SR isotopic study of ultramafic nodules from Neogene alkaline lavas of British Columbia, Canada and Josephine Peridotite, Southwestern Oregon, U.S.A.
Creators: name: Sun, Min
Publisher: name: University of British Columbia
Year: 2010

description: Twelve ultramafic nodules from Neogene alkaline lavas of British Columbia are Cr-diopside series peridotite. Nodules are depleted or undepleted with respect to Cr, Al, Ti and Na abundance per diopside or enstatite formula unit, and either Ti-metasomatised or unmetasomatised. Four samples from the Josephine Peridotite are even more depleted than the depleted nodules and lack Ti-metasomatism. Pyroxene geothermobarometry (Mercier, 1980) was modified and used for clino- and ortho-pyroxene equilibrium temperature, pressure and depth calculations. Nodules came from a mantle depth of 30-50 km (936-l008°C, 9-15.5 kb). Josephine Peridotite came from a mantle depth of 30-65 km (1003-1042°C, 10-20 kb) After establishing an average total Rb blank of 0.26 ng and Sr blank of 3.3 ng, Rb and Sr contents and Sr isotopic compositions of whole rock, acid-leached mineral separates (ol, cpx and opx) and acid leachate from the nodules, eleven host and associated whole rock "basalts, and acid leached mineral separates (ol, cpx and opx) from the Josephine Peridotite have been analysed. The host basalts have ⁸⁷Sr/⁸⁶Sr (0.70238-0.70289) similar to MORB, but very much higher Rb, Sr and ⁸⁷Rb/⁸⁷Sr ratios (12.7-62.1 ppm, 702-1514 ppm and 0.028-0.138, respectively). This is attributed to a low ⁸⁷Sr/⁸⁶Sr mantle with small degree of melting or melting after recent metasomatism by a low ⁸⁷Sr/⁸⁶Sr fluid. Nodules occur only in ne normative alkali basalts and basanites. Diopside is the main carrier of Rb and Sr (Rb = 0.125-3.47 ppm in nodules and 0.023-0.076 ppm in Josephine Peridotite, Sr = 9.3-239 ppm in nodules and 0.256-0.582 ppm in Josephine Peridotite), with low ⁸⁷Sr/⁸⁶Sr ratio (0.7022-0.7041 in nodules and 0.7054-0.7063 in Josephine Peridotite) and ⁸⁷Rb/⁸⁶Sr ratio (0.004-0.1 in nodules and 0.23-0.38 in Josephine Peridotite). Olivines contain the least Rb and Sr (Rb = 0.055-0.27 ppm in nodules and 0.084-0.102 ppm in Josephine Peridotite, Sr = 0.11-3.5 ppm in nodules and 0.153-0.305 ppm in Josephine Peridotite) and give the highest ⁸⁷Sr/⁸⁶Sr ratios (0.7036-0.7 197 in nodules and 0.7089-0.7133 in Josephine Peridotite) and ⁸⁷Rb/⁸⁶Sr ratios (0.19-2.06 in nodules and 0.8-1.73 in Josephine Peridotite). Nodules from Jacques Lake, depleted in Sr and undepleted in major elements, could be a MORB-source-type mantle. Other nodules represent somewhat less Sr-depleted mantle. Whole rock nodule data fall on or off the corresponding mineral isochrons. The latter phenomenon is due to relatively recent contamination with interstitial material having a high Rb/Sr ratio. "Synthetic" whole rocks, calculated from leached-mineral data, have higher ⁸⁷Sr/⁸⁶Sr ratios than host basalts. Together with the well defined mineral isochrons, this supports the conclusion that the nodules and host basalts are not cognate. Equigranular nodules give a mid-Proterozoic mineral isochron date (1518-1537 Ma and (⁸⁷Sr/⁸⁶Sr)₀ = 0.70185). Protogranular nodules give late Precambrian (645 Ma and (⁸⁷Sr/⁸⁶Sr)₀ = 0.7037. early-mid Paleozoic (276-576 Ma and (⁸⁷Sr/⁸⁶Sr) ₀ = 0.7024-0.7032) and Mesozoic (104 Ma and (⁸⁷Sr/⁸⁶Sr)₀ = 0.7029) isochron dates. Porphyroclastic nodules do not define reliable mineral isochrons, but also show evidence of old age (at least 560-790 Ma and (⁸⁷Sr/⁸⁶Sr)₀ = 0.7028-0.7030). Depleted Josephine Peridotite gives middle Paleozoic mineral isochron dates (366-441 Ma and (⁸⁷Sr/⁸⁶Sr)₀ = 0.7038-0.7041), in conflict with the general view that the Josephine Peridotite was generated in Late Jurassic time. This implies that the ophiolite base does not necessarily have the same age as overlying volcanic rocks and dykes.