Isotope Sources of Alkaline Magmatism of the Clarion Fracture Zone (Pacific Ocean)

B. V. Belyatsky Instituite for Precambrian Geology and Geochronology RAS,

2 Makarov emb., 199034, St.-Petersburg, Russia

G. V. Ovchinikova Instituite for Precambrian Geology and Geochronology RAS,

2 Makarov emb., 199034, St.-Petersburg, Russia

L. K. Levsky Instituite for Precambrian Geology and Geochronology RAS,

2 Makarov emb., 199034, St.-Petersburg, Russia

L. N. Kogarko Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, 1

9 Kosygin st., 117975, Moscow, Russia

Clarion fracture zone (CFZ) belongs to supergiant structures crossing almost half of the Pacific Ocean in sublatitude direction and can be consided as the typical representative of the series of transform fracture zones of the Pacific.Clarion fracture zone is about 2800 miles in length. According to its morphostructural features we devided this zone into three different parts (Kogarko et al., 1991).

Under investigations were Sr-Nd-Pb isotope systems of five alkaline basalts from the CFZ. The data show considerable variations in Pb, Sr, Nd isotope ratios for alkaline basalts from different parts of the CFZ. The samples taken in the eastern part of the zone by their geochemical characteristics belong to more primitive and more deeper alkaline magmas (Kogarko et al.,1993). However they lie in the field of the depleted reservoir of basalts from the East Pacific Rise only in Nd-Sr diagram. In all the other isotope diagrams the experimental points have more enriched 87Sr/86Sr=0.702997-0.703431, 143Nd/144Nd= 0.512964-0.513058 and 206Pb/204Pb=19.067-19.130, 207Pb/204Pb=15.604-15.642 isotope ratios and are in the field of Galapagos Islands and Easter Island basalt, e.g. the data-points are displaced towards a little EM-2 and HiMu mantle reservoirs (Zindler and Hart,1986). But more enriched according to its isotope data is the sample of subalkaline trachybasalt from the western part of the zone. It has more radiogenic 87Sr/86Sr-0.704146, 208Pb/204Pb-39.233 and less radiogenic 143Nd/144Nd-0.512926 ratios and plots in the field of the Marqueses Islands basalts and EM-2 reservoir. It is noteworthy that the magmatic source of these rocks is the most close to the Hawaiian Islands but they plot in the field of isotope signatures of Hawaiian lavas in spase plots only in the Nd-Sr isotope diagram. In all the other diagrams the Pb-Pb, Sr-Pb, Nd-Pb isotope ratios of this sample are considerably more radiogenic.The central part of CFZ is represented by andesite-basalt which differs from the other investigated basalts according to obtained isotopic characteristics. In comparison with them it displays higher 207Pb/204Pb-15.639, 87Sr/86Sr-0.704249 and lower 206Pb/204Pb-18.613, 143Nd/144Nd-0.512859 isotope ratios that is typical for the enriched reservoir EM-1.

The obtained data prove the lateral heterogeneity beneath these three investigated regions of the CFZ. Probable reasons causing considerable variations in Sr-Nd-Pb isotope signatures during formation of the transform fracture zone may be the following: melting of the incorporated and partly metasomatized subcontinental mantle (McKenzie and O'Nions, 1995; Cousens et al., 1994) or addition to the depleted mantle beneath the different parts of the fracture zone of the material from different enriched mantle reservoirs typical to the sources of intraplate magmatism.

So, the obtained isotope signatures for different parts of the CFZ are comparable with isotopic data of oceanic islands, i.e. with isotope characteristics typical for deep-seated mantle material of hot spots. The space position of plume-like endmember of the mantle source is unknown. But it is quite possible that this plume reflects the activity of the hot spot of islas Revillagigedo (Island Clarion belongs to this archipelago) which is most close to the eastern part of the CFZ.

But Nd-Sr-Pb isotope signatures prove also the considerable share of the material from reservoir like EM-2 or terrigeneous sediments into the mantle source of basalts from the western part of the fracture zone. In this case it would be possible that as a result of complicated geological history of such huge fracture zone formation there had been global displacement of crust and mantle material beneath the ocean that caused the crust contamination of the mantle source beneath the western part of the fracture zone. As for the most anomalous sample from the central part of CFZ in all diagrams it plots in the field of sediments of the Pacific Ocean (Stolz et al., 1993). The carried out calculations proved that the measured values of Sr-Nd-Pb isotope ratios for this sample could have been obtained by addition of only 0.15-1% of the Paific pelagic sediments of the Pacific to the MORB-like material. These estimations don't contradict the observations (Kogarko et al., 1993) about the presence of shallow-level long-lived magmatic chambers in the central and western parts of the fracture zone. The enriched plume-like source of basalts in the eastern part also coincides with the suggestion about the deep origin of alkaline magmas in this region.