Alteration of oceanic crust at the Tonga-Kermadec subduction front and its controlling factors

Altered oceanic crust is the significant source of the materials subducting into the mantle and cause of volcanisms.The variations in mineral and geochemical compositions that caused by the alterations are crucial for understanding the chemical cycles of the solid earth.IODP Site U1365, located in the subduction slab in front of the Tonga-Kermadec subduction zone, is selected as an ideal place for study of the basalts to reveal the subduction process of the Tonga-Kermadec zone.We analyzed the major elements of alterated products of 9 basalts with electronic probe.Combined with bulk rock geochemical data, we investigated the influencing factors of low temperature alteration, and discussed the control of alteration products on bulk rock geochemical variations.The major products of alteration are recognized as saponite, celadonite, beidellite, zeolite, chlorite, palagonite, phyrite, calcite, and Fe-oxide/hydroxide, which represent a typical type of low-temperature alteration.From the lava boundary and/or vein edge to the center of the rock, aprogressive sequence of dominant secondary mineral assemblage has been identified, which changes from Fe-oxide/hydroxide to celadonite, to saponite+phyrite, indicating a transition from oxidizing condition to reducing condition.The types of alteration include mineral replacement, vent filling, and vein filling.Based on the mineral compositions and intercalating relationships, the alteration processes could be divided into four stages, characterized respectively by palagonitization of basaltic glass, formation of oxidizing minerals (celadonite and Fe-oxide/hydroxide), formation of reducing minerals (saponite and phyrite), and the closing of fractures by calcite veins.The chemical changes of oceanic basalts during the alteration are mainly marked by rising in K₂O and Fe₂O₃, and losing of FeO, CaO and Na₂O.The alteration becomes intensified when it closes to the lava boundary, as the minimum water-rock ratio rising up.As the result, it will lead to stronger chemical variations.Electronic Microprobe data analyses show that, K₂O is mainly reserved in celadonite and zeonite; Fe₂O₃in Fe-oxide/hydroxide, palagonite, and celadonite; and CaO in calcite vains but significantly low in other alteration products.This result can explain the chemical variations of the basalts of Site U1365, for instance, the samples, which suffered stronger sea water oxidation, may have more oxydic alteration products (such as Fe-oxide/hydroxide, palagonite, and celadonite) enriched in K₂O and Fe₂O₃, indicating the control of alteration types on oceanic basalt compositions.