GEMOC ARC National Key Centre

W.L. Griffin^1,2, S.Y. O'Reilly¹ and N.I. Fisher^1,3.

1 bill.griffin@mq.edu.au, GEMOC Key Centre Macquarie University, NSW 2109, Australia.
2 CSIRO Exploration & Mining, NSW 2113, Australia.
3 CSIRO Math. & Inform. Sciences, Macquarie University, NSW 2109, Australia
 

Novel statistical methods have been applied to a database of major- and trace-element data on mantle-derived Cr-pyrope garnets (n=18K), to define 15 natural populations (classes). Comparison with data on garnets from 200 xenoliths identifies the rock types and processes corresponding to each class. These can be grouped into depleted harzburgites, depleted lherzolites, depleted rocks metasomatically re-enriched in LILE and HFSE (± phlogopite), fertile lherzolites, and low-mg# rocks affected by melt-related metasomatism. The distribution of these broad groups, and of individual classes, in the subcontinental lithospheric mantle (SCLM) varies with the tectonothermal age of the overlying crust (Table 1). The previously recognised secular evolution of SCLM composition [1] is expressed as a decrease in the degree of depletion from Archon to Tecton; the proportion of both types of metasomatised rocks is highest in Proton SCLM.

Table 1.  % of garnet-derived groups in SCLM of different tectonothermal age.

                               Archon           Proton           Tecton
                             (>2.5 Ga)      (1-2.5 Ga)        (<1 Ga)
Depl. Harz.               12.3                1.5                  0.0
Depl. Lherz.             12.4               10.0                  0.0
Depl./metasom.         21.9               28.5                 1.4
Fertile lherz.             24.1               27.6                 72.8
Melt-metasom.          29.3               32.4                 25.8

The depth distribution of each class within an SCLM section can be mapped by projecting the Ni temperature (T_Ni) of each garnet grain to the local paleogeotherm. The resulting 2D or 3D maps (Chemical Tomography) of Archon SCLM sections show a large range in the degree and type of stratification. The SCLM beneath Liaoning Province (Sino-Korean Craton) shows no variation with depth, whereas the Slave Craton (Canada) and the Limpopo Belt (Africa) are extreme examples of stratified SCLM.  In the central Slave, a sharp boundary at 140-150 km depth separates an ultradepleted upper layer from a less depleted, but still typically Archean, lower layer 30-50 km thick [2].  Beneath the Limpopo Belt highly depleted, unmetasomatised SCLM (<120-180 km) is underlain by 20 km of relatively fertile lherzolite and minor harzburgite, which may be only ca 1.5 Ga old [3]. The presence of superdeep diamonds in both the Slave and Limpopo provinces suggests that the deeper parts of these sections are plume-related material accreted from below. A similar mechanism may be responsible for the compositional stratification seen in many Archean SCLM sections.
Comparison of SCLM from the SW Kaapvaal Craton in two time slices indicates a major compositional change ca 100-150 Ma ago.  SCLM sampled by the older kimberlites is largely depleted and relatively homogeneous from 100-180 km depth.  In the younger SCLM (sampled by kimberlites <100 Ma old), there is a significant reduction in the proportion of depleted rocks, but an increase in several depleted/metasomatised classes, from 120-165 km depth. The younger section is dominated by fertile lherzolites ±phlogopite at depths of 90-110 km, and by melt-related metasomatism at >165 km. We interpret these patterns as reflecting the addition of asthenosphere (s.l.)-derived material, producing a significant change in the major-element, as well as trace-element, composition of the SCLM. Similar metasomatic "stratigraphy" is seen in other Archean sections.
Proterozoic mobile belts with remnant Archean crust (eg Botswana, Michigan, Yangtze) show SCLM stratigraphy that suggests a continuation of the processes that modified the Kaapvaal SCLM, producing an overall increase in major-element fertility, and a higher proportion of metasomatised classes (Table 1). We suggest that this SCLM mainly comprises strongly reworked Archean SCLM; this is consistent with ca 2.5 Ga Re-Os ages on xenoliths from Botswana [3]. The SCLM beneath younger belts without remnant Archean crust (eg Namaqua Fold Belt) is dominated by relatively fertile lherzolites, and has a low proportion of rocks with metasomatic signatures. This (?) newly formed Proterozoic SCLM is more similar to that found beneath Tectons (Table 1).
These observations suggest that most "primary" Archean SCLM was even more strongly depleted than considered in current models. Preliminary data suggest that it may also be extremely old (>3 Ga; [4,5]). Similar material has not been produced since (?early) Archean time, and this fact must reflect a signifcant change in Earth's geodynamics. This essentially indestructible ancient SCLM, strongly reworked, may comprise the keels beneath many Proterozoic cratonic areas. These implications can be tested by the precise dating of depletion and metasomatism ages made possible by in-situ Re-Os dating of sulfide minerals in xenoliths [4,5].
 

References:

[1] Griffin, W.L. et al. (1999) Geochemical Society Special Publication #6, 13-45
[2] Griffin, W.L. et al. (1999) J. Petrol. 40, 705-727.
[3] Carlson, R.W. et al. (1999) Proc. 7th Int. Kimb. Conf., 99-108.
[4] Pearson, N.J. et al. (2001) GCA (subm.) and this conf.
[5] Aulbach, S. et al. (2001) this conf.