Petrology and Mineralogy of Mantle Peridotites from the Southern Marianas - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 141

Fig. 4 Chemical compositions of spinel in plagioclase-bearing peridotites from the southern
Marianas, with references to those from other areas of the Philippine Sea (Ohara et al. 2002, 2003 ;
Ohara 2006 ; Michibayashi et al. 2009 ). The arrow indicates the trend for impregnating melts
in peridotites from the Mariana Trough. ( a ) Cr# (Cr# = Cr/(Cr + Al)) versus Mg# (Mg# = Mg/
(Mg + Fe 2+ )), and ( b ) Cr# versus TiO 2 wt%
These facts make it difficult to evaluate the origin of the plagioclase. However, we
propose that impregnating melts influenced the crystallization of plagioclase in
peridotites from the fault at 144°E, an idea first mentioned by Bloomer and
Hawkins ( 1983 ) and more recently by Michibayashi et al. ( 2009 ). We base this
proposal on the following arguments.
1. Spinel in abyssal peridotites usually contains low or negligible amounts (<0.25
wt%) of TiO 2 (Dick and Bullen 1984 ). Therefore, it is accepted that Ti-enrichment
in spinel results from reactions caused by impregnating melts (e.g., Dick and
Bullen 1984 ; Cannat et al. 1990 ; Girardeau and Francheteau 1993 ; Niida 1997 ;
Pearce et al. 2000 ).
2. The TiO 2 contents of spinel in peridotites from the fault at 144°E are higher than
those from the Mariana Trough, and similar or only slightly lower than those in
the P (plagioclase)-type peridotites from the Parece Vela Rift (Fig. 4 ).
3. Spinel in the P-type peridotites from the Parece Vela Rift has compositions that
are thought to have been affected by an impregnating melt, and the spinel is
richer in TiO 2 than spinel in impregnation-free F (fertile)-type peridotites (Ohara
et al. 2002 ; Ohara 2006 ). The range of TiO 2 contents in spinel from peridotites
along the 144°E fault is similar to that in the P-type peridotites from the Parece
Vela Rift (Fig. 4 ). Michibayashi et al. ( 2009 ) found both high- and low-Cr#