Geochemistry of igneous rocks recovered from a transect across the Mariana Trough, arc, fore-arc, and trench, at DSDP Leg 60 Holes and DM17

Major and trace element analyses are presented for 110 samples from the DSDP Leg 60 basement cores drilled along a transect across the Mariana Trough, arc, fore-arc, and Trench at about 18°N. The igneous rocks forming breccias at Site 453 in the west Mariana Trough include plutonic cumulates and basalts with calc-alkaline affinities. Basalts recovered from Sites 454 and 456 in the Mariana Trough include types with compositions similar to normal MORB and types with calc-alkaline affinities within a single hole. At Site 454 the basalts show a complete compositional transition between normal MORB and calc-alkaline basalts. These basalts may be the result of mixing of the two magma types in small sub-crustal magma reservoirs or assimilation of calc-alkaline, arc-derived vitric tuffs by normal MORB magmas during eruption or intrusion. A basaltic andesite clast in the breccia recovered from Site 457 on the active Mariana arc and samples dredged from a seamount in the Mariana arc are calc-alkaline and similar in composition to the basalts recovered from the Mariana Trough and West Mariana Ridge. Primitive island arc tholeiites were recovered from all four sites (Sites 458-461) drilled on the fore-arc and arc-side wall of the trench. These basalts form a coherent compositional group distinct from the Mariana arc, West Mariana arc, and Mariana Trough calc-alkaline lavas, indicating temporal (and perhaps spatial?) chemical variations in the arc magmas erupted along the transect. Much of the 209 meters of basement cored at Site 458 consists of endiopside- and bronzite-bearing, Mg-rich andesites with compositions related to boninites. These andesites have the very low Ti, Zr, Ti/Zr, P, and rare-earthelement contents characteristic of boninites, although they are slightly light-rare-earth-depleted and have lower MgO, Cr, Ni, and higher CaO and Al2O3 contents than those reported for typical boninites. The large variations in chemistry observed in the lavas recovered from this transect suggest that diverse mantle source compositions and complex petrogenetic process are involved in forming crustal rocks at this intra-oceanic active plate margin.

The alkali element and boron geochemistry of ODP Site 169-1038 in the Escanaba Trough, East Pacific

A suite of conjugate pore fluid and sediment samples were collected during Leg 169 of the ODP from within the clastic sedimentary sequences which host massive sulphides at Central Hill, Escanaba Trough (ODP Site 1038). We report the alkali element and boron, and Li and B isotope data for these samples. Relative to a reference site (Site 1037) located outside the zone of high heat flow, pore fluids from Site 1038 show a wide variation in Cl (300-800 mM), and have far higher concentrations of Li (up to 6.2 mM), B (up to 9.7 mM), Cs (up to 5.0 mM), and Rb (up to 97 mM). We show that the pore fluids are derived from hydrothermal circulation that has extended into the basement oceanic crust, with input of the alkali elements and B as the rising hydrothermal fluids interact geochemically with the overlying clastic sediments. There is, however, no marked depletion of these elements in the conjugate sediments, suggesting that there has been advective transport of fluids away from the primary hydrothermal reaction site. This is supported by modelling of the Li and B isotope systematics of the pore fluids, which shows that they record extensive formation of secondary minerals during cooling of the fluids from ~350 to ~20ºC. Precipitation of metal-rich sulphides would have occurred prior to the formation of these minerals, thus, the pore fluid Li and B isotope data can place important constraints on the locus of sulphide deposition beneath the seafloor at Escanaba.