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.

Hierarchical Earth accretion and the Hadean Eon

Abstract: Geochemical traces from the Hadean Eon and terrestrial siderophile and volatile element data are discussed in the light of the standard, or hierarchical, model of planetary accretion, which envisages growth of the planets by collisions of progressively larger protoplanets. Siderophile element depletion patterns can be explained by partial inheritance from early stages of the process, when metal cores resulted from melting in collisions of small planetesimals. The abundances and isotope data for hydrogen and nitrogen suggest a chondrite-type source rather than nebular gas, in accord with atmosphere loss in the high-temperature aftermath of the Moon-forming Giant Impact, after which late accretion replenished the atmosphere. Lead isotope patterns and anomalous 142 Nd/ 144 Nd ratios in early Archaean metasediments and 176 Hf/ 177 Hf ratios of 3.7–4.4 Ga old detrital zircons point to a vanished crust that persisted through much of the Hadean Eon. A working hypothesis is proposed that links these observations. Following the freezing of a magma ocean caused by the Giant Impact, the mantle would be gravitationally unstable and an overturn would occur, leading to the formation of a huge mafic crust. After the overturn the mantle could be stable and inactive for hundreds of million years.

Mesothermal gold lodes in the north-western Alps: A review of genetic constraints from radiogenic isotopes.

Abstract: Identifying the source of the hydrothermal fluid responsible for mesothermal gold lodes in orogenic belts has proven to be a formidable hurdle. As a consequence, several key aspects of the genesis of this worldwide class of deposits remain poorly understood. This article reviews a wide spectrum of published data on the Monte Rosa Gold District, a belt of mesothermal gold lodes in the Alpine orogen. The data include Sr-, Pb- and He-Ar isotopes, 40 Ar/ 39 Ar chronology, fluid-inclusion compositions, mineralogy, and the geological framework of the deposits. It is demonstrated that simultaneous consideration of several radiogenic isotope systems is highly valuable in reconstructing the source characteristics and processes of ore deposition in open hydrothermal systems. The genetic model suggested by these data involves prograde metamorphic devolatilization of Mesozoic calcschists during mid-Tertiary continental collision and orogenic uplift. The liberated fluids scavenged gold from metabasites interlayered with the calcschists, then ascended to form auriferous quartz-carbonate-sulphide veins in rocks undergoing retrograde metamorphism. Metamorphic hydrothermal systems of this type recurred along the district over a period of at least 20 Ma, their location and timing being controlled by the progress of differential uplift of the north-western Alps. The nature of this “temporal continuum” of mineralisation contrasts with that reported for Archean gold-lode deposits. As well as clarifying aspects of gold-lode genesis, the isotopic approaches discussed have great potential to constrain scenarios of large-scale fluid flow – with or without mineralising potential – in orogenic belts of all ages.

Constraining Holocene sea levels using U-Th ages of phreatic overgrowths on speleothems from coastal caves in Mallorca (Western Mediterranean)

Abstract: Phreatic overgrowths on speleothems (POS) are carbonate formations deposited at the water table of caves in unique karstic coastal settings having morphologies that can be directly related to sea level at the time of formation. The U-Th ages of calcite and aragonite overgrowths collected from the modern water table in coastal caves on Mallorca (Cova de Cala Varques A and Cova des Pas de Vallgornera) were determined using high-precision MC-ICPMS techniques. U-Th ages indicate that phreatic carbonate deposition occurred between ca 2·8 and at least 0·6 ka BP and are in accord with an archeologically estimated age of 3·7–3·0 ka BP for a drowned prehistoric construction at a depth of 1 m below current sea level in a cave from the same area. Speleothem δ13C and δ18O and chemical composition of cave pools provide supportive evidence that POS reflect mixing between seawater and brackish water table. Copyright © 2010 John Wiley & Sons, Ltd.

A multi-dating approach applied to proglacial sediments attributed to the Most Extensive Glaciation of the Swiss Alps

Abstract: The number and the timing of Quaternary glaciations of the Alps are poorly constrained and, in particular, the age of the Most Extensive Glaciation (MEG) in Switzerland remains controversial. This ice advance has previously been tentatively correlated with the Riss Glaciation of the classical alpine stratigraphy and with Marine Isotope Stage (MIS) 6 (186–127 ka). An alternative interpretation, based on pollen analysis and stratigraphic correlations, places the MEG further back in the Quaternary, with an age equivalent to MIS 12 (474–427 ka), or even older. To re-evaluate this issue in the Swiss glaciation history, a multi-dating approach was applied to proglacial deltaic ‘Hohenschotter’ deposits in locations outside the ice extent of the Last Glacial Maximum. Results of U/Th and luminescence dating suggest a correlation of the investigated deposits with MIS 6 and hence with the Riss Glaciation. Cosmogenic burial dating suffered from large measurement uncertainties and unusually high 26Al/10Be ratios and did not provide robust age estimates.

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.