Geochemistry of lavas from Mohns Ridge,Norwegian-Greenland Sea: implications for melting conditions and magma sources near Jan Mayen |
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Authors: | K M Haase Colin W Devey Dieter F Mertz Peter Stoffers Dieter Garbe-Schönberg |
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Institution: | Geologisch-Pal?ontologisches Institut der Universit?t Kiel, Olshausenstr. 40, D-24118 Kiel, Germany, DE Institut für Geowissenschaften der Universit?t, D-55099 Mainz, Germany, DE
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Abstract: | Mohns Ridge lavas between 71 and 72°30′N (∼360 km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted
tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts
from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears
to be tapped preferentially by low degree melts (∼5%) close to Jan Mayen but also shows its signature much further north on
Mohns Ridge. Three lava types with different incompatible element compositions e.g. chondrite-normalized (La/Sm)N<1 to >2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic
melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably
buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic
mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead
we suggest that the melting may be induced by the lowered solidus temperature of a “wet” mantle. Mohns MORB (mid ocean ridge
basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/La >10).
These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and
80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source
but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during
the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot
activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental
fragment, a fracture zone and a spreading axis in this part of the North Atlantic.
Received: 3 May 1995 / Accepted: 6 November 1995 |
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