Abstract
Alkaline fluids associated with serpentinization cause the dissolution and precipitation of high field strength elements (HFSE) and formation of rare mineral species. In this study, newly discovered baddeleyites in the Higuchi serpentinite body (HSB), Japan, are described. The HSB occurs as a 15×8 m outcrop surrounded by high-P/T metapelite, and baddeleyite was collected from only one serpentinite block. The baddeleyite appear as aggregates exhibiting angular or subrounded shapes with sizes of up to 2 mm in length. The aggregates are composed of acicular baddeleyite surrounded by porous zircon rims. Both the baddeleyite and the zircon yielded U–Pb ages of ca. 96 Ma, corresponding to the peak metamorphic age of the region. Within the aggregates, Th-enriched areas with sizes of less than 20 µm were observed. The baddeleyite aggregates show enrichment of light rare earth elements with positive Eu anomalies. The baddeleyite aggregates were likely produced with zircon megacrysts through metamorphism during serpentinization. Subsequent metasomatism associated with carbonation and silicification led to the formation of the zircon rim and trace-elemental maldistribution. This study demonstrates significant heterogeneity of milli- to micro-scales within a meter-scale serpentinite outcrop. This finding may necessitate a reassessment of the HFSE cycling in subduction zones.