Australia: The Land Where Time Began

A biography of the Australian continent 

Jack Hills - Mineral Inclusions in 4 Ga Zircons Provide Constraints on Hadean Geodynamics

In this paper Hopkins, Harrison & Manning present their findings from a study that looked at the inclusion mineralogy of 1,450 zircons with an age of more than 4 Ga from Jack Hills in Western Australia, characterising the zircons for composition and phase assemblage. The results confirmed that the inclusion populations of these zircons, which were largely of igneous origin, were dominated by muscovite and quartz (~75 %). Hopkins et al. suggest that if the inclusions are original to the igneous zircons this observation, with no other support, restricts the host melts to their formation at pressure-temperature (P-T) conditions of ~650-800^oC and greater than 4 kbar. The view that most of the mineral inclusions are primary is supported by several lines of evidence, such as their lack of association with cracks, magmatic crystal forms, and the lack of exchange with fuchsitic (Cr-rich) micas in the host conglomerates. By applying Ti-in-zircon thermometry, and phengite, Ti-in-quartz, and Al-in hornblende barometry, to these inclusion assemblages estimates of magmatic P-T conditions from 5 to greater than 12 kbar and 700 ± 40oC resulted. It is indicated by these data that zircon formation along geotherms of ≤ 60^oC/km and it is also implied by these data that that conductive heat flow near the surface of <40-85 mW/m² – a range that is substantially lower than most global Hadean heat flow estimates. Underthrusting is one of a number of possible environments that would be capable of generating melting under conditions of such locally low heat flow early in the history of the Earth. Possibly this Underthrusting was similar in manner to modern convergent margins, and this appears to be the most consistent with the many other geochemical restraints that have been derived from investigation of Hadean zircons.

1. Hopkins, M. D., T. M. Harrison and C. E. Manning (2010). "Constraints on Hadean geodynamics from mineral inclusions in 4 Ga zircons." Earth and Planetary Science Letters 298(3–4): 367-376.