Geochemistry and Pb Isotopes from the Jan Mayen Fracture Zone and the Extinct Aegir Ridge
Abstract
The Aegir Ridge appears as a major gap or "hole" in the N-Atlantic LIP, created by the Iceland hotspot. The Aegir Ridge created anomalously thick crust (8-11 km) during the first 2-4 Myr spreading, followed by a decrease in magma production and crustal thickness of 3.5-6 km (51.4-25 Ma). Possible explanations are, the lithospheric structure of the newly rifting Kolbeinsey Ridge and Jan Mayen micro-continent diverted mantle flow from the hotspot away from Aegir Ridge, and/or plume flux was low at that time. We report trace element and Pb isotope results for basalts dredged from the Jan Mayen FZ and Aegir Ridge flanks ~69-64 °N. Dredges returned Mn crust, erratic cobbles, hyaloclastite, and basalt diabase. Trace elements reveal distinct chemical groups, including very-depleted melts with very high Zr/Nb ratios (60.7) at one end, and melts of highly enriched characteristics on the other (2.7). The very-depleted compositions show significant LREE depletion relative to HREE [Ce/Yb]N=0.3), while the highly enriched compositions show LREE enrichment [Ce/Yb]N=2.2. Th/Nb ratios vary between 0.07-0.49, indicating variable Th enrichment. Trace element systematics indicate that between group elemental variations can't be solely explained by fractional crystallization and/or partial melting, the observed variations are largely source-related. Trace element systematics are consistent with a mixed MORB/OIB/SCLM mantle source, where relatively enriched samples resemble Faeroe Island lavas, and depleted ones are akin to Kolbeinsey Ridge lavas. Jan Mayen FZ rocks have initial (40Ma) 206Pb/204Pb: 207Pb/204Pb: 208Pb/204Pb =18.2-18.57:15.47-15.54:37.83-38.46 and AR, 16.59-18.75:15.16-15.53:37:36.62-38.51. Jan Mayen FZ, and Aegir Ridge samples with 206Pb/204Pb > 18.2 have higher 207Pb/204Pb and 208Pb/204Pb than the Iceland Neovolcanic lavas and are similar to the Iceland Tertiary and anomalous Öræfajökull basalts. Aegir Ridge basalts with 206Pb/204Pb <17.5 plot below the NHRL in the 206Pb/204Pb vs 207Pb/204Pb and above it in the 206Pb/204Pb vs 208Pb/204Pb diagrams, a characteristic of the British Tertiary Province basalts formed during the early stages of opening of the North Atlantic. We can't be certain that the dredged samples represent primary Aegir Ridge material, or if they were derived from elsewhere along the Iceland-Faeroe Ridge (eg, Faeroes), and transported to the dredge locations. If these rocks were erupted at the Aegir Ridge, the data show that at this time the ambient N-Atlantic upper mantle was relatively uncontaminated by the Iceland Plume, but significantly polluted by continental material, presumably during the early opening of the N-Atlantic Ocean Basin. ARRGH Cruise Scientific Team: Rolf Mjelde, Rolf Birger Pedersen, Helene Kraft, Marcus Fink, Ernst Flüh
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V31D2568S
- Keywords:
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- 1021 GEOCHEMISTRY / Composition of the oceanic crust;
- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry