A NEW EU VALENCE CALIBRATION FOR APATITE IN BASALTIC SYSTEM: AN EXPERIMENTAL STUDY COMBINING XANES, EU/EU* AND LATTICE STRAIN

In this work we report partitioning of Rare Earth Elements (REE) between apatite and basalt at different oxygen fugacities (fO₂; iron-wustite to hematite-magnetite) at 1 atmosphere and temperatures ranging from 1110-1175 °C. Partitioning data from REE³⁺ in experiments (La, Sm, Gd, Lu) show a near constant value across all conditions, while Eu shows an increasing development of a negative anomaly with decreasing fO₂. Experiments presented here define three apatite calibrations that can all be used as redox sensors:

(i) a XANES calibration that directly measures Eu valence species in apatite (i.e., Eu²⁺ and Eu³⁺) in basalts that crystallize between 1175-1100 °C.

(ii) a technique that involves measurement of Sm, Eu and Gd in co-existing apatite and basalt, such that the intensity of the negative Eu anomaly is monitored by (Eu/√(SmxGd))

(iii) a technique based on the negative Eu excursion from the lattice strain model defined by REE³⁺ between apatite and basalt.

From an application perspective it is worth noting that Eu valence partitioning techniques based on (√SmxGd) and lattice strain are indistinguishable. Application of these three calibrations is best carried out in systems where both apatite and co-existing glass are present and in direct contact. In crystalline rocks (i.e., where co-existing glass cannot be analyzed), whole rock analyses can be used as a guide but considerations/corrections must be made to ensure that the effect of plagioclase crystallization can be removed. Similarly, if the melt source has an inherited negative Eu anomaly, appropriate corrections must also be made. In addition to the reported redox calibrations, the partition coefficients for REE range from a maximum D_Eu3+= 1.67 ±v 0.25 (as determined by lattice strain) to D_Eu2+ = 0.69 ± 0.10. These partition coefficient values, as observed in the Onuma diagram, show apatite is in a fortuitous situation were the most compatible REE (Eu³⁺) is also the polyvalent element used to monitor fO₂.