Environmental Engineering Reference
In-Depth Information
201.600
201.575
201.550
201.525
201.500
201.475
2
4
6
8
Th/U
magma
Figure 4.4 Model Th-corrected
238
U
206
Pb error-weighted mean dates for the
Palisades sill for a range of Th/U magma compositions. This correction yields
results that are within uncertainty of one another, even over this broad range of
Th/U compositions. The best estimate for the age of the Palisades sill comes from
using a Th/U value observed for coarse-grained samples (
-
~
300 m, Th/U
~
4,
Figure 4.2
).
values, remain within analytical uncertainty of one another. For the Palisades sill,
the Th-correction is near maximized at Th/U magma compositions of
~
3.5
(
Figure 4.4
). This range of assumed Th/U for the parental magma composition
well exceeds the variation observed within the Palisades sill (best estimate Th/U
¼
4)
(
Figure 4.2
), indicating the sill
'
is emplacement date is both accurate and precise.
4.3 Why accuracy and precision matter
The ability to (1) determine the emplacement/eruption rates of LIP magmas and
the spatial/temporal patterns in lava/sill chemistry, all of which are crucial for
understanding their origin; and (2) critically evaluate the synchroneity and thus
potential casual role of magmatism as a trigger for environmental forcing of
biological crises requires having high-precision, accurate age models for both.
For example, the age and duration of the end-Permian extinction is constrained by
dates that are precise to better than 0.02% (Burgess
et al
.,
2014
). Current temporal
constraints on the timing and duration of the majority of LIP magmatism are
consistent with the coincidence of magmatism and mass extinction (e.g. Courtillot
and Renne,
2003
), but prevent detailed evaluation of the synchrony and relative
timing of magmatism and environmental perturbation at the timescale over which
