Trinity Peridotite Research Articles

Shallow Mantle Composition and Dynamics: Fifth International Orogenic Lherzolite Conference: Foreword

The First International Orogenic Lherzolite Conferencewas held in Montpellier, France, in 1990 (Menzies et al.,1991). Subsequent Orogenic Lherzolite Conferences wereheld in Granada, Spain in 1995, Pavia, Italy in 1999 andSamani, Japan in 2002. Avital aspect of all these meetingswas the opportunity to visit nearby exposures of mantleperidotites: to date, participants have visited Lherz inFrance; Baldissero, Balmuccia, Erro Tobio, Finero, Lanzoand Liguria in Italy; Ronda in Spain; Beni Bouserain Morocco; and Horoman in Hokkaido, Japan.Peer-reviewed papers from all four conferences were pub-lished in theJournal of Petrology in 1991, 2001and 2004 and,in one instance, in Chemical Geology in1996.The Fifth Lherzolite Conference was held in ShastaCity, California in September 2008. The conference wassponsored as a Chapman Conference by the AmericanGeophysical Union (AGU), which also provided locallogistical assistance. Funding for participants was providedby the AGU, US NSF Ridge Program, the NSF EarthSciences Division, the US Science Support Program of theConsortium for Ocean Leadership, the Royal Society UKand the Lithosphere^Asthenosphere Interactions taskforce of the International Lithosphere Program. Present,from14 countries, were108 attendees including 36 studentsand 11 postdoctoral researchers. Sixty-six oral presenta-tions and 67 posters were presented during the 5day meet-ing. As is the tradition, field trips to classic mantleperidotite localities were incorporated into the program,including a pre-meeting trip to the Josephine peridotiteled by Peter Kelemen, Henry Dick, and Greg Hirth forabout 50 people; a mid-week field trip for all participants,led by Tim Grove and Christy Till; and a 2day,post-meeting trip to the Trinity peridotite for about 50people, led by Peter Kelemen, Francoise Boudier, andKolya Stremmel.The Fifth Lherzolite Conference covered a broaderrange of topics than previous meetings, including carbonsequestration in the mantle and low-temperature hydro-thermal alteration. Underlying the presentations was therealization that previous distinctions between ‘orogenic’peridotites (e.g. Lanzo Massif, Italy), and ‘ophiolitic’peri-dotites (e.g. Troodos, Cyprus; Josephine, Oregon) and‘abyssal’peridotites from ocean ridges were in need of revi-sion. It now appears that a continuum exists fromsub-continental to oceanic mantle exposed in some com-plexes (e.g. Lanzo; Bodinier et al., 1991; Piccardo et al.,2007; Mu«ntener et al., this issue), and that much ocean

Hydrogen and Oxygen Isotope Constraints on Hydrothermal Alteration of the Trinity Peridotite, Klamath Mountains, California

The Trinity peridotite represents a rare opportunity to examine a relatively fertile plagioclase peridotite that was exhumed and later subjected to intrusive events in a seafloor environment, followed by its emplacement and incorporation into a continent. Over 250 stable isotopic determinations on whole rocks and minerals elucidate the hydrothermal evolution of the Trinity complex. All three serpentine polymorphs are present in the Trinity peridotite; these separate on the basis of their δD values: antigorite, -46 < δD < -82‰ and lizardite and chrysotile, -90 < δD < -106 and -110 < δD < -136‰, respectively. Antigorite coexists with chlorite, talc, and tremolite in contact aureole assemblages associated with Silurian/Devonian gabbroic plutons. Lizardite and chrysotile alteration carries a meteoric signature, which suggests association with post-emplacement serpentinization, or overprinting of earlier low-temperature seafloor serpentinization. Regionally, contours of δD values exhibit bull's-eye patterns associated with the gabbroic plutons, with δD maxima coinciding with the blackwall alteration at the margins on the plutons. In contrast to the hydrogen isotope behavior, oxygen isotope values of the three polymorphs are indistinguishable, spanning the range 5.3 < δ18O< 7.5, and suggesting low integrated fluid fluxes and strongly 18O-shifted fluids. Inferred primary δ18O values for peridotite, gabbro, and late Mesozoic granodiorite indicate a progressive 18O enrichment with time for the source regions of the rocks. These isotopic signatures are consistent with the geology, petrochemistry, and geochronology of the Trinity massif, which indicate the following history: (1) lithospheric emplacement and cooling of the peridotite in an oceanic environment ~472 Ma; (2) intrusion of gabbroic plutons into cold peridotite in an arc environment between 435 and 404 Ma; and finally (3) intrusion of felsic plutons between 171 and 127 Ma, long after the peridotite was incorporated into the continental crust.

Observations of Li isotopic variations in the Trinity Ophiolite: Evidence for isotopic fractionation by diffusion during mantle melting

The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit. Results show large variations in 7Li/ 6Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average δ 7Li for single thin sections along the traverse reveal a low in δ 7Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of ∼10 per mil in the low δ 7Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in 7Li/ 6Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction. We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in δ 7Li as a function of distance can be created in year to decade timescales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects.

Extreme Nd isotopic variation in the Trinity Ophiolite Complex and the role of melt/rock reactions in the oceanic lithosphere

Peridotites, dykes and gabbros from the 470–420 Ma Trinity Ophiolite Complex of northern California exhibit large geochemical rare earth element (REE) and Nd isotopic variations on the small scales which are indicative of a complex history. The Trinity Ophiolite, which covers an area of ≈1600 km2, consists of three distinct units: (1) a ∼2–4 km-thick sheet of plastically deformed peridotites, including various ultrabasic lithologies (plagioclase and spinel lherzolite, harzburgite, wherlite and dunite); the peridotite unit is a fragment of mantle lithosphere of oceanic affinity; (2) a series of small (∼1 km diameter) undeformed gabbroic massifs; (3) several generations of basic dykes. The peridotites display the largest geochemical and isotopic variations, with ɛNd(T) values ranging from +10 down to 0. In the gabbroic massifs and intrusive dykes, the variation in model ɛNd(T) values is reduced to 7 ɛNd units: 0 to +7. As a general rule, peridotites, gabbros and dykes with ɛNd(T) values around 0 or +3 give less depleted L(light)REE patterns than do those with ɛNd(T) values in the range +7 to +10. In the peridotites, the Nd isotopic variations take place over very short distances, with jumps as large as 7 ɛNd units occurring on scales of less than 20 m. Comparison with available age data indicates that the peridotites with ɛNd(T)≈+10 could be slightly older than the intrusive gabbro massifs and basic dykes (470 Ma vs. 420 Ma). Strontium isotopic data used in connection with Sm-Nd results demonstrate that the 10 ɛNd units variation displayed by the Trinity Peridotite is a primary feature and not an artefact due to REE mobility during seawater interaction. The variable Nd isotopic signatures and variable LREE patterns in the Trinity Peridotite cannot represent mantle source characteristics as there is evidence that this unit was partially melted when it rose as part of the upwelling convecting mantle. Field, petrographic, geochemical and isotopic data rather suggest that the observed heterogeneity is due to local reactions between a 470 Ma proto-peridotite with ɛNd(T)=+10 and younger (420 Ma) basaltic melts with lower ɛNd(T) values (i.e. the gabbroic massifs and the dykes). The gabbros and basic dykes of the Trinity Complex have geochemical and isotopic compositions similar to the arc basalts from the adjacent Copley Formation, so it is proposed that the younger melts are related to arc magmatism.

Formation of harzburgite by pervasive melt/rock reaction in the upper mantle

Many mantle peridotite samples are too rich in Si02 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/rock reaction zones in the Trinity peridotite provide evidence for reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.

Oxygen isotope evidence for multi-stage hydrothermal alteration at a fossil slow-spreading center: the Silurian Trinity ophiolite (California, U.S.A. )

The Trinity ophiolite consists of a widespread mantle unit (the Trinity Peridotite) and a thin crustal sequence ( ∼2 km) outcropping discontinuously. The well-exposed massifs of layered and isotropic gabbros, diabasic dikes and pillow lavas define a complete oceanic crustal sequence. The basalts, dikes and gabbros are characterized by retrograde metamorphic associations. Previous work has shown that the extent of alteration is directly correlated to the amount of circulating seawater and decreases downwards for increasing temperatures. Oxygen isotopic compositions were measured on whole-rock samples and a few mineral separates across the ophiolite section. Clearly, hydrothermal alteration in the presence of seawater has introduced large variations of '80/'60 ratios with respect to those of the starting rocks whose composition is assumed to be typical of mantle-derived materials. The evolution of δ 18O-values as a function of depth is similar to the Oman example but there are some significant differences: with respect to the mantle reference composition, the upper section (pillow lavas) is '80 enriched up to 10.1‰. Low δ 18O-values down to +4‰ are recorded by some sheeted dikes, isotropic and cumulate gabbros. However, the Trinity ophiolite strongly differs from the Oman ophiolite by the presence of 18O-enriched rocks at deep levels of the ophiolite section. Microthermometry on fluid inclusions from a few quartz-bearing samples and water/rock ratios inferred from previous Sr isotope data allow us to calculate a pattern of alteration temperatures consistent with a complex regime of water-rock interactions: low-temperature ( ∼ 110–170°C) exchange occurred pervasively through the pillow lava section but also irregularly affected deeper parts of the crust (isotropic and layered gabbros) in which a previous episode of high-temperature ( ∼ 300–390°C) interaction took place. Deep channelized penetration of seawater at low temperature could be related to the wider and deeper faulting pattern which characterizes slow-spreading centers. Such a low-temperature, off- axis interaction (pervasive in the upper section and heterogeneous at deeper levels) could destroy the δ 18) balance possibly established during the axial hydrothermal activity. The oxygen isotope composition of hydrothermally-altered rocks in the Trinity complex is consistent with a δ 18O-value of 0 ± 2‰ for Silurian seawater.

Ephemeral magma chambers in the Trinity peridotite, northern California

The Trinity Massif comprises the major lithologies of an ophiolite, as defined at the 1972 Penrose conference. Previous studies have shown, however, that it differs from the Semail (Oman) or Table Mountain (Newfoundland) ophiolitic massifs, particularly because its crustal section is thin, and because its mantle section has vertical plastic flow planes. These features have led to an interpretation of the Trinity Massif as a fragment of slow-spreading oceanic lithosphere (Le Sueur et al., 1984; Boudier and Nicolas, 1985). In this paper, we show that the Trinity gabbros occur in discontinuous, kilometre-sized pockets, intrusive into the mantle peridotites. The internal stratigraphy and the petrological characteristics of these gabbros suggest that they formed in short-lived magma chambers. These ephemeral magma chambers developed after the end of the plastic deformation in the surrounding mantle, when it had cooled down to lithospheric temperatures. We discuss the possibility that these small and ephemeral magma chambers formed at a slow-spreading oceanic ridge.

Origin and Petrogenesis of the Trinity Ophiolite Complex (California): New Constraints from REE and Nd Isotope Data

Sm-Nd isotopic results, as well as concentrations of major elements and REE, are reported for nine whole-rock samples of plagioclase lherzolite, lherzolite, harzburgite, clinopyroxene-rich dykes, layered and isotropic gabbros, and a plagiogranite dyke from the central part of the Silurian Trinity Ophiolite Complex (Toad Lake area) of northern California. The data provide new constraints on the origin and petrogenesis of this ophiolite complex and have potentially important implications with regard to the origin of mid-ocean ridge basalts (MORBs) and differentiation of the oceanic lithosphere. Although Sm-Nd isotopic data published earlier on samples from the northern (China Mountain) and southern (Gray Rock) parts of the complex yielded discordant ages and initial εNd values for peridotites [T=472 ± 32 Myr; εNd(T)=+ 10·4 ± 0·4], and crustal rocks from the overlying magma chambers or intrusive dykes [T∼440 Myr; εNd(T)∼ +7·0], all the samples investigated in this study define a single array in the Sm-Nd isochron diagram, allowing an age of 423 ± 48 Myr and an εNd(T) value of +6·4 ± 0·7 to be calculated. Furthermore, although the results published so far on peridotites and gabbroic dykes from Trinity indicated variable LREE depletion, all the basicultrabasic samples from Toad Lake are found to exhibit similar, strongly LREE-depleted patterns [0±17<(Ce/Sm)N<0·34]. As elsewhere in the Trinity Ophiolite Complex, peridotites in the Toad Lake area preserved evidence of partial melting (e.g., plagioclase lenses tracking the high-temperature flow plane) and melt extraction (e.g., depleted borders along cross-cutting dykes). Comparison of the new Sm-Nd results with U-Pb zircon ages for plagiogranite samples suggests that 423 ± 48 Myr is a geologically meaningful age, corresponding to the crystallization of plagioclase lenses in plagioclase lherzolites, as well as to the crystallization of dykes and rocks of the Toad Lake magma chamber. Because of the discrepancy in Sm-Nd ages, initial εNd(T) values, and REE patterns, peridotites and crustal rocks of the Trinity Ophiolite Complex have so far been regarded as genetically unrelated. The results from Toad Lake argue for partial reassessment of this view. As far as Sm-Nd isotopic data and REE patterns are concerned, peridotites, dykes, and gabbros from this area have features consistent with a cogenetic rock suite. Clinopyroxene-rich dykes near Toad Lake have been regarded as the remnants of the conduits which fed the magma chamber, a view which is fully confirmed by the present geochemical and isotopic results. Careful evaluation shows that these dykes have petrographic and geochemical features more representative of crystallized melts, rather than mineral segregates, hence allowing us to propose that their actual picritic composition (MgO∼ 16%) might reflect the composition of the magma which entered the Toad Lake magma chamber. Comparison of the present results with those published earlier shows the Trinity peridotite to be an isotopically heterogeneous body. Moreover, a difference in age may exist between peridotites from the northern and southern parts of the complex. It is possible that the Trinity Ophiolite Complex represents the remnant of the opening of an ocean, perhaps in an area similar to a modern back-arc or inter-arc basin.

Diversity of magma types in a lower Paleozoic island arc-marginal basin system (Eastern Klamath Mountains, California, U.S.A.)

A trace-element investigation has been conducted on Ordovician to Devonian volcanics of the eastern Klamath Mountains spatially associated to the Siluro-Ordovician Trinity peridotite and gabbros. All the studied volcanics (low-K tholeiites and calc-alkaline volcanics) present strong island-arc geochemical features (low TiO 2 contents, negative Nb, Zr and Ti anomalies, mild LREE enrichment and depletion). Some volcanics from Lovers Leap and Copley exhibit strong immature island-arc geochemical affinities (very depleted LREE patterns and very low Nb contents), suggesting that they were formed by high degrees of partial melting (20%) of a residual lithosphere. Diminution of the partial melting degree and addition of sediments to the source will explain the occurrence of less depleted or slightly LREE-enriched volcanics. The Grey Rocks basaltic pillow-lavas thrust onto the Trinity peridotitic sheet probably represent the extrusive member of the ophiolitic sequence. Their strong island-arc geochemical affinities suggest that the ophiolite was formed during the first stages of a back-arc basin development. Differentiation took place in the arc by melting of the deeper basaltic layers, leading to the formation of abundant rhyolitic and trondhjemitic magmatism representing the first stages of continental accretion.

Metamorphic evolution of the Central Metamorphic Belt, Klamath Province, California: an inverted metamorphic gradient beneath the Trinity peridotite

ABSTRACT Metabasalts and metasedimentary rocks of the Devonian Central Metamorphic Belt comprise the lower plate of the east‐dipping Trinity thrust system in the Klamath province. An inverted metamorphic gradient is preserved in the Central Metamorphic Belt; metamorphic conditions decrease from amphibolite facies adjacent to the Trinity thrust, through albite‐epidote amphibolite facies, to upper greenschist facies at the base of the Central Metamorphic Belt. Mineral chemistry, mineral assemblages and limited geothermometry suggest that peak metamorphic conditions decrease structurally downward from 650 ± 50° C at the Trinity thrust to 500 ± 50° C at the base of the Central Metamorphic Belt, under pressures of 5 ± 3 kbar. Synmetamorphic Ab + Qtz veins, up to 1 m thick, increase in abundance towards the Trinity thrust. Infiltration of H2O‐CO2 fluids derived from prograde devolatilization reactions in the Central Metamorphic Belt caused extensive hydration and metasomatism of the Trinity peridotite; the hanging wall block of the Trinity thrust zone.Geological relationships and the preserved inverted metamorphic gradient suggest that the Central Metamorphic Belt formed in an east‐dipping Devonian subduction zone in an oceanic environment. The Central Metamorphic Belt appears to represent a discrete slice of accreted oceanic crust several km thick, rather than progressively accreted material. Metamorphic pressures recorded by the Central Metamorphic Belt are intermediate between the ∼2 kbar pressures recorded in dynamothermal aureoles beneath obducted ophiolites and the 7–10 kbar preserved in subduction‐related inverted metamorphic gradients. The lack of blueschist facies mineral assemblages in the Central Metamorphic Belt may possibly be explained by an anomalously warm geotherm prior to subduction or early shear heating prior to the arrival of wet rocks at depth.

Nd-Sr-Pb systematics and age of the Kings River ophiolite, California: implications for depleted mantle evolution

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with e_Nd (483)= +10.7±0.5 and e_Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron. The 483 Ma data provide the first evidence of lower Paleozoic oceanic crust in the Sierran ophiolite belt. New U-Pb analyses of zircons from a plagiogranite lying on the 483 Ma Sm-Nd isochron yield upper and lower intercepts with the concordia of 430^(+200)_(−60) and 183±15 Ma. Published zircon ages have underestimated the primary age of the ophiolite by 200–300 m.y. due to the effects of polymetamorphism. The 483 Ma samples have initial ^(87)Sr/^(86)Sr=0.7023–0.7030, ^(206)Pb/^(204)Pb=17.14–17.82, ^(207)Pb/^(204)Pb=15.37–15.52, ^(208)Pb/^(204)Pb=36.80–37.38. The 285 Ma samples have similar initial 87Sr/86Sr, but more radiogenic Pb. The range in Sr and Pb compositions is probably due to introduction of radiogenic Sr and Pb during multiple post-emplacement metamorphic events. The high e Nd, low ^(87)Sr/^(86)Sr, ^(206)Pb/^(204)Pb, ^(207)Pb/^(204)Pb, ^(208)Pb/^(204)Pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 Ma portion of the ophiolite. Igneous activity at 285 Ma is thought to have occurred in an arc or back-arc setting, or perhaps along a leaky transform. The initial e_Nd (483)=+10.7 is indistinguishable from that of the similar age Trinity Peridotite (Jacobsen et al. 1984). This value is the highest yet reported for the Mesozoic or Paleozoic depleted mantle and requires either a mantle source that was depleted ∼ 850 m.y. earlier than average or a source more highly depleted than average. Alternatively, if such values were more typical of the early Paleozoic mantle than is currently thought, then there has been little evolution of the depleted mantle over the last ∼ 500 m.y. This requires that the modern mantle has been refluxed by material with low e_Nd, such as continental crust.

Serpentinization and infiltration metasomatism in the Trinity peridotite, Klamath province, northern California: implications for subduction zones

The Trinity peridotite was emplaced over metabasalts and metasedimentary rocks of the central metamorphic belt along the Devonian Trinity thrust zone. Three metamorphic events can be recognized in the Trinity peridotite: (1) antigorite (δD= −63 to −65%.) formation related to regional underthrusting of the central metamorphic belt; (2) contact metamorphism associated with Mesozoic dioritic plutons; and (3) late-stage formation of lizardite ± brucite and chrysotile (δD= −127 to −175%.) due to infiltration of meteoric waters. Abundant relict phases indicate incomplete reactions and strongly suggest that the availability of H2O was a controlling factor during serpentinization. Antigorite (event 1) formed as a result of infiltration into the Trinity peridotite of mixed H2O-CO2 fluids derived from the underlying central metamorphic belt. Foliation defined by magnetite veins and shear zones within antigorite serpentinites are subparallel to the Trinity thrust. The assemblage Fo + Atg + Chl + Mag ± Tr ± Carb reflects partial hydration of peridotite at 425–570° C. Talc-rich serpentinite formed along the thrust as a result of the infiltration of silica-bearing fluids. Metasomatic mass-balance calculations based on silica solubilities and the extent of antigorite serpentinization suggest that 80–175 volumes of fluid have passed through a given volume of original peridotite at the Trinity thrust. The Trinity thrust probably represents a Devonian subduction zone. Thermodynamic calculations suggest that hydration reactions account for ∼30–35% of the total heat released by the cooling Trinity peridotite. By analogy, similar hydration reactions are to be expected in the overlying mantle wedge of a subduction zone which act to retard cooling of the hanging wall, just as dehydration reactions delay heating of the downgoing slab. Metasomatic zones formed in peridotite at the Trinity thrust may reflect similar metasomatic processes to those proposed to occur in the mantle wedge above a subducting slab.

Serpentinization and infiltration metasomatism in the Trinity peridotite, Klamath province, northern California: implications for subduction zones : Peacock, S.M., 1987. Contr. Miner. Petrology, 95(1): 55–70. Dept. of Geol., Arizona State Univ., Tempe, AZ 85287, USA

Serpentinization and infiltration metasomatism in the Trinity peridotite, Klamath province, northern California: implications for subduction zones : Peacock, S.M., 1987. Contr. Miner. Petrology, 95(1): 55–70. Dept. of Geol., Arizona State Univ., Tempe, AZ 85287, USA

A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution

Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A Sm Nd mineral isochron for a plagioclase lherzolite yields an age, T = 427 ± 32 Ma and initial ε Nd = + 10.4 ∓ 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite P-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm Nd model age of the plagioclase lherzolite totalrock is T CHUR Nd = 3.4 AE . This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a Sm Nd mineral isochron age of T = 435 ± 21 Ma and ε Nd = + 6.7 ∓ 0.3 . A pyroxenite dike yields an initial ε Nd = + 7.3 ± 0.4 . The initial ε Nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. Rb Sr data do not give any coherent pattern. However, some bounds can be put on initial Sr values of ε Sr ⩽ −21 for the plagioclase lherzolite and ε Sr ⩽ −8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and Sm Nd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ∼ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (∼ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ∼ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.

The origin and significance of large, tabular dunite bodies in the Trinity peridotite, northern California

Kilometer-sized, tabular dunite bodies are contained within harzburgite, lherzolite and plagioclase lherzolite host rocks in the Trinity peridotite, northern California. An igneous origin for the dunite by crystal fractionation of olivine from a melt is suggested by their tabular shapes, clots of poikilitic clinopyroxene grains, chromite pods, and by analogy to dunite bodies in the Samail and Vourinos ophiolites (Hopson et al. 1981; Harkins et al. 1980; Moores 1969). However, structures and systematic variations in mineralogy and mineral chemistry suggest that at least the marginal few meters of the bodies are residues produced by extraction of a basaltic component from a plagioclase lherzolite protolith. A model is suggested in which a picritic melt ascended through the upper mantle in vertically oriented channels. Part of the dunite in the tabular bodies was produced by fractional crystallization of olivine from the melt. Additional dunite at the margins of the bodies was formed by extraction of a basaltic component from plagioclase lherzolite wall-rocks during partial assimilation by the picritic melt. The latter process is similar to the “wall-rock reaction” discussed by Green and Ringwood (1967) and is essentially zone refining of the the mantle wall rocks by the migrating melt. It is significant because it suggests a mechanism in addition to fractional crystallization for enrichment of incompatible elements in basalts.

Petrology and petrogenesis of the Trinity peridotite, An upper mantle diapir in the eastern Klamath Mountains, northern California

The Trinity peridotite is an enormous ultramafic massif situated in the eastern Klamath Mountains of northern California and composed of a diverse assemblage of ultramafic rocks including dunite, harzburgite, plagioclase lherzolite and clinopyroxene‐rich lithologies. These rocks preserve an excellent record of a complex mantle history involving plastic deformation, partial melting, recrystallization and reaction with transient silicate melts in a large peridotite mass as it ascended through the upper mantle. Structural data are utilized to determine the relative ages of events that affected the peridotite, and petrochemical data are utilized to place constraints on the pressures and temperatures at which the events occurred. The Trinity peridotite is inferred to have ascended through the upper mantle from an initial depth of not less than 30 km, based on evidence for replacement of spinel by plagioclase. Ariegite bands and dikes formed, and isoclinal folding occurred while the peridotite was in the spinel lherzolite stability field (P &gt; 10 kbar). The peridotite passed its solidus and partially melted at a pressure of ≤10 kbar, producing a basaltic melt with tholeiitic affinities. Plagioclase lherzolite formed where partial melt was trapped and minerals reequilibrated during subsequent low pressure cooling of the peridotite. Clinopyroxene‐rich dikes were emplaced after the peridotite had passed into the plagioclase lherzolite field (P &lt; 10 kbar). Plastic deformation, recorded by penetrative foliation and lineation, occurred continuously during the diapiric rise of the Trinity body. The emplacement of undeformed gabbro marks the arrival of the Trinity at the base of the crust. Large, tabular dunite bodies, clinopyroxene‐rich dikes and ariegite dikes mark channels through which melts passed and reacted with the peridotite en route to the surface. The interiors of the dunite bodies and the dikes are interpreted to have formed as crystal cumulates from the transient melts. The margins of the dunite bodies and depleted zones around some clinopyroxene‐rich dikes formed as a ‘restite’ by assimilation of pyroxene and plagioclase from the peridotite wall rocks by the transient melts. The melts are hypothesized to have formed by partial melting of peridotite at a great depth (e.g., 45–60 km) and to have penetrated the Trinity peridotite at shallower depths (&lt;30 km) where they were not in equilibrium with pyroxene or plagioclase. Structural and stratigraphic data suggest that these events occurred near an oceanic volcanic highland in a setting analogous to a modern island arc or back‐arc basin.