GEOCHEMISTRY OF BONINITES AND OTHER LOW TiO

Boninites are unusual island arc volcanic rocks with high MgO (>9%), high Si0 2 (>55%) contents, and exceptionally low TiO 2 (<0.4%) contents. High MgO contents, MgO/(MgO + EFeO), Ni, Cr and Co contents indicate that boninites have equilibrated with mantle peridotite. Experimental petrologic studies and high Cr/(Cr + Al) in spinels in boninites suggest that they have separated from extremely refractory residual peridotite. Based on this information and low ratios of Ti/V and Ti/Sc in boninites, which are characteristic of depleted peridotites, it is concluded that the low content of Ti and other moderately incompatible elements in boninites reflect a peridotite source which has been depleted in incompatible elements by a previous melting event. Boninites usually have high chondrite normalized LREE (light rare earth element) abundances relative to HREE (heavy rare earth element) or intermediate REE, and La/Sm ratios increase with decreasing 14 Nd/ 14 Nd. Similar to other island arc volcanics, Ba and Sr are enriched relative to LREE, but K/Ba and Rb/Ba ratios in boninites are higher than in other arc volcanic rocks. These features are not consistent with derivation from depleted peridotite. A two-stage geochemical model is proposed to explain the geochemical characteristics of boninites: 1) modification of a depleted peridotite source, residual from the generation of MORB, by LREE-enriched, mantle derived fluids or melts with low 1 4 Nd/ 14 Nd, and 2) enrichment of the source in K, Rb and to a lesser extent Sr and Ba by fluids released from subducted oceanic crust which are dominated by a component with trace element and isotopic characteristics of seawater. The Facpi Formation, Guam consists of MgO-rich arc volcanics with low TiO 2 contents comparable to boninites, including tholeiitic basalts and boninite-like lavas with slightly lower Si0 2 contents than boninites. Abundance patterns of Ti, Sc and HREE in Facpi Formation lavas are virtually identical to boninites. Based on this observation and other similarities between Facpi Formation lavas and boninites it is concluded that low abundances of Ti and other moderately incompatible elements in Facpi Formation lavas also reflect derivation from depleted mantle peridotite. LREE/HREE ratios in Facpi Formation lavas increase with decreasing 14 Nd/1 44 Nd. A LREE-enriched sample has 2 0 7 Pb/ 2 0 4 Pb similar to Pacific Ocean basalts and higher 2 0 6 Pb/ 2 0 4 Pb than Pacific Ocean sediments, while a LREE-depleted sample has similar 2 0 7 Pb/ 2 0 4 Pb, but lower 2 0 6 Pb/ 2 0 4 Pb. A mantle metasomatic origin for LREE-enrichment in Facpi Formation lavas as well as boninites is supported by this data. In addition to lower Si0 2 content, boninite-like Facpi Fm. lavas differ from boninites in their lower K/Ba and Rb/Ba ratios. Basaltic Facpi Fm. lavas have lower K/Ba and Rb/Ba than the boninite-like lavas, like the majority of island arc basalts. A model is proposed to explain these major and trace element differences in which Si0 2-rich boninites originate at shallow depths within the subarc mantle, where material derived from subducted oceanic crust is dominated by K and Rb-rich fluids released by dehydration of basaltic alteration products. Basaltic Facpi Fm. lavas are generated at greater depths, where material released by subducted oceanic crust is poorer in K and Rb compared to Ba and Sr, and boninite-like Facpi Fm. lavas are generated at intermediate depths. Shallow level melting, which is consistent with the siliceous composition of boninites, can thus be related to their trace element characteristics. Lavas from Manam Island, Bismarck Arc are low TiO 2 island arc basalts. Manam lavas have lower MgO, Ni and Cr contents than boninites, and may have evolved from a more primitive magma which equilibrated with mantle peridotite. Relative abundances of Sc, V, HREE and Ti in Manam lavas are nearly identical to boninites, and on this basis a depleted peridotite source is inferred for these low Ti0 2 basalts. Enrichments in K, Rb, Ba, Sr and Pb relative to REE in Manam lavas are among the highest found in island arcs. 8 7 Sr/ 8 6Sr, 14 3 Nd/1 4 4 Nd and 2 0 Pb/ 2 0 Pb ratios are not significantly different from oceanic volcanics, which do not show this enrichment. Relative abundances of K, Rb Ba and Sr to LREE, and Ba to K, Rb, and Sr increase with increasing 87Sr/86Sr. Decreasing 14 Nd/ 14 Nd correlates with increasing LREE-enrichment, but does not correlate with abundance ratios of K, Rb, Ba, Sr or 8 7 Sr/ 8 6 Sr. Extrapolation of possible mixing relationships leads to the conclusion that the peridotite sources of Manam lavas have been modified by two materials with high K, Rb, Ba and Sr abundances relative to REE. Inferred Sr and Pb-isotopic and trace element characteristics of these materials are consistent with their derivation from subducted MORB and sediment. Based on similarities in trace element abundance patterns and isotopic ratios in boninites, and low Ti0 2 island arc volcanics from the Facpi Fm. and Manam Island, a general model for development of sources for low TiO 2 volcanics is proposed: 1) refractory peridotite residual from MORB generation within the oceanic upper mantle is enriched in LREE by mantle derived fluids or partial melts, incorporated into the upper plate of a subduction zone; and 2) materials enriched in K, Rb, Ba and Sr relative to REE, derived from subducted oceanic crust, are added to the overlying peridotite. Materials released at shallow depths have higher K/Ba and Rb/Ba than materials released at greater depth. Si0 2-rich boninites are generated by partial melting of peridotite at shallow depths and have high K/Ba and Rb/Ba, while basaltic arc volcanics are generated by partial melting of deeper level peridotite and have lower K/Ba and Rb/Ba.