Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v53c..04i&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #V53C-04
Physics
8450 Planetary Volcanism (5480), 8121 Dynamics, Convection Currents And Mantle Plumes, 3640 Igneous Petrology, 1025 Composition Of The Mantle
Scientific paper
Mantle plumes are upwellings of large volumes of mantle material in focused conduits, the leading ends of which are referred to as plume heads. Large igneous provinces (LIPs) are suspected to form from magmatism resulting from plume head decompression melting, but, evidence for this theory for the origins of LIPs is mixed. We have now reached the point of having either to modify the theory to fit characteristics of individual LIPs or to abandon the theory and search for a more unifying explanation. A case study of the two biggest LIPs on Earth - the Ontong Java Plateau (OJP) in the western equatorial Pacific Ocean, and the Kerguelen Plateau / Broken Ridge (KPBR) in the southern Indian Ocean - allows us to examine key predictions of mantle plume theory, including: (1) subaerial eruption of large portions of oceanic LIPs (2) large extents of partial melting in the plume head, resulting in tholeiitic basalt-type magmas, coupled with (3) rapid formation of the LIP, (4) post-formation subsidence comparable to normal oceanic lithosphere, and (5) the presence of a hotspot track and/or an active hotspot. The KPBR formed largely above sea level over a protracted time period ( ˜120 Ma - present) in the growing Indian Ocean basin. Early Cretaceous melts were derived from a heterogeneous source, complicated by subsequent local assimilation of continental crust. Most lavas recovered from the plateau are tholeiitic, but alkalic and evolved volcanics occur in several, widespread locations. Subsidence of the plateau has followed predictions for normal oceanic lithosphere. A prominent hotspot track, the Ninetyeast Ridge, connects Broken Ridge with Early Cretaceous continental basalts on the eastern margin of India. The Kerguelen hotspot is still active today, creating Heard and MacDonald Islands on the central plateau. The OJP was constructed well below sea level on existing Pacific lithosphere. Nearly the entire volume of magma is believed to have been created instantaneously, at ˜120 Ma, from large degrees of partial melting ( ˜30%) of a homogeneous source. Melting extents were high, so either a large temperature anomaly, or a major decompression event is required because volatile contents were low. It has subsided relatively little, or erratically, since its formation. No known hotspot track is associated with the OJP, nor is any active hotspot. Some of these observations agree with expectations of a mantle plume head origin, but several are contrary to predictions. Alternative mechanisms for the formation of LIPs, including extraterrestrial (i.e. bolide impact) or tectonic causes, are also problematic. A few of the obvious concerns are (1) large bolide impact events might not occur often enough to account for the number of known LIPs, (2) tectonic origins for LIPs ignore the presence of LIPs on planetary bodies where evidence for plate tectonics is nil or scant. A common mechanism for the formation of LIPs is highly desirable, yet, at present, all existing hypotheses appear in some way deficient.
Coffin Millard F.
Ingle Stephanie
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