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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.