22. MELT, CRYSTAL, AND FLUID INCLUSIONS IN OLIVINE AND CLINOPYROXENE PHENOCRYSTS FROM THE SUBMARINE SHIELD STAGE HYALOCLASTITES OF GRAN CANARIA, SITES 953 AND 956 1Andrey A. Gurenko,2 Thor H. Hansteen,2 and Hans-Ulrich Schmincke2 |
ABSTRACTMiocene submarine basaltic hyaloclastites, lapillistones, and breccias drilled at Sites 953 and 956 contain relicts of olivine and fresh clinopyroxene phenocrysts with abundant primary melt, fluid, and crystal inclusions. Primary melt inclusions are represented by glassy (quenched glass ± gas bubble/s) and multiphase (glass + daughter crystals ± gas bubble/s) types. Fluid inclusions are composed of gas and liquid phases and are nearly pure CO2 in composition, as shown by low-temperature microthermometric studies. Melt and crystal inclusions and their host minerals were analyzed for major elements by electron microprobe and large (>60 µm in size) representative melt inclusions by ion microprobe for trace elements and H2O. Olivine phenocrysts from two basalt fragments are Fo80-89 and correspond to the entire range of olivine compositions known for the Miocene shield basalts on Gran Canaria. Clinopyroxene phenocrysts from basalt fragments and hyaloclastite matrix are characterized by a wide compositional spectrum of Mg/(Mg+Fetot ) = 0.74-0.90, Wo37-47 , En41-52 , Fs6-15 . Crystal inclusions are represented by olivine (Fo80-82 ), clinopyroxene (Mg/(Mg+Fetot ) = 0.79-0.82, Wo41-45 , En44-48 , Fs11 ), plagioclase (An68-83 ), high-Ti chrome spinel and titanomagnetite (2.5-18.7 wt% TiO2 , Mg/(Mg+Fe2+ ) = 0.21-0.58, and Cr/(Cr+Al) = 0.17-0.74), and ilmenite. Major element compositions of melt inclusions corrected for post-entrapment crystallization of olivine and clinopy-roxene show a broad compositional spectrum ranging from quartz-normative tholeiitic to transitional basalts (46.3-54.4 wt% SiO2 , 5.1-10.7 wt% MgO, 1.3-3.5 wt% Na2O, and 0.7-1.9 wt% K2O) and are enriched in incompatible trace and rare earth elements. We interpret this large compositional range as representing that of parental magmas, rather than being caused by crystallization of a single magma. Melt inclusions are enriched by light rare earth elements [(La/Sm)n = 1.7-3.0] and depleted in heavy rare earth [(Sm/Yb)n = 4.4-10.2] and high field strength elements [(Zr/Y)n = 4.1-5.5]. Because shallow level magma crystallization is unlikely to significantly change trace element ratios in the melt, we think that the observed discrepancy in (La/ Sm)n and (Sm/Yb)n ratios, and high (Zr/Y)n ratios resulted from the melting of a garnet-bearing mantle source. Calculated parental magmas equilibrated with Fo 90 represent a range from transitional to tholeiitic compositions (46.7-52.0 wt% SiO2 ) and are similar to olivine basalt–picrite (11.2-18.0 wt% MgO). They crystallized over the range of temperatures from 1450° to 1120°C and pressures from <0.5 to 8 kbar. Oxygen fugacity varied from the conditions corresponding to FMQ-1 or WM-1 buffers during the early crystallization stage of parental magmas, to late-stage conditions of FMQ - NNO+1. Crystallization of magmas occurred in the presence of fluid of essentially CO2 composition. |
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