spinels in a single peridotite from the southern Marianas during Shinkai 6500
Dive #973. They concluded that the low-Cr# spinel is a relict that survived the
process of melt impregnation.
4. Ohara et al. ( 2002 ) reported plagioclase-free harzburgites from the Mariana
Trough. However, some harzburgites are cut by leucocratic veins and contain
altered plagioclase and spinel. This spinel, and its associated pyroxene, is
enriched in TiO 2 compared with the same minerals in the host harzburgite. These
observations support the idea that Ti-enrichment of spinel and pyroxene pro-
vides important evidence for the existing of an impregnating melt.
5. The impregnating melt might yield plagioclase, clinopyroxene, and olivine.
Therefore, it is assumed that both the TiO 2 contents of spinel and the modal
amount of plagioclase in the peridotite relate to the effects of an impregnating
melt. Figure 5 shows the relationships between the calculated modes for each
peridotite mineral and the TiO 2 content of the spinels. There is a strong correla-
tion between plagioclase and clinopyroxene modes and TiO 2 in spinel, so that
peridotites in which spinels have a higher TiO 2 content contain more plagioclase
and clinopyroxene. Assuming that all the plagioclase crystallized from an
impregnating melt, any peridotite that was not affected by such a melt should
contain spinel with 0.145% TiO 2 . This TiO 2 content correlates approximately
with a clinopyroxene mode of 1.2%.
Based on the above observations, we suggest that the peridotites, before
impregnation, might have been harzburgites containing olivine, orthopyroxene,
clinopyroxene (ca. 1.2%), and spinel, and that plagioclase and clinopyroxene
crystallized in these rocks as a result of melt impregnation and reaction.
6. Pyroxenes in peridotites from along the fault at 144°E are also rich in TiO 2 .
Although these pyroxenes have Mg# values that are similar to those of pyroxenes
in peridotites from various back-arc basins in the Philippine Sea Plate (e.g., the
Mariana Trough and Parece Vela Rift; Ohara et al. 2002 ; Ohara 2006 ), orthopy-
roxene and clinopyroxene grains in peridotites from along the 144°E fault have
higher contents of TiO 2 (Fig. 6 ), suggesting that impregnating melts had a stronger
influence here than in the Mariana Trough and Parece Vela Rift.
Degree of Partial Melting
It is well known that the Cr# of spinel in residual mantle peridotites is a sensitive
indicator of the degree of melting (Dick and Bullen 1984 ), with a lower Cr# indicat-
ing a lower degree of melting, and vice versa. Hellebrand et al. ( 2001 ) presented a
equation that links the Cr# of spinel to the degree of melting. Adopting this equa-
tion, the degree of melting for the peridotites along the fault at 144°E in the south-
ern Marianas is calculated to be around 15%.
However, it must be remembered that the Cr# of spinel from peridotites along
the 144°E fault has been modified as a result of melt impregnation, as discussed
above. Because spinels become enriched in Cr during interaction with impregnat-
ing melts (e.g., Cannat et al. 1990 ; Niida 1997 ), the calculated degree of melting