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An inclusion suite of "granitic" phases within 3.81 Ga tonalite zircons:
Restrictions for studying Hadean crustal evolution with detrital zircons


Joe Hiess1 and Allen P. Nutman2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, PR China

 

Figure 1. Cathodoluminescence (CL) and transmitted light (T) images for a single globular polymineralic inclusion exposed at the mount surface (at least one more appears to be present at depth in other grains), interpreted as a crystallised melt inclusion. Shown to the right is an enlarged back-scattered electron image of the globular inclusion. Note that the lower edge of the biotite is altered to chlorite. Errors on SHRIMP 207Pb/207Pb ages (Ma) are 2σ.

Petrogenetic interpretation of Hadean detrital zircons is problematic, because their source rocks are no longer preserved. Tonalites represent the dominant component of Eoarchaean (3500-4000 Ma) crustal rocks and therefore form an important reference point for the interpretation of Hadean detrital zircons, and Earth's earliest crustal evolution.

We conducted an electron microprobe survey of inclusions contained within igneous zircons from the best-preserved ca. 3810 Ma meta-tonalite sample G97-18 from West Greenland. Crystalline inclusions were K-feldspar, plagioclase, quartz, hornblende, biotite, ilmenite and apatite. Additionally, one globular polymineralic inclusion interpreted as crystallised melt occurs at the surface of a polished grain mount (Fig. 1). This consists of plagioclase + quartz + K-feldspar around a biotite lath. Other similar, but rare globular inclusions were seen buried within other zircons below the polished surface. These phases, particularly the presence of K-feldspar and plagioclase in broadly equal amounts, suggest the zircons crystallised from a residual granitic (sensu-stricto) melt, as opposed to a tonalitic melt. SHRIMP U-Pb dating of zircons with inclusions indicates that they grew at ca. 3810 Ma, the accepted igneous age of the rock.

This inclusion suite is compatible with the low Ti-in-zircon temperatures for G97-18 igneous zircon (Hiess et al., 2008). Therefore, low Ti-in-zircon temperatures and "granitic" inclusions reported for Hadean detrital zircons do not necessitate crystallisation from low temperature granites. Such features could also be found within Hadean zircons that crystallised late from a higher temperature zircon-undersaturated melt of tonalitic composition.

 

 


Hiess J, Nutman AP, Bennett VC, Holden P (2008) Ti-in-zircon thermometry applied to contrasting Archean igneous and metamorphic systems. Chemical Geology 247: 323-338.