Trace elements and Sr-isotopes in some mantle-derived hydrous minerals and their significance

Abstract

New analyses of K, Rb, Sr and Ba contents and the 87Sr 86Sr ratios of eight amphiboles, one phlogopite, two diopsides and one host alkalic basalt for an amphibole are reported: The samples are mostly inclusions in alkalic basalts and occur in association with peridotite inclusions. Two of the samples are from alpine-type peridotite bodies — one from the Etang de Lhers massif in the French Pyrenees and the other from the Finero massif in the Ivrea zone in northern Italy. The kaersutites come from the following localities: Hoover Dam, Arizona; Deadman Lake, California; Massif Central, France; Queensland; Spring Mountain, New South Wales. The data indicate that kaersutitic amphiboles are genetically unrelated to their host basalts. The isotopic and trace element data of these amphiboles further strengthens the suggestion of BASU and MURTHY (1977) that kaersutites play a significant role in ocean ridge basalt genesis. In addition, pargasitic amphibole with higher 87Sr 86Sr ratios, if present, may be important in the source regions of alkalic basalts. The bulk amphibole lherzolite from Lherz has the K Rb ratio and 87Sr 86Sr ratio appropriate for source material of ridge tholeiites. If the diopside and the amphibole in this rock had isotopically equilibrated under upper mantle conditions, the data show the time of last equilibration to be approximately 735 m.y., in contrast to the young emplacement age of the ultramafic massif. The coexisting phlogopite and diopside in the spinel lherzolite inclusion from Kilbourne Hole, New Mexico, show, surprisingly, isotopic equilibration under upper mantle conditions despite their drastically different Rb Sr ratios. The data show that the phlogopite must have formed very recently in the upper mantle. This phlogopite also has a high K Rb ratio (1133), contrary to the commonly held view that mantle phlogopites have low K Rb ratios. The coexisting diopside shows high K content (778 ppm) and a lower K Rb ratio than the phlogopite. This phlogopite lherzolite has trace elemental and isotopic characteristics that may be adequate for the origin of alkalic basalts upon partial melting.