GEOCHEMICAL PROCESSES IN FOREARC PERIDOTITES: DEPLETION, ENRICHMENT, AND MELT REACTIONS IN THE MANTLE WEDGE PRESERVED IN A SUPRA-SUBDUCTION ZONE OPHIOLITE, CALIFORNIA

The question of geochemical flux in the mantle wedge during subduction is critical to our understanding of arc volcanism, and forms an important aspect of the global mass transport and cycling. The MARGINS program attempts to understand these processes by studying active subduction zones, where direct observations can be made. An alternative approach is to examine outcrops of lithospheric mantle that underlie crust known to have formed by supra-subduction zone (SSZ) magmatism. This lithospheric mantle represents, in part, the source from which the overlying crust was extracted, and its mineralogy and composition reflect the processes that have affected it through time, including melt extraction, fluid phase enrichment, and subsequent interactions with melt derived from lower in the mantle. These processes have been frozen in place by cooling and emplacement of the mantle lithosphere and its overlying crust along active or previously active margins such as coastal California (Shervais et al., 2004, 2005a). Fore-arc peridotites are characterized by spinels with much higher cr#s, which range from around 38 to over 80, indicating significantly higher fractions of partial melting compared to abyssal peridotites (Ishii et al., 1992; Arai, 1994; Gaetani & Grove, 1998). High fractions of partial melting are confirmed by whole rock incompatible trace element concentrations, which are strongly depleted when compared to abyssal peridotites. Forearc peridotites near Stonyford, California, preserve evidence for variable melt extraction under different pressure regimes and subsequent melt reactions that have re-enriched most samples in LREE. Many of these samples are less than 50% serpentinized and many of the harzburgites contain pristine, optically clear clinopyroxene in addition to orthopyroxene, olivine and spinel. Cr-spinels in harzburgite and dunite at Stonyford are characterized by a range in cr#s, consistent with up to 40% partial melting in the most Crrich rocks. Spinels from two of the harzburgites have low cr#s and high mg#s that plot at the undepleted end of the abyssal peridotite array of Dick and Bullen (1984). Three harzburgites have spinels that plot below the Cr-rich end of the abyssal peridotite array, within the less-depleted end of the Izu-Bonin-Mariana fore-arc peridotite array. Spinels in the other peridotites plot within the most depleted end of the fore-arc peridotite array of Ishii et al (1992). These Cr-rich spinels are also comparable to chromite microphenocrysts in boninite lavas associated with extended fore-arcs. Pyroxenes in the depleted harzburgites have higher mg#s and lower Al, Ti, and Na contents than pyroxenes from the abyssal harzburgites. Samples that preserve primary olivine have Cr-spinels that fall on the Olivine-Spinel Mantle Array (OSMA) and melting trend defined by Arai (1994). The fraction of melt extracted from a fertile MORB mantle, based on Arai’s model, ranges from less than 5% in one of the enriched samples to 28%; spinels from several samples with no relict olivine have cr#s that imply A40% melt extraction. Our preliminary trace element data show that melting in the depleted fore-arc peridotites initiated within the garnet facies and continued into the spinel facies. The peridotite compositions fall on the melt reaction trends (pyroxene dissolved, olivine precipitated) that are distinct from simple melt addition or subtraction trends, and imply percolation of melt through a porous medium with which it was not in equilibrium (Kelemen et al., 1995; Pearce et al, 2000). The effects of this melt reaction can be seen on an element distribution diagram with data normalized to primitive mantle, whereby the LREE are enriched in the depleted harzburgites and dunites, whereas all REE are elevated in the “enriched” harzburgites (those with aluminous spinel and enriched pyroxene). Preliminary ion probe data on pyroxene from one harzburgite suggest that this melt was an alkali basalt similar to those found within the Stonyford volcanic complex (Shervais et al., 2005b).