Australia: The Land Where Time Began

A biography of the Australian continent 

Hadean Plate Boundary Interactions are Suggested by Low Heat Flow that is Inferred From >4 Ga Zircons

The Hadean Eon, the first ~600 million years of Earth history, is still poorly known, largely as no rock record of that time has been found. In the Jack Hills, Western Australia, detrital zircons of Hadean age can potentially provide insights into the conditions prevailing at that time, according to Hopkins, Harrison & Manning. The results of geological investigations (2-13) using these ancient grains have been interpreted to suggest the presence of a hydrosphere (2-4,7,8) and continental crust (9,10) prior to 4 Ga. A diverse assemblage of mineral inclusions (14-17) are suggested in this paper to be an underexploited characteristic of these >4 Ga zircons. In this paper the results are presented of more than 400 zircons of Hadean age from Jack Hills, showing that some inclusion assemblages are conducive to thermobarometry. The conditions of magmatic formation are constrained to about 700^oC and 7 kbar by these thermobarometric analyses of zircons bearing inclusions of zircons of 4.02-4.19 Ga age. A near-surface heat flow of about 75 mW/m^-2, which is about 3-5 times lower than estimates for global heat flow in the Hadean. As above subduction zones is the only site on modern Earth that is characterised by heat flow of about ¼ of the global average, it is suggested here that the magmas from which the zircons from Jack Hills of Hadean age crystallised were largely formed in an underthrust environment, which was possibly similar to a convergent margin of the present.

Sources & Further reading

1. Hopkins, M., T. M. Harrison and C. E. Manning (2008). "Low heat flow inferred from >4[thinsp]Gyr zircons suggests Hadean plate boundary interactions." Nature 456(7221): 493-496.