Oceanic islands

Cyperus rotundus L. is a weedy species, native to India, but widely distributed in countries on the Pacific Rim and islands in the Pacific Basin. Commonly referred to as purple nut sedge, it has been known in the Hawaiian Islands since the middle of the nineteenth century. In addition to its weedy nature, the taxon has attracted attention because of the antifebrile activity of its rhizomes. Chemical studies have disclosed the presence of several sesquiterpene derivatives, some of which have been implicated in the plant s medicinal use (cyperene and cyperinerol) (Wagner et ah, 1990, p. 1399). Our interest in this species is the existence of several chemotypes with interesting patterns of occurrence involving Pacific Rim countries and several oceanic islands, including the Hawaiian Islands, islands in the southern Pacific, and the Philippines. 

Miller (1989) is the question of how such an ancient organism has come to exist on a comparatively new oceanic island. Long-distance dispersal is an unlikely explanation for the present situation. Dispersal over shorter distances from island to island seems a reasonable possibility, with subsequent disappearance of many, or apparently all in the present case, of the intermediate stepping stones. Invocation of island hopping, after all, has many precedents. As Miller (1989) pointed out, obviously some vascular plant lineages have survived, some of which may have histories that reach back to the Antarctic flora. Needless to say, deep phylogenetic analyses would be helpful in addressing this question. 

Carracedo, J. C. 1994. The Canary Islands an example of structural control on the growth of large oceanic-island volcanoes. 1. Volcan. Geotherm. Res. 60 225-241. 

Further constraints can be obtained by systematic studies of the evolution of U-series disequilibria through time in a given volcano. The number of works considering the evolution of ( °Th/ U) or ( °Th/ Th) ratios is very small, but it appears that, in the studied basaltic volcanoes from oceanic islands (Marion, Mauna Kea, Piton de la Fournaise), these ratios remain nearly constant with time. This suggests that the residence time of the magmas in these continuously replenished magma chambers is short... 

Bohrson WA, Reid MR (1998) Genesis of evolved ocean island magmas by deep- and shallow-level basement recychng, Socorro Island, Mexico Constraints from Th and other isotope signatures. J Petrol 39 995-1008... 

Thomas LE, Hawkesworth CJ, Van Calsteren P, Turner SP, Rogers NW (1999) Melt generation beneath ocean islands A U-Th-Ra isotope study from Lanzarote in the Canary Islands. Geochim Cosmochim Acta 63 4081-4099... 

Hilton DR, Barling J, Wheiler GE (1990) The effect of shallow-level contamination on the helium isotope systematics of ocean island lavas. Nature 348 59-62... 

Melting of a mixed peridotite-pyroxenite source. The observation that ocean island and intraplate basalts have >1 indicates that garnet is required as a residual... 

If pyroxenites are present in the source of ocean island lavas, then the fractionation 230rru 238tt 23ip 235y predictablv distinct (Hirschmann and Stolper 1996 ... 

Figure 7. (a) vs. Sr/ Sr diagram for mid-ocean ridge, ocean island and continental basalts... 

Figure 8. ° Pb7 Pb vs. Th/Us (derived using Eqn. 5 in the text) diagram for mid-ocean ridge and ocean island basalt based on a recent data set with mostly mass spectrometry measurements (Turner et al. 1997 Bourdon et al. 1996 Dosso et al. 1999 Claude-lvanaj et al. 1998, 2001 Sims et al. 2002). The data show a relatively well defined array that intersect a closed-system hne for the bulk Earth starting with an initial lead isotope composition equal to Canyon Diablo (T = 4.55 Ga). This intersect was used by Allegre et al. (1986) to define the Th/U ratio of the Earth.

Figure 12. Compilation of and activity ratios for ocean island basalts and mid-...

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