Mineralogy and Geology

Dr. Donggao Zhao

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My interests in mantle include a comprehensive study of mantle-derived xenoliths in kimberlites and mineral inclusions in xenocrysts/phenocrysts (diamond, chromite and garnet) from kimberlites, whihc began in 1985 with my M. Sc. thesis in the Chinese Academy of Geological Sciences, in which rock-forming minerals in kimberlite, carbonatite, and lamprophyre and relationships between these associated rocks are investigated. In 1990, my coworkers and I finished a project from the Ministry of Geology and Mineral Resources, China. We employed geochemical, geophysical, remote sensing and satellite image techniques to locate target areas for further, detailed investigation of diamond in the North China Craton. This project was awarded "Science and Technology Progress Award" by the Ministry of Geology and Mineral Resources, China. The geochemical part of the project was published in 1992 as a book entitled "Kimberlites and Diamonds in the North China Craton."

In 1997 I completed a project from the Geology Division of the Indian and Northern Affairs Canada, i.e., "Newly discovered kimberlites and mantle xenoliths from Somerset Island and Brodeur Peninsula, Canada: pressure, temperature, oxygen fugacity, volatile content and age" (Economic Geology Series 1997-5). In this project, pressures, temperatures, and oxygen fugacities of the xenoliths were calculated to evaluate diamond potential. The resultant P-T-fO₂ indicates that some xenoliths come from depths where diamond is stable. The concentrations of hydrous species in garnets are very similar to those of garnets in peridotite xenoliths from the South Africa kimberlite. The preliminary ⁴⁰Ar-³⁹Ar dating suggests that the kimberlites may have erupted in the Cretaceous but retain large quantities of excess mantle-derived ⁴⁰Ar. A new oxygen barometers has been developed for calculation of oxygen fugacity from rutile-ilmenite assemblages using the reaction 2Fe₂O₃ (in ilmenite) + 4TiO₂ (rutile) = 4FeTiO₃ (in ilmenite) + O₂ (referred to as RI). The rutile-ilmenite oxygen barometer is widely applicable to ilmenite-bearing assemblages in the crust and upper mantle, including MARID (mica, amphibole, rutile, ilmenite and diopside) suites in kimberlites, MARID-like veins in kimberlitic xenoliths, rutile and ilmenite in kimberlitic eclogite, rutile-ilmenite intergrowths in kimberlites and Granny Smith diopside megacrysts. The hypothetical end-member RI reaction is located between the magnetite-hematite and Ni-NiO buffers, but shifts down to or below NNO with typical kimberlitic ilmenite compositions. Oxygen fugacities in MARID suites and MARID-like veins lie around NNO buffer and are comparable to those in strongly metasomatized garnet-chromite lherzolites. Data on rutile-ilmenite inside Granny Smith diopside megacrysts show similar fO₂. By assuming the activity of TiO₂ in rutile is unity, this method can be used to calculate an upper limit of fO₂for a single ilmenite grain. In addition, Si contents in mantle-derived chromites may serve as a barometer.

In my Ph.D. dissertation, I finished three projects. One project is about the mineral inclusions in diamonds and in chromites from the Liaoning kimberlites in North China craton. Mineral inclusions in diamonds provide information on the source region of diamonds. Mineral inclusions in chromites, on the other hand, provide information on the source region of kimberlite itself. The results suggest that mineral inclusions in diamonds formed in an earlier stage, most likely, before the formation of kimberlitic magma, and most mineral inclusions in chromites, especially carbonates and hydrous silicates, were trapped probably during the stage of the formation of kimberlitic magma. The chemistries and species of mineral inclusions in diamonds and in chromites suggest that the metasomatism became stronger from the source of diamonds to the source of kimberlite or that the metasomatism was started from the stage of diamond growth until the generation of kimberlitic magma. A second project is the above mentioned mantle xenoliths from the Northwest Territories kimberlites, Canada. The xenoliths studied include garnet lherzolites, garnet-spinel lherzolites, spinel lherzolites, dunite, garnet websterite, spinel websterite and garnet clinopyroxenite. Coarse, protogranular lherzolites, similar to the low-temperature xenoliths from the Kaapvaal and Siberian cratons, comprise the majority of the ultramafic suite at the Nikos kimberlites. A MORID vein (mica-orthopyroxene-rutile-ilmenite-diopside±chromite) was found in a garnet-spinel lherzolite, characterized by high K, Fe, Ti and OH components and probably represents the advanced stages of mantle metasomatism. Calculated P-T of the mantle xenoliths is 25 to 60 kbar and 760 to 1180°C, following a continental geotherm. Calculated fO2 suggests that diamond and graphite tend to be destroyed by the late metasomatic event. In a third project, a new oxygen barometer was developed for calculation of oxygen fugacity from rutile-ilmenite assemblages by using the reaction 2Fe2O3 (in ilmenite) + 4TiO2 (rutile) = 4FeTiO3 (in ilmenite) + O2. The oxygen barometer is applicable to many crustal and mantle rutile-ilmenite assemblages.

Ph D THESIS:

KIMBERLITES, DIAMONDS AND MANTLE XENOLITHS FROM THE NORTH CHINA CRATON AND THE CANADIAN NORTHWEST TERRITORIES

This dissertation is mainly composed of two closely related projects, focusing on the mantle compositions and processes. The first project is about the mineral inclusions in diamonds and in chromites from the Liaoning kimberlites in North China craton (the No. 50 and No. 42 diatremes); the second project is about the mantle xenoliths from the Nikos kimberlites, Somerset Island, and the Zulu kimberlites, Brodeur Peninsula, Baffin Island, Northwest Territories, Canada. In addition, a new oxygen barometer was developed for use in mantle assemblages.

Chapter I. INTRODUCTION Chapter I (PDF)

Chapter II. MINERAL INCLUSIONS IN DIAMONDS FROM THE NO. 50 KIMBERLITE DIATREME, LIAONING PROVINCE, CHINA Chapter II (PDF)

Chapter II addresses mineral inclusions in diamonds from the No. 50 kimberlite diatreme in the Liaoning Province, China. The mineral inclusions were studied by polishing the diamond hosts. The common inclusions identified are olivine, pyrope, chromite, Ca-carbonate, native iron or goethite (?), and graphite. The inclusion assemblages belong mainly to harzburgitic. It is suggested that the diamonds were formed initially by crystallization in melt environments, and then by solid state growth in metasomatic conditions. The temperatures and pressures obtained from applicable thermobarometers indicate that the diamonds crystallized in the range of 1100°C and 50 kbar to 1220°C and 70 kbar, corresponding to a depth of 150 to 200 km and following a conductive geotherm of ~42 mW/m2, consistent with the low heat flow values observed for cratonic shield areas. Sulfide inclusions are common and most were probably initially trapped as solid MSS crystals. In general, Ni contents of peridotitic sulfides are higher than eclogitic sulfides. However, Ni contents of sulfides are overlapping or transitional, it may not always be possible to assign sulfide inclusions to peridotitic or eclogitic assemblages solely based on their Ni contents.

Chapter III. MINERAL INCLUSIONS IN CHROMITES FROM THE FUXIAN KIMBERLITES, LIAONING PROVINCE, CHINA Chapter III (PDF)

Chapter III is about mineral inclusions in chromites from the No. 50 kimberlite diatreme in Liaoning Province, China. The mineral inclusions in chromites from the Liaoning kimberlites form four distinctive groups of silicates, carbonates, hydrous silicates, and sulfides. Olivine inclusions are most abundant and usually occur as isolated grains in the chromite host. Occasionally, more than one inclusion or mineral assemblage can be observed in single chromite grains. The wide and diverse chemical compositions and mineral species of chromites and their mineral inclusions relative to those for the same minerals in the diamonds studied suggest a multiple origin of chromite macrocrysts in kimberlites at the same or different depths. The composite inclusions in chromite (carbonates plus silicates) might represent kimberlitic magma. The chromite macrocrysts can be xenocrysts or phenocrysts. The P-T conditions from the inclusion assemblages and the chromite hosts define a conductive geotherm of ~42 mW/m2, which is overlapping that derived from the mineral inclusions in diamond from the same locality. The depths where chromites came from are extended from ~100 to 180 km, which are wider and shallower than the diamond sources.

The studies on mineral inclusions in both diamonds and chromites from the same locality allow a meaningful comparison between the two assemblages associated with diamonds and chromites. Mineral inclusions in diamonds provide information on the source region of diamonds. Based on the inclusion assemblages in diamonds, diamonds were originated from two distinct sources: peridotitic or eclogitic environments. The peridotitic environments could be further divided into lherzolitic, harzburgitic or pyroxenitic environments. The mineral inclusions in chromites, on the other hand, provide information on the source region of kimberlite itself. The similarities and differences between the mineral inclusion assemblages in diamonds and in chromites (see Table 3.12) suggest: 1) that mineral phases included in diamonds and in chromites come from different sources; 2) that mineral inclusions in diamonds were formed in an earlier stage, most likely, before the formation of kimberlitic magma; 3) that most mineral inclusions in chromites, especially carbonates and hydrous silicates, were trapped probably during the stage of the formation of kimberlitic magma; and 4) that some mineral inclusions in chromites and chromite hosts themselves might be inherited from the early stage of diamond formation. The chemistries and species of mineral inclusions in diamonds and in chromites suggest that the metasomatism became stronger from the source of diamonds to the source of kimberlite or that the metasomatism was started from the stage of diamond growth until the generation of kimberlitic magma.

Chapter IV. MANTLE XENOLITHS FROM THE NIKOS KIMBERLITES ON SOMERSET ISLAND AND THE ZULU KIMBERLITES ON BRODEUR PENINSULA, BAFFIN ISLAND, CANADA Chapter IV (PDF)

Chapter IV mantle xenoliths from the Nikos kimberlites, Somerset Island, and the Zulu kimberlites, Brodeur Peninsula, Baffin Island, Canada are studied for comparison with the research on xenocrysts above. The xenoliths studied include garnet lherzolites, garnet-spinel lherzolites, spinel lherzolites, dunite, garnet websterite, spinel websterite and garnet clinopyroxenite. Coarse, protogranular lherzolites comprise the majority of the ultramafic suite at the Nikos kimberlites. This is similar to the low-temperature xenoliths from the Kaapvaal and Siberian cratons. The absence of high-temperature xenoliths might be a sampling problem. However, the absence of high-temperature xenoliths and the presence of garnet-spinel lherzolite xenoliths at Nikos might also imply that the nature of the upper mantle beneath the northern part of the North American craton is different from those beneath the Siberian and Kaapvaal cratons. A MORID vein (mica-orthopyroxene-rutile-ilmenite-diopside±chromite) is found in a garnet-spinel lherzolite from the Nikos pipes (JP1-X17), which is characterized by high K, Fe, Ti and OH components and probably represents the advanced stages of mantle metasomatism. Calculated pressures are in the range of 25 to 60 kbar and temperatures are from 760 to 1180°C, following a continental geotherm. The fO2 of the xenoliths calculated by olivine- orthopyroxene-spinel oxygen barometer is from 1.3 log units above to 0.6 log units below EMOD, suggesting that diamond may or may not be stable relative to carbonates. However, the MORID vein in the garnet-spinel lherzolite yield a much more oxidizing fO2 than EMOD. Therefore, diamond and graphite tend to be destroyed by the late metasomatic event represented by the MORID fluid or melt.

Chapter V. AN OXYGEN BAROMETER FOR RUTILE-ILMENITE ASSEMBLAGES: OXIDATION STATE OF METASOMATIC AGENTS IN THE MANTLE Chapter V in (PDF)

In Chapter V, a method is developed for calculation of oxygen fugacity from rutile-ilmenite assemblages by using the reaction 2Fe2O3 (in ilmenite) + 4TiO2 (rutile) = 4FeTiO3 (in ilmenite) + O2 (referred to as RI). An available mixing model for Fe2O3-FeTiO3-MgTiO3 is applied to calculate the activities of Fe2O3 and FeTiO3 in ilmenite. The RI reaction is applicable to crustal rutile-ilmenite assemblages, rutile-ilmenite intergrowths in kimberlites, MARID (mica-amphibole-rutile-ilmenite-diopside) suites in kimberlites, MORID veins in kimberlitic xenoliths, and Granny Smith diopside megacrysts. Oxygen fugacities in MARID suites and MORID vein lie around Ni-NiO buffer and are comparable to those in strongly metasomatized garnet-chromite lherzolites. Data on rutile-ilmenite inside Granny Smith diopside megacrysts show similar fO2. By assuming the activity of TiO2 in rutile is unity, this method can be used to calculate an upper limit of fO2 for a single ilmenite grain. Hematite-rich ilmenite megacrysts in kimberlites have an upper limit of fO2 somewhere between Ni-NiO and hematite-magnetite buffers. The rutile-ilmenite oxygen barometer is widely applicable to ilmenite-bearing assemblages in the crust and upper mantle.

Chapter VI. CONCLUSIONS Chapter VI (PDF)

Appendix 2.1 olivine in diamond in xls format
Appendix 2.2 orthopyroxene in diamond in xls format
Appendix 2.3 garnet in diamond in xls format
Appendix 2.4 spinel in diamond in xls format
Appendix 2.5 sulfide in diamond in xls format
Appendix 2.6 peridotite assemblages in diamond in xls format
Appendix 3.1 spinel/chromite host in xls format
Appendix 3.2 olivine in spinel/chromite host in xls format
Appendix 4.1 Northwest Territories Mantle Xenoliths in xls format

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