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USGS1988

Crustal contributions to arc magmatism in the Andes of Central Chile

Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280??20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO2) K2O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward - opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. 87Sr/86Sr values rise northward from 0.7036 to 0.7057, and 143Nd/144Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from -95 to -295 mgal, interpreted to indicate thickening of the crust from 30-35 km to 50-60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30-40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust. ?? 1988 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1983

The granite problem as exposed in the southern Snake Range, Nevada

A geochemically and mineralogically diverse group of granitoids is present within an area of 900 km2 in the southern Snake Range of eastern Nevada. The granitoids exposed range in age from Jurassic through Cretaceous to Oligocene and include two calcic intrusions, two different types of two-mica granites, and aplites. The younger intrusions appear to have been emplaced at progressively more shallow depths. All of these granitoid types are represented elsewhere in the eastern Great Basin, but the southern Snake Range is distinguished by the grouping of all these types within a relatively small area. The Jurassic calcic pluton of the Snake Creek-Williams Canyon area displays large and systematic chemical and mineralogical zonation over a horizontal distance of five km. Although major element variations in the pluton compare closely with Daly's average andesite-dacite-rhyolite over an SiO2 range of 63 to 76 percent, trace element (Rb, Sr, Ba) variations show that the zonation is the result of in situ fractional crystallization, with the formation of relatively mafic cumulates on at least one wall of the magma chamber. Models of trace element and isotopic data indicate that relatively little assimilation took place at the level of crystallization. Nonetheless, an initial 87Sr/86Sr value of 0.7071 and ??18O values of 10.2 to 12.2 permil suggest a lower crustal magma that was contaminated by upper crustal clastic sedimentary rocks before crystallization. The involvement of mantle-derived magmas in its genesis is difficult to rule out. Two other Jurassic plutons show isotopic and chemical similarities to the Snake Creek-Williams Canyon pluton. Cretaceous granites from eastern Nevada that contain phenocrystic muscovite are strongly peraluminous, and have high initial Sr-isotope ratios and other features characteristic of S-type granitoids. They were probably derived from Proterozoic metasediments and granite gneisses that comprise the middle crust of this region. Another group of granitoids (including the Tertiary aplites) show chemical, mineralogic, and isotopic characteristics intermediate between the first two groups and may have been derived by contamination of magmas from the lower crust by the midcrustal metasediments. ?? 1983 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1970

Distribution of rubidium between sodic sanidine and natural silicic liquid

Phenocrysts of sodic sanidine from twelve upper Cenozoic units of silicic ash-flow tuff and lava from the Western United States contain from 0.25 to 0.45 the Rb present in the associated groundmass materials. The ratios of potassium to rubidium in the sanidines are, on the average, about four times greater than those of the groundmass. Separation of phenocrystic sanidine from salic melts provides an efficient method for raising the Rb content and lowering the K/Rb ratio of the melts, although the amount of differentiation probably is limited by continuous reequilibration of the alkalis between crystal and liquid phases through ion exchange. Syenites of cumulate origin will have appreciably lower Rb contents and higher K/Rb ratios than the melts from which they precipitated. Available data on the distribution of Rb between synthetic biotite and K-sanidine demonstrate that the separation of biotite probably will not deplete salic melts in Rb relative to K.

Contributions to Mineralogy and Petrology↗
USGS1988

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano, Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K 2 O in plagioclase and the abundances of TiO 2 and Fe 2 O 3 in spinel of xenoliths and mid-ocean ridge basalt, and a single 87 Sr/ 86 Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10 −3 –1.91×10 −3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevelz

Hawaii↗
USGS1988

Garnet compositions and their use as indicators of peraluminous granitoid petrogenesis - southeastern Arabian Shield

Garnet, an uncommon accessory mineral in igneous rocks, occurs in seven small peraluminous granitoid plutons in the southeastern Arabian Shield; textural equilibrium between garnet and other host granitoid minerals indicates that the garnets crystallized from their host magmas. Compositions of the garnets form three groups that reflect host-granitoid compositions, which in turn reflect source compositions and tectonic regimes in which the host magmas were generated. Garnets from the seven plutons have almandine-rich cores and spessartine-rich rims. This reverse zoning depicts host magma compositional evolution; i.e. rimward spessartine enrichment resulted from progressive, host-magma manganese enrichment. The garnets are heavy rare-earth element enriched; (Lu/La)N ranges from 13 to 355 and one of the garnets contains spectacularly elevated abundances of Y, Ta, Th, U, Zn, Zr, Hf, Sn, and Nb. Involvement of garnets with these trace element characteristics in magma genesis or evolution can have dramatic effects on trace element signatures of the resulting magmas. Other researchers suggest that Mn-enriched magmas are most conducive to garnet nucleation. Although the garnetiferous granitoids discussed here are slightly Mn enriched, other genetically similar peraluminous Arabian granitoids lack garnet; Mn enrichment alone does not guarantee garnet nucleation. The presence of excess alumina in the magma may be a prerequisite for garnet nucleation. ?? 1988 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1984

Geochronology of Precambrian granites and associated U-Ti-Th mineralization, northern Olary province, South Australia

Proterozoic granitoids and metamorphic rocks in the Olary province of the Willyama block of South Australia host ore-grade amounts of U-Th-Ti and U-Fe-Ti-Th minerals. U-Pb-Th isotope analyses on zircons from all granitoids associated with the Crocker Well brannerite deposit indicate that these granitoids were intruded within a short time span, close to the 1579.2??1.5 m.y. age of the brannerite-bearing host-rock. Though the early Paleozoic Delamerian orogeny was intense in this region, the zircon isotopic systems remained unaffected; rather, the best-defined zircon chords on concordia plots show a welldefined lower intercept of 43.8??6.5 Ma, which can only be associated with early Tertiary block faulting. Pb-U-Th isotope analyses on brannerite from the Crocker Well deposit and davidite from the Mt. Victoria deposit and the Radium Hill deposit yield badly scattered and discordant apparent ages that suggest a primary age at least as old as the age of the Crocker Well granitoids, followed by a severe disturbance in the early Paleozoic. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1981

Stable isotope systematics in mesozoic granites of Central and Northern California and Southwestern Oregon

18O, D, and H2O+ contents were measured for whole-rock specimens of granitoid rocks from 131 localitics in California and southwestern Oregon. With 41 new determinations in the Klamath Mountains and Sierra Nevada, initial strontium isotope ratios are known for 104 of these samples. Large variations in ??18O (5.5 to 12.4), ??D (-130 to -31), water contents (0.14 to 2.23 weight percent) and initial strontium isotope ratios (0.7028 to 0.7095) suggest a variety of source materials and identify rocks modified by secondary processes. Regular patterns of variation in each isotopic ratio exist over large geographical regions, but correlations between the ratios are generally absent except in restricted areas. For example, the regular decrease in ??D values from west to east in the Sierra Nevada batholith is not correlative with a quite complex pattern of ??18O values, implying that different processes were responsible for the isotopic variations in these two elements. In marked contrast to a good correlation between (87Sr/86Sr)o and ??18O observed in the Peninsular Ranges batholith to the south, such correlations are lacking except in a few areas. ??D values, on the other hand, correlate well with rock types, chemistry, and (87Sr/86Sr)o except in the Coast Ranges where few of the isotopic signatures are primary. The uniformly low ??D values of samples from the Mojave Desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area. Even so, the ??18O values and 18O fractionations between quartz and feldspar are normal in these same rocks. This reconnaissance study has identified regularities in geochemical parameters over enormous geographical regions. These patterns are not well understood but merit more detailed examination because they contain information critical to our understanding of the development of granitoid batholiths. ?? 1981 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1984

Contrasting fluid/rock interaction between the Notch Peak granitic intrusion and argillites and limestones in western Utah: evidence from stable isotopes and phase assemblages

The Jurassic Notch Peak granitic stock, western Utah, discordantly intrudes Cambrian interbedded pure limestones and calcareous argillites. Contact metamorphosed argillite and limestone samples, collected along traverses away from the intrusion, were analyzed for ??18O, ??13C, and ??D. The ??13C and ??18O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock ??18O values of the argillites systematically decrease from 19 to as low as 8.1, and the ??13C values of the carbonate fraction from 0.5 to -11.8. The change in ??13C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated {Mathematical expression} is about 4.5 permil for the high-grade samples and less for medium and low-grade samples suggesting a range in temperatures at which most decarbonation occurred. However, the amount of CO2 released was not anough to decrease the whole rock ??18O to the values observed in the argillites. The low ??18O values close to the intrusion suggest interaction with magmatic water that had a ??18O value of 8.5. The extreme lowering of ??13C by fractional devolatilization and the lowering of ??18O in argillites close to the intrusion indicates oxgen-equivalent fluid/rock ratios in excess of 1.0 and X(CO2)F of the fluid less than 0.2. Mineral assemblages in conjunction with the isotopic data indicate a strong influence of water infiltration on the reaction relations in the argillites and separate fluid and thermal fronts moving thru the argillites. The different stable isotope relations in limestones and argillites attest to the importance of decarbonation in the enhancement of permeability. The flow of fluids was confined to the argillite beds (argillite aquifers) whereas the limestones prevented vertical fluid flow and convective cooling of the stock. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1981

Subcalcic diopsides from kimberlites: Chemistry, exsolution microstructures, and thermal history

Twenty-six subcalcic diopside megacrysts (Ca/(Ca+ Mg)) = 0.280-0.349, containing approximately 10 mol% jadeite, from 15 kimberlite bodies in South Africa, Botswana, Tanzania, and Lesotho, have been characterized by electron microprobe analysis, X-ray-precession photography, and transmission electron microscopy. Significant exsolution of pigeonite was observed only in those samples for which Ca/(Ca+Mg)???0.320. The exsolution microstructure consists of coherent (001) lamellae with wavelengths ranging from 20 to 31 nm and compositional differences between the hosts and lamellae ranging from 10 to 30 mol% wollastonite. These observations suggest that the exsolution reaction mechanism was spinodal decomposition and that the megacrysts have been quenched at various stages of completion of the decomposition process. Annealing experiments in evacuated SiO2 glass tubes at 1,150?? C for 128 hours failed to homogenize microstructure, whereas, at 5 kbar and 1,150?? C for only 7.25 hours, the two lattices were homogenized. This "pressure effect" suggests that spinodal decomposition in the kimberlitic subcalcic diopside megacrysts can only occur at depths less than ???15 km; the cause of the effect may be the jadeite component in the pyroxene. "Apparent quench" temperatures for the exsolution process in the megacrysts range from 1,250?? C to 990?? C, suggesting that decomposition must have commenced at temperatures of more than ???1,000?? C. These P-T limits lead to the conclusion that, in those kimberlites where spinodal decomposition has occurred in subcalcic diopside megacrysts, such decomposition occurred at shallow levels (<15 km) and, at the present erosion level, temperatures must have been greater than 1,000?? C. ?? 1981 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1973

Thermochemical parameters of minerals from oxygen-buffered hydrothermal equilibrium data: Method, application to annite and almandine

Reversed univariant hydrothermal phase-equilibrium reactions, in which a redox reaction occurs and is controlled by oxygen buffers, can be used to extract thermochemical data on minerals. The dominant gaseous species present, even for relatively oxidizing buffers such as the QFM buffer, are H 2 O and H 2 ; the main problem is to calculate the chemical potentials of these components in a binary mixture. The mixing of these two species in the gas phase was assumed by Eugster and Wones (1962) to be ideal; this assumption allows calculation of the chemical potentials of the two components in a binary gas mixture, using data in the literature. A simple-mixture model of nonideal mixing, such as that proposed by Shaw (1967), can also be combined with the equations of state for oxygen buffers to permit derivation of the chemical potentials of the two components. The two mixing models yield closely comparable results for the more oxidizing buffers such as the QFM buffer. For reducing buffers such as IQF, the nonideal-mixing correction can be significant and the Shaw model is better. The procedure of calculation of mineralogical thermochemical data, in reactions where hydrogen and H 2 O simultaneously appear, is applied to the experimental data on annite, given by Wones et al. (1971), and on almandine, given by Hsu (1968). For annite the results are: Standard entropy of formation from the elements, S 0 f (298, 1)=−283.35±2.2 gb/gf, S 0 (298, 1) =+92.5 gb/gf. G 0 f (298, 1)=−1148.2±6 kcal, and H 0 f (298, 1)=−1232.7±7 kcal. For almandine, the calculation takes into account the mutual solution of FeAl 2 O 4 (Hc) in magnetite and of Fe 3 O 4 (Mt) in hercynite and the temperature dependence of this solid solution, as given by Turnock and Eugster (1962); the calculations assume a regular-solution model for this binary spinel system. The standard entropy of formation of almandine, S 0 f,A (298, 1) is −272.33±3 gb/gf. The third law entropy, S 0 (298, 1) is +68.3±3 gb/gf, a value much less than the oxide-sum estimate but the deviation is nearly the same as that of grossularite, referring to a comparable set of oxide standard states. The Gibbs free energy G 0 f,A (298, 1) is −1192.36±4 kcal, and the enthalpy H 0 f,A (298, 1) is −1273.56±5 kcal.

Contributions to Mineralogy and Petrology↗
USGS1973

Vesicles, water, and sulfur in Reykjanes Ridge basalts

Dredge hauls of fresh submarine basalt collected from the axis of the Reykjanes Ridge (Mid-Atlantic Ridge) south of Iceland were taken aboard R/ V TRIDENT in 1967 and 1971. The samples show systematic changes as the water depth of collection (and eruption) decreases: radially elongate vesicles and concentric zones of vesicles appear at about 700 m depth and are conspicuous to shallow water; the smoothed volume percent of vesicles increases from 5% at 1000 m, 10% at 700 m, to 16% at 500 m, and the scatter in degree of vesicularity increases in shallower water; specific gravity decreases from 2.7±0.1 at 1000 m to 2.3±0.3 at 100 m. Bulk sulfur content for the outer 2 cm averages 843 ppm up to a depth of 200 m, then drops off rapidly in shallower water owing to degassing. Sulfur content below 200 m is independent of depth (or geographic position), and the melt is apparently saturated with sulfur, but the excess cannot escape the lava unless another vehicle carries it out. Only shallower than 200 m, where intense vesiculation of other gases occurs can excess sulfur be lost from the lava erupting on the sea floor. H 2 O +110° averages about 0.35 percent and H 2 O +150° about 0.25 percent, and both apparently decrease in water shallower than 200 m as a result of degassing. H 2 O + (below 200 m) decreases with distance from Iceland or increasing depth, presumably as a result of either adsorption of water on the surface of shallower, more vesicular rocks; or more likely due to the presence of the Iceland hot mantle plume supplying undifferentiated primordial material, relative to lavas of the Reykjanes Ridge supplied from the low velocity layer already depleted in volatiles and large lithophile elements. The H 2 O +110° /S ratio of lava erupting below 200 m water depth ranges from 3 to 5 which is comparable to reliable gas analyses from oceanic basaltic volcanoes.

Contributions to Mineralogy and Petrology↗
USGS1989

Evidence for an Early Archean component in the Middle to Late Archean gneisses of the Wind River Range, west-central Wyoming: conventional and ion microprobe U-Pb data

Gneissic rocks that are basement to the Late Archean granites comprising much of the Wind River Range, west-central Wyoming, have been dated by the zircon U-Pb method using both conventional and ion microprobe techniques. A foliated hornblende granite gneiss member from the southern border of the Bridger batholith is 2670??13 Ma. Zircons from a granulite just north of the Bridger batholith are equant and faceted, a typical morphology for zircon grown under high grade metamorphic conditions. This granulite, which may be related to a second phase of migmatization in the area, is 2698??8 Ma. South of the Bridger batholith, zircons from a granulite (charnockite), which is related to an earlier phase of migmatization in the Range, yield a discordia with intercept ages of about 2.3 and 3.3 Ga. However, ion microprobe analyses of single zircon grains indicate that this rock contains several populations of zircon, ranging in age from 2.67 to about 3.8 Ga. Based on zircon morphology and regional geologic relationships, we interpret the data as indicating an age of ???3.2 Ga for the first granulite metamorphism and migmatization. Older, possibly xenocrystic zircons give ages of ???3.35, 3.65 and ???3.8 Ga. Younger zircons grew at 2.7 and 2.85 Ga in response to events, including the second granulite metamorphism at 2.7 Ga, that culminated in the intrusion of the Bridger batholith and migmatization at 2.67 Ga. These data support the field and petrographic evidence for two granulite events and provide some temporal constraints for the formation of continental crust in the Early and Middle Archean in the Wyoming Province. ?? 1989 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1978

The potential source of lead in the Permian Kupferschiefer bed of Europe and some selected Paleozoic mineral deposits in the Federal Republic of Germany

New lead isotopic compositions have been measured for Paleozoic bedded and vein ore deposits of Europe by the high precision thermal emission (triple filament) technique. Eleven samples have been analyzed from the Upper Permian Kupferschiefer bed with representatives from Poland to England, three samples from the Middle Devonian Rammelsberg deposit and one from the Middle Devonian Meggen deposit, both of which are conformable ore lenses and are in the Federal Republic of Germany (FRG); and also two vein deposits from the FRG were analyzed, from Ramsbeck in Devonian host rocks and from Grund in Carboniferous host rocks. For Kupferschiefer bed samples from Germany, the mineralization is of variable lead isotopic composition and appears to have been derived about 250 m.y. ago from 1700 m.y. old sources, or detritus of this age, in Paleozoic sedimentary rocks. Samples from England, Holland, and Poland have different isotopic characteristics from the German samples, indicative of significantly different source material (perhaps older). The isotopic variability of the samples from the Kupferschiefer bed in Germany probably favors the lead containing waters coming from shoreward (where poor mixing is to be expected) rather than basinward (where better mixing is likely) directions. The data thus support the interpretation of the metal source already given by Wedepohl in 1964. Data on samples from Rammelsberg and Meggen tend to be slightly less radiogenic than for the Kupferschiefer, about the amount expected if the leads were all derived from the same source material but 100 to 150 m.y. apart in time. The vein galena from Ramsbeck is similar to that from Rammelsberg conformable ore lenses, both in rocks of Devonian age; vein galena from Grund in Upper Carboniferous country rocks is similar to some bedded Kupferschiefer mineralization in Permian rocks, as if the lead composition was formed at about the same time and from similar source material as the bedded deposits. Although heat has played a more significant role in the formation of some of these deposits (veins and Rammelsberg-Meggen) than in others (Kupferschiefer), there is no indication of radically different sources for the lead, all apparently coming from sedimentary source material containing Precambrian detritus. One feldspar lead sample from the Brocken-Oker Granite is not the same in isotopic composition as any of the ores analyzed. ?? 1978 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1973

Stages in the P-T path of ascending basalt magma: an example from San Quintin, Baja California

Late Pleistocene or Recent lavas from San Quintin, Baja California are basanitoids and alkali basalts. The surface quench temperatures of the lavas average 1 005° C with log f O 2 "> f O 2 fO2 =−11.4, as deduced from the groundmass Fe-Ti oxides. Spinel lherzolite xenoliths and megacrysts of augite and andesine have been found in lava flows and cinder deposits. Using analytical data on the rocks and minerals and simple thermodynamic expressions, the pressures and temperatures of equilibration of lavas and xenoliths, megacrysts and phenocrysts have been calculated. The lavas could have been in equilibrium with lherzolite at 1 330–1 410° C and 27.5–31.6 kb, the more silica-poor liquid having the higher values. The basanitoid could have equilibrated with the megacrysts at about 10.5 kb and with phenocrysts at about 1.4 kb and 1130° C. The variation in composition of the lavas may be explained by a rising zone of melting within the mantle, the most silica-poor liquid having the deepest source. The source of the San Quintin basalts is probably related to spreading of the ocean floor in the Gulf of California.

Baja California, San Quintin↗
USGS1975

The orthoenstatite to clinoenstatite transformation by shearing and reversion by annealing: Mechanism and potential applications

Clinoenstatite (CE) was produced by deforming single-crystal specimens of ortho-enstatite (OE) in several different sorta of experiments. Examination with light and trans-mission electron microscopes shows that the transformation is coherent and involves a macroscopic shear on (100) [001] through an angle of 12.8±1.3 °, in good agreement with the theoretically expected value of 13.3 °, and that the transformation is accomplished by glide on (100) of partial dislocations with b = 0.83[001]. Structural analysis provides further insight into the transformation mechanism. Reversion occurs in specimens annealed under a variety of conditions, and thin lamellae of CE in unconstrained, low-strain specimens recover their original shape during transformation back to OE. Our experiments and thermodynamic estimates both suggest that the equilibrium transition temperature is raised roughly 300 ° C per kilobar of shear stress on (100) [001]. This provides the basis of a method by which it may be possible to determine the magnitude as well as the orientation of the principal stresses that produce CE in nature during deformation of enstatite-bearing rocks.

Contributions to Mineralogy and Petrology↗
USGS1984

Samarium-neodymium data on two late Proterozoic ophiolites of Saudi Arabia and implications for crustal and mantle evolution

Whole-rock and mineral samples from the Jabal al Wask and Jabal Ess ophiolites, northwestern Saudi Arabia, yield Sm-Nd isochron ages of 743+24 Ma and 782??38 Ma, respectively. These formation ages, which provide maximum limits for possible obduction ages, are in broad but not precise agreement with the previously known geologic history of the Arabian Shield. They indicate that the ophiolitic rocks are roughly coeval with nearby volcanic and plutonic rocks, supporting a back-arc origin for the two ophiolites. We suggest that the Jabal al Wask and Jabal Ess ophiolites were parts of the same northeast-southwest trending ophiolite belt, now offset along the Najd fault system. Initial e{open}Nd values range from +6.6 to+ 7.6, indicating derivation from a mantle source that has been LIL-depleted for at least 2 Ga. Reported e{open}Nd values from the Arabian Shield that are lower than this suggest the presence of older, reworked continental crust. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1984

Age and origin of anorthosites, charnockites, and granulites in the Central Virginia Blue Ridge: Nd and Sr isotopic evidence

Rb-Sr isotopic data for anorthosites, charnockites, ferrodioritic to quartz monzonitic plutons, and high-grade gneisses of the Blue Ridge of central Virginia show evidence of post-emplacement metamorphism, but in some cases retain Grenville ages. The Pedlar River Charnockite Suite yields an isochron age of 1021 +/-36 Ma, (initial 87Sr/86Sr ratio of 0.7047 +/-6), which agrees with published U-Pb zircon ages. Five samples of that unit which contain Paleozoic mylonitic fabrics define a regression line of 683 Ma, interpreted as a mixing line with no age significance. Samples of the Roseland Anorthosite Complex show excessive scatter on a Rb-Sr evolution diagram probably due to Paleozoic (475 m.y.) metamorphism. Data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 +/-26 Ma (inital ratio=0.7058 +/-4), which is in the range of the U-Pb zircon ages of 1000-1100 Ma. The Stage Road Layered Gneiss yields an age of 1147 +/-34 Ma (initial ratio of 0.7047 +/- 5). Sm-Nd data for the Pedlar River Charnockite Suite reflect a pre-Grenville age of 1489 +/-118 Ma (e{open}Nd=+6.7 +/-1.2). Data for the Roseland Anorthosite Complex and the ferrodioritic to quartz monzonitic plutons yield Grenville isochron ages of 1045 +/44 Ma (e{open}Nd=+1.0 +/-0.3) and 1027 +/-101 Ma (e{open}Nd=+1.4 +/-1.0), respectively. Two Roseland Anorthosite samples plot far above the isochron, demonstrating the effects of post-emplacement disturbance of Sm-Nd systematics, while mylonitized Pedlar River Charnockite Suite samples show no evidence of Sm-Nd redistribution. The disparity of the Sm-Nd age and other isotopic ages for the Pedlar River Charnockite Suite probably reflects a Sm-Nd "source" age, suggesting the presence of an older crust within this portion of the ca. 1 Ga old basement. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1985

Precambrian tholeiitic-dacitic rock-suites and Cambrian ultramafic rocks in the Pennine nappe system of the Alps: Evidence from Sm-Nd isotopes and rare earth elements

Major element, trace element and Sm-Nd isotope analyses were made of polymetamorphic hornblendefelses, plagioclase amphibolites and banded amphibolites from the Berisal complex in the Simplon area (Italy, Switzerland) to determine their age, origin and genetic relationships. In light of major and rare earth element data, the hornblendefelses are inferred to have originally been pyroxene-rich cumulates, the plagioclase amphibolites and the dark layers of the banded amphibolites to have been tholeiitic basalts and the light layers dacites. The Sm-Nd isotope data yield isochron ages of 475??81 Ma for the hornblendefelses, 1,018??59 Ma for the plagioclase amphibolites and 1,071??43 Ma for the banded amphibolites. The 1 Ga magmatic event is the oldest one ever found in the crystalline basement of the Pennine nappes. The Sm -Nd isotope data support the consanguinity of the tholeiitic dark layers and the dacitic light layers of the banded amphibolites with the tholeiitic plagioclase amphibolites and the ultramafic hornblendefelses. The initial e{open}Nd values indicate that all three rock types originated from sources depleted in light rare earth elements. We suggest that plagioclase and banded amphibolites were a Proterozoic tholeiite-dacite sequence that was strongly deformed and flattened during subsequent folding. The hornblendefelses are thought to be Cambrian intrusions of pyroxene-rich material. ?? 1985 Springer-Verlag.

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