GEMOC ARC National Key Centre

ARE LITHOSPHERES FOREVER?

S.Y. O'Reilly¹, W. L. Griffin^1,2  and Y.Poudjom Djomani¹

1. GEMOC Macquarie,
2. CSIRO Exploration and Mining

The lithospheric mantle and 4-D lithosphere mapping

The subcontinental lithospheric mantle (SCLM) carries a geochemical, thermal and chronological record of large-scale tectonic events that have shaped the Earth's crust.  The SCLM is part of the continental plate, and moves with the plates over the less rigid asthenosphere.  It has long been accepted that "old" (cratonic) lithosphere is relatively deep, depleted and cold; more recently it has been recognised that "young" lithosphere is relatively thin, fertile and hot.

Development of the 4-D Lithosphere Mapping methodology has provided tools for constructing realistic geological sections of the SCLM.  Xenoliths and garnet and chromite xenocrysts from mantle-derived volcanics provide samples of the lithospheric mantle at the time of eruption.  These may allow determination of the paleogeotherm, depth to the crust-mantle boundary, detailed distribution of rock types with depth, spatial distribution of fluid-related processes and the depth to the lithosphere-asthenosphere (LAB) boundary.  Volcanic episodes of different ages in one region provide this information for different time-slices, while geophysical data (seismic, gravity, magnetic, thermal) can be used to extend the geologically-derived profiles laterally or to interpret lithospheric domains with geophysical signatures that can be matched with geologically mapped sections.

Secular Variation in Lithosphere Composition

There is a fundamental distinction between Archean cratonic mantle, represented by xenoliths in African and Siberian kimberlites, and Phanerozoic circumcratonic mantle, represented by xenoliths in intraplate basalts and by orogenic lherzolite massifs.  Archean xenoliths are not only more depleted on average, but have higher Si/Mg (higher opx/olivine); subcalcic harzburgites are well-represented in Archean xenolith and xenocryst suites, but essentially absent in younger ones.  Analysis of >13,000 garnet xenocrysts from volcanic rocks worldwide shows a clear correlation of garnet composition with the tectonothermal age of the crust penetrated by the volcanic rocks.  The xenolith and garnet data, taken together, indicate that the Archean/Proterozoic boundary represents a major change in the nature of lithosphere-forming processes.

In xenoliths, the Cr2O3 content of garnet correlates well with the Al2O3 content of the host rock and xenolith suites also show good correlations between Al2O3 contents and those of other major and minor elements. These correlations make it feasible to calculate the composition of a mantle section, given the median Cr2O3 content of garnet xenocrysts from that section.  The mean compositions of SCLM beneath terrains of Archean, Proterozoic and Phanerozoic tectonothermal age, calculated in this way,  show a clear secular evolution in all measures of depletion, such as Al, Ca, mg#, and Fe/Al .

Significance of SCLM evolution to geophysical interpretation

Average mineral compositions for each age group have been used to calculate average modes, densities and seismic velocities.  Archean SCLM is 2.5% less dense than the asthenosphere (approximated by PM); for the less-depleted Phanerozoic mantle the difference is <1%.  Thermal expansion coefficients are identical within error for all compositions, so that these differences persist to high temperatures.  At 25°C, the Vp and Vs of Archean SCLM are higher than that of Phanerozoic SCLM by ca 0.5% and 1.2%, respectively; thus accounting for ca 25% of the range observed by seismic tomography.  Typical geotherms for cratonic and Phanerozoic areas were used to calculate the difference in Vp and Vs at 100 km depth; the Archean values are higher by 4-5%, corresponding to the ranges commonly seen by seismic tomography.

Lithosphere evolution and destruction

These physical property data are important constraints on the delamination and recycling of the SCLM. Thermal expansion coefficients and bulk modulus of minerals have been used to calculate the temperature-dependent density variation with depth for typical Archean, Proterozoic and Phanerozoic mantles. The results show that the entire section of Archean lithospheric mantle is significantly buoyant relative to the underlying asthenosphere. For Proterozoic and Phanerozoic mantles, a minimum thickness of ca 30 and 60km respectively must be reached before each section becomes buoyant: these are the minimum conditions for lithosphere delamination.  This effect explains the thickness and apparent longevity of existing Archean (and thick Proterozoic) lithosphere, but suggests that no deep continental root could be constructed from Phanerozoic lithosphere.

Conclusions

Correlations between mantle type and crustal age indicate that continental crust and its underlying SCLM were formed together and remain coupled for geologically long times. Archean (and Proterozoic >30 km thick) lithosphere is forever unless there is physical disruption (eg rifting, thinning and displacement) with associated thermal and chemical erosion (metasomatism).  Phanerozoic lithosphere will delaminate unless it remains hot and thin (less than about 50 km thick).