Zr solubility in mantle minerals at zircon saturation: Implications for zircon genesis in ultramafic rocks

Metasomatic zircons in ultramafic rocks are crucial in tracking orogenic processes. Metasomatic zircons are conceived to form by local zircon saturation resulting from effective Zr transport during metamorphic events and incomplete Zr accommodation in mantle minerals. However, the accommodation capacity of Zr in mantle minerals remains unknown, hampering our understanding of zircon genesis in ultramafic rocks. Here we performed experiments at 2.0-6.0 GPa and 1050-1200 degrees C and determined Zr content (solubility) in the mantle minerals at zircon saturation (ZCZS). The results show that ZCZS values for the main mantle minerals are olivine (ol, 3.68 +/- 0.27 ppm) < orthopyroxene (opx, 4.67 +/- 0.23 ppm) < clinopyroxene (cpx, 28-1403 ppm) <= garnet (grt, 294-2222 ppm) with ZCZS for amphibole (amp, 189-546 ppm) and magnetite (mag, 263-630 ppm) overlapping that for cpx and grt under the experimental conditions. The significant variations of ZCZS for cpx and grt depends on mineral composition and P-T conditions. In detail, the ZCZS for cpx increases with temperature, Al2O3 content of cpx, and decreases with pressure; that for grt increases with temperature and X-Alm (molar fraction of almandine) in garnet. Important implications of this study include: 1) Considering the very low ZCZS for ol and opx, dunite and harzburgite are prone to zircon saturation during the infiltration of Zr-bearing metamorphic agents; 2) Because garnet is the main reservoir of Zr in garnet peridotite, metamorphic decomposition of garnets may produce inter-granular zircons; 3) The ZCZS for garnet excellently correlates with X-Alm and T, providing a potential geo-thermometer for garnet-bearing zircon saturated rocks.