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

Abstract

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 (Fo 90−92 ) 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 Fo 86−89 . 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 ironrich parts are slightly lower in X Mg [= Mg/(Mg + Fe 2+ )] than the inside of olivine grain that is homogeneous in composition and lacks iron -rich stripe patterns, suggesting that metasomatic alteration also occurred in ironpoor parts. Antigorite formation results in an extra iron component because X Mg 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.

The legacy of crystal-plastic deformation in olivine: high-diffusivity pathways during serpentinization

“…This is supported by the observation that the zoning is most pronounced where antigorite penetrates the olivine grain (Fig. 3b), implying that fluid infiltration into olivine grains is related to the striped zoning development, as also recently suggested by Murata et al (2009). Furthermore, minor element contents of Mn and Ni (Table 1) associated with the zoning exhibit an opposing signature to those found in apparently dry Uenzaru peridotite (Ando et al 2001, Table 1).…”

“…Previously investigated peridotite bodies exhibiting only Fe-enriched striped zoning have been found in rocks that are apparently unaffected by hydrothermal alteration (e.g., San Carlos xenolith, USA, Kitamura et al 1986; Uenzaru peridotite, Japan, Ando et al 2001) and in serpentinized meta-peridotites (Conical and South Chamorro Seamounts, Mariana forearc, Murata et al 2009). In contrast, the samples investigated here derive from hydrated upper mantle rocks, displaying opposing types of striped zoning reflected by (1) an enrichment and (2) a depletion in Fe.…”

“…3b). Furthermore, Fe enrichment reflected in olivine domains in meta-harzburgites (X Fe = 0.12-0.14 (Fe-enriched zones); X Fe & 0.10 (bulk)) from the Mariana forearc seamounts, western Pacific (Murata et al 2009), is enriched to the same order of magnitude as those in the FUB meta-harzburgite.…”

“…5e) and in Mariana forearc seamounts (Murata et al 2009). Formation of a solid phase at a dislocation wall would be favorable because the Gibbs free energy barrier for nucleation at defects is lower than that in a defect-free bulk crystal (Putnis 1992).…”

Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific

“…4A-4C), and seems to have formed along the once-existing conduits of fl uid. The iron-rich stripes emanate from cracks fi lled with antigorite in primary olivine (Murata et al, 2009), indicating that the formation of iron-rich olivine is closely related to serpentinization (antigorite formation). These modes of occurrence suggest a possibility that iron-rich olivines formed secondarily during serpentinization.…”