Genesis Gt-2.0 Replacement
Karleen Chura
Trace element compositionof garnet kelyphites in xenoliths from Udachnaya as evidence oftheir origin
Spetsius, Z.V1 and Griffin,W.L.2,3
1. Institute of Diamond Industry,Mirny, Yakutia, Russia2. GEMOC National Key Centre, MacquarieUniversity, NSW 2109, Australia3. CSIRO Exploration and Mining,P.O. Box 136, N. Ryde, NSW 2113, Australia
Trace elements have been determinedby ICP-MS in garnet and their kelyphitic rims in garnet peridotitesfrom the Udachnaya kimberlite pipe. Comparison of these data withthe trace element patterns of kimberlites shows significant differencesin REE distribution and suggest a deep origin for the kelyphitization.
IntroductionKelyphitic rims are widespread ongrains of garnet in kimberlites and mantle xenoliths from kimberlitepipes in Yakutia and South Africa. Usually they consist of amixture of secondary minerals such as phlogopite, spinel, clino-and orthopyroxene. Some rims are complicated especially on grainsof garnets in kimberlites and consist of two or more zones withdifferent composition. In xenoliths of garnet peridotite fromUdachnaya pipe, garnets are mantled by coronas of phlogopite,spinel and secondary pyroxenes. While major and trace elementzoning in garnet are explained as a result of metasomatic processesat depth (Griffin et al., 1989), the formation of kelyphites aroundgarnets usually is linked to interaction between garnet and ascendingkimberlitic magma. To test this idea, trace element analysis hasbeen undertaken on chemically homogeneous garnet grains with sharpkelyphitic rims more than 300-500 m thick, in two peridotitexenoliths from Udachnaya kimberlite pipe (Spetsius, 1995): U-140/78(sheared Gt - lherzolite) and U-2292 (mosaic-porphyroclastic Gt- harzburgite). Trace elements were measured by laser-ablationICP-MS at Maquarie University, with NIST 610 glass as externalstandard and Ca as internal standard; pit diameters were 40 m.
Results and discussion Observations in thin-section showno notable interaction between peridotite xenoliths and kimberlitemelt. There are no obvious relationship of the thickness of kelyphiticrims with the size of xenoliths or any increase in thickness inthe outer parts of samples. In areas where kimberlite is in contactwith the minerals of the outer zone, there is a sharp border betweengarnet grains and kimberlite minerals. As was shown by Vishnevsky(1991) kelyphitic rims on garnet in xenoliths usually representa mixture of minerals: Ol + Sp + Cpx Opx Phl. Thesame paragenesis is observed in our samples. Significant variationsbetween core and rim in major and trace element composition werenot observed in tested garnets from all samples. This suggestscomplete reequilibration during ductile shearing in the mantle. The REE compositions of the investigatedgarnets are similar, but 2292 shows enrichment in MREE, while140 does not (Fig. 1). The kelyphites in the two samples havetrace-element patterns different from one another, but essentiallyidentical to their host garnet, except for enrichment in Sr. The REE patterns are significantly flatter than would be expectedfrom equilibration with kimberlite magma (Hoal et al., 1994).The very high Sr contents probably reside in secondary clinopyroxene,and locally high Ba (not shown) in secondary phlogopite. Otherwise,the similarity between garnet and kelyphite analyses suggeststhat kelyphitization has not involved extensive exchange witha metasomatic medium. Two models can be proposed to explainthe petrographic features and trace element patterns of the kelyphites.1. PT-estimates for the kelyphiteoverprint, using secondary minerals within coronas (Vishnevsky,1991; Franz et al., 1995) indicate temperatures of 1100-1250 Cand pressures of about 20 kb. These data suggest that kelyphitisationhas occurred under upper mantle conditions. According to Franzet al. (1995), in xenoliths from the Gibeon kimberlite thesemetasomatic processes have occurred within a magma chamber locatedclose to the boundary between upper mantle and lower crust. 2. We propose an alternative explanationbased in particular on trace element data and some petrographicevidence. We emphasise that the trace element patterns of thekelyphite are not related to the kimberlitic magma, but resemblethose of the garnet being replaced (Fig. 1), with addition ofSr, K and probably Ba; HFSE such as Zr, Hf, Ti and Nb, and theREE, were not affected. Experimental trace element partitioningdata (Brenan et al., 1995) indicate that at high pressure manyof these trace elements do not partition into hydrous fluids tothe same degree as into carbonatitic and silicate melts. Thissuggests that kelyphitization of garnet was not a result of interactionwith silicate melt or carbonate-rich fluid, and that hydrous fluidsplay a more important role in formation of kelyphite on garnets(at this stage of mantle metasomatism). The fine grain size ofkelyphite implies that the metasomatic processes which causedreplacement of garnet were short-lived and probably were activenot long before eruption of kimberlites, but the eruption processis very rapid. We therefore suggest that kelyphite formed in responseto the infiltration of hydrous fluids, prior to eruption. Thesource and precise nature of the metasomatic fluids remains problematic,but we speculate that they were related to a protokimberliticmagma.Conclusion Garnet peridotite xenoliths fromkimberlite pipe Udachnaya provide spectacular evidence of themetasomatic kelyphitization of garnets. Petrographic features,mineral associations and trace element patterns in kelyphitesand garnet relicts suggest that these processes took place underupper mantle conditions in the presence of water-rich fluids.Most probably, this stage of metasomatism was caused by protokimberliticfluids shortly before eruption of kimberlites.
ReferencesBrenan, J.M., Shaw, H.F., Ryerson,F.J., and Phinney, D.L., 1995, Mineral-aqueous fluid partitioningof trace elements at 900C and 2Gpa: constraints on thetrace element chemistry of mantle and deep crustal fluids, Geochim.CosmochimActa 55, 2203-2214.Franz, L., Brey, G.P., and OkruschM., 1995. Metasomatic reequilibration of mantle xenoliths fromthe Gibeon kimberlite province (Namibia), Ext. Abstr., Sixth Intern.Kimberlite Conf., 169-171. Hoal, K.E.O., Hoal, B.G., Erlank,A.J., Shimizu, N., 1994, Metasomatism in the mantle lithosphererecorded by rare earth elements in garnets, Earth Planet. Sci.Lett., 126, 303-313. Spetsius, Z. V, 1995, Occurrenceof diamond in the mantle: a case study from the Siberian Platform,J. Geochem. Expl or., 53, p. 25-39.Vischnevsky, A.A., 1991, Kelyphiteson garnets in mantle xenoliths and kimberlites: compositions,genesis, petrological implications, Ext. Abst., Fifth Intern.Kimb. Conf., 571-572.
Back to the GEMOC Abstract Titles Page Explore Macquarie
- New & Current Students
- Future Students
- International Students
- Alumni
- Faculties
- University Administration
- Research
- Learning & Teaching
- News & Events
- Job Opportunities
- Find a Course
Site Publisher: DVC Development and External Relations Last Updated: 18 August 2008 c80f0f1006