Chemical fluxes during hydrothermal alteration of a 1200-m long section ofdikes in the oceanic crust, DSDP/ODP Hole 504B

Chemical interactions between seawater and the oceanic crust have been wide ly investigated during recent years. However, most of these studies concern the uppermost volcanic part of the crust. The contribution of the underlyi ng sheeted dike complex to the global budget of the oceans is inferred sole ly from some ophiolite studies and from the 500-m high-level dike section o f DSDP/ODP 504B which was drilled in 1981, Hole 504B is the only place where a continuous and long (1260 m) section in the sheeted dike complex has been cored, and it is now regarded as a refer ence section for the upper oceanic crust. Many petrological and chemical da ta from these dolerites are available, including the relative proportions o f veins, extensively altered adjacent rocks, and less altered "host-rocks". For these three reasons, considering the entire dike section penetrated by Hole 504B is a unique chance to study chemical fluxes related to hydrother mal alteration of this part of the oceanic crust. The calculation of any chemical flux implies knowledge of the chemical comp osition of the fresh precursor (protolith). Previously, mean compositions o f glasses (= P1a) or basalts from the Hole 504B volcanics have been used as protoliths. In this paper, we calculate and discuss the use of various pro toliths based on dolerites from Hole 504B, We show that the most adequate a nd realistic protolith is the mean of individual protoliths that we calcula ted from the acquisition, by automatic mode, of about 1000 microprobe analy ses in each thin-section of dolerite from the Hole 504B lower dikes. Conseq uently, Prm is further used to calculate chemical fluxes in the dike sectio n of Hole 504B. The chemical compositions of the host-rocks adjacent to alteration halos te nd to converge to that of PFm with depth, except for Fe(2)O(3)t and TiO2. B ecause the volume percent of alteration halos increases with depth, the tot al fluxes related to these halos increase with depth. This explains why the mean flux (host-rocks + halos + veins) of the upper dikes is roughly simil ar to the mean flux of the lower dikes. During the alteration of the entire Hole 504B dike section, the dolerites gained relatively large quantities o f Fe(2)O(3)t (+4.0 g/ 100 cm(3)) and released much SiO2 (-6.8 g/100 cm(3)), CaO (-5.8 g/100 cm(3)), and TiO2 (1.6 g/100 cm(3)), and minor Al2O3 (-0.7 g/100 cm(3)) and MgO (-0.7 g/100 cm(3)). We show the importance of the choi ce of the protolith in the calculation of chemical budget, particularly for elements showing low flux values.