A suite (V25-RD6) of gabbros showing a wide range of composition from the Mid-Atlantic Ridge is described in this paper. The gabbros may be classified into three groups representing stages of progressive fractionation: troctolite and olivine-pyroxene gabbros of the early stage of fractionation, hornblende-augite gabbros of the middle stage, and Fe-Ti oxide-rich hornblende-augite gabbros of the late stage. The major and trace element compositions and the mineralogy of gabbros in the three groups are consistent with the view that the gabbros are cumulates (adcumulates and mesocumulates) derived from abyssal tholeiite magma at various stages during advancing fractionation. Early-stage gabbros probably were formed by accumulation of crystals from abyssal tholeiite magmas that were in a normal range of composition, whereas middle- and late-stage gabbros were accumulated from magmas that were formed by extraordinarily advanced fractionation, presumably in a very large intrusive mass.
Rare earth element (REE) contents of alkaline rocks from Tahiti have been determined. Their chondrite normalized patterns are characterized by relative enrichment of the light REE, as is common in other alkaline rocks. Additionally, two distinct fine structures are seen, especially in basic rocks: An upward-concave shape of the heavy REE span and an inflection in the light REE span. The author has attempted to explain these fine structures and to derive information on the petrogenetic processes based on calculated models. It is concluded that the upward-concave shape is generated under high pressure conditions corresponding to the deeper part of the uppermost mantle. Furthermore, considerably large amounts of clinopyroxene fractionation are needed to explain both the inflectional feature of the light REE span and the upward-concave shape of the heavy REE span. The most appropriate model for the parental magma for Tahitian rocks is 6 to 10% melting of a garnet peridotite mantle followed later by 60 to 70% of clinopyroxene fractionation at high pressure.
The behaviors of copper (II) ions and zinc ions in calcium bicarbonate parent solution at the formation of calcite and the influence of fluoride ions on them have been studied through the measurement of the distribution coefficients of copper and zinc between calcium bicarbonate parent solution and calcite precipitated in the solution. The distribution coefficients of copper and zinc were measured to be abnormal at the early stage of calcite formation but constant at the late stage. Their abnormal behaviors at the early stage of calcite formation are shown and discussed in this paper.
As a result of the lack of data on other than the most common sulfides (pyrite, pyrrhotite, chalcopyrite, sphalerite, galena), the applicability of the sulfur isotope thermometer is restricted. The isotope fractionation among different metal-sulfides and sulfur has therefore been experimentally investigated in this study. The experiments have been carried out in dry systems by reacting pure metals with an excess of sulfur in evacuated silica glass tubes at temperatures from 280 to 700°C with reaction times from 0.3 to 3, 000 hours. The following fractionation factors have been determined for the sulfides synthesized in the three systems: 1000 lnαPbS-S = -4.6 (105T-2) 1000 lnαAg2S-S = -4.6 (105T-2) 1000 lnαCuS-S = 2.0 (105T-3) 1000 lnαCu9S5-S = 1.0 (105T-2)
Crystal/glass partition coefficients of rare-earth elements (REE), Ba and Sr were precisely determined for the systems prepared by partial melting and partial solidification of high-alumina basalt at 30kb and alkali olivine basalt at 20 and 30kb. At 30kb partial melting produced garnet and clinopyroxene as the solid phase, while partial solidification at the same pressures produced only clinopyroxene as an essential mineral. The difference in mineral composition of the solid phase appears to be due to the difficulty of nucleation of garnet from the melts. Because of the higher tendency of garnet to enrich heavy REE compared with clinopyroxene, the bulk solid phase produced by partial melting shows much higher enrichment of heavy REE compared with that produced by partial solidification. At 20kb both partial melting and partial solidification produced only clinopyroxene as the essential mineral. However, partition patterns for the systems produced by partial melting and those by partial solidification show small but systematic differences. REE partition patterns are terrace-shaped for clinopyroxenes, which are formed by partial solidification at 30kb and by partial melting at 20kb, while REE partition pattern is modified peak-shaped for clinopyroxene formed by partial solidification at 20kb. But, rectilinear character with atomic number is commonly observed for these two REE partition patterns for clinopyroxenes. In partial melting at 30kb which produced garnet and clinopyroxene as the solid phase, on the other hand, REE partition pattern for the total solid is smoothly curved with atomic number and schanze-shaped. The results of the present work suggest that liquid magmas produced by partial melting and partial solidification at the same temperatures and pressures from the initial material with the same chemical composition could show widely different trace element abundances. It should also be emphasized that such kinetic effect on both mineral assemblage in the solid phase and on the (bulk solid)/(liquid) partition coefficients must be taken into account in discussing the origin and evolution of magmas on the basis of trace element partitioning. Further, it should be added that, in partial solidification, formation of clinopyroxene essentially free from other minerals prevails in wide fields of chemical composition of liquid and physical conditions of pressure and temperature. It is emphasized in this study that rectilinear natures of REE partitioning for this clinopyroxene are very favorable in interpreting REE patterns of many igneous rocks.