Geochemistry of Active Hydrothermal Systems in Okinawa Trough BackArc Basin

Hitoshi Chiba

Department of Earth and Planetary Sciences, Kyushu University, Japan.

Abstract Characteristics of fluid chemistry and

mineralogy of Okinawa Trough back arc hydro-

thermal systems are summarized. Unique

chemistry of fluid mainly results from the reacted

SiOrrich rock and interaction with sediment.

Mineralogy of hydrothermal precipitates is Zn-Pb-

Ba rich and is similar to the Kuroko deposit.

1. Location of bigb temperature hydrothermalsystems in the Okinawa Trough.

The locations of the high temperature hydro-

thermal systems are shown in Fig.l. Their water

depths are relatively shallow compared to those

of the mid-ocean ridge systems and range from

710 to 1390m.

Geological setting

Okinawa Trough is a back-arc basin of the

Ryukyu arc-trench system. It is thought to be

in a rifting stage of the continental lithosphere

(e.g. Sibuet et at, 1987). Along the axes of

the southern to middle part of the trough,

several volcanic ridges intruded through thinned

continental crust. The ridges are composed of

a bimodal assemblage of young volcanic rocks:

basalt to rhyolite (Naka et al., 1989; Ishizuka et

al, 1990). High temperature active

hydrothermal systems were first discovered in

1988 at the Izena Cauldron (JADE site)

(Halbach et al., 1989) and the Iheya Ridge

(CLAM site)(Tanaka et aL 1989). Three other

high temperature hydrothermal systems were

also found at the Minami-Gnsei knoll in 1990

(Hashimoto et al., 1990), at the northern knoll

of the Iheya Ridge in 1995 (Monroa et al., in

press) and at the Hakurei site of the Izena

cauldron in 1995 (Maeda et al., 1996). This

paper summarizes the results of fluid chemistry

and mineralogy of sulfide samples so far studied.

Characteristics of Fluid Chemistry

Table X summarizes the estimated chemical

compositions of endmember fluids (Mg=0)

except for the Clam site. At the Clam site, the

possible endmember fluid contains 22mM of Mg

(Sakai et al, 1990b), so that the data of the fluid

63 -

Table 1. End Member Cbcmical Compositions of Hydrothermal Fluids in the Okinawa Back Arc Basin

1: Sakai et al. (1990b), 2: Chiba et al. (1996), 3: Sated et aJ. (1990b), 4: Chiba et al. (1993), 5: Von Damm et al. (1989)

with 22mM of Mg are shown in the Table 1.

The endmember hydrothermal fluids in the

Okinawa Trough have several common distinct

chemical characteristics compared to those of

sediment starved mid-ocean ridge systems as

follows; (1) Okinawa Trough hydrothermal

fluids are enriched in K compared to sediment-

starved mid-ocean ridge fluids (Sakai et al,

1990b). (2) They are also highly enriched in

NHt, CH4, Li and titration alkalinity (Sakai et al.,

1990b). (3) CO3 content of the Okinawa fluids

are extremely high (Sakai et al, 1990a; Chiba et

al, 1992b). And (4) "Sr/^Sr of the fluids are

extraordinarily high and are close to or above

"Sr/^Sr of the seawater (Chiba et al 1992a).

Effect of Host Rock and Sediment:

Comparison with Other Seafloor

Hydrothermal Systems

Table 2 compares fluid chemistry of back-arc

hydrothermal systems, High K concentration

seems to be common characteristics of back-arc

hydrothermal systems with bimodal volcanism.

It becomes more apparent if we compare Cl-

normalized K concentration of high temperature

fluid, (K/C1)hf, with seawater, (K/Cl)sw-

(K/C1W(K/Cl)sw of Okinawa Trough, Mariana

Trough and Lau Basin back-arc hydrothermal

systems range from 4.2 to 8.1, whereas those of

sediment starved mid-ocean ridge systems

(Pacific and Atlantic) are smaller than 3.2. K

behaves as soluble element in rock-water

interaction (Ellis, 1970), so that high K

concentration results from high K concentration

of host rock if water/rock ratios are similar in

seafloor hydrothermal systems. K

concentration of rock increases as rock becomes

Si02-rich. Rocks sampled in Okinawa Trough

and Lau Basin are bimodal in composition, so

that hydrothermal fluids of these two systems

seem to get K from Si02-rich rock. Rocks

64 -

Table 2. Comparison of End-Member Fluid Chemistry of Backarc Hydrothermal Systems

sampled in Mariana Trough are back-arc basin

basaIt(BAB) which are generally rich in K

relative to MORB. On the other hand, at the

K-poor North Fiji hydrothermal system, N-

MORB is dominant at the segment where active

hydrothermal system was found. These

relationships between rock-type and K

concentration of the hydrothermal fluid suggests

that high K concentration of Okinawa Trough

fluid is due to the reacted rock.

Alkalinity and concentrations of NH ≪, CH<

and C02 becomes high when degradation of

organic matter occurs during the fluid-sediment

interaction. High concentrations of these

components are common feature of the

sediment-hosted submarine hydrothermal

systems in the world (Gamo, 1995). Thus,

high alkalinity and high concentrations of NH4

and CH4 in Okinawa Trough hydrothermal

fluids are due to the interaction with sediment.

However, extremely high CO2 concentration is

difficult to explain by sediment-fluid interaction

alone.

Li contents of sediments in the Izena cauldron

is more than two times higher than those of

volcanic rocks in the trough (Chiba et al. 1992).

High concentration of Li in the Okinawa Trough

fluid, highest among the seafloor hydrothermal

fluids so far studied, is also due to the

interaction with the sediment. High 87Sr/*6Sr

ratio, close to or higher than normal seawater,

was found in the Okinawa Trough and Escanabe

Trough. 87Sr/"Sr of volcanic rocks in the

Okinawa Trough range from 0.7043 to 0.7049

(Honma et al., 1991). The high 87Sr/*6Sr of

hydrothermal fluids are only explained by the

interaction with high-87Sr/86Sr sediment

component. Actually, "Sr/^Sr of sediments in

65 -

the Izena cauldron exceeds 0.7100 (Chiba et al,

1992a).

Source of CO2

CO2 hydrate at the seafloor was observed at

the Jade site (Sakai et al 1990), Minami-Ensei

(Chiba et aL 1992b) and northern knoll of the

Iheya ridge (Chiba et al. 1996). Such a CO2

hydrate formation is known only at the Mid-

Okinawa Trough. The sediment-fluid

interaction only partly explains the high

concentration of CO2. A model which involves

sediment (organic matter and carbonate)-fluid

interaction, CO2 degassing from the subducting

slab (carbonate precipitated at high temperature

hydrothermal activity) and C02 degassing from

the mantle was proposed (Sakai, 1995). It

uses S13C of CO2 as a constraint for estimation

of relative contribution of three components.

Though the model was not completely

successful, it suggests that more than half of

CO2 results from carbonate in the sediment and

carbonate precipitated during high temperature

hydrothermal activity and that about 30% of

CO2 comes from the degradation of organic

+++: abundant, ++: common, +: rare. 1: Nakasbima et al. (1995), 2: Cbiba et al. (in press), 3: Haymon and Kastner

(1981)

66 -

matter in the sediment. The contribution of

CO2 degassed from the mantle which is probably

the main source of CO2 at sediment-starved

mid-ocean ridge system was estimated to be less

than 10% in the Okinawa Trough.

Mineralogy of Ores and Chimneys

The mineralogy of ore and chimney samples

in Okinawa Trough hydrothermal systems is

summarized in Table 2. Samples are rich in

Zn-Pb-Ba and poor in Fe-Cu sulfides compared

to the mid-ocean ridge systems. The low Cu

abundance may be due to the low temperature

venting compared to mid-ocean ridge systems

because cooling of hydrothermal fluid to 300°C

makes most of Cu to precipitate (Seyfried and

Ding, 1995). Shallow water depth of Okinawa

Trough hydrothermal system makes the venting

temperature low because of low confining

pressure and probably prevents the fluid to carry

Cu to the seafloor. Zn concentration is

insignificantly affected by fluid cooling during

ascent to the seafloor (Seyfried and Ding, 1995).

Thus, Zn probably concentrates in ore samples.

The sulfide mineralogy of the Okinawa Trough

hydrothermal systems except for the clam site

has similarity to the Kuroko deposits, though

the origins of Pb and Ba are not clear at present.

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