Iron-rich stripe patterns in olivines of serpentinized peridotites from Mariana forearc seamounts, western Pacific

Olivines with conspicuous iron-rich stripe patterns are found in serpentinized peridotites from Conical and South Chamorro Seamounts, Mariana forearc, western Pacific. The stable association of antigorite, diopside, and olivine in these peridotites indicates that antigorite, diopside, and olivine underwent serpentinization at approximately 450-550 °C. The iron-rich stripe patterns are formed in the olivine crystal (Fo90-92) as a parallel alignment of narrow straight parts of widths ranging from 0.5 to 2.0 μm. The iron-rich parts have compositions of Fo86-89. The iron-rich stripe patterns are well developed near the rim of the host olivine where fiber crystals of antigorite are pierced into olivine and these patterns are not found in the inside of olivine grain except in the periphery of cracks. Generally, olivine is highly deformed and has well-developed cleavages in (010), (100), and (001) directions. The stripe is commonly parallel to (100), but one olivine grain has two sets of stripes that are parallel to (100) and (001). Modes of formation of iron-rich stripe patterns in olivine suggest that the infiltration of iron-rich fluid along the cleavage trace or the subgrain boundary formed by dislocations is probably responsible for the formation of the iron-rich stripe patterns. The iron-poor parts intervened between the iron-rich parts are slightly lower in XMg[= Mg/(Mg + Fe2+)] than the inside of olivine grain that is homogeneous in composition and lacks iron-rich stripe patterns, suggesting that metasomatic alteration also occurred in iron-poor parts. Antigorite formation results in an extra iron component because XMg of antigorite (= 0.94-0.97) is significantly higher than that of host olivine. Therefore, the iron-rich fluid may have been produced by serpentinization and infiltrated through olivine crystal to form iron-rich stripe patterns.