Positive Hf Anomalies Research Articles

Hf-Nd Element and Isotope Perspective on the Nature and Provenance of Mantle and Subduction Components in Western Pacific Arc-Basin Systems

This paper develops methods for using the integrated study of Hf-Nd element and isotope covariations to define the nature and provenance of the mantle and subduction inputs to subduction systems. In particular, it can be demonstrated that (1) Hf-Nd isotope space permits discrimination between mantle of Pacific and Indian provenance, (2) displacements from mantle arrays on Hf-Nd isotope and trace element projections can be related to the magnitude, source and composition of the subduction input, and (3) Hf-Nd isotope and trace element covariations can be used to interpret high field strength element (HFSE) anomalies [specifically, Hf anomalies on extended rare earth element (REE) patterns] in subduction-related magmas. These methods are tested using published volcanic arc data coupled with new data from the many components of the Izu-Bonin-Mariana (IBM) subduction system, namely the pre-subduction marginal basins, the Eocene to Recent volcanic arcs, and the crust, volcanogenic sediments and pelagic sediments of the subducting Pacific plate. The results of the IBM study show that the mantle that fed the IBM system was always of Indian provenance and that Pacific volcanogenic sediments make the most significant, though variable, contribution to the subduction component. Modelling demonstrates that the Nd/Hf ratio of the subduction component probably lay between 40 and infinity and thus was probably the main cause of the negative HFSE anomalies that characterize much of the Recent arc. This result may further indicate that the subducting sediment lost elements to the mantle wedge mostly by dehydration rather than fusion. In contrast, the data also show that the positive Hf anomalies that characterize much of the Protoarc cannot be attributed directly to subduction. One option consistent with Hf-Nd systematics is that the positive Hf anomalies in the Protoarc boninites were caused by fusion of mafic veins in their shallow mantle sources. Comparison with published data on other arcs shows significant inter-arc variations. For example, the subduction components in near-continent arcs (Banda, Lesser Antilles) appear to have lower Nd/Hf ratios more consistent with sediment fusion, and at least one arc (Tonga-Fiji) carries evidence of temporal variations in mantle provenance.

Komatiite-Tholeiite and Boninite Series Volcanic Associations of the Abitibi Greenstone Belt: Plume-Protoarc Interaction, and Crustal Growth

Komatiite-tholeiite sequences and boninite series volcanic rocks are spatially associated at several localities along a 300 km strike sector of the late Archaean (2.7 Ga) Abitibi greenstone belt, Canada. Examples include the Kidd volcanic complex (KVC) and volcanic sequences in Tisdale and Whitney Townships near Timmins, Ontario, as well as volcanic sequences near Val d'Or, Quebec. In the field, the different types of flows interdigitate on a scale of 10's to 100's metres. Komatiites include Munro, or Al-undepleted type, and Barberton, or Al-depleted type. Spinifex textured Al-undepleted komatiites at the KVC and Whitney localities, like Munro counterparts, are characterized by near-chondritic A1203/TiO2, Ti/Zr, and Zr/Y ratios. They have (1) near-fiat HREE, (2) variable L R E E depletion (La/Smn 0.4 to 0.8), and (3) small positive Hf anomalies relative to Sm, with Zr/Hf 400 km, whereas Al-undepleted komatiites reflect melt segregation at shallow levels. Mg-, Fe-tholeiites are spatially associated with the komatiites. They have near-flat R E E patterns over a continuous range from 4 to 20x chondritic abundance. Mg#, Cr, and Ni decrease continuously with increasing REE, Th, Nb, Zr, Hf and Y. The most primitive have near-chondritic interelement ratios. This magma series is interpreted to represent dynamic partial melting in the outer envelope of the mantle plume from which the komatiitic liquids were derived. Collectively, the tholeiites have a continuous range from negative to positive anomalies of Nb relative to Th and La, similar to Phanerozoic and Recent ocean plateau basalts and some ocean island basalts (Saunders et al., 1988). Alteration and crustal contamination can be ruled out as the source of the anomalies. The negative anomalies are consistent with recycling of subduction-influenced subarc mantle, or continental crust, into the mantle, and the positive anomalies recycling of ocean crust processed through a subduction zone (Saunders et al., 1988; McDonough, 1991). Consequently, the plume from which the komatiites and tholeiitic basalts were derived was multi-component, as demonstrated from trace element and isotope systematics on Phanerozoic and Recent plume-related basalts (Mahoney et al., 1993). Compositionally, the boninite series flows range from Mg# = 81-45, TiO2 = 0.17-0.41, and Ni = 1020-310 ppm. They are characterized by high A1203 (12-16 wt.%) contents, and A1203/TiO2 (60-84) ratios. In common with Phanerozoic boninites, these flows have negatively fractionated H R E E (Gd/Ybn = 0.4-0.6), Zr/Y (1.4-2.2) less than the primitive mantle value of 2.4, and variably positive Zr and Hf anomalies, where Zr/Hf 53% and Mg# > 60 given by Crawford et al. (1989). Collectively, the boninite series overlap spinifex texatred Al-undepleted komatiites in terms of TiO2 and Zr contents, but are much more aluminous, possess higher A1203/ TiO2 ratios and Sc and Yb contents, but lower Gd/ Ybn and Zr/Y ratios. The boninites are interpreted to have formed in a protoarc from second stage metasomatic enrichment of a mantle source highly