Evaluation of a crust model of eucrites from the width of exsolved pyroxene.

Miyamoto, M.; Takeda, Hiroshi

doi:10.2343/geochemj.11.161

Cited by 52 publications

(30 citation statements)

References 13 publications

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“…This agrees with experimental data (Miyamoto and Takeda 1977;Brady and McCallister 1983;Freer et al 1982;Reitmeijer 1983) and with estimated kinetics (Ganguly and Chakraborty 1990). Thus, by using orthopyroxene instead of biotite (which has a faster diffusion rate than garnet), the thermometer will be less susceptible to the influence of retrograde cation exchange.…”

Section: Temperature (Oc)supporting

confidence: 91%

Retrograde cation exchange in garnets during slow cooling of mid crustal granulites and the P-T-t trajectories from the Mont Laurier region, Grenville province Quebec

Boggs¹

1996

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During the past decade, there have been many advances in the domain of metamorphic geology that have rendered previous P-T results questionable. For example, Spear (1991) demonstrated that the widely used gamet-biotite Fe-Mg ion-exchange thermometer can underestimate temperatures by as much as 200°C due to retrograde diffusion between the garnet and biotite. Holdaway and Mukhopadhyay (1993) noted that exchange geothermometers are easily reset at granulite-grade metamorphism. Thus, all geothermobarometers would record lower diffasion-based post-peak metamorphic pressures and temperatures, particularly those such as the biotite-garnet Fe-Mg exchange geothermometer which are expecially sensitive to retrograde cation exchange. One objective of this thesis is to quantify this effect of resetting as a function of mineral grain size.The field area is located 100 km north of the Ottawa River between Ottawa and Montreal in the Central Metasedimentary Belt (CMB) of the Grenville Province. It contains the largest granulite terrane within the CMB. The western two thirds of the field area is dominated by the Bondy Gneiss Complex (BGC), a granulitic dome, consisting principally of orthogneisses with quartzites, a tonalitic pluton and metapelites. The northsouth trending Nominingue-Chénévilie Deformation Zone (NCDZ), cuts through the structural trend of the BGC and dominates the eastern third of the field area.The morphology and textures of migmatites in the area vary greatly from layers of leucosomes with granitic composition flanked by garnet-rich layers, and biotite-rich melanosomes through thin (1mm) wispy-discontinuous leucosomes and felsic moats surrounding garnets to continuous leucosomes greater than lcm thick. Many thin sections contain embayed K-feldspar or plagioclase grains with inclusions of biotite mat formed from a melt-producing reaction. Biotite is present throughout the field area, which limits the peak temperature to below 980°C. One outcrop contains garnets with spinel inclusions, suggesting the reaction Als + Grt = Spl + Qtz, which implies peak temperatures above 850°C. The Krn-Opx-Tur-Crd-Pl assemblage occurs in the BGC implying the komerupine-forming reaction Tur + Opx + Crd = Km + PL It has been proposed that the Bondy Gneiss Complex was hydrothermally altered. The plagioclase composition from the BGC are unusually diverse (between Anjs and An88), possibly due to this hydrothermal alteration. Cordierite, orthopyroxene, and spinel were only observed in the NCDZ. The spinel analyses were similar to literature values from the reconstructed Grenville Orogen at 0.8 Ga. Calcite was only observed in metapelites from the NCDZ, possibly due to fluid flow along this deformation zone during deformation. AFM projections also demonstrate this diversity between these two tectonostratigraphic domains. Garnets in the BGC are up to 20% more Mg-rich than those in the NCDZ, and the BGC biotites are also up to 30% more Mg-rich than the NCDZ biotites. VIGarnets were the only mineral found to exhibit regular zonin...

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Section: Temperature (Oc)supporting

confidence: 91%

Retrograde cation exchange in garnets during slow cooling of mid crustal granulites and the P-T-t trajectories from the Mont Laurier region, Grenville province Quebec

Boggs¹

1996

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“…In this model, chemistry, exsolution and inversion textures (summarized in Table 2) played an important role in estimating the distribution of the varying layers of crust and cooling history of the HED crust. This model was further refined by Miyamoto and Takeda (1977) by evaluating the crust model of eucrites more quantitatively using simulation of the development of exsolution lamella of augite.…”

Section: Introductionmentioning

confidence: 99%

Mineralogical records of early planetary processes on the howardite, eucrite, diogenite parent body with reference to Vesta

Takeda

1997

Meteorit & Planetary Scien

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101

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Abstract-Mineralogical information recovered from the howardite, eucrite, diogenite (HED) meteorites was employed to reconstruct the history of the parent body and relate it to 4 Vesta. These interpreted crustal evolution processes were then compared to the expected geological features on the surface of a likely protoplanet, 4 Vesta. The original crustal materials of the HED parent body were preserved as mineral grains and lithic clasts, but in many eucrites, FeMg ratios in pyroxenes were homogenized by diffusion after crystallization. The crystallization trend of the protocrust has been deciphered by (I) examining monomict and crystalline samples and using their mineralogical and chemical information to formulate a sequence of crystallization and cooling trends; and by (2) reconstructing the original crust prior to cratering events from lithic clasts and mineral fragments in polymict breccias such as howardites and polymict eucrites. Mineral components are identical, both in the individual HED and in polymict breccias, and no remnants of primitive materials were preserved in the polymict breccias. A layered crust model reconstructed from such breccias consists of an upper crust with extrusive lava-like eucrites that have been brecciated and metamorphosed, diogenite mantle, and cumulate eucrites of varying thickness between them. This model can be used to explain the surface geological features of Vesta observed from the Hubble space telescope. A large crater with diogenitic orthopyroxene at the crater floor is consistent with the deepest diogenitic layer of the layered crust model; and an underlying olivine layer is expected from early crystallized olivine in the crystal fractionation model. The old terrain of eucritic surface materials of Vesta can be howardites, polymict eucrites, or regolith-like eucrites produced from eucrites extruded and impacted on the surface. Partial melting models of eucrites seem to be favored by the rare-earth element (FEE) chemistry and experimental studies. Unfortunately, partial melting models have not demonstrated how the HED parent body is converted to a layered crust without producing any metamorphosed primitive material in the layered crust. The origin of cumulate eucrites with systematic variation of textures and chemistries of pyroxene can be explained by the layered crust model with excavation and mixing of trapped liquid. Discovery of basaltic materials with Na-rich plagioclase and augite in iron meteorites, which are the products of partial melting, suggests that eucrites may be unique to a body that underwent large-scale differentiation and metamorphism.

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“…A number of attempts have been made to use thickness of lamellae to estimate cooling histories for the pyroxenes 207 in which they formed (e.g. Dunham et al, 1972;Huebner et al, 1975;Miyamoto and Takeda, 1977). A major assumption of these examples, which would appear in doubt due to the experimental evidence, is that volume interdiffusion is the rate controlling step.…”

Section: Lamellar Processes and Geothermometrymentioning

confidence: 99%

“…The assumption that volume diffusion controls the growth rate used by these models is suspect in that transport experiments using lamellae as the starting material give diffusivities well below those of tracer studies. Numbers used in different models were replaced with values derived from this study which significantly changes the results in cases of depth of burial for a lunar sample and meteorites in a parent planetoid (Miyamoto and Takeda, 1977). …”

Section: Applicationsmentioning

confidence: 99%

Strontium and samarium diffusion in diopside

Sneeringer

Hart

Shimizu

1984

Geochimica et Cosmochimica Acta

264

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The volume diffusion of strontium has been measured as a function of temperature, pressure, and crystallographic direction in a synthetic diopside (CaMgSi 2 06), using a Cameca IMS 3f ion microprobe. Temperatures ranged from 1100 to 1300*C for pressures to 20 Kbar. Sr diffusivities were also determined for the c direction of the synthetic crystals by a radiotracer ( 8 5 Sr) and sectioning technique and Rutherford backscattering spectroscopy; and for natural samples by the radiotracer method. The natural sample diffusion coefficients were approximately two orders of magnitude higher than those for the synthetic samples, although the activation energies were similar. The diffusion of samarium in the synthetic crystals was also studied with respect to temperature, pressure, and crystallographic direction using the ion probe. Results for the two cations were comparable (within a factor of 2 at most T's). Values of AVa were negative for strontium diffusion. An empirical relation which adequately described the temperature and pressure dependence of D was calculated for Sr in the c direction of the synthetic samples. The result is:The expression for DSr in the natural samples at one atmosphere is:A single compensation trend for all the data was evident for all values of D and AHa in the synthetic crystals. In the study temperature range at one atmosphere, transport rates for the three crystallographic directions were nearly identical due to compensation with a crossover point at 1111*C and 6 x 10-1 7 cm 2 /sec. Directional compensation was also observed in two other studies Clark and Long, 1971).Comparison of the three analytical techniques yielded excellent agreement in the study range at one atmosphere (D's within a factor of two). The comparison illustrated the fact that technique-generated effects can cause the observed concentration curves to differ from the true diffusion profiles. Treatment for removal of these modifiers was effected through variations in analytical parameters and through empirical data reduction methods. Ion probe depth profiling was proven to be a useful tool for the study of diffusion in crystalline silicates, where extremely slow transport rates limit the applicability of more established techniques such as radiotracer and sectioning.The data were examined in the context of other studies which used similar methods and/or investigated the same variables. Pressure dependence of D in most experiments resulted in positive values of AVa, as opposed to the activation volumes calculated here. The comparisons showed that there are three basic sources of differences between diffusivities for a given phase and element. Sample defect composition, experimental techniques, and analytical methods are all capable of causing variations in the measured D's.A number of models of geologic processes were investigated in light of the present data. Closure temperatures (Tc) were calculated for examples of mineral-mineral age-dating, and major and trace element geothermometry. High values of Tc indicate th...

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Evaluation of a crust model of eucrites from the width of exsolved pyroxene.

Cited by 52 publications

References 13 publications

Retrograde cation exchange in garnets during slow cooling of mid crustal granulites and the P-T-t trajectories from the Mont Laurier region, Grenville province Quebec

Retrograde cation exchange in garnets during slow cooling of mid crustal granulites and the P-T-t trajectories from the Mont Laurier region, Grenville province Quebec

Mineralogical records of early planetary processes on the howardite, eucrite, diogenite parent body with reference to Vesta

Strontium and samarium diffusion in diopside

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