Deon de Bruin

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Multiple compositional megacryst groups from the Uintjiesberg and Witberg kimberlites, South Africa

Mineral chemistry of megacrysts from the Uintjiesberg and Witberg kimberlites has been used to delineate a number of distinct populations at each locality. The broad and overall complex compositional trends of clinopyroxene megacrysts have been simplified into Low-Cr, High-Cr and High-Al populations. Most of these populations display related inter-element trends that can easily be interpreted to result from fractional crystallisation from single or similar magma batches. The Low-Cr population, at both localities, shows chemical characteristics that fall within, or close to, the compositional range for Cr-poor megacrysts from typical southern African Group I kimberlites such as the Monastery kimberlite, whereas the High-Cr populations are different, as they have compositional trends that initiate at elevated magnesium-number and Cr 2 O 3 contents. Compositions similar to the High-Al clinopyroxene megacrysts are unusual in kimberlites but are similar to megacrysts derived from alnoites and alkalic basalts. The origin of the different megacryst groups points towards the existence of multiple magmas prior to intrusion of the kimberlite. Such magmas could be generated through the differential interaction of an alkaline mafic magma with the sub-continental lithospheric mantle or originate from heterogeneous sources. Pressure and temperature calculations show that the depth of formation for megacrysts for the off-craton Uintjiesberg kimberlite is −1.0 GPa less than for the on-craton Witberg kimberlite.

Corona textures in Proterozoic olivine melanorites of the equeefa suite, Natal metamorphic province, South-Africa

SummaryOlivine-plagioclase and phlogopite-plagioclase coronas have been identified from olivine melanorites of the Mid- to Late Proterozoic Equeefa Suite in southern Natal, South Africa. Olivine, in contact with plagioclase, is mantled by a shell of clear orthopyroxene, in turn rimmed by pale green (pargasitic) clinoamphibole. Locally a third rim, composed of a fine pargasite-spinel symplectite is developed adjacent to the plagioclase. The second corona reaction has produced greenish-brown pargasite at phlogopite-plagioclase interfaces. A third, less obvious reaction, between olivine and phlogopite is also noted. Analytical data of all the mineral phases present, along with the coronas, are given. Two-pyroxene thermometry yields magmatic core temperatures (∼ 1120°C), with rim compositions indicating equilibration at ∼ 850°C. Consistent with this, the modelled olivine-plagioclase reaction occurs between 830–1050°C with awaterbetween 0.1 and 1.0 at 7 kbar. The three reactions took place during a prolonged history of cooling and partial hydration of the magmatic olivine melanorites from over 1000°C down to ∼ 600°C. The P-T conditions indicated by the reactions suggest this cooling process was essentially isobaric, indicating that the area was not subjected to rapid uplift or burial throughout this entire period.ZusammenfassungAus Olivin-Melanoriten der mittel- bis spätproterozoischen Equeefa Suite im südlichen Natal, Südafrika, wurden Olivin-Plagioklas und Phlogopit-Plagioklas-Koronartexturen beobachtet. Olivin, der mit Plagioklas im Kontakt steht, wird von einem klaren Saum vom Orthopyroxen ummantelt, der seinerseits von blaßgrünem (pargasitischem) Klinoamphibol umsäumt wird. Stellenweise ist ein dritter Saum, bestehend aus feinkörnigem symplektitischem Pargasit-Spinell im Kontakt mit Plagioklas ausgebildet. Die zweite koronabildende Reaktion resultiert in Bildung eines grünbraunen Pargasites an Phlogopit-Plagioklas Kornkontakten. Eine dritte, weniger auffällige Reaktion zwischen Olivin und Phlogopit wurde ebenfalls beobachtet. Zwei-Pyroxen-Thermometrie ergab magmatische Temperaturen der Kernbereiche (∼ 1120°C) und belegt eine Gleichgewichtseinstellung in den Randzonen bei ca. 850°C. Olivin-Plagioklas-Modellreaktionen liegen ebenfalls in einem Temperaturbereich von 830–1050°C bei Wasseraktivitäten von 0.1 bis 1.0 und einem Druck von 7 kbar. Die drei Reaktionen liefen im Zuge einer länger andauernden Abkühlung unter teilweiser Hydratisierung der magmatischen Melanorite in einem Temperaturbereich von 1000°C bis ca. 600°C ab. Die aus den Reaktionen ableitbaren P-T-Bedingungen sprechen für eine im wesentlichen isobare Abkūhlungsgeschichte und zeigen, daß dieses Gebiet wáhrend dieser gesamten Periode keiner raschen Hebung bzw. keiner Versenkung unterworfen worden ist.