Phase Relations of Basalts in their Melting Ranges at PH2O = 5 kb. Part II. Melt Compositions   总被引：1，自引：0，他引：1  

HELZ  ROSALIND TUTHILL 《Journal of Petrology》1976,17(2):139-193

This paper describes the melting relations of three basalts,a Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite,and the 1801 Hualalai alkali basalt, at 5 kb water pressure,680–1045 °C, at the oxygen fugacities of the quartz-fayalite-magnetite(QFM) and hematite-magnetite (HM) buffer. All melts producedwithin the hornblende stability field are strongly quartzo-feldspathic.All are quartz-normative, including those from the alkali basalt,and all but two of the melts are corundum-normative. Melt compositionshows very little dependence on oxygen fugacity within the hornblendestability field, as MgO and FeO contents are very low. Whenhornblende begins to melt extensively (1000°–1045°C), the TiO₂, FeO, and MgO contents of the melt increaseabruptly. In this range, melts formed on the HM buffer havemuch higher Mg/Fe ratios and lower TiO₂ than melts formed onthe QFM buffer. Melt composition is also quite insensitive to changes in basaltcomposition, within the hornblende stability field. The chiefexception is the Na/Ca ratio, which varies directly with Na/Cain the starting basalt. When projected into the Ab-An-Or-Qzquaternary system, all melts produced follow a rather narrowspiral path through the tetrahedron; they descend from the Ancorner, moving toward Qz at constant Ab/Or, moving toward Oronly when plagioclase± quartz begin to precipitate. The melting behavior of hornblende, plagioclase, and augitein these experiments has been examined closely, with the followingresults: successive partial melts may differ from each otherby compositional increments which are very different in compositionfrom the minerals contributing to the melt in the temperatureinterval under consideration. These increments can almost neverbe expressed solely in terms of members of the one or two mineralsolid solutions from which they are actually derived. In a fewcases the increments cannot be expressed in terms of any reasonablecombination of minerals. This pattern contrasts markedly withthat observed in fractional crystallization, in which the differencebetween successive melts must always correspond to present orpossible phenocryst minerals. The contrast implies that magmaseries generated by any kind of melting process, equilibriumor fractional, should be recognizably different from seriesgenerated by fractional crystallization, if minerals like hornblendeor pyroxene are involved.  相似文献   

Phase Relations of Basalts in their Melting Range at PH2O = 5 kb as a Function of Oxygen Fugacity: Part I. Mafic Phases   总被引：1，自引：0，他引：1  

HELZ  ROSALIND TUTHILL 《Journal of Petrology》1973,14(2):249-302

The phase relations of three basalts, the Picture Gorge tholeiite,the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkalibasalt, were studied at 5 kb water pressure, 680–1000°C,at the oxygen fugacities of the quartz-fayalite-magnetite (QFM)and hematite-magnetite (HM) buffers. In the range 680–850 °C, the crystalline assemblageon the QFM buffer is dominantly hornblende+ plagioclase, ±ilmenite, magnetite, sphene, fayalitic olivine, and phlogopiticmica. From 875 to 1000 °C the crystalline assemblage ishornblende+ olivine± augite+ ilmenite± magnetite.A melt phase is present from 700 to 1000 °C; a vapor phasewas present in all charges. The hornblendes formed on the QFM buffer range in compositionfrom common green hornblendes at low temperatures to kaersutitichornblendes at 1000 °C. A1(IV) and Ti increase temperature.AI(VI) passes through a maximum near 825 °C, decreasingboth above and below this temperature. AI(IV) is proportionalto the sum A1(VI)+2Ti. There is a positive linear correlationof approximately 3 : 1 between AI(IV) and the number of cationsin the A-site. The most likely explanation for this correlationat present is that the substitution of AI(VI) or Ti⁺⁴for a divalentcation creates local charge imbalances in the amphibole structurewhich can be compensated only by further A-site substitution.There also appears to be a correlation between the a-cell dimensionof hornblende and the A-site occupancy. Above a thresh holdvalue of approxmately 0.5 cations in A, a increases as A-siteoccupancy increases. Phase relations on the hematite-magnetite buffer are considerablysimpler. The hornblendes show relatively little change in compositionas temperature increases, and in the tholelitic compositionsbreak down at or below 970 °C 35–60 °C above thefirst appearance of augite±olivine. The melting of hornblendeis incongruent in all cases. The Fe-Ti oxides are pseudo-brookiteand titanohematite; at 1000 °C these oxides make up 10 percent by weight of the assemblage and contain most of the Tio₂and FeO in the charge. The patterns of hornblende variation observed in this studycompare closely with those reported in a wide range of experimentaland field data. The appearance of high-TiO₂ kaersutitic hornblendesin the tholeities at 1000° C, P_H2O= 5 kb on the QFM bufferimplies that the restricted occurence of kaersutite in nature(where it is associated only with mafic to intermediate alkalicrocks) is controlled by volatile content (H₂O_,F₂)rather thanby differences in condensed bulk composition.  相似文献