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Most recently, another
Shinkai 6500
dive (6K#973) has recovered residual peri-
dotites with mylonitic, porphyroclastic, and coarse, moderately deformed second-
ary textures (Michibayashi et al.
2009
). Michibayashi et al. (
2009
) analyzed the
olivine crystal preferred orientations, and concluded that the peridotites represent a
ductile shear zone within the lithospheric mantle of the overriding plate (i.e., the
Mariana Back-Arc Basin), suggesting that back-arc basin mantle is exposed along
the West Santa Rosa Bank Fault, even in its fore-arc setting.
Along the inner wall of the Mariana Trench, southwest of Guam, mantle perido-
tites have been recovered from several locations (Ohara and Ishii
1998
; Michibayashi
et al.
2007
). Petrologic and mineralogical studies reveal that these peridotites rep-
resent the highly refractory residue left behind after extensive mantle melting
related to island-arc magmatism (Ohara and Ishii
1998
). Therefore, along the
Mariana Trench, southwest of Guam, mantle peridotites associated with both back-
arc magmatism and island-arc magmatism are exposed.
3
Petrographic Descriptions of the Peridotites
During cruise KH-98-1, along the fault at 144°E in the southern Marianas, serpen-
tinized peridotites weighing approximately 20 and 35 kg were recovered from sites
KH-98-1D1 and KH-98-1D2, respectively.
Modal compositions were measured with a point-counter (Table
2
). For each
specimen, more than 1,000 (usually 2,000) points were counted on a 0.50 × 0.50
mm grid. Computations were then made, fitting mineral compositions (average
composition for each thin section) to bulk rock compositions in terms of SiO
2
, TiO
2
,
Al
2
O
3
, FeO (total Fe as FeO), MnO, MgO, CaO, Na
2
O, Cr
2
O
3
, and NiO contents.
For bulk rock compositions, rock samples were initially split and crushed, and then
pulverized in an agate ball mill. H
2
O and loss on ignition were determined at 110°C
and 950°C (over 6 h), respectively. Major elements were determined by X-ray fluo-
rescence (XRF), using a RIGAKU 3270 at the Ocean Research Institute, the
University of Tokyo, Japan. The mixture used for major element analysis was ~0.5
g of each powdered sample and 5 g of anhydrous lithium tetraborate (Li
2
B
4
O
7
).
Bulk rock compositions are listed in Table
2
.
Comparing the two methods, point-counting gives higher proportions of olivine
and lower proportions of orthopyroxene and clinopyroxene than do the computa-
tions, possibly due to: (1) difficulties and errors in identifying small grains of
pyroxene and olivine; (2) the problem of dealing with serpentinized minerals where
the original mineral could not be identified (our approach was to count such ser-
pentinized grains as olivine); and (3) the inability of our computations to determine
the degree of serpentinization. Therefore, in this study, we adopt the figures from
point-counting for the degree of serpentinization, and adopt the computations in
determining the original lithology.
Based on modes, most of the peridotites from sites KH-98-1D1 and KH-98-1D2
are classified as lherzolite and lherzolitic harzburgite which contain significant
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