Neil R. Banerjee

The 500 metres of gabbro drilled in ODP Hole 735B provide a unique opportunity to study the chemical effects of hydrothermal alteration resulting from seawater penetration into the lower ocean crust. Previous studies identified five major styles of alteration / metamorphism ranging from anhydrous granulite facies assemblages accompanying brittle-ductile deformation to lower greenschist and zeolite facies assemblages resulting from circulation of cold oxygenated seawater. Many veins contain sequential mineral assemblages indicating progressive alteration.

This study focuses on the geochemical effects of alteration in and around inclined veins on a variety of scales: within minerals; between minerals; in thin sections; and in altered whole-rock samples a few centimetres from veins. Microprobe traverses from primary clinopyroxene to secondary amphibole are characterized by abrupt decreases in CaO and less abrupt, variable changes in other oxides. A traverse from primary plagioclase to secondary sodic plagioclase is characterized by an abrupt increase in SiO₂ and Na₂O, and an abrupt decrease in Al₂O₃ and CaO. A similar traverse from primary plagioclase to secondary epidote is characterized by an abrupt decrease in SiO₂ and Na₂O, and an abrupt increase in FeO and CaO. Primary minerals, secondary plagioclase, and secondary epidote have relatively constant compositions but secondary amphibole is more variable. Overall, the mineral traverses suggest that CaO and MgO were lost, FeO and Na₂O were gained, Al₂O₃ remained relatively constant, and SiO₂ was variable during alteration.

Element mobility was determined by comparison of major and trace element bulk-rock compositions of altered host rock adjacent to veins and fresh equivalents. Element fluxes based on constant aluminum indicate element mobility within a few centimetres of each vein. Calculated volume factors are used to explain different element-flux trends in the same lithology and suggests that alteration probably occurred under conditions of constant volume. Modal mineralogy is used to explain element-flux variations between lithologies. Element-flux trends, particularly observed discrepancies in CaO and MgO fluxes reflect vein mineral assemblages. Qualitative comparison of element fluxes from Hole 735B samples with similar studies of oceanic Layer 2, indicate that for Si, Na, and Rb, trends appear to be similar; however, Mg trends are reversed.

Gridded electron microprobe traverses provide chemical data for entire thin sections. Equivalent fresh mineral compositions are subtracted to produce residual chemical maps of alteration. These show chemical gradients near veins resulting from chemical exchange between the vein and host rock which are interpreted as alteration halos. Gradients are only observed in certain oxide residual maps, namely SiO₂, CaO, and Na₂O, suggesting that alteration affected different elements by different amounts. In general, alteration halos are restricted to areas within a few millimetres of veins suggesting limited chemical modification of the host rock. Residual maps show very similar trends to microprobe traverses for alteration of both clinopyroxene and plagioclase. In addition, estimates of chemical changes in the host rock from residual maps are almost the same in direction and magnitude as calculated element fluxes for the same samples. However, residual maps also provide spatial information on alteration trends which whole-rock analyses do not. Residual chemical maps, therefore, provide a new method to quantify the effects of alteration in samples at the thin section scale.

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Pages: 233
Supervisor: Paul Robinson