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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.
Topics: Metamorphism, Craton, Cave, Granulite, Zircon


Papers
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Journal ArticleDOI
01 Nov 1997-Geology
TL;DR: In this paper, Re-Os isotope results on chromites separated from age-constrained Archean (27 to 35 Ga) ultramafic intrusions in the Zimbabwe craton show that chromites average a present-day 187Os/188Os ratio of 1 066.
Abstract: Re-Os isotope results on chromites separated from age-constrained Archean (27 to 35 Ga) ultramafic intrusions in the Zimbabwe craton show that chromites average a present-day 187Os/188Os ratio of 01066, significantly below the bulk silicate Earth value The source reservoir is interpreted to reflect the Zimbabwean subcontinental lithospheric mantle (SCLM) Archean Re depletion of the SCLM has previously been postulated on the basis of xenolith studies from southern Africa, Siberia, and the Wyoming craton Because our samples are age constrained, however, the Re-Os data also shed light on the evolution of the Archean SCLM through time They indicate that the Zimbabwean SCLM began to be separated from asthenospheric mantle before 38 Ga and grew quasi-continuously throughout the Archean This result is reconciled with present-day SCLM data in forward modeling

61 citations

Journal ArticleDOI
TL;DR: In this article, structural, petrologic, and age data show that tectonic styles and metamorphic histories in the West Alldays area of the Central Zone of the Limpopo Complex, southern Africa, can be linked to neo-Archean and Paleoproterozoic high-grade (granulite facies) tectonometamorphic events.
Abstract: New field, structural, petrologic, and age data show that tectonic styles and metamorphic histories in the West Alldays area of the Central Zone of the Limpopo Complex, southern Africa, can be linked to neo‐Archean and Paleoproterozoic high‐grade (granulite facies) tectonometamorphic events. The styles comprise (1) a regionally developed high‐grade (S2) gneissic fabric that evolved into a regional (D2) system of sheath folds mapped as circular to oval‐shaped structures with lineations plunging steeply to the WSW and (2) a system of mainly N‐S‐trending and north‐verging (D3) shear zones characterized by high‐grade tectonites (“straight gneisses”) with well‐developed (S3) gneissic fabrics. In the West Alldays area, the superimposition of D3 onto the regional D2 fold pattern produced the kilometer‐scale N‐S‐trending Baklykraal shear zone, which, before this study, was mapped as the Baklykraal fold and interpreted as part of the regional D2 fold pattern in the Central Zone. The Paleoproterozoic high‐...

57 citations

Journal ArticleDOI
TL;DR: It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.
Abstract: A combined procedure for separating Lu, Hf, Sm, Nd, and rare earth elements (REEs) from a single sample digest is presented. The procedure consists of the following five steps: (1) sample dissolution via sodium peroxide sintering; (2) separation of the high field strength elements from the REEs and other matrix elements by a HF-free anion-exchange column procedure; (3) purification of Hf on a cation-exchange resin; (4) separation of REEs from other matrix elements by cation exchange; (5) Lu, Sm, and Nd separation from the other REEs by reversed-phase ion chromatography. Analytical reproducibilities of Sm-Nd and Lu-Hf isotope systematics are demonstrated for standard solutions and international rock reference materials. Results show overall good reproducibilities for Sm-Nd systematics independent of the rock type analyzed. For the Lu-Hf systematics, the reproducibility of the parent/daughter ratio is much better for JB-1 (basalt) than for two analyzed felsic crustal rocks (DR-N and an Archaean granitoid). It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.

54 citations

Journal ArticleDOI
TL;DR: In this article, major and trace element analyses and Nd-Pb isotope measurements on different components of two migmatite outcrops in the Limpopo Belt (Lose Quarry, Botswana, and Palala, South Africa) were carried out in order to assess the effect of migmatization during a high grade metamorphic event on Sm-Nd systematics and nd model ages.

52 citations


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01 Jan 1989
TL;DR: In this article, trace-element data for mid-ocean ridge basalts and ocean island basalts are used to formulate chemical systematics for oceanic basalts, 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.
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.

19,221 citations

Journal ArticleDOI
TL;DR: In this paper, the authors compared the relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites and found that the most consistent composition of the Earth's core is derived from the seismic profile and its interpretation, compared with primitive meteorites, and chemical and petrological models of peridotite-basalt melting relationships.

10,830 citations

Journal ArticleDOI
TL;DR: In this paper, a new calculation of the crustal composition is based on the proportions of upper crust (UC) to felsic lower crust (FLC) to mafic lower-crust (MLC) of about 1.6:0.4.

5,317 citations

Journal ArticleDOI
TL;DR: In this paper, the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses are reported.
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.

4,845 citations