摘要:

AbstractNatural, pure CO2inclusions in quartz and olivine (c.Fo90) were exposed to controlledfH2conditions atT= 718–728°C andPtotal= 2 kbar; their compositions were monitored (before and after exposures) by microsampling Raman spectroscopy (MRS) and microthermometry. In both minerals exposed at the graphite–methane buffer (fH2= 73 bar), fluid speciations record the diffusion of hydrogen into the inclusions. In quartz, room‐temperature products in euhedral isolated (EI type) inclusions are carbonic phases with molar compositions ofc.CO2(60) + CH4(40) plus graphite (Gr) and H2O, whereas anhedral inclusions along secondary fractures (AS type) are Gr‐free and contain H2O plus carbonic phases with compositions in the rangec.CO2(60) + CH4(40) to CO2(10) + CH4(90). EI type inclusions in olivine evolved toc.CO2(90–95) + CH4(5–10) without Gr, whereas AS type inclusions have a range of compositions from CO2(90) + CH4(10) ± Gr to CH4(50) + H2(50) ± Gr; neither H2O nor any hydrous species was detected by optical microscopy or MRS in the olivine‐hosted products. Differences in composition between and among the texturally distinct populations of inclusions in both minerals probably arise from variations in initial fluid densities, as all inclusions apparently equilibrated with the ambientfH2. These relations suggest that compositional variability among inclusions in a given natural sample does not require the entrapment of multiple generations of fluids. In addition, the absence of H2O in the olivine‐hosted inclusions would require the extraction of oxygen from the fluids, in which case re‐equilibration mechanisms may be dependent on the composition and structure of the host mineral.Many of the same samples were re‐exposed to identicalP–Tconditions using Ar as the pressure medium, yielding ambientfH2= 0.06 bar. In most inclusions, the carbonic fluids returned to pure CO2and graphite persisted in the products. Reversal of the mechanisms from the prior exposure atfH2= 73 bar did not occur in any inclusions but the AS types in olivine, in which minor CO2was produced at the expense of CH4and/or graphite. The observed non‐reversibility of previous mechanisms may be attributed to: (1) slower fluid–solid reactions compared to reactions in the homogeneous fluid phase; (2) depressed activities of graphite due to poor ordering; and/or (3) low ambientfO2at the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00137.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractMineral equilibria in the system CaO–MgO–Al2O3–SiO2–H2O provide a basis for mapping of four reaction isograds and one bathograd in the low‐pressure transition from subgreenschist to greenschist facies. Most of the Matachewan area of the Abitibi greenstone belt is in the lower‐pressure bathozone, as indicated by the widespread occurrences of the subassemblage Prh–Chl. The higher‐pressure bathozone is indicated by two occurrences of Pmp–Act–Ep–Qtz, but in these samples the bathograd is displaced to anomalously low pressure by the high Fe content of the coexisting minerals. This illustrates the need to analyse coexisting minerals, calculate activities of end‐member species, and computeP–Tcurves for individual samples before interpreting the isograd/bathograd pattern.Petrographic and microprobe analysis indicates that great care must be taken in the selection of ‘equilibrium’ assemblages. Pyroxene phenocrysts in one sample are replaced by the assemblage Pmp–Act–Ep–Chl–Qtz, whereas Prh–Act–Ep–Chl–Qtz occurs in the groundmass. Compositional variation may be more cryptic, as in a sample of metabasaltic hyaloclastite that contains two spatially distinct ‘univariant’ assemblages, Prh–Pmp–Ep–Chl–Qtz and Prh–Act–Ep–Chl–Qtz, within the devitrified matrix. Whereas chlorite compositions are similar in both assemblages, prehnite and epidote in the latter assemblage are significantly richer in Fe and poorer in Al. Accordingly, the rock is interpreted to contain two distinct ‘univariant’ assemblages, rather than one ‘invariant’ assemblage (Prh–Pmp–Act–Ep–Chl–Qtz). The displaced ‘univariant’ curves for this sample intersect at 2.2 kbar and 250°C.Taking account of all thermobarometric implications, the low‐grade limit of the greenschist facies is at 250–270°C and 2–2.5 kbar, corresponding to depths of 7–8 km. Comparison of apparentP–Tconditions on both sides of the Larder Lake – Cadillac break, a regional CO2‐metasomatized fault zone that is spatially associated with many Archaean gold deposits, p

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00138.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Contact Metamorphism. Edited by D. M. Kerrick.Oxide Minerals: Petrologic and Magnetic Significance. Edited by D. H. Lindsley.Geology and Tectonics of the Karakoram Mountains. By M. P. Searle.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00139.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00140.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY