Page 365 - Global Tectonics

348 CHAPTER 11

those which occur presently in regions of elevated
geotherms. By contrast, geophysical surveys and iso-
400
topic studies of mantle nodules suggest that the cra-
tonic mantle is strong and cool and that the geotherm
has been relatively low since the Archean (Section
11.3.1). Some of the most compelling evidence of
300
Heat flow (mW m –2 ) 200 ric studies of silicate inclusions in Archean diamonds,
cool mantle lithosphere comes from thermobaromet-
which suggest that temperatures at depths of 150–
200 km during the Late Archean were similar to the
present-day temperatures at those depths (Boyd et al.,
1985; Richardson et al., 2001). Although geoscientists
have not yet reconciled this apparent inconsistency,
100 the relationship provides important boundary condi-
tions for thermal models of Archean and Proterozoic
tectonic processes.
In addition to allowing estimates of ancient mantle
0 geotherms, the evidence from mantle xenoliths indi-
4.0 3.0 2.0 1.0 Present
Age (Ga) cates that the cool mantle roots beneath the cratons
quickly reached their current thickness of ≥200 km
Fig. 11.1 Variation of surface heat flow with time. Solid during Archean times (Pearson et al., 2002; Carlson et

line, based on a chondritic model; dashed line, based al., 2005). This thickness is greater than that of old
on a K/U ratio derived from crustal rocks (redrawn from oceanic lithosphere but much thinner than it would be
McKenzie & Weiss, 1975, with permission from Blackwell if the lithosphere simply had cooled from above by
Publishing). conduction since the Archean (Sleep, 2005). Progressive
thickening by conductive cooling also can be ruled out
because the mantle roots do not display an age progres-
sion with depth (James & Fouch, 2002; Pearson et al.,
tion in the early Earth. If the heat loss mostly occurred 2002). Instead, the relatively small thickness and long-
by the relatively inefficient mechanism of conduction term preservation of the cratonic roots indicate that

then the lithosphere would have been warmer. they must have been kept thin by convective heat trans-
However, if the main mechanism of heat loss was fer from the underlying mantle (Sleep, 2003). Once the
convection beneath oceanic lithosphere, which is very cratonic roots stabilized, the heat supplied to the base
effective at dissipating heat, then the continental of the lithosphere from the rest of the mantle must have

lithosphere need not have been much hotter (Lenardic, been balanced by the heat that flows upward to the
1998). Clarifying these aspects of the Archean thermal surface. In this model, a chemically buoyant layer of
regime is essential in order to reconstruct tectonic lithosphere forms a highly resistant lid above the con-
processes in the ancient Earth and to assess whether vecting mantle, allowing it to maintain nearly constant
they were different than they are today. thickness over time. These considerations illustrate how
Another part of the challenge of determining the the formation and long-term survival of the cool mantle
Precambrian thermal regime is to resolve an apparent roots beneath the cratons has helped geoscientists
inconsistency that comes from observations in the constrain the mechanisms of heat transfer during
crust and mantle parts of Archean lithosphere. Geo- Precambrian times.
logic evidence from many of the cratons, including Differences in the inferred mechanism of heat loss
an abundance of high temperature/low pressure met- from the Earth’s interior have resulted in contrasting
amorphic mineral assemblages and the intrusion of views about the style of tectonics that may have oper-
large volumes of granitoids (Section 11.3.2), suggest ated during Precambrian times (e.g. Hargraves, 1986;
relatively high (500–700 or 800°C) temperatures in Lowman et al., 2001; van Thienen et al., 2005). A con-
the crust during Archean times, roughly similar to ventional view suggests that an increased heat supply in