Petrology of an alkali-olivine basalt sill from site 169 of DSDP Leg 17: The Central Pacific Basin

Abstract

An 8-m-thick alkali-olivine basalt sill intrudes sediments that overlie oceanic tholeiitic flows. The average modal mineralogy is plagioclase (46.9% fresh, 5.1% zeolite and smectite replaced), clinopyroxene (23.8%), olivine (4.8% fresh, 6.0% smectite replaced), interstitial smectite (7.1%), titanomagnetite (3.4%), interstitial zeolite (2.4%), amphibole (0.3%), apatite (0.1%), chromian spinel (<0.1%), and sulfides (<0.1%). The cooling of the sill was slow enough to permit minor olivine settling but rapid enough to produce extreme compositional zoning in plagioclase and ferromagnesian minerals. Late-stage deuteric zeolitization was locally extensive. Development of an ophitic texture is related mainly to the relative position of clinopyroxene in the paragenetic sequence. Variation in Fe + Mg in plagioclase is sensitive to clinopyroxene crystallization. Clinopyroxenes are calcic aluminous augites showing an overall trend of Fe enrichment, with no Ca depletion, and a decrease in components other than Wo-En-Fs. Amphiboles (arfvedsonites, Ti arfvedsonites, ferrorichterites, ferroactinolites, and grunerites) crystallized as late-stage deuteric phases and are not the result of postcrystallization alteration. The paragenetic sequence is chromian spinel + olivine (Fo80-Fo65(?)) + plagioclase (An80-An65) → olivine (Fo65(?)-Fo15) + plagioclase (<An65) + clinopyroxene (Wo46En39Fs15-Wo45En9Fs46) + titanomagnetite → plagioclase (<An5) + amphibole + apatite + smectite + zeolite + calcite ± titanomagnetite ± sulfides.