Secular Evolution of Sub-Continental Mantle

W.L Griffin1,2, Suzanne Y. O'Reilly2, Dmitri A. Ionov2 and C.G. Ryan1

1. CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113

2. GEMOC, School of Earth Sciences, Macquarie University, Sydney NSW 2109

Lithosphere mapping using garnet concentrates from kimberlites and other volcanic rocks has produced images of the thermal state and lithostratigraphy of the lithospheric mantle to depths of 150 to 250 km in >35 localities worldwide. These sections show consistent differences between Archean and younger lithosphere. Archean mantle sections contain depleted garnet harzburgites, typically concentrated between 140 to 180 km depth and interspersed with variably depleted lherzolites; the proportion of harzburgite ranges from 10 to 60%. At shallower levels depleted lherzolites make up >90% of the column, while at greater depths more fertile lherzolites may occur. In Proterozoic mantle sections, harzburgitic rocks are very rare, and the garnets of the dominant lherzolites are on average less depleted in LIL and HFSE elements. There is no consistent difference in the paleogeotherm between the Archean and Proterozoic parts of cratons.

Garnet concentrates from volcanic rocks also record secular changes in lithospheric mantle composition. Lherzolitic garnets from Archean sections have high mean Zr/Y (³5) and low mean Y/Ga (<3), while similar garnets from Phanerozoic areas have low mean Zr/Y (²1) and high mean Y/Ga (³4); garnets from Proterozoic sections have intermediate values. Comparisons with xenolith data and numerical modelling based on partition coefficients indicate that these differences reflect an increase in the average Cpx/Gnt ratio of lithospheric mantle from Archean to Phanerozoic time. Garnet peridotite xenoliths from areas with Phanerozoic tectonothermal ages have, on average, high modal (Cpx+Gnt) as well as high Cpx/Gnt. This reflects a high average bulk (Ca+Al), which is shared by spinel lherzolites from many localities. These xenoliths, like oceanic peridotites, have Mg# vs Mg/Si relationships consistent with an origin as residues from extraction of basaltic melts, at degrees ranging from moderate to very low.

Garnet peridotite xenoliths in kimberlites from Archean areas, by contrast, are extremely depleted in (Ca+Al), have low Cpx/Gnt, and have Mg/Si too low (relative to Mg#) to allow their derivation by extraction of basic or ultrabasic melts (Boyd and Mertzman, 1989). Limited data on xenolith suites from areas with Proterozoic tectonothermal ages show that the peridotites are intermediate in their degree of depletion and Cpx/Gnt, consistent with the more abundant garnet trace-element data.

These observations suggest that major changes have occurred in the processes that have produced subcontinental mantle through time; the lithospheric mantle has become progressively less depleted. Archean continental roots retain the products of processes that have not operated since ca 2.5 Ga, while most Phanerozoic lithospheric mantle appears to have formed by subduction of oceanic material. The intermediate nature of Proterozoic lithospheric mantle worldwide suggests that it reflects transitional processes. These might include both generation of "primary" Proterozoic lithosphere in a different convection regime than at present, with more extensive melting at spreading centres, and tectonic/magmatic reworking of Archean lithospheric mantle.

The older (>1.7 Ga) parts of many cratons have keels or roots with high Vs, extending to depths of 250-450 km (Polet and Anderson, 1995). This material is believed to be both cooler and compositionally different than that underlying younger cratons and Phanerozoic mobile belts, which have no significant Vs anomalies. A large harzburgitic component will contribute to the Vs anomaly beneath the >2.5 Ga cratons, but the shallow (<200 km) roots of Early Proterozoic cratons do not contain such rocks; this suggests that moderately depleted lherzolites can provide a Vs anomaly. What of the deeper parts of the cratonic roots? Where the Taihang Fault Zone disrupts the Archean North China Craton, kimberlites carry garnets with Zr-Y-Ga relations typical of Phanerozoic mantle, and derived from depths of <100 km. This material may have underlain the Archean mantle of the craton, and risen to shallow depths as that mantle was disrupted and displaced. We suggest that the deep cratonic keels consist of relatively fertile peridotite, capable of generating both kimberlites and flood basalts; this material must be significantly cooler than otherwise similar Phanerozoic mantle, to provide a deep seismic velocity anomaly.