TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

3.6 Discussion
3.6.1 Evidence of Crystal Sorting
In each instance where I measured multiple CSDs in a single flow, I observed
evidence of plagioclase sorting. For example, my observations that the top of the
Site 1203 Unit 3 pillow basalt has greater modal abundance plagioclase and an
upward curved plagioclase CSD are consistent with crystal sorting and concen-
tration (Fig. 3.4b). Plagioclase phenocrysts were concentrated near the top of
this ∼ 25 m thick pillow flow most plausibly by flotation. I observed evidence of
plagioclase sorting in Unit 8 from Site 884, where the plagioclase CSD from the
top of the ∼ 4 m thick flow is linear and has lower modal plagioclase, and CSDs
from the middle and bottom of the flow are nearly identical, curved upward, and
have greater modal plagioclase (Fig. 3.5a; Table 3.2). Faster cooling of the flow
top could have lead flowing lava to concentrate larger crystals toward the bottom
of the flow. Simple gravitational sinking of plagioclase can also explain concentra-
tion of large crystals near the flow bottom, however the density contrast between
basaltic lava and plagioclase would likely have minimized significant plagioclase
settling. Plagioclase sorting is also evident in the Unit 10 pillow flow (∼ 7m thick)
from Site 884 (Fig. 3.5a). The plagioclase CSD I measured from the center of the
flow displays the greatest upward curvature and highest modal plagioclase abun-
dance (Table 3.2), which is consistent with greater accumulation of plagioclase at
the flow center. This type of flow sorting of crystals was documented by Marsh
[98] on a larger scale in basaltic sills.
The two CSDs I measured on samples taken from near the top and bottom
of the ∼ 62 m thick Site 1203 Unit 23 alkalic compound pahoehoe flow appear
to fan (or rotate) about a point at low L (Fig. 3.4i). Two samples provide only
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