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1.
Genesis of high Mg# andesites and the continental crust 总被引：38，自引：2，他引：36
Peter B. Kelemen 《Contributions to Mineralogy and Petrology》1995,120(1):1-19
The continental crust has an andesitic composition with high Mg/(Mg+Fe) and Ni contents which may be too high to have formed by differentiation of basaltic magmas. Instead, mantle-derived, high Mg# andesites (HMA) may form a substantial component of the crust. HMA may be produced by partial melting of previously depleted, subsequently metasomatised mantle peridotite. However, they are more likely produced by reaction between ascending melts and mantle peridotite. HMA are less common than basalts among lavas in modern island arcs, but may have been more common in the past, may be produced in specific environments (such as ridge subduction), may be more common among plutonic rocks in the lower and middle crust than among lavas at the surface, and may be selectively preserved during later erosion and subduction processes. 相似文献
2.
Magmas erupted at mid-ocean ridges (MORB) result from decompression melting of upwelling mantle. However, the mechanism of melt transport from the source region to the surface is poorly understood. It is debated whether melt is transported through melt-filled conduits or cracks on short time scales (<∼ 103 yrs), or whether there is a significant component of slow, equilibrium porous flow on much longer time scales (>∼ 103-104 yrs). Radiogenic excess 226Ra in MORB indicates that melt is transported from the melting region on time scales less than the half life of 226Ra (∼1600 yrs), and has been used to argue for fast melt transport from the base of the melting column. However, excess 226Ra can be generated at the bottom of the melt column, during the onset of melting, and at the top of the melt column by reactive porous flow. Determining the depth at which 226Ra is generated is critical to interpreting the rate and mechanism of magma migration. A recent compilation of high quality U-series isotope data show that in many young basalts, 226Ra excess in MORB is negatively correlated with 230Th excess. The data suggest that 226Ra excess is generated independently of 230Th excess, and cannot be explained by “dynamic” or fractional melting, where observed radiogenic excesses are all generated at the base of the melt column. One explanation is that the negative correlation of activity ratios is a result of mixing of slow moving melt that has travelled through reactive, low-porosity pathways and relatively fast moving melt that has been transported in unreactive high-porosity channels. We investigate this possibility by calculating U-series disequilibria in a melting column in which high-porosity, unreactive channels form within a low-porosity matrix that is undergoing melting. The results show that the negative correlation of 226Ra and 230Th excesses observed in MORB can be produced if ∼60% of the total melt flux travels through the low-porosity matrix. This melt maintains 226Ra excesses via chromatographic fractionation of Ra and Th during equilibrium transport. Melt that travels through the unreactive, high-porosity channels is not able to maintain significant 226Ra excesses because Ra and Th are not fractionated from each other during transport and the transport time for melt in the channels to reach the top of the melt column is longer than the time scale for 226Ra excesses to decay. Mixing of melt from the high porosity channels with melt from the low-porosity matrix at the top of the melting column can produce a negative correlation of 226Ra and 230Th excesses with the slope and magnitude observed in MORB. This transport process can also account for other aspects of the geochemistry of MORB, such as correlations between La/Yb, αSm/Nd, and Th/U and 226Ra and 230Th excess. 相似文献
3.
Othmar Müntener Peter B. Kelemen Timothy L. Grove 《Contributions to Mineralogy and Petrology》2001,141(6):643-658
Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable H2O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H2O concentration in the liquid changes the crystallization sequence. Low H2O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H2O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas. Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm3. The combination of high pressure and high H2O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO2, Al2O3) of igneous crust in arcs. 相似文献
4.
Stefan Bernstein Peter B Kelemen Christian Tegner Mark D Kurz Jurek Blusztajn C.Kent Brooks 《Earth and Planetary Science Letters》1998,160(3-4):845-862
Mafic and ultramafic intrusions in East Greenland adjacent to the offshore Greenland–Iceland ridge were emplaced 5–9 My after continental breakup at 55 Ma [1]. Rare earth element (REE) concentrations determined by secondary ion mass spectrometry are reported for cumulus clinopyroxene from these intrusions, and the data are used to estimate REE abundance in equilibrium melts using available partitioning data. Estimated equilibrium melts from intrusions have strongly fractionated REE patterns with Nd/Dy(N) in the range 2 to 5.6 and Yb/Dy(N) 0.55 to 0.92, similar to values for coeval basalts. These melts have markedly higher Nd/Dy(N) than earlier breakup related flood basalts. The moderately low Yb/Dy(N) for the post-breakup volcanism is indicative of residual garnet in the source, while their high Nd/Dy(N) ratios can best be explained by aggregating low degree melts from a light-REE-enriched garnet- and spinel-bearing mantle source. We also report He, Sr, and Nd isotopic data for the intrusions. The highest 3He/4He ratios (>10 R/Ra) are found in the samples whose REE data reflect the largest proportion of melts from a garnet-bearing source, and having Sr and Nd isotopic compositions identical with the radiogenic Sr and unradiogenic Nd isotope end of the Iceland compositional field. There is no indication of a MORB-type mantle in the source of the intrusions. We postulate that post-breakup volcanism along the East Greenland coastline reflects the increasing proximity of the mantle plume to the East Greenland continental margin. The low degree of melting at high mean pressure inferred for the parental melts for the intrusions may reflect re-thickening of the lithosphere, which in turn was caused by the vigorous volcanism during breakup, with accompanying depletion of upper mantle and underplating of the crust at the continental margin. 相似文献
5.
Peter B. Kelemen 《Journal of the Geological Society of India》2009,74(2):280-282
Discussion
Paleoproterozoic Boninite-Like Rocks in an Intracratonic Setting from Northern Bastar Craton, Central India by D.V. Subba Rao, V. Balaram, K. Naga Raju and D.N. Sridhar. Jour. Geol. Soc. India, v.72, 2008, pp.373–380 相似文献6.
CO2, CH4, and N2 adsorption and gas-induced swelling were quantified for block Blind Canyon, Pittsburgh #8 and Pocahontas Argonne Premium coals that were dried and structurally relaxed at 75 °C in vacuum. Strain measurements were made perpendicular and parallel to the bedding plane on ~ 7 × 7 × 7 mm3 coal blocks and gravimetric sorption measurements were obtained simultaneously on companion coal blocks exposed to the same gaseous environment. The adsorption amount and strain were determined after equilibration at P ≤ 1.8 MPa. There is a strong non-linear correlation between strain and the quantity of gas adsorbed and the results for all gases and coals studied follow a common pattern. The dependence of the coal matrix shrinkage/swelling coefficient (Cgc) on the type and quantity of gas adsorbed is seen by plotting the ratio between the strain and the adsorbate concentration against the adsorbate concentration. In general, Cgc increases with increasing adsorbate concentration over the range of ~ 0.1 to 1.4 mmol/g. Results from the dried block coals are compared to CO2 experiments using native coals with an inherent level of moisture as received. The amount of CO2 adsorbed using native coals (assuming no displacement of H2O by CO2) is significantly less than the dried coals. The gas-induced strain (S) and adsorption amount (M) were measured as a function of time following step changes in CO2, CH4, and N2 pressure from vacuum to 1.8 MPa. An empirical diffusion equation was applied to the kinetic data to obtain the exponent (n) for time dependence for each experiment. The data for all coals were pooled and the exponent (n) evaluated using an ANOVA statistical analysis method. Values for (n) near 0.5 were found to be independent on the coal, the gas or type of measurement (e.g., parallel strain, perpendicular strain, and gas uptake). These data support the use of a Fickian diffusion model framework for kinetic analysis. The kinetic constant k was determined using a unipore diffusion model for each experiment and the data were pooled for ANOVA analysis. For dry coal, statistically significant differences for k were found for the gases (CO2 > N2 > CH4) and coals (Pocahontas >Blind Canyon > Pittsburgh #8) but not for the method of the kinetic measurement (e.g., strain or gas uptake). For Blind Canyon and Pittsburgh #8 coal, the rate of CO2 adsorption and gas-induced strain for dry coal was significantly greater than that of the corresponding native coal. For Pocahontas coal the rates of CO2 adsorption and gas-induced strain for dry and native coal were indistinguishable and may be related to its low native moisture and minimal amount of created porosity upon drying. 相似文献
7.
Recycled crust controls contrasting source compositions of Mesozoic and Cenozoic basalts in the North China Craton 总被引：7，自引：0，他引：7
We explore Fe/Mn and Nb/Ta ratios of basalts as potential tracers for differentiating melts of recycled mafic crustal lithologies from peridotitic melts. Trace elements and Fe/Mn ratios of the Mesozoic and Cenozoic basalts from East China were analyzed by ICP-MS. Low Nb/Ta ratios (15.4 ± 0.3 (2σ, n = 45)), high Nb and Ta contents (60.1 and 4.01 ppm) and high Fe/Mn ratios (64.7 ± 1.5 (2σ, n = 45)) characterize the <110 Ma basalts. Mesozoic basalts with ages >110 Ma are characterized by superchondritic Nb/Ta ratios (20.1 ± 0.3 (2σ, n = 25)), low Nb and Ta contents (10.8 and 0.54 ppm) and slightly lower Fe/Mn ratios (60.0 ± 1.1 (2σ, n = 25)). Both the Mesozoic and Cenozoic basalts have Fe/Mn ratios higher than basaltic melt formed by partial melting of peridotite at the same MgO and CaO levels. Although both the Mesozoic and Cenozoic basalts are characterized by highly fractionated REE patterns, the >110 Ma basalts have island arc-type trace element patterns (i.e., depletion in Nb and Ta), whereas OIB-type trace element patterns (e.g., no depletion in Nb and Ta) are characteristic of the <110 Ma basalts. Based on DFe/Mn values for olivine, clinopyroxene, orthopyroxene and garnet, high Fe/Mn ratios and negative correlations of Fe/Mn with Yb (Y) of the <110 Ma basalts suggest clinopyroxene/garnet-rich mantle sources. The lower Fe/Mn ratios and positive correlations of Fe/Mn with Y and Yb in the >110 Ma basalts suggest orthopyroxene/garnet-rich mantle sources. Combining these data with Sr-Nd isotopes, we present a conceptual model to explain the Nb/Ta ratios and PM-normalized trace element patterns of the >110 and <110 Ma basalts. Preferential melting of recycled ancient lower continental crust during Mesozoic lithospheric thinning resulted in (1) peridotite-melt/fluid reaction that formed the orthopyroxene/garnet-rich mantle sources for the >110 Ma basalts, and (2) peridotite + rutile-bearing eclogite mixing that formed the clinopyroxene/garnet-rich mantle sources for the <110 Ma basalts. The choice of models may indeed be arbitrary and non-unique, but the goal is to seek relatively simple forward models that explain the characteristics of the lavas, and the differences between the >110 and <110 Ma basalts, in a relatively consistent geodynamic framework. 相似文献
8.
Kay L. Achenbach Michael J. Cheadle Ulrich Faul Peter Kelemen Susan Swapp 《Earth and Planetary Science Letters》2011,301(1-2):199-212
Eleven harzburgites and one dunite from Ocean Drilling Program Leg 209 Hole 1274A preserve high-temperature mantle textures. Electron backscatter diffraction (EBSD) analysis shows moderately developed crystal lattice preferred orientations (LPOs) in olivine and orthopyroxene (M-indices ≈ 0.1) indicative of crystal-plastic deformation at ~ 1250 °C. These rocks preserve a protogranular texture with a weak olivine foliation, a very weak or absent orthopyroxene foliation that may be decoupled from the orthopyroxene LPO, and minor interstitial clinopyroxene and spinel. Olivine grain size distributions, along with melt-related microstructures in orthopyroxene, clinopyroxene and spinel suggest that high-temperature deformation textures have been overprinted by pervasive post-deformation melt-rock interaction. Paleomagnetic data constrain the olivine [100] axes to be subhorizontal and oriented at low angle (≤ 28.6° ± 10.6°) to the ridge axis at the onset of serpentinization. This orientation is consistent with either complex 3-D mantle upwelling or 2-D mantle upwelling coupled with complex 3-D tectonic emplacement to the seafloor. 相似文献
9.
Elisabeth Streit Peter Kelemen John Eiler 《Contributions to Mineralogy and Petrology》2012,164(5):821-837
Tectonically exposed mantle peridotite in the Oman Ophiolite is variably serpentinized and carbonated. Networks of young carbonate veins are prevalent in highly serpentinized peridotite, particularly near low-temperature alkaline springs emanating from the peridotite. An unusual feature in some samples is the coexistence of serpentine and quartz, which is not commonly observed in serpentinites. This assemblage is unstable with respect to serpentine?+?talc or talc?+?quartz under most conditions. Serpentine in the carbonated serpentinites in this study is more iron rich than in most serpentinites reported in previous studies, and samples with co-existing quartz contain the most iron-rich serpentines. Calculations of thermodynamic equilibria in the MgO–SiO2–H2O–CO2 system suggest that serpentine?+?quartz may be a stable assemblage at low temperatures (e.g., <~15–50?°C) and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Based on these calculations, serpentine?+?quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater for all samples analyzed, while the δ18O of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater for the majority of samples analyzed. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment. 相似文献
10.