Abstract Present-day continental lithospheric mantle (CLM) heat production estimates vary considerably and likely overestimate heat generation due to the infiltration of the host magma (i.e., kimberlite), mantle metasomatism or variable heat-producing element (HPE) ratios. We present estimates of heat production in the CLM beneath Jagersfontein, from bulk rock reconstruction of 11 peridotitic xenoliths based on in-situ analyses of primary mineralogy, to avoid kimberlite contamination. Higher concentrations of Th and U are observed in the reconstructed bulk rocks at shallower depths (< 5 GPa) and decrease towards the deepest parts of the CLM (Th: 0.5–26 versus 1–5 ppb; U: 0.4–19 versus 1–3 ppb). Moreover, the reconstructed samples have a broad range of bulk K/U (~ 70-16500) and Th/U ratios (~ 0.2–3.8), outside the expected range of the modern convecting mantle. A crucial factor is garnet, as it can control the U budget, has Th/U < 1 and is present across the CLM in the garnet stability field. The differences of the CLM with the convecting mantle challenge the use of assumedly constant HPE ratios to calculate the heat production. Our estimates of present-day heat generation from reconstructed bulk data yield ~ 0.0002–0.008 µW/m3 at shallow depths, decreasing down to ~ 0.0005 µW/m3 near the lithosphere-asthenosphere boundary, lower than typical heat generation values used in most previous models. The variable heat production in the CLM derives from the metasomatism and re-fertilization near the base caused by rising asthenospheric melts, which react and fractionate as they ascend, potentially carrying most of the HPE in a fluid phase to shallower depths.

Abstract Petrographic and other microscopy analyses which require the use of polarised microscope, is an essential tool for the geologist and materials scientists, and thus, there is an effort to advance it technologically by introducing digital automation and digital image acquisition and processing. Studies with the polarising microscope now become faster, easier and quantitative. However, still the combination of the microscope’s optical components, such as lenses, filters, polarisers, mirrors, and often light sources, can introduce chromatic aberrations, light intensity variations, different degrees of polarisation, which can significantly affect digital image acquisition and processing, especially when sensitive birefringence colours are acquired. In this paper, we describe a method to characterise the full polarising microscope setup, by registering the parameters that affect image acquisition. Image corrections can then be dynamically applied during the operation of the microscope, increasing the precision of digital image processing techniques. The suggested procedure involves the use of a spectrometer to quantify the applied corrections. Different modes in transmitted light conditions, such as under plane polarised, crossed polarised, and circularly polarised light conditions are studied and the Correlated Colour Temperatures (CCT) are computed, also during rotation of the specimen under the polarising light. We demonstrate this on petrographic thin rocks sections, however, the same method can also be used when inspecting technological materials that inhibit birefringence, such as liquid crystals, polymeric fluids, thin films, plastics, optical fibres, and biological samples such as collagen and proteins.

Abstract Trace element and Sr-Nd-Pb isotope compositions of high-density fluids (HDFs) trapped in diamonds provide key insights into mantle processes and diamond formation. This study focuses on diamonds containing different HDF types from the Voorspoed carbonate-rich olivine lamproite (CROL) in the Kroonstad cluster, South Africa. Their trace elements reveal signatures varying between primitive mantle-normalized incompatible enriched fractionated patterns mostly characterizing saline HDFs, and overall flatter patterns for silicic-carbonatitic compositions. The HDFs Sr-Nd-Pb isotope compositions vary markedly; 87Sr/86Sr = 0.70647–0.71556, 143Nd/144Nd = 0.5113–0.5122, 206Pb/204Pb = 17.36–18.77, 207Pb/204Pb = 15.41–15.71 and 208Pb/204Pb = 37.47–39.39. A Rb–Sr age of 780 ± 220 Ma recorded by the saline HDFs does not correspond with the timing of their host diamonds formation (~ 160–220 Ma; based on nitrogen aggregation estimates). The age records an earlier metasomatic event associated with formation of the silicic-carbonatitic HDFs and diamond (~ 330–730 Ma; based on nitrogen aggregation estimates), that likely took place during the Pan-African Orogeny. We suggest that Neoproterozoic subduction-related saline fluids infiltrated different lithologies in the Kroonstad lithospheric mantle. Upon interaction with eclogite, melting occurred and diamonds crystallized, forming the older silicic-carbonatitic HDF-bearing diamonds with lower alkalis and La/Nb, Th/Nb, La/Sm ratios. Concurrently saline fluids that penetrated harzburgite had little interaction with the host rock and were stored as metasomes. These metasomes were locally re-melted during subsequent thermal event/s, potentially the Karoo flood basalt volcanism (~ 180 Ma), to form saline HDFs and their host diamonds. Later metasomatism that involved high-Mg carbonatitic HDFs was smaller in scale than the previous diamond-forming events and took place at < 160 Ma (< 30 Myr before the Voorspoed CROL erupted). The similarities in trace element and isotope compositions between Voorspoed HDFs and Kroonstad CROLs, support some degree of shared lithospheric origin or similar metasomatic processes that controlled their compositions.