Josephine Peridotite Research Articles

Experimental measurements of anisotropic viscosity in naturally sourced dunite with a preexisting CPO

Naturally deformed dunite cores with a preexisting crystallographic preferred orientation (CPO), collected from a shear zone in the Josephine peridotite (southwest Oregon, USA), were deformed experimentally in triaxial compression. The compression axis was varied relative to the CPO geometry to measure the anisotropy in viscosity of these rocks. Deformation experiments were performed in a gas-medium apparatus at three constant displacement-rate steps, at a temperature of 1250 °C and a confining pressure of 300 MPa. Cores were dehydrated at 1200 °C in a controlled CO/CO2 atmosphere prior to deformation to achieve nominally dry conditions. Data from individual experiments were fit by a power-law, yielding a stress exponent of n ≈ 3.6, indicative of deformation by dislocation creep. The maximum difference in viscosity was a factor of 2.6 at constant stress. The CPO of the olivine grains was measured after deformation by electron backscatter diffraction (EBSD). The flow stress correlated with the mean Schmid factor (or resolved shear stress) on the easiest dislocation slip systems, calculated from grain orientations determined from EBSD analyses. Analysis of our data with a simplified effective-medium model, which incorporated constraints from published single-crystal flow laws, demonstrated the link between CPO and anisotropy in viscosity. Predictions from viscoplastic self-consistent (VPSC) models support our simplified analysis of the measured anisotropy.

Evolution of the Josephine Peridotite Shear Zones: 2. Influences on Olivine CPO Evolution

AbstractSeismic anisotropy arises in the upper mantle due to the alignment of olivine crystal lattices and is often used to interpret mantle flow direction. Experiments on the evolution of olivine crystal‐preferred orientation (CPO) have found that the texture that develops is dependent on many factors, including water content, differential stress, preexisting CPO, and deformation kinematics. To evaluate the role of these factors in naturally deformed samples, we present microstructural transects across three shear zones in the Josephine Peridotite. Samples from these shear zones exhibit a mixture of A‐type textures, which have been associated with dry conditions and primary activation of the olivine [100](010) slip system, and of E‐type textures, which have been associated with wetter conditions and primary activation of the [100](001) slip system. CPOs with characteristics of both A‐type and E‐type textures are also present. CPO type does not evolve systematically as a function of either strain or water content. We used a micromechanical model to evaluate the roles of preexisting texture and kinematics on olivine CPO evolution. We find that the preexisting texture controls CPO evolution at strains up to 5 during simple shear. Kinematics involving a combination of simple shear and pure shear can explain the olivine CPOs at higher strain. Hence, preexisting CPOs and deformation kinematics should be considered in the interpretation of CPOs measured in naturally deformed rocks and of large‐scale patterns in upper‐mantle seismic anisotropy.

Evolution of the Josephine Peridotite Shear Zones: 1. Compositional Variation and Shear Initiation

AbstractShear localization in the upper mantle, a necessity for plate tectonics, can have a number of causes, including shear heating, the presence of melt, the development of a strong crystal preferred orientation, and the presence of water. The Josephine Peridotite of southwestern Oregon contains shear zones that provide an excellent opportunity to examine the initiation of shear localization. These shear zones are relatively small scale and low strain compared to many shear zones in peridotite massifs, which typically have extreme grain size reduction indicating extensive deformation. We use major, trace, and volatile element analyses of a large suite of harzburgites from the Fresno Bench shear zones to evaluate the mechanisms leading to shear localization. Lithological evidence and geochemical transects across three shear zones show a complex history of melting, melt addition, and melt‐rock interaction. The distribution of aluminum and heavy rare earth elements across the shear zones suggest that melt flow was focused in the centers of the studied shear zones. Water concentrations in orthopyroxene grains of 180–334 ppm H2O indicate a comparatively high degree of hydration for nominally anhydrous minerals. The correlation of water with aluminum and ytterbium in orthopyroxene is consistent with a melt source for this hydration, suggesting that water equilibrated between the melt and peridotite. The presence of melt and hydration of the host rock provide mechanisms for initial weakening that lead to localized deformation.

Serpentinite with and without brucite: A reaction pathway analysis of a natural serpentinite in the Josephine ophiolite, California

A partially serpentinized peridotite from the Josephine ophiolite has been studied in detail in order to characterize the chemical processes of its serpentinization. The original rock was harzburgite, and its olivine and orthopyroxene are partially replaced by veins and patches of lizardite serpentine and magnetite; brucite and talc are completely absent from the serpentinite, regardless of whether the precursor mineral was olivine or pyroxene. Petrographic and mineral-chemical data suggest at least two phases of serpentinization. Incipient serpentinization produced lizardite and magnetite veinlets, from preferential dissolution of orthopyroxene, and/or infiltration of a silica-rich fluid. No talc or brucite was produced, which suggests this serpentinization happened in a chemically open system. Later serpentinization was from a fluid closer to Fe-Mg-Si chemical equilibrium with the harzburgite, which should in theory favor formation of a brucite-bearing serpentinite. Brucite is absent from late serpentine veins, but they have some porosity which could represent former brucite that was dissolved out or was reacted out after serpentinization. Isocon modeling suggests that Si, Fe, and K were added during serpentinization and that Ca was lost; i.e., the serpentinization was not isochemical (except for H2O). Results of petrographic observations, thermodynamic modeling, and mass balance calculations were used to constrain the reactions for global serpentinization of the studied sample. These reactions indicate that water with a concentration of H2 up to two times that of deep sea vent fluids may have been produced during the serpentinization of the Josephine peridotite, which could then have been a potential host for significant biomass.

Quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone

AbstractThe influence of pyroxene on upper mantle viscosity remains unclear but may occur either through its difference in strength relative to olivine or through its effect on olivine microstructure. To determine the role of pyroxene in moderating peridotite viscosity, we analyzed microstructures in paired harzburgites and dunites from a natural shear zone exposed in the Josephine Peridotite. Olivine crystallographic textures evolve similarly in harzburgites and dunites with increasing strain, indicating the operation of similar deformation mechanisms in olivine. The mean olivine grain sizes are ~1.5 times larger in dunites than in harzburgites, whereas the mean olivine subgrain sizes are a factor of ~1.2 smaller in dunites than in harzburgites. The average stresses in olivine, estimated with a subgrain‐size piezometer, are inversely correlated with pyroxene volume fraction and directly correlated with the mean olivine grain size. The calculated ratio of pyroxene viscosity to olivine viscosity for each harzburgite/dunite pair varies from 1.2 to 3.3. Notably, our data indicate that olivine viscosity is non‐Newtonian with a finite grain‐size sensitivity. We suggest that at the conditions of the shear zone, pyroxene is more viscous than olivine but that the increase in aggregate strength with larger pyroxene fractions is offset by correspondingly smaller olivine grain sizes. Thus, major rheological weakening associated with pyroxene content may only be possible if (1) temperatures are higher than those characterizing this shear zone, increasing the viscosity contrast between olivine and pyroxene or (2) stresses are high enough that grain‐size reduction promotes a transition to a deformation mechanism with extreme grain‐size sensitivity, such as diffusion creep.

Pyroxenes as tracers of mantle water variations

AbstractThe concentration and distribution of volatiles in the Earth's mantle influence properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H2O, P, and F were measured in olivine, orthopyroxene, and clinopyroxene in mantle peridotites by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H2O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and coexisting pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high‐temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral‐mineral partition coefficients. Clinopyroxenes have 60–670 ppm H2O, while orthopyroxenes have 10–300 ppm, which gives calculated olivine concentrations of 8–34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 1019 Pa s at 1250°C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 1021 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20–220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late‐stage melt addition, during which only hydrogen diffuses fast enough for reequilibration.

The influence of water and LPO on the initiation and evolution of mantle shear zones

We present data from the Josephine Peridotite (SW Oregon, USA) that constrain the underlying physical processes responsible for the initiation of shear localization and the evolution of ductile shear zones in Earth's mantle. Field measurements of narrow (2–60m wide) ductile shear zones in harzburgite were used to construct strain profiles, which have maximum shear strains ranging from γ=5.25 to γ>20. Measurements of pyroxene water concentrations from harzburgite samples within and immediately adjacent to the shear zones indicate that gradients in water concentration exist on a 10–100m scale, even after exhumation. Water concentration measurements are correlated with olivine lattice-preferred orientation (LPO), corroborating experimental results on the influence of water on slip system activity. Using empirical olivine flow laws and the diffusivity of water in olivine, we model initiation of a ductile shear zone through localized water weakening. We demonstrate that this mechanism can readily generate spatial perturbations in both effective viscosity and strain. However this model is not able to reproduce both the observed shear strain gradients and water concentration data from the Josephine shear zones. We evaluate other plausible localization mechanisms, which may amplify this initial strain perturbation. The most relevant at these conditions is the development of viscous anisotropy associated with the evolution of olivine LPO. Using recent experimental results, we demonstrate that progressive rotation of olivine LPO into the shear plane enhances deformation within a shear zone. We conclude that feedback between at least two microphysical processes is needed to account for observed outcrop-scale shear localization.

Helium distribution in a mantle shear zone from the Josephine Peridotite

A previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high-temperature ductile deformation in the mantle. In order to test this result, this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small (6m width) ductile mantle shear zone from the Josephine Peridotite, Oregon, USA. All measurements were made by coupled in vacuo crushing and melting, demonstrating that most of the helium (>80%) resides within the solid phases rather than fluid or melt inclusions. The present study confirms the influence of deformation on helium contents, but only at the highest shear strain (γ>20) are helium contents significantly higher. The highest helium concentration, by roughly a factor of two, is found in the center-most sample, which also has grain size reduction by a factor of ∼4. Dislocations and sub-grain boundaries are present in all samples and do not correlate with helium concentrations. Mineralogy also appears to have a negligible influence in this shear zone, as modal mineralogy is relatively homogeneous, with all samples being harzburgites. These observations suggest that the increase in helium concentration is related to grain size reduction, with grain boundaries proposed as an additional storage site for helium in the mantle.The present data also characterize the isotopic composition of the Josephine Peridotite: 3He/4He=6.7±0.2Ra (n=33, between 6.3 and 7.1Ra). The presence of cosmogenic 3He in the matrix is indicated by the helium isotopic composition released by melting: 3He/4He=8.5±0.3Ra (n=10; from 7.9 to 10.9). This corresponds to an exposure age of 10Kyr, which is approximately concordant with the end of the last glacial maximum. Very little radiogenic helium is present in the samples, suggesting extremely low uranium and thorium contents ([U]<0.3ppb). Helium isotope measurements in four samples outside the shear zone suggest that mineralogy and melt infiltration are also important factors for understanding helium storage in the mantle.

Trapped Melt in the Josephine Peridotite: Implications for Permeability and Melt Extraction in the Upper Mantle

Tabular dunites in the Josephine peridotite of southwestern Oregon represent conduits for melt extraction from the upper mantle. The amount of melt trapped within these channels during cooling and exhumation of the peridotite massif was calculated using a mass-balance approach. These calculations suggest that no more than 0·1–0·2 vol. % melt could have been trapped within these dunites after crystallization. This observation indicates that the threshold porosity for melt extraction is ≤0·2 vol. %. We use this constraint to evaluate published permeability models for partially molten dunite. We then use constraints from these permeability models to calculate lower bounds on the bulk viscosity of partially molten dunite and conclude that the bulk viscosity is of the order of 1023 Pa s at melt contents of ∼0·1 vol. % and reasonable melt extraction column heights. The small amount of trapped melt in these dunites is consistent with geochemical evidence for near fractional melting in the upper mantle.

Microstructural and Rheological Evolution of a Mantle Shear Zone

We conducted a microstructural study of a high-strain mantle shear zone from the Josephine Peridotite, SW Oregon, USA. The goal of this study is to understand how microstructural evolution at large strains leads to transitions in rheological behavior. The shear zone we investigated exhibits higher strain and greater localization than previously studied shear zones in the Josephine Peridotite. The margins of the shear zone have a homogeneous microstructure, characterized by moderately strong olivine fabrics, fairly weak orthopyroxene fabrics, and grain sizes of 2^3 mm. The highly deformed samples from the center of the shear zone display two distinct microstructural domainsca relatively coarse-grained domain (� 550m) that contains only olivine and a finer-grained domain (� 250m) that contains both olivine and orthopyroxene. The coarse-grained domain has a strong E-type olivine lattice-preferred orientation (LPO). Within the fine-grained domain the olivine LPO is also E-type, but significantly weaker.The E-type fabrics are rotated slightly past the shear plane, providing the first field-based confirmation of similar experimental observations. The presence of E-type fabrics, which form in the presence of moderate quantities of water, also highlights the potential importance of water to shear zone evolution.The orthopyroxene in the fine-grained domains has no LPO, suggesting that a transition to grain-size sensitive deformation occurred. The microstructural transition in orthopyroxene may have resulted in a marked weakening of the rock, suggesting that orthopyroxene plays a critical role in shear localization. These samples provide a crucial microstructural link between moderately localized shear zones and highly deformed ultramylonites.

Evolution of olivine lattice preferred orientation during simple shear in the mantle

Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO in harzburgites re-orients from a pre-existing LPO outside the shear zone to a new LPO with the olivine [100] maximum aligned sub-parallel to the shear direction between 168% and 258% shear strain. The strain at which [100] aligns with the shear plane is slightly higher than that observed in experimental samples, which do not have an initial LPO. While our observations broadly agree with the experimental observations, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, olivine re-alignment appears to be dominated by slip on both (010)[100] and (001)[100], due to the orientation of the pre-existing LPO. Fabric strengths, quantified using both the J- and M-indices, do not increase with increasing shear strain. Unlike experimental observations, our natural samples do not have a secondary LPO peak. The lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re-alignment. Harzburgites exhibit girdle patterns among [010] and [001] axes, while a dunite has point maxima. Combined with the observation that harzburgites are finer grained than dunites, we speculate that additional phases (i.e., pyroxenes) limit olivine grain growth and promote grain boundary sliding. Grain boundary sliding may relax the requirement for slip on the hardest olivine system, enhancing activation of the two easiest olivine slip systems, resulting in the [010] and [001] girdle patterns. Overall, our results provide an improved framework for calibration of LPO evolution models.

Oceanic versus emplacement age serpentinization in the Josephine ophiolite: Implications for the nature of the Moho at intermediate and slow spreading ridges

We present field, petrographie, and geochemical evidence for oceanic serpentinization in the 162 Ma Josephine ophiolite of NW California and SW Oregon. Undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation. The dikes intruding serpentinites are of two types: (1) Fe‐Ti enriched dikes with normal mid‐ocean ridge basalt (NMORB) magmatic affinity which are geochemically linked to the uppermost lavas and a late Fe‐Ti dike within the crustal sequence of the ophiolite; and (2) hornblende‐bearing, calc‐alkaline dikes intruded during ophiolite emplacement between 150 and 146 Ma. Based on crosscutting relationships between dikes and serpentinites, serpentinization of upper mantle peridotites took place at or near the ridge axis, during periods of amagmatic structural extension in the absence of a magma chamber. Lizardite‐bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of &lt;350°C within the oceanic upper mantle. Oceanic, lizardite‐bearing, serpentinized shear zones, from the ultramafic cumulate section to the basal sole, indicate that the entire Josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement The ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement, and we suggest that the paleo‐Moho in the Josephine ophiolite is a serpentinization boundary. The basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement. Oceanic serpentinization in the Josephine ophiolite took place beneath a 2–3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid‐ocean ridges, as well as at slower spreading ridges.

Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon

Results of a field study demonstrate that melt flow in the shallow mantle was focused along actively deforming ductile shear zones in the Josephine peridotite in SW Oregon. Intergranular flow of ascending liquids dissolved pyroxene and precipitated olivine, forming zones of dunite replacing harzburgite. Syntectonic formation of dunite was focused along near‐vertical shear zones, with both vertical and dextral displacement, in the shallow mantle at less than 30 km depth and temperatures between 950 and 1100°C. The migrating magmas which formed dunite and related pyroxenite included high‐Mg, calc‐alkaline andesites characteristic of subduction‐related magmatic arcs. Explanations for focusing of melt flow along shear zones include the presence of a pressure gradient between strong and weak material during active deformation (Stevenson, 1989) and rapid infiltration of liquid when grain boundaries are aligned in mantle tectonites (Waff and Faul, 1992). Focused flow of melt may, in turn, lead to localized deformation and the development of shear zones. It is likely that melt‐lubricated shear zones provide an important locus for both melt extraction and strain in the shallow mantle beneath spreading ridges and in subduction‐related magmatic arcs.

The effects of serpentinization on density and magnetic susceptibility: a petrophysical model

Abstract New measurements are presented of initial magnetic susceptibility and density for serpentinized harzburgites from the Josephine Peridotite (Oregon). An inverse correlation between density and susceptibility, which is typical of these rocks and similar rocks from other localities, is not explained by simple serpentinization reactions. Calculations of density and susceptibility as a function of reaction progress for reactions that produce magnetite show susceptibilities that are larger than those observed at low degrees of serpentinization. Susceptibilities of Fe-bearing serpentine and brucite are calculated from a molecular field model initially developed for the olivine and orthopyroxene solid solutions. At high degrees of serpentinization, susceptibilities that are lower than those observed are predicted by reactions where iron is sequestered by serpentine and brucite. Observed density-susceptibility trends of serpentinized harzburgites fall between the values predicted by these limiting reactions. The observed properties may be modelled by multiple reactions involved in multi-stage serpentinization processes such that the production of magnetite increases with the degree of serpentinization. The empirical and calculated properties provide a complementary characterization of density and susceptibility values of serpentinites.

Microstructure of serpentinite mylonites from the Josephine ophiolite and serpentinization in retrogressive shear zones, California

The Josephine Peridotite has experienced deformational episodes over a wide range of temperatures. Deformation within the stability field of a serpentine group mineral appears invariably accompanied by retrogressive metamorphism (serpentinization). Two general types of deformed serpentinites have been identified: (1) incohesive serpentinites (most common), which appear to have experienced low-temperature, predominantly brittle deformation and (2) serpentinite mylonites, which have experienced plastic flow near the upper limit of antigorite stability. The distinction between these two is based on textural and mineralogical criteria. Serpentinite mylonites are cohesive, homogeneously foliated, and typically strongly lineated rocks composed primarily of antigorite and magnetite. They occur in planar-bounded enclosed within the Josephine Peridotite that were the loci of both noncoaxial deformation and serpentinization. We discuss the development of two important shear-sense indicators, porphyroclast systems and oblique shear-band foliations as observed in these rocks. Porphyroclast systems in serpentinite mylonites are often comprised of cores and mantles of different mineralogy. Cores are usually composed of primary pyrogenetic minerals, whereas mantles, including tails, are composed of products of the serpentinization reaction. These systems are therefore termed porphyroctasts with neocrystallization tails.. Oblique shear bands are common micro-structures in serpentinite mylonites. Shear bands which offset the mylonitic foliation along narrow, discrete fractures are termed f-type, whereas those which offset the mylonitic foliation along narrow, ductile micro-shear zones are termed d-type. The latter are considered more reliable for shear-sense determinations. Strains accomplished by movement on shear bands are probably small. Strain resulting from a volume increase dur-ing serpentinization while holding the shear-zone width constant is sufficient to produce the shear bands. Therefore, we suggest that oblique shear bands may develop to accom-modate volume increases associated with deformation.

The Josephine ophiolite, northwestern California

Research Article| September 01, 1984 The Josephine ophiolite, northwestern California GREGORY D. HARPER GREGORY D. HARPER 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1984) 95 (9): 1009–1026. https://doi.org/10.1130/0016-7606(1984)95<1009:TJONC>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GREGORY D. HARPER; The Josephine ophiolite, northwestern California. GSA Bulletin 1984;; 95 (9): 1009–1026. doi: https://doi.org/10.1130/0016-7606(1984)95<1009:TJONC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The 157-m.y.-old Josephine ophiolite is a complete ophiolite consisting of the following units: harzburgite tectonite, cumulate ultramafics, cumulate gabbro interlayered with cumulate ultramafics, cumulate gabbro, high-level gabbro and diorite, a 1.5-km-thick sheeted dike complex, and 0.4 km of pillow lava and breccia. The ophiolite is conformably overlain by the “Galice Formation,” consisting of a thin, siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental-margin sources. The ophiolite and overlying “Galice Formation” were intruded by numerous 150- to 151-m.y.-old calc-alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite-pumpellyite to lower-greenschist facies.The Josephine Peridotite makes up the base of the ophiolite and covers >800 km2. It is an alpine-type peridotite that represents upper-mantle material and the residue of partial melting. Ultramafic rocks overlying the Josephine Peridotite are olivine + spinel ± clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene ± olivine ± plagioclase cumulates, and finally to clinopyroxene + plagioclase cumulates. High-level gabbros and diorites are characterized by highly variable textures, complex intrusive relationships, strongly zoned plagioclase, and abundant mafic dikes; plagiogranites occur at this level as dikes, small intrusions, and screens between mafic dikes, but they are rare.The sheeted dike complex consists of mafic dikes averaging 0.6 m in thickness. Chilled margins are locally well preserved, and many of the dikes have been split and intruded by younger dikes, resulting in “half” and “marginless” dikes. The dike complex is composed of 100% sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts. The dikes are inclined at ∼ 40° to bedding after correcting for folding and are inferred to have been rotated shortly after formation. The extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas. Some of the breccias formed as talus deposits, whereas others are graded and matrix-supported, indicating transport and redeposition, possibly in an area of rugged submarine topography. The dikes and lavas have geochemical affinities to island-arc tholeiites but are in some aspects transitional to mid-ocean–ridge basalts.The dikes and lavas are cogenetic with the underlying plutonic sequence, as indicated by similarity in crystallization sequence, contradictory intrusive relationships between sheeted dikes and high-level plutonic rocks, and lack of diabase dikes in the lowermost cumulates and harzburgite. The upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center, resulting in downward changes in mineralogy, texture, and δ18O; in addition, metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs. A geothermal gradient of 250–350 °C/km is inferred from mineral assemblages.On the basis primarily of its regional setting, the ophiolite is inferred to have formed in a back-arc basin; in particular, it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex (remnant arc). This interpretation is consistent with abundant volcanic-arc detritus in graywackes overlying the ophiolite. The island-arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back-arc spreading. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Josephinite: Crystal structures and phase relations of the metals

Josephinite, a complex, metal-bearing rock from the region of the Josephine Peridotite in southwest Oregon, contains FeNiCo metal alloy phases having exsolution textures. Scanning and transmission electron microscopy have revealed Widmanstaetten patterns in which the lamellae are polysynthetically twinned, ordered, face-centered-cubic FeNi 3 surrounded by untwinned, ordered, face-centered-cubic FeNi 3 and body-centered-cubic FeCo. These exsolution textures require a temperature in excess of 500°C for their formation. This is consistent with a mantle derivation of josephinite prior to obduction of the peridotite in one of the Klamath ophiolites.

Anomalous noble gases in josephinite and associated rocks?

In this paper we report Ne, Ar, Kr and Xe analyses of josephinite, Josephine Peridotite, and serpentinized Josephine Peridotite. In all three samples the elemental abundance patterns resemble patterns associated with surface waters, the Ne data do not exhibit the large 21Ne enrichments observed earlier, and the Kr and Xe compositions are indistinguishable from atmospheric composition at all isotopes, including 129Xe. Our data thus offer no significant evidence for isotopic anomalies in the noble gases. We also argue that the previous claims for primordial atmospheric-like Ar, anomalous Kr and Xe, excess 129Xe, and 4.6 × 10 9-year age are all questionable interpretations which cannot be defended against more prosaic alternatives. This leaves excess 21Ne as the only noble gas argument for exotic origin; we suggest that this might be an experimental artifact. Until the 21Ne question can be settled by more definitive experimentation, we feel that noble gas data cannot be used to support arguments that the origin of josephinite is more exotic than crustal serpentinization.

Origin of josephinite.

Josephinite is a nickel-iron alloy-bearing rock from Josephine County, southwestern Oregon. Josephinite had been thought to be the same as the metallic nickel-iron mineral awaruite (KRISHNARAO, 1962, 1964; RAMDOHR, 1950), but recently it has been shown to be a complex assemblage of minerals (BIRD and WEATHERS, 1975a). Although most samples of josephinite are somewhat altered, least-altered josephinite consists of several nickel-iron alloys, one intergrown with andradite garnet, an iron-cobalt alloy, and minor amounts of iron-nickel arsenides, iron-nickel-copper sulphides, and various unidentified phases. The intergrown metal-garent assemblage is apparently unique to josephinite. Alteration phases include serpentine minerals and magnetite. Josephinite occurs as detritus in several streams that originate and flow on the Josephine Peridotite; bedrock includes sheared serpentinite zones and basaltic to dioritic dikes within the peridotite. It is a reasonable assumption that the josephinite pebbles must be associated with one or more of these rocks; however, no unequivocal, large josephinite samples have been found within bedrock. BIRD and WEATHERS (1975a) proposed that because of its remarkable and apparently unique composition and textures, and its association with obducted mantle, josephinite has been derived from the mantle, conceivably the deep-mantle. Subsequently, DICK (1974), THORNBER and HAGGERTY (1976) and BOTTO and MORRISON (1976), following RAMDOHR (1950), argued that josephinite was formed by reduction of silicates, or sulphides, during serpentinization of the Josephine Peridotite. Currently, there are two views concerning the origin of josephinite: 1) josephinite is a product of low-temperature, synserpentinization processes and is comparable with awaruite 2) because of its mineral composition, textures, and rock associations, josephinite pre-dated serpentinization of the Josephine Peridotite (obducted ophiolite), was related to the origin of the Josephine Peridotite, and was derived from the primitive mantle, conceivably from as deep as the core-mantle boundary. Excesses of 3He, 21Ne and 129Xe have recently been found in josephinite and have been interpreted to indicate a mantle residence and, perhaps, a primordial age for this rock (DOWNING et al., 1977; BOCHSLER et al., 1978). We continue to propose a mantle derivation of josephinite; additionally, we suggest that josephinite was primitive material incorporated into the earth during earth accretion.

Partial melting in the Josephine Peridotite; I, The effect on mineral composition and its consequence for geobarometry and geothermometry

Partial melting in the Josephine Peridotite; I, The effect on mineral composition and its consequence for geobarometry and geothermometry