Supra-subduction zone (SSZ) ophiolites have the geochemical characteristics of island arcs but the structure of oceanic crust and are thought to have formed by sea-floor spreading directly above subducted oceanic lithosphere. They differ from ‘MORB’ ophiolites not only in their geochemistry but also in the more depleted nature of their mantle sequences, the more common presence of podiform chromite deposits, and the crystallization of clinopyroxene before plagioclase which is reflected in the high abundance of wehrlite relative to troctolite in their cumulate sequences. Most of the best-preserved ophiolite complexes in orogenic belts are of this type. 

Geological reconstructions suggest that most SSZ ophiolites formed during the initial stages of subduction prior to the development of any volcanic arc. Evidence from these ophiolites suggests that the first magma to form in response to intra-oceanic subduction is boninitic in composition, derived by partial melting of hydrated oceanic lithosphere in the ‘mantle wedge’. As subduction proceeds, the magma composition changes to island-arc tholeiite, probably because the hydrated asthenosphere of the ‘mantle wedge’ eventually becomes the dominant mantle source. Other SSZ ophiolites formed in the early stages of back-arc spreading following splitting of a pre-existing arc. Nonetheless the more common mechanism for formation of SSZ ophiolites appears to have been pre-arc rather than back-arc spreading

Characteristics and tectonic significance of supra subduction zone ophiolites

Geochemical data have been interpreted as requiring that a significant fraction of the melting in MORB source regions takes place in the garnet peridotite field, an inference that places the onset of melting at ≥80 km. However, if melting begins at such great depths, most models for melting of the suboceanic mantle predict substantially more melting than that required to produce the 7 ± 1 km thickness of crust at normal ridges. One possible resolution of this conflict is that MORBs are produced by melting of mixed garnet pyroxenite/spinel peridotite sources and that some or all of the “garnet signature” in MORB is contributed by partial melting of garnet pyroxenite layers or veins, rather than from partial melting of garnet peridotite. Pyroxenite layers or veins in peridotite will contribute disproportionately to melt production relative to their abundance, because partial melts of pyroxenite will be extracted from a larger part of the source region than peridotite partial melts (because the solidus of pyroxenite is at lower temperature than that of peridotite and is encountered along an adiabat 15–25 km deeper than the solidus of peridotite), and because melt productivity from pyroxenite during upwelling is expected to be greater than that from peridotite (pyroxenite melt productivity will be particularly high in the region before peridotite begins melting, owing to heating from the enclosing peridotite). For reasonable estimates of pyroxenite and peridotite melt productivities, 15–20% of the melt derived from a source region composed of 5% pyroxenite and 95% peridotite will come from the pyroxenite. Most significantly, garnet persists on the solidus of pyroxenite to much lower pressures than those at which it is present on the solidus of peridotite, so if pyroxenite is present in MORB source regions, it will probably contribute a garnet signature to MORB even if melting only occurs at pressures at which the peridotite is in the spinel stability field. Partial melting of a mixed spinel peridotite/garnet pyroxenite mantle containing a few to several percent pyroxenite can explain quantitatively many of the geochemical features of MORB that have been attributed to the onset of melting in the stability field of garnet lherzolite, provided that the pyroxenite compositions are similar to the average composition of mantle-derived pyroxene-rich rocks worldwide or to reasonable estimates of the composition of subducted oceanic crust. Sm/Yb ratios of average MORB from regions of typical crustal thickness are difficult to reconcile with derivation by melting of spinel peridotite only, but can be explained if MORB sources contain ∼5% garnet pyroxenite. Relative to melting of spinel peridotite alone, participation of model pyroxenite in melting lowers aggregate melt Lu/Hf without changing Sm/Nd ratios appreciably. Lu/Hf-Sm/Nd systematics of most MORB can be accounted for by melting of a spinel peridotite/garnet pyroxenite mantle provided that the source region contains 3–6% pyroxenite with ≥20% modal garnet. However, Lu/Hf-Sm/Nd systematics of some MORB appear to require more complex melting regimes and/or significant isotopic heterogeneity in the source. Another feature of the MORB garnet signature, (^(230)Th)/(^(238)U)>1, can also be produced under these conditions, although the magnitude of (^(230)Th)/(^(238)U) enrichment will depend on the rate of melt production when the pyroxenite first encounters the solidus, which is not well-constrained. Preservation of high (^(230)Th)/(^(238)U) in aggregated melts of mixed spinel peridotite/garnet pyroxenite MORB sources is most likely if the pyroxenites have U concentrations similar to that expected in subducted oceanic crust or to pyroxenite from alpine massifs and xenoliths. The abundances of pyroxenite in a mixed source that are required to explain MORB Sm/Yb, Lu/Hf, and (^(230)Th)/(^(238)U) are all similar. If pyroxenite is an important source of garnet signatures in MORB, then geochemical indicators of pyroxenite in MORB source regions, such as increased trace element and isotopic variability or more radiogenic Pb or Os, should correlate with the strength of the garnet signature. Garnet signatures originating from melts of the garnet pyroxenite components of mixed spinel peridotite/garnet pyroxenite sources would also be expected to be stronger in regions of thin crust.

A possible role for garnet pyroxenite in the origin of the “garnet signature” in MORB

Geochemistry of subduction zone serpentinites: A review

The earth9s mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo2) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo2 greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo2 of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO2 and H2O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO2 and H2O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe3+ contents are stabilized in the garnet structure and that fo2 deareases with increasing depth.

https://science.sciencemag.org/content/sci/248/4953/337.full.pdf

Mantle Oxidation State and Its Relationship to Tectonic Environment and Fluid Speciation

[1] Suprasubduction zone (SSZ) ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites. This sequence includes the following: (1) birth, which entails the formation of the ophiolite above a nascent or reconfigured subduction zone; this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex; (2) youth, during which is continued melting of refractory asthenosphere (depleted during birth) occurs in response to fluid flux from the subducting slab, with extensional deformation of the older plutonic suite, eruption of refractory lavas, and the intrusion of wehrlite-pyroxenite; (3) maturity, with the onset of semistable arc volcanism, typically calc-alkaline, as the subduction zone matures and stabilizes, and the intrusion of quartz diorite and eruption of silicic lavas; and (4) death, which is the sudden demise of active spreading and ophiolite-related volcanism, which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis; expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites; (5) resurrection, with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction. The early stages (1–3) may be diachronous, and each stage may overlap in both time and space. The existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the SSZ tectonic setting.

/pdf/birth-death-and-resurrection-the-life-cycle-of-2uj9u0878i.pdf

Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites

Origins of the mafic and ultramafic rocks in the Ronda peridotite

Bay of Islands ophiolite suite, Newfoundland: Petrologic and geochemical characteristics with emphasis on rare earth element geochemistry

The low-level waste source term model development project has adapted/developed two computer codes to predict the migration of radionuclides emplaced in shallow land burial facilities The computer code FEMWATER is used to predict water flow and moisture content The computer code BLT is used to predict container Breach, waste form Leaching, and contaminant Transport Recent work on this project focused on two areas One involved improvements to the leaching models incorporated in BLT In particular, this report describes an additional model that was added to BLT which simulates the waste form using the method of finite differences and treats the contacting solution as a mixing bath This model improves upon the previous models in BLT in three areas: (a) it treats the release processes of diffusion, dissolution, and surface rinse simultaneously; (b) it allows for partitioning between the waste form and solution; and (c) it permits solution feedback effects to influence diffusive releases Verification studies of the finite difference/mixing bath model are discussed in detail The second area of research involved comparing BLT model predictions to experimental data This report presents the results of modeling laboratory scale wet/dry cycle leach experiments and lysimeter experiments conducted at Pacific Northwest Laboratories Basedmore » on this modeling work, recommendations for future areas of study are given 23 refs, 19 figs, 15 tabs« less

Low-level waste source term model development and testing

This report discusses the seismic hazards associated with living in various urban areas of California, particularly the Fresno area. According to this assessment and other studies, the Fresno metropolitan area is relatively safe from the threat of a large destructive earthquake, due to its location away from major earthquake-prone fault zones. Unlike other major metropolitan areas in California such as San Francisco and Los Angeles, the Fresno area has no known active faults that are capable of causing destructive tremors. Several maps are included which indicate the location of earthquake epicenters and magnitudes in California from 1769 to the present.

Are all major California cities seriously threatened by earthquakes

Performance assessment models typically account for the processes of sorption and dissolution-precipitation by using an empirical distribution coefficient, commonly referred to as K{sub d} that combines the effects of all chemical reactions between solid and aqueous phases. In recent years, however, there has been an increasing awareness that performance assessments based solely on empirically based K{sub d} models may be incomplete, particularly for applications involving radionuclides having sorption and solubility properties that are sensitive to variations in the in-situ chemical environment. To accommodate variations in the in-situ chemical environment, and to assess its impact on radionuclide mobility, it is necessary to model radionuclide release, transport, and chemical processes in a coupled fashion. This modeling has been done and incorporated into the two-dimensional, finite-element, computer code BLT-EC (Breach, Leach, Transport, Equilibrium Chemistry). BLT-EC is capable of predicting container degradation, waste-form leaching, and advective-dispersive, multispecies, solute transport. BLT-EC accounts for retardation directly by modeling the chemical processes of complexation, sorption, dissolution-precipitation, ion-exchange, and oxidation-reduction reactions. In this report we: (1) present a detailed description of the various physical and chemical processes that control the release and migration of radionuclides from shallow land LLW disposal facilities; (2) formulate the mathematical models that represent these processes; (3) outline how these models are incorporated and implemented in BLT-EC; and (4) demonstrate the application of BLT-EC on a set of example problems.

/pdf/blt-ec-breach-leach-transport-and-equilibrium-chemistry-a-ewub2tyf9f.pdf

BLT-EC (Breach, Leach Transport, and Equilibrium Chemistry), a finite-element model for assessing the release of radionuclides from low-level waste disposal units: Background, theory, and model description

C.J. Suen

Papers

Origins of the mafic and ultramafic rocks in the Ronda peridotite

Bay of Islands ophiolite suite, Newfoundland: Petrologic and geochemical characteristics with emphasis on rare earth element geochemistry

Low-level waste source term model development and testing

Are all major California cities seriously threatened by earthquakes

BLT-EC (Breach, Leach Transport, and Equilibrium Chemistry), a finite-element model for assessing the release of radionuclides from low-level waste disposal units: Background, theory, and model description