Environmental Engineering Reference
In-Depth Information
Below we summarize evidence linking zircons that form in basaltic rocks with a
residual liquid or mesostatis, an observation that implies that zircons crystallize from
a highly enriched residual liquid whose Zr concentrations likely far exceed the
whole-rock values.
c rocks
Zircon crystals isolated from diabase or dolorite sills, basalts and gabbros com-
monly have distinct morphologies. Rather than being clear, faceted, doubly ter-
minated crystals as are common in felsic rocks, these grains are more often blocky,
brittle, and are easily fragmented during mineral separation. When not fragmented,
grains commonly exhibit aspect ratios of 7:1 or greater (e.g.
Figure 4.3a
-2).
In plane light, these grains are translucent to brownish and if unaltered
(e.g.
Figure 4.3a
-1,2,3) can be transparent and faceted. In the more common case
(e.g.
Figure 4.3a
-4), grains are fragmented and translucent brown. Grains often
have crystal surfaces that are tightly corrugated, a morphology that is likely
evidence of having crystallized in contact with a preexisting plagioclase crystal
(e.g. Corfu
et al
.,
2003
). Our experience suggests that grains which depart from
this seemingly characteristic morphology are often xenocrysts.
Melt inclusions are common within LIP zircons and are often found radiating from
a central point near the middle of the long axis of the crystal (e.g.
Figure 4.3a
-2,3), and
also as a single, dark band in a similar orientation (e.g.
Figure 4.3a
-4). In rare
cases where the entire zircon crystal is recovered, these opaque melt inclusions
appear to narrow in the middle and
4.2.2 Zircon morphology in ma
ed
melt-inclusion compositions fall into two distinct categories: (1) feldspars that are
unmixed into (cathodoluminescent) CL-bright and -darker zones
-
with the precision
afforded by wavelength dispersive spectrometry, these zones are chemically indistin-
guishable (
Figure 4.3d
-2,3); and (2), an Fe-rich composition that is also separated
into CL-light and -dark domains with no measurable compositional difference.
Melts rich in silica, alkalis, and Fe are expected in the late stages of crystallization
(e.g. Walker,
1969
; Philpotts,
1979
; Charlier and Grove,
2012
).
Mineral inclusions are found primarily along the long axis of crystals, although
the boundary between inclusion and host crystal is sharp rather than undulatory, as
is the case with melt inclusions. Wavelength dispersive spectrometry suggests that
these mineral inclusions have bulk compositions similar to the devitri
flare towards the end of the crystal. Devitri
ed melt,
either an Fe-rich phase or a feldspar, such as albite. Partial removal of inclusion
material during polishing (e.g.
Figure 4.3d
-1) exposes evidence that suggests that
the zircon nucleated on a pre-existing mineral, such as feldspar, and subsequently
overgrew it, corroborating late zircon crystallization. Both mineral and melt
inclusions are often surrounded by a halo of CL-bright zircon, which is then
surrounded by CL-dark zircon. In grains without mineral and/or melt inclusions,
