Mathematics – Logic
Scientific paper
Jun 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..68..485m&link_type=abstract
Earth and Planetary Science Letters, Volume 68, Issue 3, p. 485-498.
Mathematics
Logic
5
Scientific paper
Narrow reversed rims on plagioclase are ubiquitous in troctolites and olivine gabbros of the Kiglapait intrusion and may be a common feature of all such cumulates. The rims occur at plag/plag, plag/ol, and less strongly at plag/aug grain boundaries. They are optically obvious at ΔAn < 10 mol.% and can reach ΔAn = 32 mol.% or more. In parallel, K/Na drops sharply. Although ubiquitous from sample to sample, the reversed rims are only locally present at grain boundaries even for the same pair of crystals in contact; they are prominent in linear networks suggesting the last trace of intercumulus liquid. A subsolidus origin is ruled out by the absence of reactants at plag/plag and plag/ol boundaries and by the local rather than pervasive development of rims. The rims are required to grow from intercumulus liquid, in which the partition of An component between crystals and liquid increases with the trapped augite component of the liquid. Calculations from published experimental data show that ΔAn > 30 can easily be achieved by such a process. It is also probable that the trapped liquid is part of an An-rich boundary layer generated by solute rejection during adcumulus growth. The ability of the rims to sustain steep K/Na gradients despite a long subsolidus cooling history proves that the K-Na exchange rate is vanishingly small over a geologic time scale in An-rich feldspar, suggesting that at low K content the potassium is site-bound to the tetrahedral Al/Si distribution. Reversed rims therefore provide important information on diffusion limits as well as on the late-stage solidification history of plagioclase-rich cumulates. Moreover, they demonstrate that plagioclase geothermometry cannot be divorced from effects of liquid composition and structure as monitored, for example, by augite content.
Morse Stephen A.
Nolan Kathleen M.
No associations
LandOfFree
Origin of strongly reversed rims on plagioclase in cumulates does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Origin of strongly reversed rims on plagioclase in cumulates, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Origin of strongly reversed rims on plagioclase in cumulates will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1853906