GEMOC ARC National Key Centre

S. Aulbach^1,*, T. Stachel¹, K.S. Viljoen², G.P. Brey¹ and J.W. Harris³,

1. Institut fur Mineralogie, Universität Frankfurt, Senckenberganlage 28, 60054 Frankfurt, Germany,
2. De Beers GeoScience Centre, PO Box 82232, Southdale, 2135, South Africa,
3. Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK,
* present address: GEMOC ARC National Key Centre, Macquarie University, NSW 2109, Australia
 

Introduction:  Websterites occurring as xenoliths or within orogenic peridotite massifs have been intensively studied. They are commonly interpreted as having crystallized from a melt, e.g. as segregates in magma conduits [e.g. 1] or as cumulates from basaltic melts [e.g. 2]. In contrast, little is known about websteritic (W-) inclusions in diamond. Their transitional character, with major element compositions intermediate between eclogitic (E-) and peridotitic (P-) inclusions, has already been established [3]. We use this characteristic combined with trace element data for coexisting garnet (grt) and clinopyroxene (cpx) inclusions in diamonds to propose an alternative model for the formation of diamondiferous websterites.

Major and Trace Elements:
Grt. W-grt inclusions have pyrope contents be-tween 53.0 and  66.9 mol%, a compositional range not shared by E- or P-garnets in diamonds from Venetia. Equilibrium with both cpx and opx restricts the Ca contents in W-garnets to values between 3.6 to 4.6 wt%. Cr₂O₃ is < 1 wt%. REE_N (N=5) have a positive LREE/HREE slope, with some garnets showing La enrichment (La/Ce > 1) and two having small positive Eu-anomalies. Ba and Sr abundances are ~ 0.01 chondritic, Nb, Ti, Zr and Hf are present in near-chondritic abundances.
Cpx. Mg-#'s in W-cpx's show a bimodal distribution, with a first mode coinciding with E-cpx (~80) and a second  (~85) being intermediate between E- and P-values. Na₂0 (1.2-2.1 wt%) and Cr₂O₃ (0.1-0.4 wt.%) also lie between concentrations found in E- and P-samples. REE_N (N=5) patterns are almost flat, peaking at Eu or Gd. Some cpx show enrichment in La to Pr. Small negative and positive Eu-anomalies were determined for three cpx. All cpx have near chondritic abundances of Ti, Hf and Y and a trough at Zr, for three cpx a trough at Nb was observed.
Calculation of hypothetical melts. Hypothetical melt compositions in equilibrium with W-grt's (W-Liquids) were calculated using D-values of [4]. Calculated W-liquids have strongly fractionated REE patterns ranging from La_N = 100 to Lu_N = 1. Of all the incompatible elements, Nb is most strongly enriched in the melt (Nb_N = 1000).

Discussion:  Current models of websterite genesis all require formation from primary mantle melts. A comparison of mantle-derived melts from different settings with W-liquids shows that this model is unsatisfactory, at least for W-inclusions in diamonds. For OIB's the degree of REE fractionation is much lower, whereas kimberlites have similarly fractionated REE but at much higher total abundances. An additional constraint comes from the occurrence of Eu anomalies, suggesting a precursor that experienced a stage of pla-gioclase accumulation, e.g. gabbro. Radiogenic and stable isotopic systematics and the occurrence Eu-anomalies for some websterites suggest crystallisation directly from slab melts [5]. Formation of websteritic parageneses directly from partial melting of eclogites should result in a more fertile composition of the former relative to the latter. However, the opposite is the case.
The intermediate major element composition of W-inclusions relative to P- and E-inclusions could have formed by mixing. This may occur during melting of subducted oceanic crust, with subsequent infiltration of the highly silicic, LREE-enriched melt into overlying peridotite. Reaction between such melts and surrounding peridotite should produce lithologies with intermediate major element content but a trace element budget dominated by the percolating melt.
Recent melting experiments using layers of eclogite and peridotite as starting materials [6,7] resulted in run products with major element compositions very similar to websteritic grt and cpx. We modelled an eclogite melt by applying an enrichment factor, given in [7] for the calculation of a 30% partial melt from any eclogitised basalt, to an assumed gabbroic precursor. Comparison of the calculated eclogite melt with W-Liquids shows a very good fit of both relative and absolute abundances. Nb_N is an exception, being much lower in the eclogite melt due to experimental conditions. At lower degrees of melting and in the absence of residual rutile in the source, the eclogite melt should become sufficiently enriched in Nb to compensate for this discrepancy.
In summary, major and trace element characteristics of the W-diamond source at Venetia are consistent with formation by reaction of slab-derived melts with peridotite.
 

References:

[1] Frey F.A. and Prinz M. (1978) Earth Planet. Sci. Lett., 38, 129-176.
[2] Kornprobst J. et al. (1990) J. Petrol., 31, 717-745.
[3] Moore R.O. and Gurney J.J. (1989) Kimberlites and Related Rocks, Blackwell Publ., 1029-1041.
[4] Green T.H. et al. (2000) Lithos, 53, 165-187
[5] Pearson D.G. et al. (1993) J. Petrol., 34, 125-172
[6] Yaxley G.J. and Green D.H. (1997) Schweiz. Mineral. Petrogr. Mitt., 78, 243-255.
[7] Rapp R.P. et al. (1999) Chem. Geol., 160, 335-356.