|Title:||Constraints on Hadean geodynamics from mineral inclusions in >4 Ga zircons|
|Authors:||M. D. Hopkins, T. M. Harrison, and C. E. Manning|
|Publication:||Earth Planet. Sci. Lett., v. 298, p. 367‐376.|
The inclusion mineralogy of 1450 zircons over 4 billion years in age from the Jack Hills,Western Australia, was characterized for composition and phase assemblage. Results confirm that the inclusion population of these largely igneous zircons is dominated bymuscovite and quartz ( 75%). If the inclusions are original to the igneous zircons, this observation alone restricts the host melts to formation at pressure–temperature (P–T) conditions of 650–800 °C and >4 kbar. Several lines of evidence support the viewthatmost of the analyzedmineral inclusions are primary, including their lack of association with cracks, magmatic crystal forms, and lack of exchange with fuchsitic (Cr–rich) micas in the host conglomerate. The application of Ti–in–zircon thermometry, and phengite, Tiin– quartz, and Al–in‐ hornblende barometry to these inclusion assemblages yields estimates of magmatic P–T conditions from5 to N12 kbar and 700± °C. These data indicate zircon formation along geotherms of<60 °C/ km and imply conductive near–surface heat flow of b40 to 85mW/m2– a range that is substantially lower than most estimates of global Hadean heat flow. Of the possible environments capable of generating melting under such locally low heat flow early in Earth history, underthrusting, possibly in a manner similar to modern convergent margins, appears most consistent with numerous other geochemical constraints derived from investigation of Hadean zircons.