Masato Nohara

Structural stability of marine 10 Å manganates from the Ogasawara (Bonin) Arc: Implication for low-temperature hydrothermal activity

Abstract Hydrothermal 10 A manganate deposits with distinct differences in structural stability when exposed to air were found on the Kaikata Seamount of the Ogasawara Arc, Western Pacific. The most stable 10 A manganates are concentrated near the center of the hydrothermal field while the unstable ones occur towards the field margins. All manganates are characterized by very high Mn Fe ratios and extremely low contents of transition elements such as Cu, Co and Ni. The variation in 10 A stability of the hydrothermal manganates is not regulated by mineral chemistry or aging, but appears to be determined by other factors, e.g., the hydrothermal fluid temperature at mineral deposition. We propose a structural model for marine manganates showing two continuous series of increasingly stabilized 10 A manganates. One series is of hydrothermal origin, structurally similar to todorokite with a tunnel structure fixed at deposition, and the other is a buserite-like series of diagenetic origin comprising phyllomanganates. All the marine 10 A manganates can be progressively stabilized in two different manners; the todorokite-like series by the establishment of smaller tunnels, and the buserite-like series by post-depositional uptake of 10 A-stabilizing interlayer cations. Both series are linked together with a common end member, the most unstable 10 A manganate which appears to be formed at low temperature and easily transforms to a 7 A manganate in air.

Submarine hydrothermal manganese deposits from the Ogasawara (Bonin) Arc, off the Japan Islands

Abstract Numerous submarine hydrothermal sulfide and oxide deposits have been found in the mid-ocean ridge systems, but until now, no activity has been found in an island arc system. The Geological Survey of Japan (GSJ) has started a geological, geophysical, and geochemical investigation program in the Bonin-Mariana Arc since 1984, aiming at evaluation of submarine hydrothermal metal deposits and understanding of their geological background. During the reconnaissance survey by R/V “Hakurei-maru” in June 1984, thick hydrothermal manganese crusts were recovered from three seamounts of the Bonin Arc at a depth of ca. 1000 m. The chemical and mineralogical features and characteristic structure, and occurrence of associated calcareous fossils and nontronite claystones suggest the Quaternary low-temperature hydrothermal activity forming well-crystallized todorokite, pyrolusite, goethite, and nontronite deposits. Thin hydrogenous ferromanganese encrustations on the hydrothermal crusts from a guyot of the northern arc indicate that the recent very active hydrothermal activities around this area may be declining or already finished; whereas the fresh hydrothermal manganese crusts from the Kaikata Seamount suggest recent hydrothermal activity.

North Fiji Basin basalts and their magma sources:Part I.Incompatible element constraints

A systematic compilation of the geochemical data collected during the starmer programme is presented, in addition to all published data gathered along the North Fiji Basin (NFB) spreading system. From the 194 samples selected, the geochemical variations observed along the different spreading segments can be related to uni- or multi-modal origin of distinctive magma sources. These variations are interpreted as directly linked to the geodynamical features which surround the NFB. In the southern NFB, on the N174°E segment, N-MORB are present, but the low rate (or dead) subduction along the Hunter ridge still marks its influence as several basalts show a significant subduction-related contamination with negative Nb anomaly and high LOI. Several others are marked by a weak E-MORB source contribution, that may be related to subducted-OIB seamounts from the South Fiji Basin. In the central NFB, along the N-S segment which represents the most active and morphologically regular spreading ridge of the NFB, the magma source produces only N-MORB, with depleted LILE, HFSE, and LREE patterns, except in its northernmost part where magmatic signatures similar to that of the N15° segment appear. Along the N15° segment, three distinctive sources coexist and produce a wide range of geochemical signatures on the basalts collected; (1) a N-MORB source signature; (2) a transitional towards E-MORB source signature with a negative Nb anomaly indicating that some subduction related contamination still exists beneath the NFB (the basalts derived from this source might also have been called BABB previously); and (3) a source signature transitional towards E-MORB or OIB marking the start of an influence that increases towards the northern NFB as the Rotuma-Samoan hot spot lineament is approched. Around 17°S, on the Kaiyo station 4 site explored and sampled by the Nautile deep-sea submersible, as well as in the triple junction area, the same variability derived from three distinctive mantle sources is observed. Along the N160° segment, the three sources still coexist, but the influence of the E-MORB or OIB source (hot spot-related) increases, whereas the influence of the subduction-related source decreases. Along the Pandora-Rotuma ridge, the OIB-derived lava type is the only one present, the eventual contribution from other sources being completely diluted. Thus, the geochemistry of the NFB basalts is directly influenced by (1) the regional geodynamic environment, such as subduction zones and/or hot spot trails, (2) the geodynamical regime and stability of this type of back-arc system. The influence of the New Hebrides subduction, located some 500 km west of the active spreading ridge, is still perceptible, although weak, in the whole northern half of the NFB. However, this infuence is not directly linked to the presently active subduction, but originates rather from the partial melting of an upper mantle source that suffered subduction contamination during the clockwise rotation of the New Hebrides arc leading to the opening of the NFB during the past 12 Myr. In the northern NFB, many basalts result from the mixing of an N-MORB and a OIB source, similar to transitional alkalic lavas from oceanic intra-plate magmatism. This sources mixing increases northwards from 18°20′S to 12°S.

The North Fiji Basin basalts and their magma sources: Part II. Sr-Nd isotopic and trace element constraints

Abstract Sr-Nd isotope and trace element data are reported for basalts from the North Fiji Basin (NFB). NFB basalts are characterized by extreme variations in isotopic ratios and trace element abundances which are related to mantle heterogeneities. Values of 87 Sr 86 Sr for basalts from the northern segments, N160° and triple junction range from 0.7029 to 0.7041, whereas 143 Nd 144 Nd values vary from 0.51281 to 0.51313. Most of the basalts from these segments are characterized by strong relative enrichments in Rb, Ba, Sr, K, Nb, Ta La, Ce and Ti that are comparable to OIB components. The central segments, N15° and N-S have 143 Nd 144 Nd ratios between 0.51298 and 0.51363 and 87 Sr 86 Sr ratios between 0.7029 and 0.7033, and are depleted in large ion lithophile, light rare-earth and high field strength elements similar to N-MORB. Covariation between trace element and isotopic ratios among NFB basalts supports a model in which melts from the NFB rift system are derived by mixing of OIB-type and depleted N-MORB mantle components. The Sr-Nd isotopic and trace element variability indicates that the NFB basalt source is heterogeneous on the scale of individual melt batches.

Petrology and geochemistry of the central North Fiji Basin spreading centre (Southwest Pacific) between 16°S and 22°S

Abstract The North Fiji Basin (NFB) is a 12 m.y. old back-arc basin that has a complex multi-stage history. The presently active spreading system can be divided into four segments between 16°S and 22°S, which from north to south trend N160, N15 and N-S (the fourth segment is the N-S trending segment located near 174E). The main N-S segment is morphologically similar to other medium-rate oceanic ridges, whereas the other segments have rougher morphologies which have been severely disturbed by a triple junction at 16°45′S and several instability features such as overlapping spreading centres (OSCs) and propagating rifts. The spreading rate seems to diminish from 7.8 cm/yr near 20°S to 4.6 cm/yr near 18°S. Mineralogical, pertological and geochemical data were obtained on 24 new stations located along all the four segments. The petrogenesis of the basalts collected is essentially controlled by low-pressure crystal fractionation of plagioclase±olivine±clinopyroxene (plagioclase>olivine>clinopyroxene) with 52% of the NFB basalts reaching the four-phase cotectic. Locally, some magma mixing occurs, but this is limited to magma batches of closely related composition, as might be expected to occur inside a magma reservoir. The N-S segment, which, since 3 m.y., is the only steady-state segment, is also petrologically and geochemically very comparable to other medium-rate oceanic spreading centres, producing moderately evolved LILE and LREE-depleted N-MORB. In contrast, the three other segments produce basalts of much more variable petrology and geochemistry characterized by LILE and slightly LREE-enriched magmas cf back-arc basin basalt (BABB) affinity (but not as enriched as, for example, the Mariana BABB); MORB is, however, also found on the N160, N15 and 174E segments. Diagrams using Ba, Rb, K/P and (K/Ti)N (Normalized to the chondrites) plotted against latitude clearly show along-strike variations. Beneath the recently formed segments, the mantle source is heterogeneous, and locally has some BABB affinities, whereas beneath the more steady-state N-S segment the magma source is more homogeneous, being generally depleted in LILE and REE as is the case for classical N-MORB mantle sources. Simple evolution from an early stage of BABB production to MORB described in the Lau basin and proposed for the NFB does not seem to occur. Present-day activity still produces large amounts of BABB along the less-stable and more recently created segments, and MORB was produced in the earlier stages of the development of the NFB.

Fluctuation of biogenic and abiogenic sedimentation on the Shatsky Rise in the western North Pacific during the late Quaternary

Abstract Sedimentation of biogenic and abiogenic components was studied in cores NGC108 (36°36.85′N, 158°20.90′E; water depth 3390 m) and S2612 (32°19.84′N, 157°51.00′E; water depth of 2612 m) from the Shatsky Rise to understand fluctuations in primary productivity and abiogenic sedimentation in the mid-latitude of the western North Pacific during the late Quaternary. The mean C Organic /N atomic ratio of 6.0–7.8 in both cores indicates that organic matter is mainly marine in origin. Organic carbon is positively correlated with biogenic opal in core NGC108 in contrast to a weak correlation in core S2612. Although the maxima of paleoproductivity estimates in both cores generally occur during glacial times, the paleoproductivity estimates, biogenic opal/carbonate ratios and the C Organic /C Carbonate ratios have always been higher in core NGC108 than in core S2612 during the last 180 kyr, suggesting that the surface water at site NGC108 could have been influenced more by Subarctic water mass than at site S2612. However, the opal/carbonate ratio in core S2612 remains fairly constant relative to that in core NGC108, which might mean that the transition zone between Subarctic and Central water was narrower in latitude in at the oxygen isotope stage (OIS) 2/3 boundary, OIS 4 and OIS 6. Sedimentation of 13 inorganic elements has been measured in both cores. These elements are classified into four groups based on correlation between each element in content: (1) terrigenous components (Al, Ti, Fe), (2) biogenic calcareous material (Ca, Sr), (3) biogenic-scavenged elements (Mg, Zn, Cr, Be), and (4) the other elements (Mn, Ba, Cu, Ni). The terrigenous mass accumulation rates were elevated in OIS 2, 3 and 4 and late OIS 6 in core NGC108 while they were higher in early OIS 1, OIS 2, 4 and 6 in core S2612. MnO 2 and Ba might be redistributed during the sub-surface reduced condition. Especially precipitation of particle-reactive Be, which could be accelerated by both enhanced terrigenous input and biogenic vertical transport, has fluctuated largely in response to climatic change because of its short residence time (on the order of the oceanic mixing time).