The primary objective of Leg 185 was to determine the geochemical composition of the sediments and upper volcanic section of oceanic crust being subducted into the western Pacific arc system. These data are required as part of the subduction equation, which involves quantifying the inputs and outputs, both into the arc and back into the mantle, of the subduction factory (Fig. 1). These processes are important, as it is in the subduction factory that the majority of chemical recycling is currently taking place on Earth. These "factories" were probably the main sites of crustal production through geological time (Armstrong, 1968; Karig and Kay, 1981; Reymer and Schubert, 1984; McLennan, 1988), and despite the fact that there is good evidence for transport of fluid and melt from the subducted plate to the arc system (Morris et al., 1990; Hawkesworth et al., 1997; Elliott et al., 1997), there are few quantitative constraints on the recycling equation and its effect on the dynamics of crust formation and destruction. The ODP program since the late 1980s has, as a part of several drilling legs, tackled this problem (see "Historical Perspectives" section), but Leg 185 was the first ODP leg for which the objectives were specifically applied to coring oceanic crust and sedimentary sections representative of the different inputs into the subduction factory; in this case the Mariana and Izu-Bonin arcs of the west Pacific ocean (Fig. 2).
Many of the key elements that are important in understanding crustal growth (e.g. Th, rare earth elements [REEs], Ba, and Be) are sequestered in the sedimentary column and in the uppermost oxidized portions of the volcanic section of oceanic basement (K, B, U, CO2, and H2O). The input of these and other elements may vary as a function of sediment composition (Plank and Langmuir, 1998) or the nature of the volcanic basement (Staudigel et al., 1986 ; Alt and Teagle, 1999). For example, the absence of significant carbonate in the sediments may influence the CO2 content of an arc. The presence of organic-rich sediments or hydrothermal sediment may influence the input of metals in different arcs. Alkaline off-axis volcanics and associated volcaniclastic sediments, when subducted, may significantly affect the alkali inventory to the subduction factory.
The igneous section of ocean crust inherits many of its physical and geochemical characteristics at the spreading ridge. Significant differences in eruption style, ridge morphology, and structure occur depending on the rate of spreading (see review in Perfit and Chadwick, 1998). Similarly, the hydrothermal systems vary as a function of the longevity and depth of the magma chamber (e.g., Gillis, 1995; Haymon et al., 1991), and, on average, the alteration characteristics and, therefore, the geochemical inventory of crust, must vary as a function of spreading rate. As crust ages and moves away from the spreading axis, it is initially cooled by hydrothermal activity and later warms again as it equilibrates with the geothermal gradient. The chemical changes occurring during this transition are important, not only in controlling the compositions of the oceans (Staudigel et al., 1986; Alt and Teagle, 1999), including the retro actions between continental erosion and oceanic composition, but also in fixing key elements that will later be recycled into the subduction factory. Some of these elements will migrate into the arc crust, whereas others will be recycled into the mantle, possibly to return to the oceanic crust as hot-spot magma. Although the chemical maturation of crust must continue for several tens of millions of years after its formation (Stein and Stein, 1994) and probably throughout its history, the most significant alteration is at the ridge axis and for about 10-30 m.y. following crustal accretion (Staudigel et al., 1986; Alt et al., 1986; Alt et al., 1992).
In an analogous fashion, the history of sedimentation on the oceanic crust as it transits different oceanic regimes will influence the composition of the input into the subduction factory (Plank and Langmuir, 1998). The sedimentary sequence in subduction regimes in nonequatorial zones will differ significantly in composition from those where the oceanic crust has "resided" mainly in equatorial regimes. The presence of intraplate volcanoes may result in a significant flux of volcaniclastic material into the sedimentary sequence. This may have very different characteristics in key isotope ratios, especially Pb, that the arc may inherit or that may be recycled back into the mantle. When proximal to active margins, the upper sediments may contain significant quantities of terrigenous turbidites. As the oceanic plate approaches the trench, the final contribution to the sedimentary pile will include ash from the volcanic arc. For margins that are not accreting sediments, this component will be recycled into the mantle or created beneath the forearc.
The oldest oceanic crust on Earth is subducting into the Izu-Mariana arc system, and in addition to providing geochemical data to input into the subduction equation, the two sites studied provide important geochemical constraints on the nature and history of Mesozoic ocean crust. Both sites are shown in Figure 2. Site 801 is in the Pigafetta Basin, which is in the Jurassic Quiet Zone (JQZ) and is dated as ~170 Ma (Pringle, 1992). It is the oldest crust drilled by ODP or the Deep Sea Drilling Project (DSDP). The second site, Site 1149 in the Nadezhda Basin, is on the same flow line as Site 801 but is on magnetic Anomaly M11 and, as such, has an estimated age of ~135 Ma. Both sites originated at spreading centers in the Southern Hemisphere and then migrated northward, but at different times and durations. Thus, in addition to the "Subduction Factory experiment," Leg 185 scientists had an unparalleled opportunity to (1) assess the paleoequatorial sedimentation history of the Pacific Ocean since Mesozoic time, (2) place limits on the ages of the oldest magnetic anomalies in the ocean basins, and (3) study the nature of the JQZ.
With the exception of relatively soft oozes and clays, drilling in oceanic crust rarely recovers the entire sedimentary and igneous section; thus, calculating the geochemical inventory is problematic. The gaps in the data have to be filled in by combining detailed core description and logging the drill hole both for physical parameters, such as resistivity, porosity, and velocity, and for geochemical composition. In addition to the regular inventory of ODP logs, the geochemical logging tool was used during Leg 185.
The ultimate long-term goal of studies of the subduction factory is to create a complete geochemical mass balance of the inputs, outputs, and residues lost from the system. Geochemists and geophysicists argue strongly for the recycling of oceanic crust and sediments to the mantle (Hofmann, 1997; Van der Hilst et al., 1997). Given adequate control on the subduction equation, it may ultimately be possible to identify the recycled products of the factory, not only in the arc volcanoes, but as they reappear as mantle plumes on the Earth's surface after being recycled into the mantle. The Izu-Mariana system was chosen as the first of these studies because it is relatively simple:

It is characterized only by limited sediment accretion.
It has a well-defined subduction geometry, which is relatively steep in the Mariana arc, and penetrates the 670 km discontinuity and is shallower in the Izu-Bonin arc (Van der Hilst et al. (1997).
It has a wide aperture from forearc across the arc to the backarc.
The region has already been the subject of ODP drilling in serpentinite mounds associated with forearc dewatering (Leg 125; Fryer, 1992) and has a well-studied deep-sea ash and volcanic record (Legs 125 and 126)

Too often postcruise research on ODP samples produces a data set dispersed among many individual investigators. During Leg 185 a novel approach was that the investigators chose to work on a common set of samples. The geochemical database thus developed for Leg 185 will be a unique contribution to the Geochemical Earth Reference Model (GERM) and to the MARGINS Program initiative, and the communal samples will be a legacy of the leg. The two sites drilled during Leg 185 provide critical information on the input to the "subduction factory" system. In particular, Hole 801C, now at a depth of 934 mbsf remains as an ODP legacy hole in the oldest ocean crust on Earth.
An important objective not directly related to the problem of geochemical recycling involved the study of the deep biosphere at both sites. Bacteria have been located in association with ridge axis hydrothermal systems within the sediment column as deep as 800 m (Parkes et al., 1994). In addition, textural evidence suggests that bacteria living off nutrients associated with basaltic glass alteration may thrive in the basaltic crust (Thorseth et al., 1995; Fisk et al., 1998; Furnes and Staudigel, 1999). The fascinating possibility that bacterial activity may persist in oceanic crust as old and as deep as that at Sites 801 and 1149 provided the motivation for sampling the basement for bacteria culturing and DNA extraction in the search for extremophile life. To control the extent of contamination from surface waters, drilling mud, and drilling tools, a series of tests for contaminants were undertaken as part of the operations at Sites 801 and 1149.

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