Electronic Supplementary Marials: Petrographical and mineral chemical
description of KPR volcanics
Petrography
The 2-pyroxene volcanics from the Miyazaki SMT (KT94-10 D02)
2-px volcanics was recovered only from the Miyazaki SMT (KT94-10 D02). These
are basaltic to andesitic and exhibit porphyritic texture, with up to 10 % phenocrysts
(Fig. A1a, b). The phenocrysts are mainly plagioclase (pl), orthopyroxene (opx),
clinopyroxene (cpx) and minor amount of magnetite (mt). The typical ratio of modal
percentage of phenocryst of opx, cpx, and pl is about 1: 1: 3.
Pl phenocrysts are the most common type of mineral. They are euhedral to subhedral
and usually less than 1 mm in length, through up to 3mm. They show grained and short
to long prismatic shape and commonly have lamellar twins. Some large phenocrysts
have complex zoning and glass inclusions. The number of opx phenocrysts is about 1/3
of that of pl phenocrysts. They are euhedral to subhedral. They show grained to short
prismatic shape and columnar or lath-shaped and slightly brownish color under the
plane polarized light. Cpx phenocryst numbers are roughly the same as that of opx.
They are euhedral to subhedral and exhibit short prismatic or grained shape with no
clear pleochroism.
Groundmass minerals are mostly pl, and minor amounts of both px. Groundmass
glass shows dark gray to black color under the plane polarized light. Pl groundmass
minerals show a columnar to acicular shape, and most of groundmass minerals have
twin. Pigeonite is only present as groundmass minerals.
Small amounts of euhedral to anhedral mt are observed and are commonly found as
an opaque phase among the groundmass and/or microphenocrysts. Microphenocrysts,
up to 0.3 mm, are common in some samples.
The clinopyroxene volcanics from the Nichinan SMT (KT94-10 D03) and other
seamounts
Cpx volcanics is the main rock type in the northern KPR. Most volcanic rocks are
basalt and classified into two petrographic types based on mineral assemblages, cpx
basalt (Fig. A1c) and hornblende (hbl)-cpx basalt (Fig. A1d). Hbl-cpx basalt was only
recovered from the Nichinan SMT (KT94-10 D03). Both volcanic rocks exhibit
porphyritic texture and contain about 30 % phenocrysts. The modal ratio of phenocryst
is higher, and the sizes of phenocrysts are larger than that of 2-px volcanic suites.
In the cpx basalt at the Nichinan SMT, phenocrysts are mainly consisting of pl, cpx
and minor amount of mt and olivine (ol). A ratio of modal percentage of cpx and pl is
about 3: 2. Large phenocrysts (up to 3 mm long) are richer than in 2-px volcanic rock
samples. Cpx phenocrysts are the most common mineral in cpx volcanic suites. They
are euhedral to subhedral, fine to coarse, and up to 3 mm long. They exhibit a short to
long prismatic shape and appear greenish under crossed nicols with high birefringence
and oblique extinction. One set of cleavage planes is usually prominent. Some
phenocrysts have rare zoning under the crossed nicols. The number of pl phenocrysts is
2/3 amount of that of cpx phenocrysts. They are euhedral to subhedral, fine to coarse,
and up to 2 mm long. They exhibit a short to long prismatic shape and are colorless with
low relief and low birefringence. They commonly have lamellar twins and complex
zoning patterns, reflecting the morphology during stages of growth, and some glass
inclusions.
Groundmass mineral is mostly pl, and some mt and cpx. Groundmass glass exhibits a
dark gray to black color under the plane polarized light. Pl in the groundmass has a
columnar to acicular shape and projects because of its low relief, and most of
groundmass minerals have twins.
The amount of euhedral to anhedral mt is small but more than that of 2-px suite rocks
and commonly present as an opaque phase in the groundmass or as microphenocrysts.
Microphenocrysts, up to 0.5 mm long and larger than in the 2-px suite, are common in
some samples.
Ol is present in trace amounts in these rocks. Ti is commonly euhedral, fine-grained
microphenocrysts up to 0.5 mm long, that is clear and colorless under the plane
polarized light, with high birefringence, which invariably has a touch of green or brown.
The terminal faces are very well developed. They are replaced to carbonate with
hematite, or reddish iddingsite. Relic olivine is distinct from relics of pyroxene in shape
and mineralogy.
Characteristics of hbl-cpx basalt are similar to that of cpx basalt. Total amount of hbl
phenocryst is about 1/5 to 1/10 of that of cpx. Hbl is euhedral to subhedral and long
prismatic phenocrysts and groundmass. They exhibit prismatic texture, and columnar or
lath-shaped, and appear brownish under the open nicol (Fig. A1d).
Cpx suite rocks at the other seamounts show similar characteristics to that of the
Nichinan SMT (Fig. A1e, f). The mode of phenocrysts is higher in the samples at the
Bungo SMT (KT94-10 D08) and lower in the samples at the northern peak of the
Komahashi-Daini SMT (KT94-10 D04). The samples at the Komahashi SMT (KT98-19
D04) show lower cpx content relative to other cpx suites. The size of phenocryst in the
samples at the Nichinan SMT is the largest in the northern KPR. The other cpx suites
exhibit a smaller phenocryst size than that of the Nichinan SMT, but are similar to that
of the 2-px suite rocks at the Miyazaki SMT.
Mineral chemistry
The chemical composition of the constituting minerals are measured by EPMA on the
selected 39 rock samples, i.e., 15 samples for the 2-px volcanics from the Miyazaki
SMT (KT94-10 D02), 12 samples for the cpx volcanics from the Nichinan SMT
(KT94-10 D03). Samples from the both two sites exhibit the typical characteristics of
the 2-px and cpx volcanics. The other analyzed samples are 1 sample from the site
KT94-10 D04 and 1 sample from the site KT98-19 D11 at the northern peak in the
Komahashi-Daini SMT, 1 sample from the Bungo SMT (KT94-10 D08), 3 samples
from the Buzen Knoll (KT94-10 D09), 3 samples from the Hyuga SMT (KT98-19 D02),
1 sample from the Higo SMT (KT98-19 D03) and 2 samples from the Komahashi SMT
(KT98-19 D04). All of these samples are classified into the cpx suite.
The 2-px volcanics
Chemical compositions of px phenocrysts in samples from the Miyazaki SMT
(KT94-10 D02) are plotted in pyroxene quadrilateral diagrams (Fig. A2a) Core
compositions of typical opx phenocrysts are En (enstatite)63-74 (Mg#: cation
Mg/(Mg+Fe) = 65-75) and Wo (wollastonite)3.0-4.0, and exhibit bronzite and hypersthene.
That of cpx phenocrysts is Wo36-39En43-47 (Mg# = 66-78), and exhibit augite. Rim
compositions of typical both pyroxene phenocrysts are similar to core compositions.
Pigeonite occurs in peripheral part of phenocrysts and groundmass minerals;
compositions are Wo6.5-12.5En54-62 (Mg# = 60-66). The one felsic sample, KT94-10
D02-15, exhibits acidic both px compositions.
Pl and px compositions are plotted in composition vs. frequency diagrams, that is, pl
is plotted anorthite (An) content and pyroxenes are plotted Mg# vs. frequency (Fig.
A3a). Pl phenocryst core composition is from An95 to An80 in typical samples. The
dacite sample, KT94-10 D02-15, exhibit low An content, An45-55. Some samples have
low An composition of some phenocrysts, and these compositions are similar to pl
phenocrysts in the dacite samples. This characteristic indicates the existence of internal
magma mixing in the magma process of the 2-px suite rocks.
The cpx volcanics
Chemical compositions of px phenocrysts of the cpx volcanics are plotted in
pyroxene quadrilateral diagrams (Fig. A2b). Core compositions of typical cpx
phenocrysts exhibit Wo43-45En40-43 (Mg# = 70-78). Rim compositions are similar to core
compositions. Some samples have mafic cpx phenocrysts, Wo45-47En45-49 (Mg# = 91-73),
classified into diopside.
Besides, core compositions of pl phenocrysts in the cpx basalt are from An95 to An70,
and those in the hbl-cpx andesite exhibit low An content, about An50 (Fig. A3b). This
composition of pl phenocrysts in the hbl-cpx andesite exhibits extremely felsic
compared to bulk composition. This characteristic indicates the existence of felsic end
member during the magma genesis in the Nichinan SMT.
Compositions of cpx phenocrysts core in the 2-px and cpx suite rocks are plotted in
Al2O3, TiO2, and Cr2O3 content vs. Mg# diagrams (Fig. A4). Al2O3 and TiO2 content
exhibit gradually increasing, and Cr2O3 content exhibits rapidly decreasing with
decreasing of Mg#. The 2-px suite rocks at the Miyazaki SMT exhibit low Al2O3 and
TiO2 content of the cpx phenocrysts than the cpx suite rocks at the Nichinan SMT. The
cpx phenocrysts in the other cpx suite rocks exhibit similar TiO2 content to that of the
Nichinan SMT volcanics. However, these volcanics exhibit variable Al2O3 contents
between the Miyazaki and the Nichinan seamounts volcanics. Cr2O3 contents are low in
the volcanics of the Miyazaki SMT and higher in the mafic phenocryst of the volcanics
of the Nichinan SMT. Many other cpx suite rocks exhibit similar Cr2O3 content to the
Nichinan SMT volcanics. However, the Komahashi SMT volcanics have high Cr2O3
content cpx phenocrysts at the given Mg# compared to other volcanics.
References
Ishii T (1975) The relations between temperature and composition of pigeonite in some
lavas and their application to geothermometry. Mineral J 8: 48-57