Jan Kramers

Bio: Jan Kramers is an academic researcher from University of Johannesburg. The author has contributed to research in topics: Metamorphism & Craton. The author has an hindex of 56, co-authored 175 publications receiving 20428 citations. Previous affiliations of Jan Kramers include University of Liverpool & Leipzig University.

Noble gases, K, U, Th, and Pb in native gold

Abstract: We present determinations of the noble gas and Pb isotopic abundances and of K, Th, and U concentrations of native gold. Our results demonstrate that gold is an excellent carrier for crustal volatiles, but direct dating of gold using the U, Th-4He, 40K–40Ar, and U fission Xe methods was not successful for various reasons. The main significance of this work is the great sensitivity of gold for trapped gases as well as for gases that were produced in situ which gives the prospects of using gold and its fluid and solid inclusions for the study of paleogas composition. Numerous nuclear effects characterize the noble gas inventory of placer gold from Switzerland and Italy, vein gold from Italy, South Africa, and Venezuela, and lode gold from South Africa. The degassing patterns obtained by mass spectrometry show a low-temperature release of volatiles around 500°C from fluid inclusions mainly in vein gold and a high-temperature release from solid inclusions and the gold itself. The low-temperature volatiles represent species that were trapped when the gold crystallized. We investigated the following trapped species: the isotopes of He, Ne, Ar, Kr, Xe, and Pb, and the abundances of K, U, Th, H2O, and CO2. The crustal gases trapped by gold comprise 3He from 6Li(n,α)3H → β− → 3He, 4He and 40Ar from the U, Th, and K decay, and Xe from 238U fission. We observe 4He/40Ar = 3.9 for the radiogenic trapped gases of tertiary gold and a ratio of 1.4 for Archean gold. These ratios are consistent with the production ratios from U and K at the respective times and demonstrate that gold can be used as a sampler of ancient atmospheric gases. The concentrations of U and Th range from a few parts per billion to a few parts per million, and those of K and Pb range up to some tens of parts per million. The antiquity of trapped Pb is indicated by the Pb-Pb model age of about 3000 Ma for the lead extracted from vein gold and quartz of the Lily gold mine (South Africa). Gold also contains noble gases produced in situ by U, Th, and K decay. We obtained 4He up to the extremely high concentration of 6.8×10−2 cm3 STP/g (STP: atmospheric pressure and 0°C) for gold separated from a specimen of the Witwatersrand West Rand gold fields, indicating that this gold has a large U content.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Abstract: Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Three natural zircon standards for u‐th‐pb, lu‐hf, trace element and ree analyses

Abstract: We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.