TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

The common occurrence of population B crystals in rounded and embayed crys-
tals in Site 1203 Units 3, 14, and 31 and Site 884 Unit 8 basalts is a qualitative
indication that these crystals were commonly out of equilibrium with their host
basalts, which is also consistent with plagioclase accumulation.
Huang et al. [72] specifically noted the significance of plagioclase accumulation
in Unit 14 and 31 lavas from Site 1203 (i.e. Fig. 13 of [72]). My observations
of strongly upward kinked plagioclase CSDs for these two Units (Fig. 3.4e,l) are
consistent with their assertions of plagioclase accumulation. Although I provide
seemingly independent evidence of plagioclase accumulation in these two lavas,
the exact origin of the non-linear CSDs of these lavas is not straightforward. Both
magma mixing and crystal accumulation can produce a non-linear CSDs (Fig. 3.3)
[67]. Higgins [67] discussed a number of explanations for curved CSDs, including
size dependent crystal growth rate, destruction of small crystals (i.e., fines destruc-
tion), changes in cooling rate, or changes in the dominant crystallizing phases.
Cashman and Marsh [20] and Marsh [99] presented sound arguments against size
dependent growth. Fines destruction and changes in cooling rate cannot be ruled
out based upon textural evidence alone. I have noted distinct compositional dif-
ferences between population A and B crystals from Site 1203 Units 14 and Units
31, which is evidence against these processes as noted by Higgins [67]. Huang et
al. [72] suggested a significant role for olivine and plagioclase fractionation during
DSM basalt petrogenesis, and there is little other evidence to indicate there was
a change in the dominant crystallizing phases. Although I cannot rule out minor
roles for any of these processes, I suggest accumulation and/or magma mixing are
responsible for the majority of the CSD curvature I have noted.
Higgins [67] noted that magma mixing would lead to compositional differences
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