Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for di€usion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr2O3, and 20 to 220 ppm H2O. Thermobarometric calculations show that pyrope crystals with di€erent Cr contents formed at di€erent depths ranging from 50 km (where T » 600 °C and P ˆ 15 kbar) to 95 km (where T » 800 °C and P ˆ 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM21O38, where A ˆ Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi2O6), and a new oxide mineral, carmichaelite (MO2)x(OH)x, where M ˆ Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a signi®cant Nb (up to 2.7 wt% Nb2O5), Cr (up to 6 wt% Cr2O3), and OH (up to 0.9 wt% H2O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needleand blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped ̄uid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high ®eld strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic ̄uids/melts. Such ̄uids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks.