Mantle-crust interactions in a paleosubduction zone: Evidence from highly siderophile element systematics of eclogite and related rocks

Substantial differences in 187Os/188Os and absolute and relative abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) in mantle peridotites compared to oceanic crust indicate that this suite of elements may prove useful in assessing mechanisms and pathways of mantle and slab mass transfer within the subduction channel. Currently, however, information regarding the mobility of the HSE in subduction zone environments is limited. To better understand the systematics of highly siderophile elements in subduction settings, we measured Os isotopic compositions and HSE abundances of cores and rinds of meter-scale blocks of eclogite, blueschist and garnet amphibolite from subduction-related mélanges within 1) the Franciscan Complex, CA, and a related terrane in Oregon; 2) the Catalina Schist, CA; and 3) the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Rinds composed of amphibole, chlorite, ± phengite, talc, titanite and rutile partly enclose blocks at each location. Mineralogic, petrologic and geochemical data suggest that these rinds are metasomatic features that formed by fluid-mediated interaction between mafic blocks and an ultramafic matrix.The cores of high-grade blocks are characterized by high (Pt + Pd)/(Os + Ir + Ru), and variably radiogenic Os; initial 187Os/188Os ranges from 0.197 to 4.30. These characteristics are consistent with the HSE compositions of subducted basalts. In contrast, (Pt + Pd)/(Os + Ir + Ru) of rinds are generally much lower than cores, approaching values typical of mantle peridotites. The initial 187Os/188Os of most rinds are also typical of upper mantle peridotites; values for most rinds fall between 0.125 and 0.14. The similarities of the 187Os/188Os and HSE abundances between rinds and mantle peridotites indicate that the HSE contained within rinds reflect a contribution by mantle peridotite, whether by fluid-mediated transfer or through mechanical processes. If the HSE signatures of the rinds were generated by fluid-mediated metasomatic processes, these data require remarkably efficient and uniform transference of mantle HSE from peridotitic mélange matrix to the exteriors of mélange blocks during rind formation. More likely, the rinds are coatings of materials that contain HSE derived from peridotite precursors. These results suggest that centimeter-scale mass transfer by physical mechanisms may be important for some elements within the subduction channel.

► High P/T blocks have HSE contents and radiogenic Os similar to subducted basalts. ► Reaction rinds on blocks have HSE and low 187Os/188Os similar to peridotites. ► Rinds likely derived from peridotites, suggesting physical transfer of matter.