Midoceanic ridge basalt

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. 

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. 

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. 

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. 

There are various terrestrial reservoirs that have distinct volatile characteristics. Data from midocean ridge basalts (MORBs) characterize the underlying convecting upper mantle, and are described here without any assumptions about the depth of this reservoir. Other mantle reservoirs are sampled by ocean island basalts (OIBs) and may represent a significant fraction of the mantle (Chapter 2.06). Note that significant krypton isotopic variations due to radiogenic additions are neither expected nor observed, and there are no isotopic fractionation observed between any terrestrial noble gas reservoirs. Therefore, no constraints on mantle degassing can be obtained from krypton, and so krypton is not discussed further. Comparison between terrestrial and solar system krypton is discussed in Chapter 4.12. 

A number of estimates have been made of the trace element content of the Earth s mantle. These are based on trace element concentrations in midocean ridge basalts, OIBs (see Fig. 3.5a), arc basalts, and continental basalts, and these are explored more fully later in this chapter. 

The trace element composition of the subcontinental lithosphere In contrast to the depleted major-element character of Archaean subcontinental lithosphere it is often enriched in trace elements, relative to a midocean ridge basalt mantle source (Richardson et al., 1985 Jordan, 1988). A resolution of this apparent paradox can be found in the timing of the two events. Major element depletion is thought to have taken place during the early formation of the subcontinental lithosphere whereas the trace element enrichment reflects later melt infiltration. 

One of the most striking features of midocean ridge basalts (MORB) is that they are chemi-... 

Although this idea has now been largely superceded by the polybaric melting model a new possibility has recently emerged whereby the uppermost mantle may behave as a chemical filter. Midocean ridge basalt (MORB) is silica saturated and is in equilibrium with orthopyroxene at pressures greater than about 8 kb. At the lower pressures commensurate with the base of the oceanic crust MORB melts are undersaturated in silica and have the capacity to dissolve orthopyroxene (Braun Kelemen, 2002). There is field evidence from... 

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