| Title: | Geochemisty of the oldest Pacific and Atlantic oceanic crusts: tectonic and geodynamic implications |
| Author: | Pat R Castillo |
| Date Submitted: | 05/01/2002 |
| Address: | Scripps Institution of Oceanography
UCSD
La Jolla
CA
USA
92093-0212
|
| Phone: | (858) 534-0383 |
| Email: | pcastillo@ucsd.edu |
| Co-Authors: | |
| Affiliation: | Scripps Institution of Oceanography, UCSD |
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| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725594 |
| Keywords: | MORB, LIPS, CAMP, mid-Cretaceous volcanic event, Ontong Java |
| Abstract: | Complete major and trace element and Sr-Nd-Pb isotopic compositions for samples of the oldest Pacific and Atlantic oceanic crusts collected by Deep-Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) have recently become available. Data for the Jurassic-Early Cretaceous (170-130 Ma) mid-ocean ridge basalts (MORB) recovered in the central and northwestern Pacific Ocean indicate that these basalts were generated by degrees and pressures of melting identical to those of modern Pacific mid-ocean ridges, and are geochemically identical to Pacific normal-MORB. In general, these Mesozoic Pacific MORB are depleted in highly incompatible elements and display a wide range in Nd and Pb isotopic ratios (eNd(T) = 8.4-11.6; 206Pb/204Pbi = 17.9-18.6) but have a low and uniform Sr isotopic composition (87Sr/86Sri = 0.7023-0.7026). Compared to the older and modern Pacific MORB, mid-Cretaceous Pacific MORB (=115-100 Ma) are moderately to strongly enriched in highly incompatible elements with an "enriched mantle" isotopic affinity. The shift in MORB composition coincides with the onset of effusive mid-Cretaceous intraplate volcanism in the Pacific and reflects widespread contamination of the Pacific upper mantle with materials derived from the plumes or plume heads responsible for mid-Cretaceous oceanic plateaus and seamount chains. Data for the oldest central Atlantic oceanic crust, on the other hand, show that MORB generated from about 160 to 120 Ma are the ones that display clear isotopic and chemical signals of plume contamination (e.g., 87Sr/86Sri = 0.7032 to 0.7036, eNd(T) = +6.2 to +8.2, incompatible element patterns with positive Nb anomalies). Plume contamination signals are muted or absent in crust generated between 120 and 80 Ma, which resembles modern Atlantic normal-MORB. The plume-affected pre-120 Ma Atlantic MORB are isotopically similar to lavas from the Ontong Java Plateau and mid-Cretaceous Pacific MORB. The strongest plume signature is displayed near the center of the Central Atlantic Magmatic Province (CAMP), a region covered by a brief, but massive flood volcanism at ~200 Ma (near the Triassic-Jurassic boundary) on what are now the continental margins of eastern North America, southwestern Europe, West Africa, and northeastern South America. However, the hotspots presently located nearest this location in the mantle reference frame do not appear to be older than the Late Cretaceous and are isotopically distinct from the oldest Atlantic MORB. The widespread plume contamination of the nascent Atlantic upper mantle, but with a lack of evidence for a long-lived volcanic chain and/or oceanic plateau associated with this plume, indicates an origin different from that of the Pacific upper mantle that produced mid-Cretaceous MORB. The enriched signature of the early Atlantic crust, and possibly the eruption of the CAMP, were caused by a relatively short-lived, but large volume plume feature that was not rooted at a mantle boundary layer. |
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