Geochemistry
- Petrology
- Earth Science
Background:
- Earth's continental crust and depleted mantle exhibit subchondritic ratios for key elements like Niobium (Nb), Tantalum (Ta), and Titanium (Ti).
- This elemental imbalance suggests the presence of an unaccounted reservoir with superchondritic ratios.
Purpose of the Study:
- To investigate the role of refractory eclogite in resolving Earth's observed mass imbalance for Nb, Ta, and Ti.
- To determine the mineralogical and bulk-rock geochemical characteristics of eclogites that could explain these elemental discrepancies.
Main Methods:
- Analysis of trace element compositions in minerals within xenolithic eclogites from cratonic lithospheric mantle.
- Geochemical modeling to quantify the required mass and composition of an eclogite reservoir.
Main Results:
- Rutile in xenolithic eclogites significantly controls the abundance of Nb and Ta.
- Eclogites exhibit superchondritic Nb/Ta, Nb/La, and Ti/Zr ratios, consistent with the required reservoir.
- Approximately 1-6% eclogite by weight, with specific elemental concentrations and ratios, is needed to balance Earth's budget.
Conclusions:
- Refractory eclogite, particularly rutile-rich varieties, is a viable candidate for the missing reservoir resolving Earth's Nb, Ta, and Ti mass imbalance.
- The required eclogite mass suggests a significant portion may reside in the lower mantle, potentially near the core-mantle boundary.
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