|These photos were taken using Nomarski lenses, after
the slide was etched with fluorboric acid for 75 seconds. Nomarsik lenses
uses reflected light instead of refracted light (which has been used to
take the pictures ppreviously seen). It splits the light beam so that there
will be different travel times of the beams of light depending on the area
of the slide that the light hits. This reveals a micro- topography depending
on the differentially etched zones. The more calcic zones will have a lower
relief than the more sodic zones.The lower the relief the longer time it
takes the light to be reflected, and this is how the zoning stands out.
By using this method, we can get a qualitative observation of the history of the magma chamber through chemical variations of the plagioclase crystal. Also any truncations of zoning due to resorbtion indicate these subtle changes.