Rectilinear partition coefficient function of REE pertaining to the formation of basaltic rocks from the New Georgia Group, the Solomon Islands.
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Abstract:
Nine basaltic rocks from the New Georgia Group, the Solomon Islands, have been analyzed for REE, Ba and Sr abundances, Sr and Nd isotopic ratios, and major element compositions. Nd and Sr isotopic ratios of four olivine basalts and two picrite basalts fall in restricted ranges, suggesting that these rocks have a common source. REE patterns of six of these basaltic rocks have the following features: (1) The patterns have no intersection with each other, (2) All of the patterns possess a common fine structure of convex curve around Nd-Gd-Dy, and (3) Inclination of light REE span increases with increase of REE concentration. REE pattern analysis of the basalts leads to inferences that (A) a linear REE partition coefficient function with a sharp break at Ho has operated in the generation of the olivine basalts by the Rayleigh fractional crystallization of a picrite basalt magma, and (B) the absolute magnitudes of the partition coefficients did not change substantially during the magmatic differentiation.Keywords:
Fractional crystallization (geology)
Peridotite
Incompatible element
Peridotite
Trace element
Incompatible element
Anhydrous
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Abstract The proton microprobe has been used to determine contents of Ca, Ti, Ni, Mn and Zn in the olivine of 54 spinel lherzolite xenoliths from Australian and Chinese basalts. These data are compared with proton-probe data for Ni, Mn and Zn in the olivine of 180 garnet peridotite xenoliths from African and Siberian kimberlites. Fe, Mn, Ni and Zn contents are well-correlated; because the spinel lherzolite olivines have higher mean Fe contents than garnet peridotite olivines (average Fo 89.6 vs. Fo 90–92 ) they also have lower Ni and higher Mn contents. Zn and Fe are well-correlated in garnet peridotite olivine, but in spinel peridotites this relationship is perturbed by partitioning of Zn into spinel. None of these elements shows significant correlation with temperature. Consistent differences in trace-element contents of olivines in the two suites is interpreted as reflecting the greater degree of depletion of Archean garnet peridotites as compared to Phanerozoic spinel lherzolites. Ca and Ti contents of spinel-peridotite olivine are well correlated with one another, and with temperature as determined by several types of geothermometer. However, Ca contents are poorly correlated with pressure as determined by the Ca-in-olivine barometer of Köhler and Brey (1990). This reflects the strong T -dependence of this barometer: the uncertainty in pressure (calculated by this method) which is produced by the ±50°C uncertainty expected of any geothermometer is ca ± 8 kbar, corresponding to the entire width of the spinel-lherzolite field at 900–1200°C.
Peridotite
Xenolith
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Peridotite
Metasomatism
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Peridotite
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Olivine is one of the earliest crystallization minerals from basaltic magma, and its composition can directly give the metallogenesis information on parent magma composition, processes of magmatic fractional crystallization, and sulfide segregation degree. In this study, the authors selected the Tudun mafic-ultramafic complex to conduct electron microprobe analysis of olivine minerals. According to the Fo and Ni values of olivine, he Mg O content of parent magma, which contains the highest Fo olivine,was estimated to be 12.95%, indicating that the parent magma of the Tudun complex is a high-Mg O basalt. The modeling results show that sulfide segregation and fractional crystallization almost took place at the same time. A slight sulfide segregation(about0.2%) occurred during the early stage(0- 2%) of olivine fractional crystallization process, and less sulfide segregation(about0.01%) occurred during the later stage(6%-7%), which suggests that the metallogenic potential of the Tudun complex is limited. In addition, some data implying the negative relationship between Fo and Ni indicate that Fe-Ni exchange reaction happened between Fe-rich olivine and trapped sulfide liquid, which caused remarkable differences in olivine composition.
Fractional crystallization (geology)
Magma chamber
Ultramafic rock
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Peridotite
Pyroxene
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Forsterite
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