TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

When diffusive equilibrium has been reached between two adjacent plagioclase
zones with An contents A and B the condition in equation 4.4 is met.
C A/B
n /D A/B
n = 1 (4.4)
This test was applied to each plagioclase crystal examined in this study for Sr
and La at texturally distinct transitions between intermediate zones and crystal
rims (Fig. 4.15). In case of Unit 4 crystal P, where an intermediate zone to rim
transition examination was not possible, a core-intermediate zone transition was
tested. The partition coefficient values listed in tables 4.4 and 4.5 were used for
the test. Two of the ten Unit 4 crystals (Q and P) are within error of the Sr
diffusive equilibrium line (i.e., equation 4.4; Fig. 4.15a), whereas only one Unit 4
crystal (P) is within error of the La diffusive equilibrium line (Fig. 4.15c). These
observations are consistent with the findings of Cherniak [27] that trivalent REE
diffuse slower in plagioclase than divalent cations like Sr.
Six of the seven Unit 10 crystals are within error of the Sr diffusive equilib-
rium line (Fig. 4.15b), whereas five of the seven crystals are within error of the
La diffusive equilibrium line (Fig. 4.15d). The greater proportion of diffusively
re-equilibrated plagioclase crystals in the Unit 10 basalt qualitatively suggests
that relative to the Unit 4 crystals, Unit 10 crystals were held at magmatic tem-
peratures for greater periods of time based upon the fact that diffusion occurs
more rapidly when crystals are held at magmatic temperatures (i.e., [27]). This
consistent with the working hypothesis that the majority of the Unit 4 plagioclase
phenocrysts bear a close genetic relationship with their host basalt, and the ma-
jority of the Unit 10 crystals were stirred up from a cumulate layer or other mush
pile.
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