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reflect processes such as weathering and
erosion. The zircon trace element data are,
therefore, consistent with the hypothesis
that increases in the proportion of evolved
magmas along convergent margins have
had an important influence on radiogenic
Sr input into Earth’s oceans during these
time intervals.
PATTERNS OF CRUSTAL
THICKENING
Increases in magmatic reworking of pre-
existing radiogenic crust should occur asso-
ciated with thermal maximums as the crust
thickens (DeCelles et al., 2009). Garnet is a
mineral found in crustal magmas that is
highly sensitive to pressure and incorporates
heavy rare earth element (HREE)+Yb rela-
tive to other trace elements (Ducea et al.,
2015). Therefore, changes in Yb/Gd ratios
in zircon, for example, are thought to cor-
relate with changes in the crustal thickness
during magmatism (Barth et al., 2013). The
trace element record retained within the zir-
con data shows that the lowest Yb/Gd ratios
in the data set (Fig. 2C) correlate well with
the Paleoproterozoic and Neoproterozoic
Th/Yb and εHf peaks. These crustal thick-
ness patterns are similar to those presented
recently based on La/Yb ratios for a global
compilation of 5587 detrital zircons (Balica
et al., 2020). In particular, both analyses
show Paleoproterozoic and Neoproterozoic
peaks in crustal thickness that are separated
by an intervening interval from ca. 1.8 Ga
to 0.8 Ga during a period of environmen-
tal stasis known as the “boring billion”
(Holland, 2006). The trace element data are
therefore consistent with increased assimi-
lation of radiogenic crust during periods of
increased crustal thickness along conver-
gent margins. Increases in crustal thickness
are in turn associated with mountain build-
ing driven by tectonic shortening along
Earth’s major convergent plate boundaries
involving advancing states of subduction and
collisions. Thus, the patterns in the zircon
trace element data are also consistent with
the hypothesis that increases in the propor-
Figure 2. (A) Average Th/Yb (crustal input proxy); (B) εHf (crustal input proxy; note axis
reversal); and (C) Yb/Gd (crustal thickness proxy) with their 95% confidence envelopes tion of radiogenic rocks (e.g., older base-
determined by Monte Carlo bootstrap resampling of zircons in 0.1-Gyr time brackets ment) uplifted and exposed along conver-
compared to normalized marine Sr/ Sr evolution. εHf data from Puetz and Condie (2019).
87
86
Age of “boring billion” from Holland (2006). UHP—ultrahigh-pressure. gent margins have had an important
influence on radiogenic Sr input into Earth’s
oceans (Richter et al., 1992).
large detrital zircon data set (n = 70,656) normalized to model Sr/ Sr ratios of the
87
86
show that significant negative deviations global river and mantle inputs, shows CRUSTAL THICKNESS AND Sr FLUX
of εHf values correlate with these Th/Yb increases that post-date the peaks in crustal Geologists have long recognized that the
peaks (Fig. 2B). The 87 Sr/ Sr isotope assimilation indicated by zircon Th/Yb and widespread generation of continental topo-
86
curve shown in Figures 2A–2B, which is εHf values—a time lag that may in part graphic relief, which increases the overall
6 GSA TODAY | February 2022
