TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

these magmas chemically evolved in the shallow crust. Clues to these processes and
relatively primitive magma compositions may be best recorded in early liquidus
mineral phases [5]. Coffin ad Eldholm [32] suggested that some LIPs are emplaced
at rates greater than 10 5 km 2 m.y. −1 , which underscores the possibility that LIP
emplacement can have dramatic effects on the global environment. Large Igneous
Provinces are more buoyant than normal oceanic crust and are relatively resistant
to subduction, which indicates they may have an important role in the genesis of
continents throughout geologic time [32].
1.4 Crystal Size Distributions (CSDs): Identification and Examination of Crystal
Populations
The origins of phenocrysts are an often under-appreciated aspect of petrologic
investigations. It is becoming increasingly apparent that they commonly represent
a more complex formation than simple crystallization from their current host
magmas [98]. This complexity is well documented by CSDs, which measure the
number of crystals of a characteristic size per unit volume of rock (e.g., [20, 96,
98]). Higgins [67] used CSDs to identify two distinct populations of plagioclase in
porphyritic dacites from Kameni Volcano, Greece, which he interpreted to be a
result of mixing between two phenocryst-carrying magmas (Fig. 1.2b). Crystal size
distributions are customarily displayed on a log-normal plot of population density
(number of crystals per unit volume rock) versus crystal size (L) (Fig. 1.2) [20].
The utility of the CSD is best illustrated by considering a simple example,
emplacement of a thin lava flow where magma arrives at the surface in a wholly
liquid state. Crystal nucleation rate increases as the flow cools [18]. If nucleation
and growth continue uninterrupted, the final rock will yield a linear or near linear
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