Basalts ratios

Seawater/basalt ratio Chlorite high (up to 40) Calcite low (0.1-1.8) Chlorite-quartz-rich rock high (more than 50) Epidote-actinolite, albite- quartz, chlorite assemblage low (1.6)... 

Owing to the stability of the uranyl carbonate complex, uranium is universally present in seawater at an average concentration of ca. 3.2/rgL with a daughter/parent activity ratio U) of 1.14. " In particulate matter and bottom sediments that are roughly 1 x 10 " years old, the ratio should approach unity (secular equilibrium). The principal source of dissolved uranium to the ocean is from physicochemical weathering on the continents and subsequent transport by rivers. Potentially significant oceanic U sinks include anoxic basins, organic rich sediments, phosphorites and oceanic basalts, metalliferous sediments, carbonate sediments, and saltwater marshes. " ... 

Figure 1.34. Frequency histogram for MgO/FeO ratios (in wt%) of chlorite from the basalt studied (A) and MORE (B). Data sources are Shikazono and Kawahata (1987), Humphris and Thompson (1978) (M Mid-Atlantic Ridge) and Kawahata (1984) (C Costa Rica Rift, Galapagos Spreading Centre). The data on chlorite from MORE are taken from typical metabasalt and not from quartz-chlorite breccia and veins which formed in a hydrothermal upflow zone (Shikazono et al., 1987).

Sverjensky (1984) calculated the dependency of Eu +/Eu + in hydrothermal solution on /oj (oxygen fugacity), pH and temperature. According to his calculations and assuming temperature, pH and /oj for epidote-stage alteration of basalt and Kuroko ores (Shikazono, 1976), divalent Eu is considered to be dominant in the rocks and hydrothermal solution. Thus, it is reasonable to consider that Eu in the rocks was removed to hydrothermal solution under the relatively reduced condition more easily than the other REE which are all tiivalent state in hydrothermal solution. Thus, it is hkely that Eu is enriched in epidote-rich altered volcanic rocks. Probably Eu was taken up by the rocks from Eu-enriched hydrothermal solution which was generated by seawater-volcanic rock interaction at relatively low water/rock ratio. 

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

Honma et al. (1991) have shown that the Okinawa Trough basalts have significantly high K, Rb and Sr contents and D/H, 0/ 0 and Sr/ Sr ratios than N-Morb have and these are due to generation of magma from normal-type mantle peridotite modified by component from the subducted slab and crustal contamination. 

Fig. 2.34. (A) Chondrite-normalized REE pattern of fresh basaltic andesite. (B) Chondrite-normalized REE pattern of altered basaltic andesite. (C) Ratios of REE content in the altered rock normalized to the fresh basaltic andesite (298-R-02). The dashed line is the ratio line of one. (D) Chondrite-normalized REE patterns of hydrothermal fluids from Vienna Wood, Pacmanus and Desmos, Tamagawa and Kusatsu-Yubatake (Gena et al., 2001).

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... 

Fig. 2.43. Graphical illustration of sulfur isotope values of HiS (left axis and. solid line) produced during basalt-seawater interaction at various water/rock ratios. Calculations assume that seawater sulfate is mostly removed as anhydrite, that any residual sulfate is reduced by iron oxidation in reacting basalt, and that there is quantitative leaching of basaltic sulfide and homogeneous mixing of both sulfides. Dashed line...

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). 

Fig. 2.47. Model predicting mineral assemblages and proportions produced when basalt reacts with seawater in different water/rock mass ratios. The model is based on experimental data but is close to actual observed assemblages in recovered greenschist facies metabasalts (Mottl, 1983).

Watanabe et al. (1993) have determined a whole rock isochron age of 107 15 Ma (yi b = 1.42 X 10 Vy) for well-preserved pillowed basalts in Western Shikoku with their basalt initial ratio of 0.70401. 

Fig. 3.25 Left signal-to-noise ratio (SNR) of the Mbssbauer spectra of a basalt taken with MIMOS II (full SI-PIN detector system black data-points) and MIMOS IIA (1/4 of full SDD system red data-points) respectively. Right XRF spectra of low Z elements measured with MIMOS IIA (SDDs) at —20°C. The Compton scattered 14.4 keV line (at 13.8 keV) and the resonant 14.4 keV Mossbauer line are well separated...

Fig. 8.37 Left, spectrum of an accumulation of hematite rich spherules (Blueberries) on top of basaltic soil (Sol 223-228 of the mission 1 Sol = 1 Martian day). The spectrum is dominated by the hematite signal. Estimations based on area ratios (bluebeiries/soil) and APXS data indicate that the blueberries as composed mainly of hematite. Right MI picture (3x3 cm ) of hematitic spherules (blueberries) on basaltic soil at Meridiani Planum...

On a still shorter timescale, Sigmarsson (1996) used Pb- Ra disequilibria in Surtsey and Heimaey (Iceland) lavas to infer the differentiation time from the Surtsey alkali basalt erupted in 1963-1967 to the Heimaey hawaiites and mugearites (1973 eruption). The ( Pb/ Ra) ratios increase from the basalts to the evolved lavas, which can be explained by a closed system differentiation of about 10 years, a value similar to the time elapsed between the two eruptions. Thus, the Heimaey eruption is interpreted as a consequence of the injection in the crust of a small volume of basaltic magma, 10 years before eruption, a duration long enough to allow its differentiation towards hawaiite and mugearite compositions. 

Another recent study on historic, basaltic lava flows (1848 to 1977 AD) from Karthala volcano (Comores, Indian ocean) by Claude-Ivanad et al. (1998) reveals significant variations of the ( Ra)o/Ba ratios, correlated with Sr/ Sr ratios, which are explained by mantle source heterogeneities preserved by short transfer times of the magmas (a few centuries at most). 

Other variations in ratios observed in the alkali basalts of the 1718 AD... 

Variations in ( Ra/ Th) ratios are also apparent in the Ardoukoba (Asal Rift) 1978 basaltic eruption studied by Vigier et al. (1999). ( Ra/ jj ratios decrease with increasing Th content, as do the ( Ra)/Ba ratios. These variations cannot be entirely explained by plagioclase fractionation, which led the authors to propose a model of a laterally zoned fissural reservoir, with several injections of basaltic magmas. In this case, the oldest magma would have stayed in the reservoir for about 1.9 ka. However small Sr... 

Figure 23. Measured ( °Th/ Th) ratios in basalts from Piton de la Fournaise (Reunion Island) as a function of their eraption ages deduced from mineral isochrons. These ratios decrease with increasing emption ages as a result of post-eraptive radioactive decay. The curve shows the theoretical evolution by radioactive decay for a rock with a Th/U ratio of 3.95 and a ( °Th/ Th) ratio of 0.93, similar to the values measured in presently erapted lavas. An approximate age can thus be obtained from the measured ( °Th/ Th) ratio of an old sample. Part of the dispersion around the theoretical curve are due to small source heterogeneities (slightly variable ( °Th/ rh) and Th/U ratios), also evidenced by Sr/ Sr ratios (Condomines et al. 1988, and unpublished results).

Further constraints can be obtained by systematic studies of the evolution of U-series disequilibria through time in a given volcano. The number of works considering the evolution of ( °Th/ U) or ( °Th/ Th) ratios is very small, but it appears that, in the studied basaltic volcanoes from oceanic islands (Marion, Mauna Kea, Piton de la Fournaise), these ratios remain nearly constant with time. This suggests that the residence time of the magmas in these continuously replenished magma chambers is short... 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Volpe AM, Olivares JA, Murrell MT (1991) Determination of Radium isotope ratios and abundances in geologica samples by thermal ionization mass spectrometry. Anal Chem 63 916-919 Volpe AM, Goldstein SJ (1993) Ra- °Th disequilibrium in axial and off-axis mid-ocean ridge basalts. Geochim Cosmochim Acta 57 1233-1241... 

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