Showing papers by "Jan Kramers published in 2003"

Holocene forcing of the Indian monsoon recorded in a stalagmite from southern Oman.

Abstract: A high-resolution oxygen-isotope record from a thorium-uranium-dated stalagmite from southern Oman reflects variations in the amount of monsoon precipitation for the periods from 10.3 to 2.7 and 1.4 to 0.4 thousand years before the present (ky B.P.). Between 10.3 and 8 ky B.P., decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores, indicating that early Holocene monsoon intensity is largely controlled by glacial boundary conditions. After approximately 8 ky B.P., monsoon precipitation decreases gradually in response to changing Northern Hemisphere summer solar insolation, with decadal to multidecadal variations in monsoon precipitation being linked to solar activity.

Indian Ocean Climate and an Absolute Chronology over Dansgaard/Oeschger Events 9 to 13

Abstract: Oxygen-isotope ratios of a stalagmite from Socotra Island in the Indian Ocean provide a record of changes in monsoon precipitation and climate for the time period from 42 to 55 thousand years before the present. The pattern of precipitation bears a striking resemblance to the oxygen-isotope record from Greenland ice cores, with increased tropical precipitation associated with warm periods in the high northern latitudes. The largest change, at the onset of interstadial 12, occurred very rapidly, in about 25 years. The chronology of the events found in our record requires a reevaluation of previously published time scales for climate events during this period.

Importance of water for Archaean granitoid petrology: a comparative study of TTG and potassic granitoids from Barberton Mountain Land, South Africa

Abstract: A new model for Archaean granitoid magmatism is presented which reconciles the most important geochemical similarities and differences between tonalite-trondhjemite-granodiorite (TTG) and potassic granitoids. Trace element abundances reveal a strong arc magmatism signature in all studied granitoids from Barberton Mountain Land. Characteristic features include HFSE depletion as well as distinct enrichment peaks of fluid-sensitive trace elements such as Pb in N-MORB normalisation, clearly indicating that all studied granitoids are derived from refertilised mantle above subduction zones. We envisage hydrous basaltic (s.l.) melts as parental liquids, which underwent extensive fractional crystallisation. Distinctive residual cumulates evolved depending on initial water content. High-H2O melts crystallised garnet/amphibole together with pyroxenes and minor plagioclase, but no olivine. This fractionation path ultimately led to TTG-like melts. Less hydrous basaltic melts also crystallised garnet/amphibole, but the lower compatible element content indicates that olivine was also a liquidus phase. Pronounced negative Eu-anomalies of the granitic melts, correlating with Na, Ca and Al, indicate plagioclase to be of major importance. In the context of our model, the post-Archaean disappearance of TTG and concomitant preponderance of granites (s.l.), therefore, is explained with secular decrease of aqueous fluid transport into subduction zones and/or efficiency of deep fluid release from slabs.

The Source of the Great Dyke, Zimbabwe, and Its Tectonic Significance: Evidence from Re‐Os Isotopes

Abstract: Re‐Os data for chromite separates from 10 massive chromitite seams sampled along the 550‐km length of the 2.58‐Ga Great Dyke layered igneous complex, Zimbabwe, record initial 187Os/188Os ratios in the relatively narrow range between 0.1106 and 0.1126. This range of initial 187Os/188Os values is only slightly higher than the value for the coeval primitive upper mantle (0.1107) as modeled from the Re‐Os evolution of chondrites and data of modern mantle melts and mantle derived xenoliths. Analyses of Archean granitoid and gneiss samples from the Zimbabwe Craton show extremely low Os concentrations (3–9 ppt) with surprisingly unradiogenic present‐day 187Os/188Os signatures between 0.167 and 0.297. Only one sample yields an elevated 187Os/188Os ratio of 1.008. Using these data, the range of crustal contamination of the Great Dyke magma would be minimally 0%–33% if the magma source was the primitive upper mantle, whereas the range estimated from Nd and Pb isotope systematics is 5%–25%. If it is assumed...

Geochemistry of metabasalts from ophiolitic and adjacent distal continental margin units: Evidence from the Monte Rosa region (Swiss and Italian Alps)

Abstract: In this paper we present new whole rock analyses of amphibolites from the ophiolitic and adjacent continental tectonic units in the Monte Rosa region. Mg numbers and Ni contents indicate that these amphibolites were derived from fractionated magmas with compositions ranging from Eto N-MORB. Based on their Ni, Ti, REE and Nb systematics, the metabasalts from the ophiolitic Zermatt-Saas and Antrona units and from the continental units of the Furgg zone and the Portjengrat unit are ascribed to a common origin. They represent a coherent suite ranging from Tto N-MORB. In contrast, amphibolites from the continental Siviez-Mischabel and Monte Rosa nappes were derived from enriched MORB and/or gabbroic precursors, which are not related to the metabasalts from the ophiolites, the Furgg zone or the Portjengrat unit. The geochemical differences between the basalts of the ophiolitic Zermatt-Saas and Antrona units and the adjacent continental Furgg zone and the Portjengrat unit are very subtle. Most mafic rocks were derived from low to moderate degrees of melting of an N-MORB type mantle source. Some compositional parameters such as (Ce/Sm)n, Zr* and (Nb/Zr)n indicate a transition from T-MORB compositions in the continental units towards less enriched compositions in the ophiolitic units. Y, Ti, V and Zr concentrations are highly correlated in the metabasalts from the Furgg zone, whereas such inter-element correlations are less well defined in the metabasalts from the ophiolitic units. This renders the previously proposed interpretation of the Furgg zone amphibolites as tectonically incorporated ophiolitic fragments unlikely. Our data rather suggest that the distal continental units (Portjengrat unit and Furgg zone) and the nearby ophiolitic units were intruded by similar magmas. Portjengrat unit and Furgg zone are interpreted as a formerly continuous tectonic unit which, based on structural grounds, represents the ocean-continent transition zone of the Briançonnais to the immediately adjacent oceanic Antrona unit. However, the ambiguity in the paleogeographic provenance of the Antrona unit (Valais vs. Piemont-Liguria ocean) cannot be resolved with the existing geochemical data.