The secondary mineralogy of altered diabase in the lower sheeted dikes cored during Leg 148 comprises an earlier high-temperature assemblage of amphibole, diopside, anorthitic plagioclase, chlorite, titanite, and apatite, overprinted by a subsequent alteration at lower temperatures that led to actinolite, albite, chlorite or chlorite-smectite, talc, epidote, and hydrous calc-silicate (now laumontite). Secondary minerals occur as a pervasive replacement of original igneous minerals; however, they are modally most abundant within irregular patches, centimeter-scale halos of hydrothermal veins, and millimeter-scale veins. The lower sheeted dikes of Leg 148 may have been altered initially within the "reaction zone" of an axial hydrothermal system.
Secondary plagioclase compositions range from anorthite to albite: all are K2O poor (less that or equal to 0.08 wt%). Secondary calcic plagioclase occurs in association with hydrothermal hornblende as the dominant, high-temperature, early alteration assemblage in the halos of many amphibole veins. In practically every instance, the secondary plagioclase is more calcic than the primary igneous plagioclase. The secondary plagioclase is also clouded with solid and fluid inclusions, contains depressed concentrations of MgO and FeO, and exhibits bright cathodoluminescence. Hydrothermal amphibole ranging in composition from magnesio-hornblende to actinolite occurs as a major vein-forming mineral, as well as a replacement of clinopyroxene in vein halos and in matrix diabase. Secondary clinopyroxene is diopsidic augite, and it forms rare syntaxial rims on igneous augite along the walls of some amphibole veins. The compositions of secondary plagioclase, amphibole, pyroxene, chlorite, chlorite-smectite, talc, and epidote are similar to those reported for the slightly shallower sheeted dike section sampled during Leg 140 at Hole 504B.
Geothermometric evidence suggests that the earliest alteration assemblage of hornblende plus calcic plagioclase represents high temperatures (greater than or equal to 400ŚC) characteristic of the mid-ocean-ridge reaction zone. Later generations of actinolite + chlorite + albite and epidote + quartz most likely formed at lower temperatures during the waning of axial hydrothermal activity. The presence of calcic plagioclase in the high-temperature alteration assemblage challenges the widely held notion, based upon oft-sampled altered basalts, diabases, and spilites, that the dominant plagioclase of the mid-ocean-ridge reaction zone is albite. Evidence from Hole 504B, as well as recent experimental studies, suggests that albite is a later plagioclase in the reaction zone, which formed during waning hydrothermal conditions.
Date of initial receipt: 17 August 1994
Date of acceptance: 29 January 1995
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