Australia: The Land Where Time Began

A biography of the Australian continent 

Jack Hills and Mt. Narryer, Earth’s Oldest Detritus – Tracing provenance and recrystallisation processes with an In Situ Sm-Nd study of monazite 

Metasedimentary rocks from Jack Hills and Mt. Narryer in the Narryer Gneiss Complex of the Yilgarn Craton contain zircons that have been dated to up to 4.4 Ga, which makes them the oldest crustal materials found on Earth, and monozites up to 3.6 Ga. For this study Tsuyoshi et al. investigated ¹⁴⁷Sm-¹⁴³Nd systematics of detrital and metamorphic monazites that were recovered from these sedimentary rocks by the use of laser ablation-multicollector inductively coupled plasma mass spectrometry (LS-MC-ICPMS). All detrital monazites have negative initial εNd(t) values, which indicates their parental magmas were formed by the remelting of older crustal materials. A comparison between the initial εNd(t) values of the detrital monazites and granitoids in the Narryer Gneiss Complex indicates that the Mt. Narryer and Jack Hills sediments were partially derived from the isotopically enriched surrounding granitoids with ages of about 3.6 and 3.3 Ga. The metamorphic monazites generally have negative initial εNd(t) values when compared to the detrital monazites. The detrital and metamorphic monazites, however, show similar εNd(t) distributions at the ages the sediment was deposited, 3.28 Ga for Mt. Narryer and 3.05 Ga for Jack Hills. Also, it was shown by multiple analyses of single monazite grains which had core-rim structures that the cores and the recrystallised rims had Nd isotopic compositions that were identical at the time of recrystallisation. Older monazites are indicated by these findings to be a source for light rare earth elements for the formation of younger metamorphic monazites and, therefore, that monazite can inherit its primary Sm-Nd isotopic signature during recrystallisation. The Nd model ages for all monazites that were analysed were calculated to estimate the residence time in the crust of their source materials. The results showed that no igneous monazites older than 4.0 Ga were recrystallised to form the monazites. The lack of Hadean monazites is implied by this to not be due to recrystallisation of ancient monazites during later metamorphism, instead being due to high-Ca compositions of the parental magmas of Hadean detritus, which prevent growth of magmatic monazite.

Sources & Further reading

1. Tsuyoshi, I., N. Oliver and T. M. Malcolm (2011). "Tracing the provenance and recrystallization processes of the Earth's oldest detritus at Mt. Narryer and Jack Hills, Western Australia: An in situ Sm–Nd isotopic study of monazite." Earth and Planetary Science Letters 308(3–4): 350 - 358.