Environmental Engineering Reference
In-Depth Information
analysis of small amounts of radiogenic Pb in single zircons is routine (e.g. Schoene
et al
.,
2013
), and Pb-loss can in most cases be eliminated. Rocks of nominally
basaltic composition do not typically yield single zircon crystals that are large
enough (
>
20
μ
m) to be dated by high-precision methods. Many LIPs, however,
do contain thick flows with relatively coarse-grained interiors as well as pegmatitic
segregations, both of which can contain zircon and baddeleyite (Kouvo,
1977
). In
our experience, zircons from these rocks have distinctive morphologies, high
U contents, and easily fracture, requiring specialized separation and handling.
Below we describe occurrences and guidelines for
c
rocks from LIPs based on our observations from the Siberian, Karoo, Ferrar,
and CAMP (Central Atlantic Magmatic Province) LIPs, outline the petrographic
setting of zircons, describe their behavior during the chemical abrasion process,
and discuss the importance of accurate correction for initial Th exclusion.
finding primary zircon in ma
4.2.1 Zircon crystallization in ma
c rocks
The vast majority of ma
c LIP rocks, both intrusive and extrusive rocks, are
fine-grained with limited textural variation and contain rare zircon and baddeleyite
crystals in excess of 10
μ
m. As a general rule, zircons are most common in
the interior, coarsest-grained parts of a
flow or intrusion or where fractional crystal-
lization and physical segregation work to concentrate melts with high volatile and
incompatible element concentrations. Much of this remaining melt buoyantly rises
through either conduits or diapirs (Puffer and Horter,
1993
; Philpotts
et al
.,
1996
)to
concentrate below the upper quench surface of a
flow or sill. Other enriched
segregations occur as irregularly bounded pods of coarse-grained plagioclase,
pyroxene and oxides surrounded by a matrix of finer-grained material. These pods
are seemingly quenched during their upward migration (Kontak,
2008
;
Figure 4.1
).
Consistent with this model are crystal size data for the Palisades sill, a
300-m-thick
intrusive basaltic member of the CAMP, which shows that crystal size is
~
nest at
both the top and bottom boundaries, and increases towards the sill center. Maximum
crystal size is
fluids
have buoyantly risen and collected beneath the upper quench surface. Mimicking the
observed trend in crystal size is the concentration of incompatible elements, such as
Zr, which is shown to increase in concentration within the center of the Palisades sill,
with the very highest concentrations corresponding to the coarse-grain horizon
(
Figure 4.2
). A seminal increase in incompatible element concentration is seen
within coarse-grained segregations in the CAMP basalt
~
40 m below the sill top, where we presume that volatile-rich
flows (Puffer and Horter,
1993
; Philpotts
et al
.,
1996
). In the Pallisades sill, large zircon crystals were found in
multiple samples collected at the stratigraphic height of
~
300 m, and yielded a
weighted mean
206
Pb/
238
U date of 201.520
0.034 Ma (Blackburn
et al
.,
2013
).
