# Petrogenesis of garnet and spinel peridotites : a study with particular reference to the role of chromium in geothermometry and geobarometry

Nickel, KG 1983 , 'Petrogenesis of garnet and spinel peridotites : a study with particular reference to the role of chromium in geothermometry and geobarometry', PhD thesis, University of Tasmania.

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## Abstract

Spinel and garnet peridotites are the dominant rock types of the upper mantle. Petrography, mineralogy, bulk rock and mineral chemistry of spinel lherzolites from inclusions in basanites in Victoria, Australia were studied to provide evidence for their petrogenesis and relation to host rocks and to other xenolithic rock types occurring within these host rocks. Experimental studies in ultramafic systems were carried out to infer P,T conditions of the origin of garnet peridotites, particularly those of xenolithic occurrence in kimberlites reported in the literature, to investigate the relation between garnet and spinel peridotites and to give clues to the relation between host rocks, garnet peridotites and simultaneously occurring xenoliths, megacrysts and minerals.
Ultramafic inclusions from two neighbouring localities in Victoria, Australia (Lake Bullenmerri and Mt Leura) include spinel lherzolites with and without hydrous phases, wehrlites, pyroxenites and hornblendites. Mineral chemistry provides evidence for equilibrium crystallization for individual nodules at a depth near 45 km but over a range of temperatures. Temperature estimates yield systematic differences between anhydrous assemblages > phlogopite-bearing assemblages > amphibole-bearing assemblages. Bulk rock variation of MgO, CaO, Al_2O_3 and compatible element contents in lherzolite and harzburgite has been modelled as an early partial melting event, giving rise to various degrees of depletion. The extracted liquid was of picritic composition.
Amphiboles in spinel lherzolites are developed independently of the early partial melting event and postdate it They crystallized as a response to near-isochemical metamorphic reaction, consequent on addition of water. The hydration events predate but are not precursor conditions for production of basanite. Hydration-metasomatism occurs in the uppermost mantle above the LVZ, but this is most probably not the region of formation of the alkaline magams. The emplacement and passage of alkaline magmas through the lithosphere/upper mantle may be the cause of local metasomatism and hydration.
Wehrlites, pyroxenites, some lherzolites, and hornblendites are precipitates from magmas fractionating and/or crystallizing at mantle depths. Observable wallrock reaction is extremely restricted (about 1 cm) as evidencedby composite xenoliths.
Experiments in the systems CaO-MgO-Al_2O_3-SiO_2 (CMAS) and SiO_2-MgO- Al_2O_3-CaO-Cr_2O_3 (SMACCR) show that spinel and garnet in peridotitic assemblages are related by an univariant reaction in the CHAS system, but coexist with each other over a range of P,T conditions in the SMACCR system. Both the width and location of this field in P,T space is dependent on the Cr/Cr+Al ratio of the bulk composition.
Temperature estimates for assemblages containing two pyroxenes by the method of Wells (1977) give satisfactory results for the range of 1000- 1400°C and 15-40 kb in all systems studied (CMS, SMACCR and multicomponent "natural" systems). Doubts on the accuracy of this thermometer exist particularly at loWtemperatures (<900°C), but for most xenolithic occurrences of natural garnet lherzolites,'temperatures may be estimated by this method with considerable confidence.
Cr-Al distributions between ortho- and clinopyroxene as well as between pyroxenes and spinel show a regular behaviour in the SMACCR system, independent of pressure and temperature. The Cr-Al exchange between garnet and spinel is sensitive to temperature in the SMACCR system above 1200°C and a thermometer based on this, reaction has been calibrated for the simple system. . Experiments in multicomponent systems show however strong compo-sitional dependencies of this reaction, rendering it as of no use for practical geothermometry.
The solubility of both Al_2O_3 and Cr_2O_3 in orthopyroxene coexisting with garnet and clinopyroxene is dependent on pressure, temperature and composition of the coexisting phases. By both thermodynamic reasoning and empirical curve-fitting these dependencies have been modelled in the CMAS and SMACCR systems, resulting in barometric expressions for these systems. The results have been combined with recent experimental investi-gations in Fe-bearing systems and two independent empirical geobarometers were calibrated, giving satisfactory and consistent results when applied to experiments in multicomponent systems and natural garnet lherzolites.
P,T estimates for various suites of garnet lherzolite xenoliths show differing distributions of P and T comditions of equilibration for different provinces and host rocks. Low temperature xenoliths (<1100°C) from South Africa and Russia outline a geotherm comparable to steady-state geotherms for continental shields. High-temperature xenoliths (>1100°C) from South Africa are derived from a narrow pressure interval around 48-51 kb, covering a range of temperatures from 1100-1400°C. Russian samples with transitional temperatures (1050-1200°C) indicate a parallel trend to South African'xenoliths. Garnet lherzolites from lanprophyre (Four-Corners area, southwestern U.S.A.) equilibrated over a range of =1000-1200°C at a narrow pressure range around 36-39 kb. Samples from oceanic environments (Solomon Islands), young continental areas (N.S.W., Australia) and the Ile Bizard locality (Canada) show P,T conditions of equilibration in agreement for estimates of P,T conditions for oceanic upper mantle based on heat-flow data or convecting mantle models.
The data on P,T estimates for garnet lherzolites from South Africa have been combined with melting studies on peridotitic systems and lquidus studies on kimberlite to infer conditions of 160-180 km depths and 1400-1500°C for the origin of South African kimberlites. Megacrysts and cumulates found in those kimberlites are precipitates from pre-dating magmas fractionating and/or crystallizing at mantle depths. Diamonds crystallized from pre-dating magmas at conditions close to the graphite-diamond boundary (900-1300°C, 40-55 kb). Megacrysts, cumulates and diamonds are probably related to kimberlitic magmas, but are xenolithic/ xenocrystic to the host kimberlite.

Item Type: Thesis - PhD Nickel, KG Peridotite, Garnet, Petrogenesis, Ultrabasic rocks Copyright 1983 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Thesis (PhD) - University of Tasmania, 1983. Bibliography: leaf 278-294 View statistics for this item

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