Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 sink in the Early Archean

Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO₂ flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO₂ fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity.Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ¹³C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K₂O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K₂O, Rb, and Ba, and are depleted in Na₂O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO₂ enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO₂ in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO₂ ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite.The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 10¹³ mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10^-3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 10¹¹ cm²/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO₂ in the Early Archean.     

Geochemical Features and Tectonic Setting of Greenstones from Kunimiyama, Northern Chichibu Belt, Central Shikoku, Japan

Abstract. We report whole‐rock chemical data for the greenstones from the Kunimiyama area in the Northern Chichibu Belt and their implications on the tectonic setting of these rocks. The Kunimiyama greenstones are associated with stratiform fer‐romanganese deposits or bedded cherts in the northern part of the study area, but are closely associated with a thick limestone block or bedded cherts in the southern part. The constituent minerals of greenstones are albitized plagioclase, clinopy‐roxene, chlorite, calcite, epidote, pumpellyite, prehnite, quartz, celadonite, sericite and opaque minerals such as iron oxyhy‐droxide and hematite. These mineral assemblages, epidote + pumpellyite + chlorite and chlorite + pumpellyite + prehnite, suggest that the metamorphic grade of greenstones from the Kunimiyama area is prehnite‐pumpellyite facies. The whole‐rock chemical compositions of greenstones associated with ferromanganese deposits are generally similar to those of normal mid‐ocean ridge basalt (N‐MORB). In contrast, the chemical compositions of the greenstones associated with the limestone block are comparable to those of ocean island alkaline basalt. Greenstones associated with bedded cherts are of enriched MORB and ocean island basalt, as well as N‐MORB origins, suggesting they probably formed as a result of plume‐related MOR volcanism in the Panthalassa Ocean in Early Permian and by tectonic mixing of ocean island basalts with oceanic ridge crustal fragments during accretion/subduction processes. These geological and geochemical lines of evidence suggest that the Kunimiyama greenstones are allochthonous blocks of accreted oceanic crust and seamounts. The ferromanganese deposits are frequently accompanied by reddish greenstones. Compared to common greenish greenstones, the reddish greenstones are characterized by high MnO and rare earth element contents and distinct negative Ce anomalies, implying a slight contribution of hydro thermal component forming the ferromanganese deposits.     

Two Modes of Salinity and Temperature Variation in the Surface Layer of the Pacific Warm Pool

The characteristics of temperature and salinity variation in the Pacific warm pool were investigated using Empirical Orthogonal Function (EOF) analysis on one year's temperature and salinity data in the surface layer (0–50 m) obtained from the Triangle Trans-Ocean Buoy Network (TRITON) buoy array. Two dominant modes of surface temperature and salinity variation were found. One is a positive correlation mode where temperature and salinity were scattered almost parallel to isopycnal lines in a T-S diagram, which has little effect on the density field. The other is a negative correlation mode where temperature and salinity were distributed across isopycnal lines, which has a substantial impact on the density field. In particular, we found that the negative correlation mode at 5°N, 156°E was predominant on a seasonal time scale and contributed to the surface dynamic height variation, and therefore to surface geostrophic current. This revised version was published online in July 2006 with corrections to the Cover Date.     

Petrological and Geochemical Characteristics of Intrusive Rocks Related to Porphyry Copper Mineralization and the Implications for the Genesis of Deposits in the Namosi area, Viti Levu, Republic of the Fiji Islands

In the Namosi district, of Viti Levu Island, Republic of the Fiji Islands, we conducted chemical analysis on intrusive rocks and attempted to clarify the relationship between petrological characteristics and mineralization. Samples were collected from the Waisoi copper deposit, Waivaka corridor, Waisomo, Wailutelevu and Wainabama Southwest localities. The intrusive rocks in the district show the overall characteristics of calc-alkaline magnetite series. Using the normative composition diagram for granite classification, most of the intrusive rocks plot in the tonalite, granodiorite, quartz monzodiorite and quartz diorite fields. Rare earth element (REE) analysis revealed that at Waisoi the intrusive rocks tend towards an Eu positive anomaly, whereas most of those at Waisomo tend to have a negative anomaly. The (Ce_cn/Yb_cn)/Yb_cn ratio shows that the intrusive rocks in the Namosi district may be derived from a mantle origin magma possibly contaminated by crustal materials. Waisoi rocks have less contamination with continental crust and show higher gold content compared to those of the Waivaka corridor. The observed differences between Waisoi and Waivaka corridor in the Cu/Au ratio might also be caused by varying amounts of contamination by crustal materials. There is no clear correlation between the La/Yb ratio and K–Ar ages, however, the La/Yb ratio of the Waivaka corridor rocks has a wide range, suggesting a variable amount of crustal contamination of the magma from 7 to 5 Ma. The less crustal contamination and generation of relatively high silica contents under relatively oxidized conditions in the Waisoi magmas is correlated with Au-rich porphyry copper mineralization and in contrast more crustal contamination produced Au-poor porphyry copper mineralization at the Waivaka Corridor in the Namosi district.     

Cumulative Benioff strain-release, modified Omori's law and transient behaviour of rocks

Irreversible thermodynamic theories with internal state variables can be used to derive a general constitutive law for both transient and steady-state behaviours of rocks. This constitutive law can represent the concepts of damage and damage evolution in either the fibre-bundle model or continuum damage mechanics. We have previously proposed an empirically based constitutive law for both the transient and steady-state behaviours of rocks ultimately derived from laboratory experimental data. We show here that this law is concordant with the general constitutive law derived from irreversible thermodynamic theories, and that the relaxation modulus has a temporal power–law that depends on a structural fractal property of rocks. Our constitutive law predicts forms for the cumulative Benioff strain-release for precursory seismic activations and the modified Omori's laws of aftershocks, both aspects of the temporal fractal properties of seismicity. These seismic properties can also be derived by the fibre-bundle model or continuum damage mechanics. Our model suggests that these time-scale invariant processes of seismicity may be regulated by the fractal structures of crustal rocks.     

Stability of a circular tunnel in cohesive-frictional soil subjected to surcharge loading

The stability of circular tunnels in cohesive-frictional soils subjected to surcharge loading has been investigated theoretically and numerically assuming plane strain conditions. Despite the importance of this problem, previous research on the subject is very limited. At present, no generally accepted design or analysis method is available to evaluate the stability of tunnels/openings in cohesive-frictional soils. In this study, continuous loading is applied to the ground surface, and both smooth and rough interface conditions are modelled. For a series of tunnel diameter-to-depth ratios and material properties, rigorous lower- and upper-bound solutions for the ultimate surcharge loading are obtained by applying finite element limit analysis techniques. For practical use, the results are presented in the form of dimensionless stability charts with the actual tunnel stability numbers being closely bracketed from above and below. As an additional check on the solutions, upper-bound rigid-block mechanisms have been developed and the predicted collapse loads from these are compared with those from finite element limit analysis. Finally, an expression that approximates the ultimate surcharge load has been devised which is convenient for use by practising engineers.     

Terrestrial heat flow at Hirabayashi on Awaji Island, south-west Japan

Abstract Terrestrial heat flow at Hirabayashi in Awaji Island, south-west Japan, was investigated using the deep borehole penetrating through the Nojima Fault, which was activated during the 1995 Hyogo-ken Nanbu earthquake, by measuring the thermal conductivity of basement rocks. Using the temperature logging data, the value of terrestrial heat flow in Hirabayashi was found to be 56.6 ± 5.2 mW/m². The relationship between cut-off depth of aftershocks of the Hyogo-ken Nanbu earthquake in Hirabayashi and terrestrial heat flow are discussed. The cut-off depth roughly corresponds to isotherms of 300°C.     

Geochemical constraints on the depositional environment of the 1.84 Ga Embury Lake Formation,Flin Flon Belt,Canada

The Flin Flon Belt of Canada contains Paleoproterozoic volcanic–sedimentary sequences that are related to the Trans‐Hudson Orogeny. The sequences include island arc volcanic and volcaniclastic rocks (Amisk Group) that are unconformably overlain by subaerial sedimentary rocks (Missi Group), and younger deep facies sediments. In the Flin Flon area, several north–south trending faults divide the sequences into blocks and obscure the depositional environment of the deep facies sediments. Locally, within the Flin Flon area, the Embury Lake Formation is in fault contact with island arc volcanic–sedimentary sequences of the Amisk and Missi Groups. To identify the depositional environment of the Embury Lake Formation, we used lithologic and geochemical approaches. Here, we report carbon isotopic values in organic matter (δ¹³C_org) and sulfur isotopes (δ³⁴S), as well as total organic carbon and total sulfur measurements for the black shale in the formation. Samples were taken from a drill core that contains alternating bands of sandstone and black shale. Pyrite in the black shale is divided into four textural types: euhedral, vein‐type, elliptical, and microcrystalline. Microcrystalline pyrite is typically generated by microbially mediated sulfate reduction. An extremely low S/C ratio (avg. = 0.04) is consistent with lacustrine deposition. The ranges of δ¹³C_org (?36 ‰ to ?27 ‰) and δ³⁴S (+3.0 ‰ to +7.7 ‰) values can be explained by bacterial photosynthesis that involved Calvin cycle and acetyl CoA pathways, and sulfate reduction in a low‐sulfate environment. Considering the depositional age reported in a previous study of < 1.84 Ga, the Embury Lake Formation was likely emplaced in a lacustrine setting during the Trans‐Hudson Orogeny.     

Juxtaposition of adakite, boninite, high-TiO2 and low-TiO2 basalts in the Devonian southern Altay, Xinjiang, NW China

We present petrographic and geochemical data on representative samples of the Devonian adakite, boninite, low-TiO₂ and high-TiO₂ basalts and associated rocks in the southern Altay areas, Xinjiang, NW China. These volcanic rocks mostly occur as tectonic blocks within suture zones between the Siberian and Junggar plates. Adakite occurs in the Suoerkuduke area ca. 40 km south of Fuyun, and actually represents a poorly-sorted massive volcaniclastic deposit, mostly consisting of a suite of hornblende andesite to pyroxene andesite. The geochemical features of the adakite suggest its generation by melting of subducted oceanic crust. Boninite occurs in the Saerbulake area ca. 20 km southwest of Fuyun, as pillowed lava or pillowed breccia. It is associated with high-TiO₂ basalt/gabbro and low-TiO₂ basalt. The boninites are metamorphosed, but contain relict clinopyroxene with Mg# (=100*Mg/(Mg+Fe)) of 90–92, and Cr₂O₃ contents of 0.5–0.7 wt% and chromian spinel with Cr/(Cr+Al) ratio of 0.84. The bulk rock compositions of the boninites are characterized by low and U-shaped REE with variable La/Yb ratios. They are classified as high-Ca boninite. The Cr-rich cpx phenocryst and Chromian spinel suggests that the boninites were formed by melting of mildly refractory mantle peridotite fluxed by a slab-derived fluid component under normal mantle potential temperature conditions. Basaltic rocks occur as massive flows, pillowed lavas, tuff breccia, lapilli tuff and blocks in tectonic mélanges. Together with gabbros, the basaltic rocks are classified into high-TiO₂ (>1.7 wt%) and low-TiO₂ (<1.5 wt%) types. They show variable trace element compositions, from MORB-type through transitional back-arc basin basalt to arc tholeiite, or within plate alkalic basalt. A notable feature of the Devonian formations in the southern Altay is the juxtaposition of volcanic rocks of various origins even within a limited area; i.e. the adakite and the boninites are associated with high-TiO₂ and low-TiO₂ basalts and/or gabbros, respectively. This is most likely produced by complex accretion and tectonic processes during the convergence in the Devonian–Carboniferous paleo-Asian Ocean between the Siberian and Junggar plates.