Geochemistry and Intrusive History of the Ashland Pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial ⁸⁷Sr/⁸⁶Sr = 0{dot}7044, ¹⁸O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (La_N/Lu_N = 7) and then by a suiteof K₂O- and P₂O₅ rocks of quartz monzodioritic affinity (La_N/Lu_N= 13–21; La_N/Sm_N = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(La_N/Lu_N= 13–19; La_N/Sm_N = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately P_total = 2{dot}3kb,P_H2O between 1{dot}5 and 2{dot}2 kb, and f_O2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO₂ togetherwith increasing ¹⁸O and decreasing initial ⁸⁷Sr/⁸⁶Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr₁ and high (¹⁸O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in ¹⁸O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P₂O₅-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P₂O₅-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.     

The Perseverance Ultramafic Complex, Western Australia: The Product of a Komatiite Lava River

The Perseverance ultramafic complex is a body of olivine-richkomatiitic rocks spatially associated with the Agnew nickeldeposit, in the Agnew-Wiluna greenstone belt of the ArchaeanYilgarn Block in Western Australia. The complex consists ofa central lenticular body, up to 700 m thick, of olivine adcumulates,flanked by laterally extensive sheet-like bodies of olivineorthocumulates and spinifextextured komatiite flows. Rocks progressivelyfurther away from the central lens have chemical compositionsreflecting higher original proportions of komatiite liquid tocumulus olivine. Parent liquids had MgO contents between 25and 32% MgO, approximately chondritic Al/Ti ratios and HREEpatterns, and moderate depletion in LREE. Olivines within the adcumulate lens show a progressive increasein forsterite content from Fo₉₃ at the bottom to Fo_94?5, atthe top. Calculated original olivine compositions in the flankingrocks are similar to those at the base of the central lens.Original olivine nickel contents show a symmetrical variationfrom maximum values of 3500 ppm at the top of the central lens,through minimum values of 1000 ppm at the base and margins ofthe central lens to intermediate values in the distal rocks.The complex as a whole shows evidence for nickel depletion relativeto other komatiite suites. These observations are explained in terms of prolonged eruptionand flow of komatiitic lava down a major flow channel or lavariver. Adcumulates crystallized on the floor and sides of thecentral channel, which was formed at an early stage by thermalerosion of floor rocks. Episodic overflow of the central channelproduced distal ‘flood plain’ rocks consisting ofolivine orthocumulates and layered flows. Lavas became moremagnesian and nickel-rich with time, giving rise to the observedspatial variation in primary olivine composition. Nickel depletionof the earliest lavas is attributed to pre-eruption segregationof large volumes of immiscible Fe-Ni-sulfide, which were concentratedto form the underlying Agnew nickel deposit.     

Instrumental Neutron Activation Analysis by Collecting Only One Spectrum: Results For International Geochemical Reference Samples

Instrumental neutron activation analysis of rocks and minerals is currently done on basis of collecting 2 or 3 energy spectra. We have found that, by collecting one spectrum one week after irradiation, satisfactory results for the USGS and CRPG geochemical reference samples can be obtained.     

Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism

Chromite compositions in komatiites are influenced by metamorphicprocesses, particularly above 500°C. Metamorphosed chromiteis substantially more iron rich than igneous precursors, asa result of Mg–Fe exchange with silicates and carbonates.Chromite metamorphosed to amphibolite facies is enriched inZn and Fe, and depleted in Ni, relative to lower metamorphicgrades. Relative proportions of the trivalent ions Cr³⁺, Al³⁺and Fe³⁺ are not greatly modified by metamorphism up to loweramphibolite facies, although minor Fe³⁺ depletion occurs duringtalc–carbonate alteration at low temperature. SignificantAl is lost from chromite cores above 550°C, as a resultof equilibration with fluids in equilibrium with chlorite. ElevatedZn content in chromite is restricted to rocks with low (metamorphic)Mg/Fe ratios, and is the result of introduction of Zn duringlow-temperature alteration, with further concentration and homogenizationduring prograde metamorphism. Cobalt and Mn also behave similarly,except where carbonate minerals are predominant in the metamorphicassemblage. Chromite at amphibolite facies is typically extensivelyreplaced by magnetite. This is the result of incomplete metamorphicreaction between chromite and chlorite-bearing silicate assemblages.Magnetite compositions at the inner chromite–magnetiteboundary are indicators of metamorphic grade. KEY WORDS: chromite; komatiite; spinel; metamorphism; Zn     

Sheet slides and rotational failures on a convergent margin: the Kidnappers Slide, New Zealand

The Kidnappers Slide, on the upper continental slope of the convergent margin off eastern North Island, New Zealand, has been re-examined using a grid of high-resolution seismic reflection profiles. The slide is not a single feature but a complex of sheet slides and rotational failures ranging from 20 to 140 m thick, and covering a total area of 720 km². Failures occurred in several phases, on slopes of 1–5°, in late Quaternary, muddy, shelf-edge clinoforms that have prograded into an accretionary, trench-slope basin. Piston cores and seismic stratigraphy show that the main failure probably occurred in early Holocene times but that movements ranged from mid last glacial to late Holocene times. The sheet slides exhibit tensional collapse via numerous listric normal faults that sole out on glide planes; there is no clear evidence of compressional structures anywhere within the complex. The glide planes occur at progressively deeper stratigraphic levels towards the northeastern end of the complex, and near the steep slope that defines the seaward edge of the trench-slope basin. There is retrogressive failure at the top of the slope. The surficial slides are being deformed by growth of active tectonic faults and folds associated with the convergent plate margin. This type of slope failure may be partially related to metastable sandy layers within the last glacial age progradational sequence, and possibly to formation of bubble phase gas at shallow depths. Failure was probably triggered by earthquake loading of sediments in this highly seismic region.     

Petrology and Geochemistry of the Late Eocene Harrison Pass Pluton, Ruby Mountains Core Complex, Northeastern Nevada

The late Eocene Harrison Pass pluton was emplaced in the transitionzone between the infrastructure and suprastructure of the RubyMountains core complex. Emplacement was at     

Comment on "Two New Carbonate Stable Isotope Standards"

Friedman et al (1) propose that NBS 19 calcium carbonate substitute for PDB. This is in apparent conflict with the recommendation of the September 1976 consultants' meeting convened by the International Atomic Energy Agency on stable isotope standards. Furthermore, Friedman et al suggest ɛ-values be adopted for NBS 19 that do not take into account the ɛ-values of NBS 20 (Solenhofen limestone). We believe that adoption of these proposals may lead to confusion in the stable isotope literature and to lack of correspondence between historic and future isotopic results.
In sharp contrast to the results of Friedman et al (1), we find NBS 20 to be a satisfactory reference material if it is handled properly.     

The Concentration of the Platinum-Group Elements in South African Komatiites: Implications for Mantle Sources, Melting Regime and PGE Fractionation during Crystallization

We have analysed 18 samples of komatiite from five consecutivelava flows of the Komati Formation at Spinifex Creek, BarbertonMountain Land. Our samples include massive komatiite, varioustypes of spinifex-textured komatiite, and flow-top breccias.The rocks have low platinum-group element (PGE) contents andPd/Ir ratios relative to komatiites from elsewhere, at 0·45–2ppb Os, 1–1·4 ppb Ir, <1–5 ppb Ru, 0·33–0·79ppb Rh, 1·7–6 ppb Pt, 1·6–6·1ppb Pd, and Pd/Ir 3·3. Pt/Pd ratios are c. 1·1.Platinum-group elements are depleted relative to Cu (Cu/Pd =15 300). They display a tendency to increase in the less magnesiansamples, suggesting that the magmas were S-undersaturated uponeruption and that all PGE were incompatible with respect tocrystallizing olivine. Komatiites from the Westonaria Formationof the Ventersdorp Supergroup and the Roodekrans Complex nearJohannesburg have broadly similar PGE patterns and concentrationsto the Komati rocks, suggesting that the PGE contents of SouthAfrican ultrabasic magmas are controlled by similar processesduring partial mantle melting and low-P magmatic crystallization.Most workers believe that the Barberton komatiites formed byrelatively moderate-degree batch melting of the mantle at highpressure. Based on the concentration of Zr in the Komati samples,we estimate that the degree of partial melting was between 26and 33%. We suggest that the low PGE contents and Pd/Ir ratiosof all analysed South African komatiites are the result of sulphideshaving been retained in the mantle source during partial melting.The difference in Pd/Ir between our samples and Al-undepletedkomatiites from elsewhere further suggests that the PGE arefractionated during progressive partial melting of the mantle.Thus, our data are in agreement with other recent studies showingthat the PGE are hosted by different phases in the mantle, withPd being concentrated by interstitial Cu-rich sulphide, andthe IPGE (Os, Ir, Ru) and Rh resting in monosulphide solid solutionincluded within silicates. Pt is possibly controlled by a discreterefractory phase, as Pt/Pd ratios of most komatiites worldwideare sub-chondritic. KEY WORDS: platinum-group elements; komatiites; Barberton; mantle melting; South Africa