Geology, geophysics, geochemistry, and deep-sea mineral deposits, Federated States of Micronesia: KORDI-USGS R.V. Farnella Cruise F11-90-CP

Abstract

Ifalik Atoll-Tarang Bank and trends east-west. This segment consists of large carbonate(?) banks and atolls and is generally of less than 2500 m water depth. The western third of Caroline Ridge extends from Ifalik Atoll-Tarang Bank to the Yap trench and trends northwestward. This segment consists of a large shallow-water (<2500 m) ridge bounded by, and cut by, narrow troughs that represent strike-slip faults (southern margin; Hamiltonj 1985), normal faults (northern margin; Andrews, 1971), and small spreading basins. Seismic profiles presented here show that both the north and south flanks of Caroline Ridge are block faulted. The origin of the western two-thirds of Caroline Ridge is unknown. West Caroline Ridge was proposed to be a relict island arc by Bracey and Andrews (1974). However, Hamilton (1985) disagreed with their interpretation and speculated that the ridge represents a leaky transform fault that connects the Mussau and Mariana trenches. Perfit and Fornari (1982) called on a combination of leaky transform fault and hot spot volcanism to form the ridge. Hegarty and Weissel (1988) suggested that the western part of Caroline Ridge, as well as Eauripik Rise, formed when a melting anomaly passed beneath the Pacific plate during the late Oligocene. Our work indicates that west Caroline Ridge, Sorol Trough, and associated topographic features may represent an extinct(?) spreading centertransform fault system. Vogt et al. (1976) suggested this possibility in passing, but provided no corroborative evidence. Eauripik Rise may also be an extinct spreading center, one of three in the Caroline Basin, which is located south of Caroline Ridge (Winterer et al., 1971; Erlandson et al., 1976; Mammerickx, 1978). The Yap arc and trench represent an Oligocene(?) and Neogene convergent plate margin, but one that is distinct in many ways from other west anq southwest Pacific arcs (Cole et al., 1960; Johnson et al., 1960; Hawkins and Batiza, 1977). For example, the distance between the arc summit and trench axis is very narrow and subduction may have ended in the late Miocene. Also, many of the rocks recovered from the arc (inner trench wall, summit, and summit islands) are metamorphic rocks of greenschist and amphibolite grade (Johnson et al., 1960; Shiraki, 1971). Many of the volcanic rocks have an oceanic crust compositional signature and thus may be obducted oceanic crust (Hawkins and Batiza, 1977; Woridng group, 1977). Other rocks belong to the calc-alkaline and island arc tholeiite series (Beccaliva et al., 1980; Crawford et al., 1986). Rocks dredged from the outer trench slope have a MORB-like composition and are about 7 m.y. old (Beccaluva et al., 1980). Hydrothermal mineralization of Quaternary sandstones collected by us from the central Yap arc indicate that the suggestions that subduction ended in the late Miocene and that back-arc basin crust was obducted onto the volcanic arc need to be modified, or at least must account for Quaternary hydrothermal activity at the! summit of the arc (Hawkins and Batiza, 1977). | ...

Hydrothermal Manganese Deposits from the Tonga-Kermadec Ridge and Lau Basin Region, Southwest Pacific

“…Hydrothermal enrichment of Zn in Mn crusts has been reported from the Galapagos Spreading Center (Moore and Vogt, 1976;Cronan, 1986;Rogers, 1996), the Mariana-Bonin-Yap Arc (Schulz and Hein, 1991;Hein et al, 1992), and previously from the TKR . The overall Zn concentrations reported here for hydrothermal crusts are lower than those reported for crusts from the other areas.…”

Diffuse flow hydrothermal manganese mineralization along the active Mariana and southern Izu‐Bonin arc system, western Pacific

“…However, high Ni plus Cr contents probably reflect the incorporation of a detrital ultramafic component or the leaching of ultramafic rocks, which for the IBM and Yap arcs would include serpentinites. Stratabound Mn deposits from the Yap arc are especially enriched in those two elements (Figure 12) and serpentinite was recovered in the same dredge haul as the hydrothermal Mn [ Hein et al , 1992]. Unfortunately, Ni also occurs in sulfides and therefore has multiple sources; if high Ni is not accompanied by high Cr, then an ultramafic component may not be indicated.…”

“…Most samples show a mixture of the Mo proxy and sulfide proxy end‐members, which is more pronounced if Ni is not plotted with Cr, because Ni can have multiple sources (see text for discussion). Only the Yap samples have a strong contribution from ultramafic rocks (Yap arc samples from Hein et al [1992]).…”

“…[66] Hydrothermal Mn deposits have been found along all active volcanic arcs in the west Pacific such as the Kuril arc [Gavrilenko, 1997;Glasby et al, 2006], Izu-Bonin arc [Usui et al, 1986[Usui et al, , 1989, Yap-Belau arc [Hein, 1992;Hein et al, 1999], Tonga/Kermadec arc [Cronan et al, 1982;Moorby et al, 1984;Hein et al, 1990], and the Mariana arc, as described here. Hydrothermal, diffuse flow Mn mineralization is perhaps the most widespread type of mineralization in active arcs, at least in the near-surface environment.…”

Composition of Co-rich ferromanganese crusts and substrate rocks from the Marshall Islands, cruise KODOS 97-4

“…Bulk Crusts: The mean Fe and Mn contents of the 27 bulk crust samples are 15.1% and 21.4%, respectively (Tables 13,15). The mean Fe/Mn ratio (0.71) is comparable to the mean ratio for the entire central Pacific region (0.72; Hein et al, 1992), but is higher than the ratio for the entire Marshall Islands EEZ (0.64; Hein et al, 1998), and lower than the ratio for the area to the northwest of the Marshall Islands EEZ (0.76; Hein et al, 1997a). The mean Mn content is less and the Fe content somewhat less compared to bulk crusts from the entire Marshall Islands EEZ, 16.9% and 26.3%, respectively.…”