Hf Isotopes in Basic and Ultrabasic Xenoliths from the Kerguelen Archipelago: New Insights Into Relationships Between Lava Sequences and Deep Cumulates
Abstract
Oceanic plateaus contain important thicknesses of volcanic rocks and plutonic cumulate rocks that remain often hidden, except when transported as xenoliths. Seismic refraction studies have shown that the crust beneath the Kerguelen Archipelago (Indian Ocean), the emergent part of the Northern Kerguelen Plateau is unusual. Beneath a quite typical but thickened oceanic crust (8 to 9-km-thick layer 2 with Vp = 5.5 km/s and a 6 to 8-km-thick layer 3 with Vp = 6.6 km/s) is an atypical "low-velocity" zone with Vp = 7.2-7.5 km/s that has been interpreted as representing underplated magmas and/or cpx accumulation. Basic and ultrabasic xenoliths found within basanites on the Kerguelen Archipelago provide direct evidence of underplated basaltic magmas and deep cumulates (equilibration pressures = 0.8 to 1.8 GPa). We have determined high-precision Hf isotopic compositions by MC-ICP-MS (Nu-Plasma with DSN desolvator) of a set of basic and ultrabasic xenoliths (clinopyroxenites and cpx-, garnet-, sapphirine-bearing metagabbros) from the Kerguelen Archipelago to better understand the accumulation process at the mantle-crust boundary and relationships with erupted products. Isotopic compositions of the xenoliths lie within fields reported for the Cenozoic Northern Kerguelen Plateau (Site 1140) and Kerguelen Archipelago lavas. Recent systematic studies of volcanic sections from the archipelago indicate the presence of three distinct groups: 30-26 Ma tholeiitic-transitional lavas, 25-24 Ma mildly alkalic lavas, and <10 Ma highly alkalic lavas and plugs. The 24-25 Ma mildly alkalic lava series coincide with a change in isotopic composition (high 206Pb/204Pb and intermediate Hf, Nd and Sr isotopic ratios) and depth of melting (increased alkalinity Index and La/Sm) relative to the older >26 Ma flood basalts. No xenoliths record the isotopic compositions typical of <10 Ma alkalic lavas. The metagabbros display Hf-Nd isotopic compositions (ɛ Hf: 12.1 to 4.6 for ɛ Nd: 4.6 to 0) that overlap with those of the >26 Ma flood basalts, whereas clinopyroxenites show systematically less radiogenic isotopic compositions (ɛ Hf: 2.2 to 0.5 for ɛ Nd: -0.4 to -1.1) and plot within the field of the 24-25 Ma mildly alkalic basalts. The decrease of the "depleted" signature in Cenozoic Kerguelen lavas over time (Hf, Nd isotopic ratios decrease for increasing Sr, Pb isotopic ratios) in part reflects changes in mixing between the enriched Kerguelen plume source and the ambient depleted Indian MORB source whose proportion decreased as the distance between the ridge and the Kerguelen Archipelago increased. One important physical consequence of these changes (i.e. depth of melting) at ~25 Ma is the first appearance of discrete differentiated intrusions associated with the 24-25 Ma volcanic rocks, which may reflect a decrease in the volume of magmatic products reaching the surface of the archipelago. This apparent decrease magma flux may be due to either a decrease in melt productivity and/or more significant accumulation and fractionation of magmas at the mantle-crust boundary. The Lu-Hf isotopic systematics should help to constrain the age of this underplating event and therefore the age of formation of the Northern Kerguelen Plateau and Kerguelen Archipelago.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V43B..07M
- Keywords:
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- 1025 Composition of the mantle;
- 1035 Geochronology;
- 1040 Isotopic composition/chemistry;
- 3640 Igneous petrology;
- 9340 Indian Ocean