CHEMICAL COMPOSITIONS OF CLINOPYROXENE, ORTHOPYROXENE, AND AMPHIBOLE FROM THE HIGH-TEMPERATURE MICROSCOPIC VEINS AND THE RELATED TEXTURES

In this section, we compare the chemical compositions of mafic silicates (clinopyroxene, orthopyroxene, and amphibole) from the high-temperature microscopic veins and the related textures. Representative microprobe analyses are given in Tables T4, T5, T6, T7, and T8.

Chemical compositions of clinopyroxene are plotted in Figures F8 and F9. Clinopyroxenes with intergrowths both close to the high-temperature microscopic veins and in the interfingering textures are higher in Wo content (Ca/[Ca + Mg + Fe]) than magmatic clinopyroxenes. This suggests that the former clinopyroxenes, hereafter referred to collectively as secondary clinopyroxenes, were formed under lower-temperature conditions than the latter clinopyroxenes (Fig. F8). The difference in Wo content between magmatic clinopyroxene and secondary clinopyroxene is rather gradational, suggesting that the conditions are also gradational. This is also indicated by the calculated temperatures discussed in a later section. The secondary clinopyroxenes near the high-temperature microscopic veins are also clearly lower in Al₂O₃ content than magmatic clinopyroxenes, although the difference is rather gradational. The secondary clinopyroxenes with interfingering textures plot between the magmatic clinopyroxenes and the clinopyroxenes near the high-temperature microscopic veins. TiO₂ contents show a similar tendency to that of Al₂O₃ contents. TiO₂ contents of the interfingering clinopyroxenes are nearly the same or slightly lower than those of the magmatic clinopyroxenes. Clinopyroxenes hosting the intergrowths of brown amphibole + orthopyroxene near the high-temperature microscopic veins are clearly lower in TiO₂ than those of magmatic clinopyroxenes. Magmatic clinopyroxene has exsolution lamellae of Fe-Ti oxide, and therefore the primary TiO₂ content would have been even higher. On the other hand, as mentioned earlier, the secondary clinopyroxenes lack these lamellae and look clear under the microscope, indicating that their lower Ti contents are not due to the exsolution of Fe-Ti oxide. These features of the secondary clinopyroxenes, lower in Ti and Al contents than magmatic clinopyroxenes (Fig. F9C), may be related to the formation of brown, Ti-rich pargasitic amphibole blebs. These chemical and textural features are also common in secondary clinopyroxenes from other oceanic gabbros (e.g., Mével, 1987; Manning and MacLeod, 1996). So the decreases in Al and Ti of secondary clinopyroxene are a very common phenomena in the oceanic crust. Al- and Ti-poor secondary clinopyroxenes are also reported from the Skaergaard layered intrusion (Manning and Bird, 1986) and are interpreted as reflecting exchange with hydrothermal fluids (Manning and MacLeod, 1996).

Orthopyroxene is a subordinate mineral in the olivine gabbros, occurring as magmatic rims on cumulus grains of olivine and as blebs in intergrowths with brown amphibole in clinopyroxene grains both near the high-temperature microscopic veins and within the interfingering textures. Based on textures, orthopyroxene can be divided into four categories: magmatic rims, blebs in the interfingering clinopyroxene, blebs near the high-temperature microscopic veins, and filling high-temperature microscopic veins penetrating olivine grains. There is no detectable difference in composition among these orthopyroxenes (Fig. F8), except for the orthopyroxene filling high-temperature microscopic veins in olivine grains, which are characterized by lower contents of CaO, TiO₂, and Al₂O₃ than those of other orthopyroxenes (Fig. F10). It is likely that these compositional features are attributed to the Ca-, Ti-, and Al-poor nature of olivine, which is a precursor of the orthopyroxene.

Microprobe analyses of amphibole are shown in Figure F11. The procedures for the calculation of amphibole formula used are given in the "Appendix." Terminology of amphibole is after Leake et al. (1997). In Figure F11, we classify amphiboles into five categories (i.e., those of magmatic rims, blebs in the interfingering clinopyroxene, blebs near the high-temperature microscopic veins, amphiboles from the high-temperature microscopic veins within olivine grains, and amphiboles within clinopyroxene grains). It is noted that, of course, these categories do not include all varieties of amphibole from Hole 735B cores, such as the later amphiboles mentioned above.

In a (Na + K)_A-site vs. Al^IV diagram, most of the analyses are plotted on the join between pargasite and tremolite (Fig. F11A). Thus, amphiboles rich in (Na + K)_A-site (0.5) are rich in Al^IV (1.0) and vice versa. In a plot of Mg# vs. Al^IV (Fig. F11B), amphiboles from the high-temperature microscopic veins in clinopyroxene grains are slightly lower in Mg# at a given Al^IV than other amphiboles. Magmatic amphiboles are pargasite to edenite. Amphibole blebs in clinopyroxene from the interfingering areas are plotted in the field of pargasite, and those near the high-temperature microscopic veins are in the fields of pargasite, edenite, and magnesiohornblende. Amphiboles within the high-temperature microscopic veins are plotted in a wider area of pargasite, edenite, magnesiohornblende, and actinolite. These wider compositional ranges may be partly attributed to overprinting and modification at lower-temperature conditions.