The Effects of Partial Melting and Re-fertilization in the Zr Stable Isotope Composition of Earth's Upper Mantle

Recently reported variations in the Zr stable isotope composition of Zr-rich phases from an igneous cumulate [1] and bulk volcanic rock samples [2] suggest that mass-dependent Zr stable isotope fractionations can be induced by intra-crustal magmatic fractional crystallization. Yet, the Zr stable isotope composition—and variability—of Earth's mantle has not been measured directly, nor have the effects induced by peridotite partial melting and refertilization been explored. Orogenic peridotite massifs preserve an unparalleled record of the petrologic, geochemical, and structural development of the subcontinental lithospheric mantle (SCLM) [3], and are key to our understanding of Earth's mantle and bulk chemical/isotopic composition. Therefore, exploring the isotopic effects induced by petrologic processes that affect mantle peridotites is necessary for understanding the Zr isotope composition of the silicate Earth.

The Beni Bousera peridotite massif in the Betic-Rif belt, western Mediterranean, preserves an outstanding record of the petrologic processes that shape the SCLM. Correlations between major and trace elements, as well as variable modal abundances of Ol-Cpx-Opx, have been ascribed to variable degrees of melt extraction and/or secondary re-fertilization via melt infiltration and melt-rock reactions [4]. Clinopyroxene, the main phase consumed during partial melting of fertile lherzolites, is also the main reservoir of Zr in mantle peridotites. Therefore, reactions that consume or produce Cpx are likely to have an influence in the Zr isotope systematics of variably depleted peridotites.

We will present δ^94/90Zr_NIST compositions of peridotite samples representing a continuous 'depletion' trend. Samples include refractory residues produced by extensive partial melting (i.e., harzburgites and subordinated dunites) as well as fertile lherzolites formed at the expense of harzburgites via melt-rock reactions. Altogether, these results allow us to constrain the roles that peridotite partial melting and rejuvenation have played in the Zr stable isotope evolution of the terrestrial upper mantle.

[1] Ibanez-Mejia and Tissot (In revision)

[2] Inglis et al. (2019) GCA 250, 311

[3] Garrido and Bodinier (1999) J. Pet. 40, 729

[4] Varas-Reus et al. (2016) J. Pet. 57, 2251