Showing papers in "Contributions to Mineralogy and Petrology in 1988"
TL;DR: In this article, 15 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.
Abstract: 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.
2,013 citations
TL;DR: In this article, the melting of a natural metapelite under fluid-absent conditions was studied experimentally and a series of P-T, T-XH2O, and liquidus diagrams were proposed.
Abstract: In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als. The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C. We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites. The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.
1,017 citations
TL;DR: In this article, the authors provide a formalism for the macroscopic study of kinetic and physical processes affecting crystallization, within which the explicit affect of chemical and physical process on the CSD can be analytically tested.
Abstract: Crystal-size in crystalline rocks is a fundamental measure of growth rate and age. And if nucleation spawns crystals over a span of time, a broad range of crystal sizes is possible during crystallization. A population balance based on the number density of crystals of each size generally predicts a log-linear distribution with increasing size. The negative slope of such a distribution is a measure of the product of overall population growth rate and mean age and the zero size intercept is nucleation density. Crystal size distributions (CSDs) observed for many lavas are smooth and regular, if not actually linear, when so plotted and can be interpreted using the theory of CSDs developed in chemical engineering by Randolph and Larson (1971). Nucleation density, nucleation and growth rates, and orders of kinetic reactions can be estimated from such data, and physical processes affecting the CSD (e.g. crystal fractionation and accumulation, mixing of populations, annealing in metamorphic and plutonic rocks, and nuclei destruction) can be gauged through analytical modeling. CSD theory provides a formalism for the macroscopic study of kinetic and physical processes affecting crystallization, within which the explicit affect of chemical and physical processes on the CSD can be analytically tested. It is a means by which petrographic information can be quantitatively linked to the kinetics of crystallization, and on these grounds CSDs furnish essential information supplemental to laboratory kinetic studies. In this three part series of papers, Part I provides the general CSD theory in a geological context, while applications to igneous and metamorphic rocks are given, respectively, in Parts II and III.
726 citations
TL;DR: Experimental investigation of the fluid absent melting reaction has shown that the reaction has a positive dP/dT at least up to 10 kbar, compared to a calculated backbending in a simplified system.
Abstract: Experimental investigation of the fluid absent melting reaction biotite+plagioclase+A12SiOs+quartz~ garnet + K-feldspar + melt at 10 kbar, indicated that melt- ing began between 760 and 800~ and was extensive at 850 ~ C This reaction apparently has a positive dP/dT at least up to 10 kbar, compared to a calculated backbending in a simplified system Natural biotites are stabilised to higher dehydration melting temperatures probably by Ti Consequently, metapelites undergo two-stages of fluid-ab- sent melting even in thickened, continental crust The two stages firstly as muscovite, and secondly as biotite react, appear to persist to at least 17 kbar, depending upon biotite composition This lies well within the stability field of kya- nite migmatites Thus, complex petrogenetic grids can be computed from the little available data over a wide range of P-T-Xroek-XH20 conditions Microprobe analyses indicate good correlations with the Fe/Mg garnet + biotite exchange thermometer and the garnet +plagioclase +A12SiO5 + quartz barometer at 850 ~ C, 10 kbar Furthermore, micro- probe analyses show that vGar'" Liq ~yBio in fluid-absent ~x Fe f Fe t- l Fe as well as in HzO-saturated pelite anatexis Thus intermedi- ate Fe/Mg compositions, containing Bio, Gar, Als, Plg, Ksp, Qtz will eutectically melt before Fe-richer or Mg-richer metapelites If the partition VG,r/vLiq reverses with increas- " Fe /~x Fe
720 citations
TL;DR: Experimental data on the partitioning of Fe2+ and Mg between garnet and clinopyroxene (Raheim and Green 1974; Mori and Green 1978; Ellis and Green 1979) have been used to construct a new expression for the garnet-clinopyroxenes geothermometer, including a curvilinear relationship between In Kd and XCa(ga): T(°C)=((-6173(XCa)2+6731 XCa+1879 +10 P(kb))/(lnKd+1.393))−273
Abstract: Experimental data on the partitioning of Fe2+ and Mg between garnet and clinopyroxene (Raheim and Green 1974; Mori and Green 1978; Ellis and Green 1979) have been used to construct a new expression for the garnet-clinopyroxene geothermometer, including a curvilinear relationship between In Kd and XCa(ga): T(°C)=((-6173(XCa)2+6731 XCa+1879 +10 P(kb))/(lnKd+1.393))−273
701 citations
TL;DR: In this paper, the authors use Nd and Sr isotopic data together with previous chemical and petrographic observations to identify a northern belt of continent margin I-type granitoids grading southwards to inner continent S-type plutons in the eastern half of the fold belt.
Abstract: Apparent crustal residence ages indicated by Nd model age data for metamorphic rocks, sediments and granitoids of the Hercynian Fold Belt of Europe vary from 1.3 Ga to 3.0 Ga, but are mainly in the range 1.4–1.7 Ga. 2 Ga basement inliers have been documented previously in northern Spain and islands off northwestern France but, in addition to these, old (∼2–3 Ga) model ages are found along the southern margin of the fold belt. These do not identify old inliers but are interpreted to represent Archeanearly Proterozoic crustal components recycled from a southern source. The Nd data, when considered together with the surface geology and recent single-grain zircon ion microprobe data, argue against a binary mixing of Archean components with new Palaeozoic crustal additions to generate the main 1.4–1.7 Ga model age population. Hercynian Europe comprise mainly recycled Proterozoic components although significant new Palaeozoic additions as well as Archean contributions are indicated. Nd and Sr isotopic data together with previous chemical and petrographic observations allow the recognition of a northern belt of continent margin I-type granitoids grading southwards to inner continent S-type plutons in the eastern half of the fold belt. This felsic plutonic association is used to infer a Hercynian plate configuration involving the attachment of the fold belt to a southern parent cratonic block that the model age data suggest may be of early Proterozoic-Archean age.
680 citations
TL;DR: In this article, the parent parent scoria compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents.
Abstract: The climactic eruption of Mount Mazama has long been recognized as a classic example of rapid eruption of a substantial fraction of a zoned magma body. Increased knowledge of eruptive history and new chemical analyses of ∼350 wholerock and glass samples of the climactic ejecta, preclimactic rhyodacite flows and their inclusions, postcaldera lavas, and lavas of nearby monogenetic vents are used here to infer processes of chemical evolution of this late Pleistocene — Holocene magmatic system. The 6845±50 BP climactic eruption vented ∼50 km3 of magma to form: (1) rhyodacite fall deposit; (2) welded rhyodacite ignimbrite; and (3) lithic breccia and zoned ignimbrite, these during collapse of Crater Lake caldera. Climactic ejecta were dominantly homogeneous rhyodacite (70.4±0.3% SiO2), followed by subordinate andesite and cumulate scoriae (48–61% SiO2). The gap in wholerock composition reflects mainly a step in crystal content because glass compositions are virtually continuous. Two types of scoriae are distinguished by different LREE, Rb, Th, and Zr, but principally by a twofold contrast in Sr content: High-Sr (HSr) and low-Sr (LSr) scoriae. HSr scoriae were erupted first. Trace element abundances indicate that HSr and LSr scoriae had different calcalkaline andesite parents; basalt was parental to some mafic cumulate scoriae. Parental magma compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents.
563 citations
TL;DR: In this paper, a steady state crystal population balance equation is used to estimate the effective growth time (τ) of a sample from Kilauea Volcano, Hawaii, where the intercept of the intercept is J/G where J is nucleation rate.
Abstract: Crystal size distribution (CSD) theory has been applied to drill core samples from Makaopuhi lava lake, Kilauea Volcano, Hawaii. Plagioclase and Fe-Ti oxide size distribution spectra were measured and population densities (n)were calculated and analyzed using a steady state crystal population balance equation: n=n0 exp(-L/Gτ). Slopes on ln(n) versus crystal size (L) plots determine the parameter Gτ, a. product of average crystal growth rate (G) and average crystal growth time (τ). The intercept is J/G where J is nucleation rate. Known temperature-depth distributions for the lava lake provide an estimate of effective growth time (τ), allowing nucleation and growth rates to be determined that are independent of any kinetic model. Plagioclase growth rates decrease with increasing crystallinity (9.9−5.4×10−11 cm/s), as do plagioclase nucleation rates (33.9−1.6×10−3/cm3 s). Ilmenite growth and nucleation rates also decrease with increasing crystallinity (4.9−3.4 ×10−10 cm/s and 15−2.2×10−3/cm3 s, respectively). Magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus (G =2.9×10−10 cm/s, J=7.6×10−2/cm3 s). Moments of the population density function were used to examine the change in crystallization rates with time. Preliminary results suggest that total crystal volume increases approximately linearly with time after ∼50% crystallization; a more complete set of samples is needed for material with <50% crystals to define the entire crystallization history. Comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling. This interpretation is consistent with proposed thermal models of magmatic cooling, where heat loss is balanced by latent heat production to maintain equilibrium cooling.
518 citations
TL;DR: In this paper, the authors considered that Coronites formed near T = 900°C and P = 10 Kb by two stages of subsolidus reaction from an anorthositic gabbro parent.
Abstract: Coronite-bearing anorthositic granulites consisting of olivine, orthopyroxene, clinopyroxene, garnet and plagioclase assemblages are particularly well preserved at Gaupas and Holsnoy in the Bergen Arc of West Norway. The coronites are considered to have formed near T= 900° C and P=10 Kb by two stages of subsolidus reaction from an anorthositic gabbro parent. The first reaction involved ol+plag→cpxI+opxI+sp and the second cpxI+opxI+sp+pl→cpxII+opxII+gn. The incomplete reaction products are preserved to varying extents in different corona structures. Sm-Nd isotopic data for each of four coronas yield precise isochrons, and demonstrate isochronism both between the constituent phases of the corona assemblages and dispersed ground mass phases. Three individual coronas not associated with shear zones yield ages of 907±9 my, 912±18 my and 905±37 my. Eclogite facies mineralogy is developed locally in shear zones, which are shown by Sm-Nd and Rb-Sr analyses to be Caledonian in age. Where relict corona structures survive unsheared within these zones, Nd exchange between the constituent phases cannot be resolved. This observation together with sympathetic Ca/Mg and Sm/Nd zoning preserved at T∼900° C in the garnet mantles of coronites places a limit on the diffusivity of Nd in pyropic garnets which is no higher than published experimental values for Mg in pyrope-almandine garnets. Consequently even in slowly-cooled granulite terrains, garnet grains are expected to yield Sm-Nd chronologies very close to the time of mineral growth.
298 citations
TL;DR: In this paper, a minimum of three components are required to describe the geochemistry of subduction zone basalts: high Sr/Nd material is derived from the dehydration of subducted basaltic ocean crust, and a low Sr /Nd component is thought to be from subducting terrigenous sediment.
Abstract: Subduction related basalts display wide ranges in large ion lithophile element ratios (e.g., Rb/Ba and Rb/ Sr) which are unlikely to result from mixing, but suggest a role for small degree partial melting of a relatively Rb-poor mantle wedge source. However, these variations do not correlate with other trace element criteria, such as the depletions of high field strength elements (HFSE) and light rare earth elements (LREE) relative to the LILE, which characterise subduction related magmatism. Integration of radiogenic isotope and trace element data demonstrates that the elemental enrichment cannot be simply related to two component mixtures inferred from isotopic variations. Thus a minimum of three components is required to describe the geochemistry of subduction zone basalts. Two are subduction related: high Sr/Nd material is derived from the dehydration of subducted basaltic ocean crust, and a low Sr/Nd component is thought to be from subducted terrigenous sediment. The third component is in the mantle wedge, it is usually similar to the source of MORB, particularly in its isotopic composition. However, in some cases, notably continental areas, more enriched mantle wedge material with relatively high 87Sr/86Sr, low 143Nd/144Nd and elevated incompatible trace element contents may be involved Mixing of these three components is capable of producing both the entire range of Sr, Nd and Pb isotope signatures observed in destructive margin basalts, and their distinctive trace element compositions. The isotope differences between Atlantic and Pacific island arc basalts are attributed to the isotope compositions of sediments in the two oceans.
265 citations
TL;DR: In this paper, a population density function n =dN/dL is used to measure the number of crystals per unit volume and L is a linear crystal size. And the authors use the slope and intercept of these linear plots in conjunction with growth rate estimates to infer minimum mineral growth times of less than 100 years at ΔT <10°C, and nucleation rates between 10−4 and 10−1/cm3/s.
Abstract: Crystal size distributions (CSDs) measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates, growth times, and the degree of overstepping (ΔT) of reactions during metamorphism. CSDs are described through use of a population density function n=dN/dL, where N is the cumulative number of crystals per unit volume and L is a linear crystal size. Plots of ln (n) vs. L for olivine+pyroxene and magnetite in high-temperature (1000° C) basalt hornfelses from the Isle of Skye define linear arrays, indicating continuous nucleation and growth of crystals during metamorphism. Using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at ΔT<10° C, and nucleation rates between 10−4 and 10−1/cm3/s. Garnet and magnetite in regionally metamorphosed pelitic schists from south-central Maine have CSDs which are bell-shaped. We interpret this form to be the result of two processes: 1) initial continuous nucleation and growth of crystals, and 2) later loss of small crystals due to annealing. The large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth. The extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known. Finally, the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs. contact metamorphosed rocks: because contact metamorphism involves high temperatures for short durations, resulting CSDs are linear and unaffected by annealing, similar to those produced by crystallization from a melt; because regional metamorphism involves prolonged cooling from high temperatures, primary linear CSDs are later modified by annealing to bell shapes.
Abstract: Recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive Gibbs free energies for hedenbergite and pyrope. These were used to calculate the geobarometric equilibria Hedenbergite+Anorthite=Grossular+Almandine +Quartz: “HD barometer”, Diopside+Anorthite=Grossular+Pyrope+Quartz: “DI barometer”. We have compared the pressures obtained from these equilibria for garnet-clinopyroxene-orthopyroxene-plagioclasequartz assemblages with the geobarometer Ferrosilite+Anorthite=Almandine+Grossular+Quartz: “FS barometer”. Pressures calculated for 68 samples containing the above assemblage from a variety of high grade metamorphic terranes indicate that, in general, the HD and DI barometers yield values that are in good agreement with the FS barometer, and that the three barometers are generally consistent with constraints from aluminosilicate occurrences. However, in some samples the HD barometer yields pressures up to 2 kbar greater than constraints imposed by the presence of an aluminosilicate phase. Relative to the FS barometer, the HD barometer overestimates pressure by an average of 0.2±1.0 (1σ) kbar and the DI barometer underestimates pressure by an average of 0.6±1.6 (1σ) kbar. The pressure discrepancies for the HD and DI barometers are likely to be a result of imprecision in thermodynamic data and activity models for silicates, and not a result of resetting of the clinopyroxene equilibria. The relative imprecision of the DI barometer relative to the FS barometer results from overestimates of pressure by the DI and FS barometers in Fe-rich and Mg-rich systems, respectively. Application of the HD and DI barometers to high grade Cpx-Gt-Pg-Qz assemblages yields pressures that are generally consistent with other petrologic constraints and geobarometers. It is concluded that the HD and DI barometers can place reasonable constraints on pressure (±1 kbar relative to the FS barometer) if not extrapolated to mineral assemblages whose compositions are extremely far removed from the end member system for which the barometers were calibrated.
TL;DR: In this paper, the ages for four ophiolites and three crosscutting arc-related plutons from the Norwegian Caledonides were reported, with a best-fit line defined by plagiogranite and clinopyroxene-phyric gabbro best fit lines.
Abstract: U/Pb zircon ages are reported for four ophiolites and three crosscutting arc-related plutons from the Norwegian Caledonides. Plagiogranite differentiated from gabbro of the Karmoy ophiolite is dated at 493+7/-4 Ma whereas arc-related trondhjemite cutting this ophiolite crystallized at 485+/−2 Ma. A crosscutting clinopyroxene-phyric gabbro intrusion is dated at 470+9/−5 Ma by near concordant magmatic titanite (sphene) and discordant U-rich (2903–6677 ppm) zircon. Lower intercepts of 247+/−68 and 191+/−70 Ma defined by the plagiogranite and clinopyroxene-phyric gabbro best-fit lines may reflect a real low-T alteration/rift-related event.
TL;DR: Vapor-saturated experiments at 200 MPa with peraluminous, lithophile-element-rich rhyolite obsidian from Macusani, Peru, reveal high miscibility of H2O and silicate melt components as mentioned in this paper.
Abstract: Vapor-saturated experiments at 200 MPa with peraluminous, lithophile-element-rich rhyolite obsidian from Macusani, Peru, reveal high miscibility of H2O and silicate melt components. The H2O content of melt at saturation (11.5+-0.5 wt.%) is almost twice that predicted by existing melt speciation models. The corresponding solubility of melt components in vapor decreases from 15 wt.% dissolved solids (750°–775° C) to 9 wt.% at 600° C. With regard to major and most minor components, macusanite melt dissolves congruently in vapor. Among the elements studied (B, P, F, Li, Rb, Cs, Be, Sr, Ba, Nb, Zr, Hf, Y, Pb, Th, U, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), only boron has a vapor/melt partition coefficient (D[B]) consistently ≥1 at superliquidus temperatures (>645° C). Phosphorus and fluorine behave similarly, with D[P] and D[F]<0.5. Little or no significant vapor/melt fractionation is evident among most periodic groups (alkalis, alkaline earths, Zr/Hf, or the REE). The temperature dependence of vapor/melt partition coefficients is generally greatest for cations with charge ≥ +3 (except Nb and U); most vapor/melt partition coefficients for trace elements increase with decreasing temperature to the liquidus. Crystallization proceeds by condensation of crystalline phases from vapor; most coexisting melts are aphyric. Changes in the major element content of melt are dominated by the mineral assemblage crystallized from vapor, which includes subequal proportions of white mica, quartz, albite, and orthoclase. The volumetric proportion of (mica + or-thoclase)/albite increases slightly with decreasing T, creating a sodic, alkaline vapor. Vapor deposition of topaz (T≤500° C), which consumes F from melt, returns K/Na ratios of melt to near unity with the vapor-deposition of albite. The abundances of most trace elements in residual melt change little with the crystallization of major phases, but in some cases are strongly controlled by the deposition of accessory phases including apatite (T≤550° C), which depletes the melt in P and REE. Below the liquidus, boron increasingly favors the vapor over melt with decreasing temperatures.
TL;DR: In this article, diffusivities for calcium, iron, magnesium, manganese and aluminum have been measured for St John's olivine undergoing cation exchange with synthetic basaltic melts.
Abstract: Diffusivities for calcium, iron, magnesium, manganese and aluminum have been measured for St. John's olivine undergoing cation exchange with synthetic basaltic melts. The variety of temperature, pressure and fO2 conditions under which the diffusivities were measured complement the equilibrium-partitioning study of calcium in olivine-bearing basalts by Jurewicz and Watson, 1988. Olivine was found to be anisotropic with respect to the diffusion of calcium, iron, magnesium and manganese. This anisotropy is a weak function of temperature, but strongly dependent upon oxygen fugacity.
TL;DR: In this article, the Sanbagawa schists in central Shikoku, Japan have been studied using the K-Ar method and the results show that the ages of the schists are consistently older with increasing metamorphic grade.
Abstract: Seventy muscovites from schists in the Sanbagawa terrain in central Shikoku were dated by the K-Ar method. The muscovite ages are consistently older with increasing metamorphic grade. Within the same zone the ages are significantly younger in schists which have been more severely deformed. These K-Ar age variations could be due to systematic argon depletion during deformation i.e., to the dynamic recrystallization of muscovites during ductile deformation that formed a large-scale recumbent fold during the uplift and cooling. Argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low-temperature deformation than the higher grade zone. The relationships between age and grain size in a pelitic schist suggest that coarse-grained muscovites lost more argon than the finegrained ones. There was no significant resetting of ages in the vicinity of major strike-slip faults, such as the Median Tecotonic Line or near thrust faults. The combination of geochronological and geological data constrains the cooling rate of the Sanbagawa schists to 9–12° C/Ma in the oligoclase-biotite zone in central Shikoku, Japan.
TL;DR: In the high grade gneiss terranes of the Grenville Province in Ontario, metagabbro with silicate coronas around primary olivine and Fe-Ti oxide contains trace amounts of both baddeleyite and zircon as discussed by the authors.
Abstract: In the high grade gneiss terranes of the Grenville Province in Ontario, metagabbro with silicate coronas around primary olivine and Fe-Ti oxide contains trace amounts of both baddeleyite and zircon. Single baddeleyite crystals with radiating columnar coronas of polycrystalline zircon, in places with outer rims of garnet, are analogous to the coronas around olivine and ilmenite. Single crystals of zircon and baddeleyite without zircon coronas also occur in the same rocks. Both zirconium minerals have been isotopically dated at three widely separated localities. In each case baddeleyite records an igneous crystallization age of ca.1170 Ma, 125 Ma older than the ca.1045 Ma age of the zircons, interpreted to be the age of metamorphism. These data agree well with U-Pb ages obtained from other rocks in the same region and refute the alternative interpretation that silicate coronas in these metagabbros formed during cooling following magmatic crystallization. The age does not support direct correlation with diabase dykes of either the Sudbury or Abitibi swarms northwest of the Grenville Front.
TL;DR: The Miocene-Pliocene Macusani ash-flow tuffs from SE Peru, containing magmatic andalusite and muscovite, have homogeneous major element compositions, with a narrow range of SiO2 (71-74 wt%), high Al2O3 (normative corundum >2%; A/CNK>1.2) and alkalis, and low FeOt, MgO, CaO, TiO, etc. as discussed by the authors.
Abstract: The Miocene-Pliocene Macusani ash-flow tuffs from SE Peru, containing magmatic andalusite and muscovite, have homogeneous major element compositions, with a narrow range of SiO2 (71–74 wt%), high Al2O3 (normative corundum >2%; A/CNK>1.2) and alkalis, and low FeOt, MgO, CaO, TiO2. P2O5, F, Li2O, and B2O3 are also high. The associated obsidian glasses are more felsic and peraluminous and extremely enriched in F, P, Li and B compared to the ash-flow tuffs. These are compositionally similar to Himalayan or Hercynian two-mica granites and the obsidian glasses to some rare fractionated members of the two-mica granite series. Both ash-flow tuffs and obsidian glasses show enrichments in lithophile trace elements (Be, Zn, As, Rb, Nb, Sn, Sb, Cs, Ta, W, U) and depletions in Cl, S, Sc, V, Cr, Co, Ni, Cu, Y, Mo, Hf. REE patterns for the ash-flow tuffs are fractionated (La/Lun=13-26) with a moderate Eu anomaly and they contrast with patterns for the obsidian glasses characterized by lower total REE, lower La/Lun and Eu/Eu*. Sr(87Sr/86Sr initial ratio= 0.721–0.726), Pb (206Pb/204Pb=18.74–19.45; 207P/204Pb= 15.66–15.72) and Nd isotopic compositions (ɛNd=-8.96 to-9.35) are typically crustal. Oxygen isotopic compositions are high in 18O (glasses:δ18O=+12‰; quartz:δ18O=+ 11.5 to +12.7‰). Batch melting of isotopically heterogeneous source rocks is suggested by the Sr and Pb data. In contrast to major elements, trace elements demonstrate compositional differences between erupted magmas. The last erupted magmas are less fractionated relative to the first erupted.
TL;DR: In this paper, the authors show that at Medicine Lake volcano, California, andesite of the Holocene Burnt Lava flow has been produced by fractional crystallization of parental high alumina basalt (HAB) accompanied by assimilation of granitic crustal material.
Abstract: At Medicine Lake volcano, California, andesite of the Holocene Burnt Lava flow has been produced by fractional crystallization of parental high alumina basalt (HAB) accompanied by assimilation of granitic crustal material. Burnt Lava contains inclusions of quenched HAB liquid, a potential parent magma of the andesite, highly melted granitic crustal xenoliths, and xenocryst assemblages which provide a record of the fractional crystallization and crustal assimilation process. Samples of granitic crustal material occur as xenoliths in other Holocene and Pleistocene lavas, and these xenoliths are used to constrain geochemical models of the assimilation process. A large amount of assimilation accompanied fractional crystallization to produce the contaminated Burnt lava andesites. Models which assume that assimilation and fractionation occurred simultaneously estimate the ratio of assimilation to fractional crystallization (R) to be >1 and best fits to all geochemical data are at an R value of 1.35 at F=0.68. Petrologic evidence, however, indicates that the assimilation process did not involve continuous addition of granitic crust as fractionation occurred. Instead, heat and mass transfer were separated in space and time. During the assimilation process, HAB magma underwent large amounts of fractional crystallization which was not accompanied by significant amounts of assimilation. This fractionation process supplied heat to melt granitic crust. The models proposed to explain the contamination process involve fractionation, replenishment by parental HAB, and mixing of evolved and parental magmas with melted granitic crust.
TL;DR: The U-Pb isotopic systems of zircon, monazite, titanite and some apatite and the Pb isotope composition of K-feldspar were investigated in three areas of the Winnipeg River Subprovince (WRS) of the Superior Province, Canada, in order to define the timing of magmatic and metamorphic processes in this Archean gneissic-granitoid terrain this article.
Abstract: The U-Pb isotopic systems of zircon, monazite, titanite and some apatite and the Pb isotopic composition of K-feldspar have been investigated in three areas of the Winnipeg River Subprovince (WRS) of the Superior Province, Canada, in order to define the timing of magmatic and metamorphic processes in this Archean gneissic-granitoid terrain.
TL;DR: Trace element analysis of Phlegrean Fields volcanic products shows that they belong to a homogeneous series whose evolution is due mainly to fractional crystallization as discussed by the authors, and the enrichment is strongest in products resulting from protracted differentiation (trachybasalts-latites).
Abstract: Trace element analysis of Phlegrean Fields volcanic products shows that they belong to a homogeneous series whose evolution is due mainly to fractional crystallization. However quantitative modelling of crystal fractionation using measured mineral/groundmass and computed bulk distribution coefficients singles out K, Sb, Cl and F as elements which are selectively enriched in the differentiation process with respect to proven hygromagmaphile (HYG) elements. This “anomalous” enrichment involving elements that are easily transported in a magmatic fluid phase is thought to be due to percolation of such a fluid through a stored magma body. This enrichment is strongest in products resulting from protracted differentiation (trachybasalts-latites). Isotopic data from the literature are reinterpreted in terms of this model and show good agreement. Trace elements concentrations in clinopyroxenes provide evidence that complex differentiation paths were followed to produce latitic magmas. Their origin by mixing of magmas from different parentages is not indicated by the data. However, mixing of batches of the same parentage but of different degree of evolution seems likely. Because HYG elements ratios are not modified in the evolution of the magmas, they record these ratios in the partial melting zone. For example, the Th/Ta ratio in the source areas of the magmas has apparently been quite heterogeneous.
TL;DR: In this article, the authors use the slope of the trend on a Pearce element ratio diagram to test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites.
Abstract: Pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites. Comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity. In basaltic systems, the incompatible elements, Ti, K, and P, are often conserved. The slope of the trend on a Pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases. A judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions. In basaltic rocks, diagrams with Ti/K vs P/K can provide a test of the comagmatic hypothesis. Diagrams with 0.5 [Mg + Fe]/K vs Si/K have trends that are distinct for each comagmatic suite and different mineral assemblage. Different suites are distinguished by the intercepts in diagrams, whereas mineral assemblages are recognized by the slopes of the trends. For example, if olivine is the sole crystallizing and segregating phase, the trend will have a slope of 1. Diagrams with [2Ca + Na]/K vs Al/K distinguish plagioclase from augite assemblages and, in conjunction with 0.5 [Mg + Fe]/K diagrams, unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation. Analyses from the Uwekahuna laccolith, Kilauea, the 1955 and 1967–68 eruptions of Kilauea, Diamond Craters Volcanic Field, Oregon, and experimental data on MORB glasses provide illustrations of the interpretations that can be obtained from Pearce element ratios.
TL;DR: In this paper, two eclogite-bearing megalenses within the Seve Nappe Complex in Norrbotten (northern Sweden) have been dated with the Sm-Nd method.
Abstract: The Seve Nappes consist of long-transported thrust sheets belonging to the Upper Allochthon within the Scandinavian Caledonides. Eclogites from two different megalenses within the Seve Nappe Complex in Norrbotten (northern Sweden) have been dated with the Sm-Nd method. The two eclogite-bearing megalenses have been subjected to different pressure and temperature conditions during the high-pressure metamorphism. Garnet + omphacite + whole-rock from TS2 within the Tsakkok Lens give an age of 505±18 Ma (2σ), I=0.512231±0.000024, MSWD=0.10. Garnet and whole-rock from TS5 and whole-rock TS3 also fall on the above isochron. The composite isochron gives an age of 503±18 Ma (2σ). These results are interpreted to date the peak of the high-pressure metamorphism (500–630° C and 12–15 kbars) for the Tsakkok Lens. The eclogites in the Vaimok Lens (at Grapesvarre) were subjected to higher pressure and temperature conditions and more extensive reequilibration during the early cooling stages. Retrograde breakdown-reactions accompanied by retrograde zoning of relict garnet seems to be associated with disturbance of the Sm-Nd isotopic systems. In contrast, a sample with unzoned garnet and well preserved high-pressure mineralogy from Grapesvarre gives a Sm-Nd garnet + omphacite age of 503±14 Ma (2σ), I = 0.512010±0.000038 (2σ). The ages for the Seve eclogites are significantly older than the Sm-Nd eclogite dates from the Western Gneiss Region of Norway (WGR), suggesting the existence of at least two eclogite-forming events in the Caledonide Orogen. The younger event has been related to the main continent-continent collision stage of the Caledonian Orogeny, while the older event that led to the production of the Norrbotten eclogites must have taken place several hundred kilometres to the north and in a different tectonic setting more oceanward to the WGR. It appears that the older event (ca. 505 Ma) was restricted to the subduction of dyke-intruded sedimentary cover rocks which are thought to represent the rifted edge of the Baltic continent.
TL;DR: Isotopic data were collected on lower crustal xenoliths to constrain the Mexican lower crust as source material for the mid-Tertiary Sierra Madre Occidental, which is one of the largest silicic volcanic piles known as discussed by the authors.
Abstract: Isotopic data were collected on lower crustal xenoliths to constrain the Mexican lower crust as source material for the mid-Tertiary Sierra Madre Occidental, which is one of the largest silicic volcanic piles known. The xenoliths are predominantly pelitic gneisses and mafic orthogneisses that were brought to the surface on the eastern edge of the Sierra Madre Occidental by recent alkalic basalts. The pelitic gneisses are uniform in mineral assemblage and contain garnet+quartz+plagioclase+sanidine+rutile +sillimanite/kyanite+graphite. The orthogneisses are plagioclase, garnet and/or spinel bearing two pyroxene granulites. Available geothermometric and geobarometric data show that the xenoliths equilibrated at temperatures and pressures consistent with those of the mantle/crust boundary in those areas. The xenoliths range from 46.2 to 67.2 SiO2. Paragneisses are in general more silicic than the orthogneisses. The xenoliths have Rb concentrations between 0.4 and 97 ppm but most samples are very low, with less than 3 ppm Rb. The Sr isotopic ratios of orthogneisses from the lowermost crust throughout most of northern Mexico are very similar and range from ca. 0.705 to 0.706. Previous studies indicate that these rocks have measured ɛ(inNd) values between+2 and −5. Paragneiss xenoliths are generally more radiogenic in Sr isotopic ratio, up to 0.730, and have lower ɛ
Nd values of−11. The Nd and Sr isotopic characteristics of the orthogneisses are similar to those of the voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental. The xenoliths cannot represent cumulate material produced during the mid-Tertiary volcanism because they are Paleozoic or older. Consequently, based on Sr and Nd isotopic data, the silicic ignimbrites could comprise up to 100% lower crustal material.
TL;DR: Pb, Sr and Nd isotope variations are correlated in diverse lavas erupted at small seamounts near the East Pacific Rise as discussed by the authors, and the isotope variability covers ∼80% of the variability for Pacific MORB due to the presence of small-scale heterogeneity in the underlying mantle.
Abstract: Pb, Sr and Nd isotope variations are correlated in diverse lavas erupted at small seamounts near the East Pacific Rise. Tholeiites are isotopically indistinguishable from MORB (206Pb/204Pb=18.1–18.5; 87Sr/86Sr=0.7023–0.7028; 143Nd/144Nd=0.51326-0.51308); associated alkali basalts always show more radiogenic Pb and Sr signatures (206Pb/204Pb=18.8–19.2; 87Sr/86Sr=0.7029–0.7031) and less radiogenic Nd (143Nd/144Nd=0.51289–0.51301). The isotopic variability covers ∼80% of the variability for Pacific MORB, due to the presence of small-scale heterogeneity in the underlying mantle. Isotope compositions also correlate with trace element ratios such as La/Sm. Tholeiites at these seamounts have 3He/4He between 7.8–8.7 R
A(R
A= atmospheric ratio), also indistinguishable from MORB. He trapped in vesicles of alkali basalts, released by crushing in vacuo, has low 3He/4He (1.2–2.6 R)Ain conjunction with low helium concentrations ([He] 2 Ga) isolation of the source from the depleted upper mantle, similar to Dupal components which are more prevalent in the southern hemisphere mantle. 3He/4He at Shimada Seamount is between 3.9–4.8 R
A. Because the helium concentrations range up to 1.5×10−6, the low 3He/4He can not be due to radiogenic accumulation of 4He in the magma for reasonable volcanic evolution times. The low 3He/4He may be due to the presence of “enriched” domains within the lithosphere with high (U + Th)/He ratios, possibly formed during its accretion near the ridge. Alternatively, the low 3He/4He may be an inherent characteristic of an enriched component in the mantle beneath the East Pacific. Collectively, the He-Pb-Sr-Nd isotope systematics at East Pacific seamounts suggest that the range of isotope compositions present in the mantle is more readily sampled by seamount and island volcanism than by axial volcanism. Beneath thicker lithosphere away from the ridge axis, smaller degrees of melting in the source regions are less efficient in averaging the chemical characteristics of small-scale heterogeneities.
TL;DR: The Koloa Volcanics of the island of Kauai as mentioned in this paper are generated by variable small degrees of partial melting of a compositionally heterogeneous garnet-bearing source that has about two-thirds the concentration of P2O5, rare-earth elements, and Sr of the source of the Oahu Volcano.
Abstract: At the top of the Waimea Canyon Basalt on the island of Kauai, rare flows of alkalic postshield-stage hawaiite and mugearite overlie tholeiitic flows of the shield stage. These postshield-stage flows are 3.92 Ma and provide a younger limit for the age of the tholeiitic shield stage. The younger Koloa Volcanics consist of widespread alkalic rejuvenated-stage flows and vents of alkalic basalt, basanite, nephelinite, and nepheline melilitite that erupted between 3.65 and 0.52 Ma. All the flows older than 1.7 Ma occur in the west-northwestern half of the island and all the flows younger than 1.5 Ma occur in the east-southeastern half. The lithologies have no spatial or chronological pattern. The flows of the Koloa Volcanics are near-primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet-bearing source that has about two-thirds the concentration of P2O5, rare-earth elements, and Sr of the source of the Honolulu Volcanics on the island of Oahu. The same lithology in the Koloa and Honolulu Volcanics is generated by similar degrees of partial melting of distinct source compositions. The lavas of the Koloa Volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt, respectively. Phases that remain in the residue of the Honolulu Volcanics, such as rutile and phlogopite, are exhausted during formation of the Koloa Volcanics at all but the smallest degrees of partial melting. The mantle source for Kauai lava becomes systematically more depleted in 87Sr/86Sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage: at the same time, the lavas become systematically more enriched in incompatible trace elements. On a shorter timescale, the lavas of the Koloa Volcanics display the same compositional trends, but at a lower rate of change. The source characteristics of the Koloa Volcanics, considered along with those of the Honolulu Volcanics, support a mixing model in which the source of rejuvenated-stage lava represents large-percent melts of a plume source mixed with small amounts of small-percent melts of a heterogeneous mid-ocean-ridge source.
TL;DR: The spinel lherzolite suite is more refractory than the pyroxenite suite [Fo71-85] and is composed of olivine, orthopyroxene, Cr-diopside, and spinel.
Abstract: Three major types of xenoliths, namely, dunite, spinel lherzolite, and pyroxenite suites, occur. The spinel lherzolite suite [ol: Fo86–92] is more refractory than the pyroxenite suite [Fo71–85], and is composed of olivine, orthopyroxene, Cr-diopside, and spinel. Spinel lherzolites represent metasomatically modified mantle residues that constitute the lithosphere underneath Oahu. Metasomatism has induced significant heterogeneity in terms of [Na]cpx in the spinel lherzolitic lithosphere: compared to other vents, Salt Lake xenoliths are anomalously high in [Na]cpx. The fluids responsible for such a process may have been released after crystallization of the hydrous phases in pyroxenite suite veins intrusive into the spinel lherzolites.
TL;DR: In this paper, peridotite xenoliths from the Pello tuff cone in the Rift Valley of northern Tanzania, bear witness to upper mantle veining and metasomatism.
Abstract: Peridotite xenoliths from the Pello tuff cone in the Rift Valley of northern Tanzania, bear witness to upper mantle veining and metasomatism. Veins of katungite composition, with an asthenospheric signature, have imposed K, Fe, Ti, OH and REE metasomatism upon previously depleted peridotite. Chemical and mineralogical gradients are present in the peridotite wall rocks, and hydrous phases developed in the peridotite are generally lower in Ti and Fe, but higher in Mg and Cr, than those in the veins. The metasomatism has reduced the density of affected peridotite by up to 4.5%, supporting earlier geophysical models for low-density mantle beneath the Rift Valley. Age constraints for the metasomatically-induced density decrease permit correlation with Recent faulting in the Rift Valley, but not with the major upwarp of the Kenya Dome in the late Tertiary.
TL;DR: In this paper, the authors describe migration of stable isotopes via a fluid phase infiltrating porous media, using the formalism of continuum fluid mechanics to deal with the problem of microscopic inhomogeneity.
Abstract: Equations are developed describing migration of stable isotopes via a fluid phase infiltrating porous media. The formalism of continuum fluid mechanics is used to deal with the problem of microscopic inhomogeneity. Provision is made explicitly for local equilibrium exchange of isotopes between minerals and fluids as well as for kinetic control of isotopic exchange. Changing characteristic parameters of transport systems such as porosity, permeability, and changes in modal proportions of minerals due to precipitation or dissolution are taken into account.