Early Earth Plate Tectonics; A Universal or Craton Specific Feature?
Implication from Poisson’s Ratio Calculation of the Primary Melts
Abstract
Primary melt compositions and mantle potential temperature (Tp) was
calculated for c.a. 3.4 Ga -2.21 Ga volcanic rocks e.g., Basalts, Dikes,
Komatiites, Komatiitic-basalts from different Indian cratons, using the
data sourced from previously published literature. The results are then
compared with the previously published values of Tp from different
cratons of the world. In addition, we have also calculated the Poisson’s
ratio value for the Primary melts from Indian cratons and other cratons
of the world as well. A volume normative mineralogy was used for the
input to calculate the Poisson’s ratio. For temperature, we have used
the Tp value of the corresponding Primary melts, and the pressure was
kept constant at 1 bar. The measured values of Poisson’s ratios are then
plotted against their respective ages. Poisson’s ratio estimates from
different cratons range between 0.2764-0.2840 for 3.5 Ga to 2.6 Ga time
window, and records a rise at 2.5 Ga, reaching 0.288 at 2 Ga. The
average value of Poisson’s ratio from 3.5 Ga to 2.6 Ga is 0.2791±
0.0021(SD), whereas it is 0.282923±0.00283 from 2.5 Ga to 1.3 Ga.
However, the number of data points from 1.3 Ga to 500 Ma is very less,
and the modern world (i.e., 500 Ma- Present) records an average
Poisson’s ratio of 0.285 ± 0.004. Nevertheless, the scenario flips
around for a few cratons e.g., Superior craton of Canada, the Baltic
shield of eastern Europe, and Bundelkhand craton of India; as these
cratons are showing an extraordinarily high Poisson’s ratio between 2.8
Ga-2.7 Ga. The average Poisson’s ratio of 0.287±0.0021 with a high Tp
value of 1550°C-1630°C is recorded from Superior craton and the Baltic
shield. Tp value from the Bundelkhand craton, on the other hand, is low
(1388°C). Similar to the present day, subduction dominated Poisson’s
ratio from Superior craton and Baltic shield with high Tp value at 2.7
Ga suggests a possibility of manifestation of subduction even at higher
temperature conditions. These phenomena, however, appear to be
craton-specific rather than a general approach, as no such values of
Poisson’s ratio have been documented from any other craton in the world
between 3.5 Ga-2.7 Ga. The other way around for the above condition
might be explained by a proto-tectonic activity such as drip-tectonics
or sagduction. This is because, although the Poisson’s ratio is high for
these cratons during 2.8 Ga- 2.7 Ga, rigid plate subduction at such a
high temperature seems to be a difficult task. On the other hand,
multiple peaks in Tp values with a certain interval between 3.5 Ga-2.5
Ga indicate cyclic mantle overturn events, which might have terminated
the possibility of subduction. However, a lower Tp with a high Poisson’s
ratio from Bundelkhand might suggest a very similar rigid plate motion
during the end of Archaean. Key Word- Mantle Potential Temperature,
Poisson’s Ratio, Primary Melt