Midocean ridge basalt

Wolery, T.J. (1978) Some chemical aspects of hydrothermal processes at midoceanic ridges — A theoretical study I, Basalt-seawater reaction and chemical cycling between the oceanic crust and the oceans. II, Calculation of chemical equilibrium between aqueous solutions and minerals. Ph.D. Thesis, Northwestern U. 

A few REE data on hydrothermal solutions are available (Fig. 2.34). Chondrite normalized REE patterns of hydrothermal solutions from Vienna Wood, Pacmanus and Desmos, Manus Basin exhibit positive Eu anomaly and LREE enrichment are similar to midoceanic ridge solution and Kuroko ore fluids. This positive Eu anomaly (Fig. 2.35) may have been caused by the selective leaking of Eu due to the interaction of an ascending hydrothermal solution and footwall volcanic rocks (Gena et al., 2001). It is interesting to note that altered basaltic andesite has a negative Eu anomaly and this feature is the same as that found in the Kuroko mine area (Shikazono, 1999). 

Gena et al. (2001) reported advanced argillic alteration of basaltic andesite from the Desmos caldera, Manus back-arc basin which was caused by interaction of hot acid hydrothermal fluid originated from a mixing of magmatic gas and seawater. It is noteworthy that the acid alteration is found in back-arc basins (Manus, Kuroko area) but not in midoceanic ridges. 

Kawahata and Shikazono (1988) summarized S S of sulfides from midoceanic ridge deposits and hydrothermally altered rocks (Fig. 2.42). They calculated the variations in 5 " S of H2S and sulfur content of hydrothermally altered basalt as a function of water/rock ratio (in wt. ratio) due to seawater-basalt interaction at hydrothermal condition (Fig. 2.43) and showed that these variations can be explained by water/rock ratio. The geologic environments such as country and host rocks may affect S S variation of sulfides. For example, it is cited that a significant component of the sulfide sulfur could... 

Li values of Mariana hydrothermal solution (—8.5%o) are similar to average value (—9%c) of midoceanic ridge hydrothermal solution (Elderfield and Schultz, 1996). This value can be explained by the constant mixing ratio of basaltic Li (8 Li = —4%o) and seawater Li (S Li = —32.3%o) (Elderfield and Schultz, 1996). 

Chemical composition of Kuroko ore and MORB (midoceanic ridge basalt) (logarithmic unit in wt%) (Shikazono, 1988)... 

As already noted, intense bimodal volcanic activity occurred in the Kuroko mine area at middle Miocene age and dacitic and basaltic rocks suffered hydrothermal alteration. The midoceanic ridges basalt (MORE) is widespread and sometimes hydrothermally altered. Shikazono et al. (1995) compared hydrothermally altered basalt from the Kuroko mine area and MORE and clarified the differences in the characteristics of these basaltic rocks. 

Although several differences are observed in both basalts from Kuroko mine area and midoceanic ridges, the hydrothermal alteration mineral assemblages as a function of... 

Fig. 2.50. Plot of amphibole compositions on the Al203-Fe0 -Mg0 (AFM) diagram, together with the compositional range of amphibole in the hydrothermally metasomatized basalt from midocean ridges (Shinozuka et al., 1999).

Average bulk compositions of samples from seafloor sulfide deposits at sediment-starved midocean ridges in host basalts... 

It is shown in Fig. 2.57 that the lead isotopic variation of the Besshi-subtype is similar to that of midoceanic ridge basalt, suggesting the lead in the Besshi-subtype was derived from mantle. The data from the Shimokawa, and Yanahara deposits (Group B) are slightly more radiogenic than Group A, suggesting that crustal lead was involved in the formation of the Shimokawa deposit, and lead isotopic values for the Shimokawa and Yanahara plot between MORB and Cretaceous-Tertiary deposits in Japan (Kuroko, skarn, vein-type deposits). 

The studies on the hydrothermal systems at midoceanic ridges during the last three decades clearly revealed that the seawater-basalt interaction at elevated temperatmes (ca. 100-400°C) affects the present-day seawater chemistry (Wolery and Sleep, 1976 Edmond et al., 1979 Humphris and Thompson, 1978). For example, a large quantity of Mg in seawater is taken from seawater interacting with midoceanic ridge basalt, whereas Ca, K, Rb, Li, Ba and Si are leached from basalt and are removed to seawater (Edmond et al., 1979 Von Dammet al., 1985a,b). 

Bulk rock chemistry of hydrothermally altered midoceanic ridge basalt has been well studied and used to estimate the geochemical mass balances of oceans today (Wolery and Sleep, 1976 Humphris and Thompson, 1978 Mottl, 1983). In contrast, very few analytical data on hydrothermally altered volcanic rocks that recently erupted at back-arc basins are available. However, a large number of analytical data have been accumulated on the hydrothermally altered Miocene volcanic rocks from the Green tuff region in the Japanese Islands which are inferred to have erupted in a back-arc tectonic setting (section 1.5.3). 

The Mg content of hydrothermally altered volcanic rocks is reflected by the extent of seawater-volcanic rock interaction at elevated temperatures, because it has been experimentally and thermodynamically determined that nearly all of the Mg in seawater transfer to volcanic rocks, owing to the reaction of the cycled seawater with volcanic rocks at elevated temperatures (Bischoff and Dickson, 1975 Mottl and Holland, 1978 Wolery, 1979 Hajash and Chandler, 1981 Reed, 1983 Seyfried, 1987). It has been shown that the CaO content of hydrothermally altered midoceanic ridge basalt is inversely correlated with the MgO content with a slope of approximately — 1 on a molar basis (Mottl, 1983). This indicates that Ca of basalt is removed to seawater and Mg is taken up from seawater by the formation of chlorite and smectite during the seawater-basalt interaction. This type of reaction is simply written as ... 

Kaiho and Saito (1994) estimated 20 x 10 km /m.y. and 2x 10 km /m.y. for present-day midoceanic ridge crustal production rate and back-arc basin crustal production rate, respectively. If their estimates are correct. Mg removal to midoceanic ridge basalt during early-middle Miocene age is estimated to be 2.6 1 x 10 g/year. Although estimates of annual Mg removal by interaction of circulating seawater with midoceanic ridge basalt are uncertain, it seems likely that Mg removal by seawater-volcanic rock interaction at back-arc basins corresponds to that of Mg removal at midoceanic ridge axis. 

The main alteration minerals surrounding Kuroko ore body are K-mica, K-feldspar, kaolinite, albite, chlorite, quartz, gypsum, anhydrite, and carbonates (dolomite, calcite, magnesite-siderite solid solution), hematite, pyrite and magnetite. Epidote is rarely found in the altered basalt (Shikazono et al., 1995). It contains higher amounts of ferrous iron (Fe203 content) than that from midoceanic ridges (Shikazono, 1984). 

Wood, D.A., Jordon, J.L., Marsh, N.G., Tamey, J. and Greuil, M. (1980) Major and trace element variations drilled in basalts from the North Philippine Sea during Deep Sea Drilling Project Leg 58 A comparative study of back-arc basin basalts with lava series from Japan and midocean ridges. Deep Sea Drilling Project, Initial Reports, 58, 827—894. 

Similar results have been reported by Mattern et al. (2002), using more recent equations of state for lower-mantle minerals and incorporating the solubility of alumina in silicate perovskite. They also used a three-layered slab model (midocean ridge basalt (MORE) over harzburgite over pyrolite), but with a MORE composition (Si/(Mg - - Fe) = 2.29) intermediate between our extreme end-members of the Helffrich et al. (1989) eclogite (1.65) and the Helffrich and Stein (1993) gabbro (2.58). 

Schiano P., Birck J.-L., and Allegre C. J. (1997) Osmium-strontium-neodymium-lead isotopic covariations in midocean ridge basalt glasses and the heterogeneity of the upper mantle. Earth Planet. Sci. Lett. 150, 363-379. 

Salters V. J. M. and Dick H. J. B. (2002) Mineralogy of the midocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites. Nature 418, 68-72. 

Graham D. W. (2002) Noble gas isotope geochemistry of midocean ridge and ocean island basalts characterization of mantle source reservoirs. Rev. Mineral. Geochem. 47, 247-317. 

Kinzler R. J. and Grove T. L. (1992a) Primary magmas of midocean ridge basalts 1. Experiments and methods. J. Geophys. Res. 97(B5), 6885-6906. 

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