Element abundances and Nd isotopic composition in boninite and tholeiite from DSDP Hole 60-458

Boninites are unusual high MgO-high SiO2 volcanic rocks found in several western Pacific island arcs. Their high Mg/(Mg + total Fe) (0.55-0.83) and compatible element contents (Ni = 70-450 ppm, Cr = 200-1800 ppm) indicate equilibration with mantle peridotite, but their low TiO2 contents (0.1-0.5%) indicate severe depletion of this source. K, Rb, Sr and Ba abundances in boninites are typical of primitive arc basalts, but ratios such as Ti/Zr and La/Yb are variable (Ti/Zr = 23-67, (La/Yb)e.f. = 0.6-4.7). Evidence for both enrichment and depletion of incompatible elements suggests that boninites are derived from refractory peridotite which has been metasomatically enriched in LREE, Zr, Sr, Ba and alkalis. Wide variations in 143Nd/144Nd (0.51262-0.51296) are correlated with La/Sm, Sm/Nd and Ti/Zr, which enables identification of components in the boninite source. Possible LREE depleted components have relative REE and Ti abundances like those in depleted peridotites and high 143Nd/144Nd ratios which reach MORB-like values. Possible LREE enriched components have relative REE abundances similar to those in metasomatized mantle peridotite nodules, and low 143Nd/144Nd ratios which indicate either sedimentary sources or mantle sources with recent to ancient LREE enrichment. Relative abundances of Ba and Sr in boninites decrease with increasing LREE enrichment and suggest a non-sedimentary source for the LREE enriched material. Enrichment in Ba, Sr and alkalis may result from a third component derived from subducted oceanic crust. Two models can account for the successive generation of boninites and arc tholeiites within a single area: 1) boninites can be derived from the peridotite residue of earlier arc tholeiite generation which is metasomatically enriched in LREE before boninite volcanism, or 2) arc tholeiites and boninites can be derived from a variably depleted peridotite source which has been pervasively enriched in LREE. Areas of fertile peridotite would yield tholeiites while refractory areas would yield boninites.

Petrography and geochemistry of basement rocks from five DSDP Leg 60 Sites

Bulk chemistry and trace elements data were measured in 72 samples, selected from 5 basement sections, which have been recovered by Leg 60 drilling (Sites 453, 454, 456, 458, and 459). According to analytical results a metagabbro- metabasalt breccia, deposited about 5 Ma at the westernmost flank of the Mariana Trough (Site 453), was derived from an island arc source. Basalts from the Mariana Trough (Sites 454 and 456) are similar in many respects to midoceanic ridge basalts (MORB). Yet rocks of unusual geochemistry, reflecting the possible influence of arc volcanism, were found among the pillow lavas at the easternmost trough (Site 456). The acoustic basement in the Mariana fore-arc region was formed by submarine eruptions of arc tholeiites (Sites 458 and 459) and peculiar high-MgO andesites related to the boninite suite.

Geochemistry at DSDP Leg 60

Eocene to Pleistocene volcanogenic sediments from the Mariana Trough and the Mariana arc-trench system have been studied by X-ray diffraction, X-ray fluorescence, and atomic absorption, and with a scanning electron microscope with an X-ray-energy-dispersive attachment. The mineralogical composition of the volcaniclastic sediments (tuffs) is the same as that of the other associated sediments (mudstones). Diagenetic alterations are significant and seem to result from two processes. The first (low-temperature alteration) develops with age and depth; it consists of the genesis of pure smectite, coupled with zeolites (phillipsite, clinoptilolite). The second is limited to sediments immediately overlying basalts and to the altered basalts themselves. It consists of the massive development of palygorskite, and seems to be linked with hydrothermal activity in the igneous basement.

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.