Charles V. Guidotti

Thermal modelling in low-pressure/high-temperature metamorphic belts

Abstract Low-pressure/high-temperature metamorphic belts occur throughout the world. They display a characteristic metamorphic style which has remained unchanged with time. They are found in disparate tectonic settings, including magmatic arcs, regions of extension, regions of thickened crust in continent-continent collision zones, and accretionary wedges. They are commonly characterized by an abundance of granitoid plutons. Pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point (approximately 4 kbar), while temperatures are in the range 500–750 °C. With rare exceptions, inferred PTt paths are nearly isobaric and recrystallization was rapid and prograde. Metamorphic thermal gradients are commonly in the range 60–150 °C/km. These conditions necessitate the presence of a large component of advective heat transfer and/or anomalously high basal heat flow during metamorphism. The large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism (LPM). Lithospheric extension is likely to produce LPM only if the degree of thinning is large ( β > 3) and there is additional heat from magmas. The generation of LPM by crustal thickening requires either a large component of advective heating by magmas, anomalously high basal heat flow, or pre-thickening lithospheric extension. LPM in this environment must necessarily be accompanied by plutonic activity. Advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but, acting along, is unlikely to produce LPM. Regional LPM induced by advective sources, rarely occurs along a geotherm which increases monotonically with depth, or synchronously across the region at a given depth; it occurs over time as a series of overlapping temporally separate and spatially localized thermal events.

Titanium in biotite from metapelitic rocks: Temperature effects, crystal-chemical controls, and petrologic applications

Abstract An extensive natural biotite data set from western Maine constrains the temperature and crystalchemical controls on the saturation Ti levels in biotites from metapelites. The geologically and petrologically well-characterized metamorphic terrain associated with the M3 metamorphism of the Acadian Orogeny of western Maine is ideal for this approach in that metamorphism occurred at roughly isobaric conditions of 3.3 kbar, and chemical equilibrium was closely approached. The data set from these metapelites exhibits systematic variations in Ti contents over a continuum of metamorphic grades (garnet through sillimanite-K-feldspar zones), mineral assemblages, and bulk compositional ranges. Samples were selected so that competing substitutions are restricted to those in metapelites with quartz, aluminous phases (chlorite, staurolite, or sillimanite), Ti phases (ilmenite or rutile), and graphite. Due to crystal-chemical factors, in any given metamorphic zone, an inverse linear relationship exists between Ti and Mg contents. Decreasing octahedral Ti and increasing tetrahedral Si in Mg-rich biotite helps alleviate size disparity between octahedral and tetrahedral sheets. For a biotite with a given Mg content, Ti most dramatically increases above staurolite zone conditions. Our constrained data set allows us to calculate a Ti saturation surface for natural biotite as a function of temperature and Mg content at 3.3 kbar. The Ti saturation surface can be used to establish several important metamorphic features in similar metamorphic settings. These include a general approach to equilibrium, local and/or subtle departures from equilibrium due to minor alteration to chlorite, and relative and absolute geothermometry based on Ti in biotite inclusions in refractory minerals and in matrix biotite.

Fe3+ and Fe2+ partitioning among silicates in metapelites: A synchrotron micro-XANES study

Abstract Synchrotron micro-XANES (X-ray absorption near-edge structure) spectroscopy (SmX) was used to measure Fe3+ and Fe2+ distribution among minerals in standard thin sections with an X-ray beam size of 10 ¥ 15 mm. Measurements were made at beamline X26a, National Synchrotron Light Source, Brookhaven National Lab. Samples studied included metapelites from garnet to upper sillimanite zone rocks that coexist with graphite, graphite/ilmenite, or varying combinations of ilmenite, magnetite, and hematite. SmX results are compared with Mössbauer spectroscopic measurements of Fe3+ on mineral separates from the same rocks. Results show excellent agreement (within ±5-10%) between Mössbauer and SmX for the Fe-rich phases biotite, chlorite, staurolite, and garnet. Mössbauer spectra of muscovite typically show lower values than SmX for Fe3+/SFe (especially at low grades), suggesting contamination of the muscovite separates by fine-grained chlorite. However, heterogeneity of Fe3+ and Fe2+ is probably the chief source of discrepancy between the bulk and micro-scale measurements. The %Fe3+ (relative to total Fe) in these samples ranges from a high of 90% in muscovite to a low of 0-2% in garnet, with Ms > St = Bt = Chl > Tur > Grt in graphite/ilmenitebearing rocks. SmX measurements suggest that the averaged Fe3+/SFe in each mineral species does not change as a function of grade, but varies as a function of buffering assemblage, especially on a very localized scale. This effect shows that the oxygen buffering capacity of mineral assemblages is very large compared to the oxidizing/reducing potential of metamorphic fluids. However, distribution of Fe atoms among phases at each grade also reflects crystal chemistry. Across all grades, the Fe2+/Mg ratio is such that Grt >> St > Bt > Chl > Tur > Ms. Fe3+/Al is highest in biotite in garnet and sillimanite- zone rocks. For Fe3+/Al, Bt > Chl > Tur > St > Ms, and for Fe3+/Fe2+, partitioning behaves as Ms >> Bt ≈ St ≈ Chl > Tur > Grt in graphite/ilmenite-bearing rocks. Overall, partitioning of Fe3+ in any of the minerals of interest reflects both oxide assemblage and crystal-chemical constraints on the amount of Fe3+ that can be substituted into the structure.

The paragonite-muscovite solvus: II. Numerical geothermometers for natural, quasibinary paragonite-muscovite pairs☆

Three parametric (nonthermodynamic) equations have been developed to calculate final equilibration temperatures for natural, quasibinary paragonite-muscovite (Pg-Ms) pairs (mole fraction margarite in Pg < 0.05, Si/formula unit ≤ 6.2 and/or Σ(Mg + Fe2+ + Fe3+) ≤ 0.35 for coexistent Ms). The first two equations are paragonite-based and muscovite-based, because they yield equilibration temperatures that depend on the Na-K compositions of K-saturated Pg and Na-saturated Ms, respectively. The third equation is closure-based, because it yields equilibration temperatures that vary with the width of the gap between the Na-K compositions of coexisting Pg and Ms (i.e., the degree of solvus closure). Equilibration temperatures can be estimated, using just one of the geothermometers, or by averaging the temperatures obtained from any combination of the three equations. Comparison of the three calculated temperatures yields information on the mutual consistencies of the Na-K compositions of coexisting Pg and Ms with respect to their utility in Pg-Ms solvus thermometry. Geothermometric inconsistencies in the Na-K compositions of quasibinary Pg-Ms pairs can be caused by (1) inaccuracies in determinations of the Na-K compositions of the coexisting micas, (2) H3O+ → K+ and vacancy → K+ substitution in muscovite and/or paragonite (3) crystallization and preservation of quasibinary Pg-Ms pairs with metastable Na-K compositions. Due to the effects of pressure on the stability relations of Pg-Ms pairs and the K/Na ratios of Nasaturated muscovites, practical applications of Pg-Ms solvus thermometry are restricted to quasibinary Pg-Ms pairs that equilibrated at pressures between approximately 2 and 8 kbar. Within this pressure range, the utility of Pg-Ms solvus thermometry is limited thermally to approximately 300 ≤ T ≤ 700°C by (1) the decomposition of K-saturated paragonite at temperatures between approximately 580 and 700°C, and (2) steepening of the solvus limbs at temperatures below 300°C. Graphical and calculated Pg-Ms solvi based on solvus data for synthetic, binary Pg-Ms micas are inconsistent with solvus data for natural, quasibinary Pg-Ms micas. Consequently, these solvi should not be used to estimate equilibration temperatures for natural, quasibinary Pg-Ms pairs.

Evidence for extensive Hercynian metamorphism in western Maine

40 Ar/ 39 Ar release spectrum ages for five hornblendes from two Acadian plutons in western Maine and petrologic observations for nearby metamorphic rocks are interpreted to indicate that high-grade K-feldspar + sillimanite metamorphism occurred during Carboniferous time, not Devonian time as previously believed. The metamorphism was closely associated in time with the emplacement of the Sebago batholith, 325 Ma. Petrologic data suggest that rocks as far as 30 km north of the Sebago batholith were affected. Release spectrum ages for hornblendes from the Songo pluton indicate that by 300 Ma high-grade metamorphic conditions had ended and cooling through 500 °C had occurred.

Miscellaneous isomorphous substitutions in Na-K white micas: a review, with special emphasis to metamorphic micas

Numerous new studies have been done over the last ten years on the minor isomorphous substitutions in Na-K white micas. These results, from over a hundred papers, are reviewed and critically discussed here. Special emphasis is given to the new advances which the authors perceive as having a direct petrogenetic significance for understanding metamorphic rocks.RiassuntoNegli ultimi dieci sono stati compiuti molti studi sulle sostituzioni isomorfe minori nelle miche sodico-potassiche. I relativi risultati sono contenuti in un centinaio di pubblicazioni. Il presente lavoro è una rassegna critica di questi risultati, e pone particolare attenzione a quelli che vengono ritenuti rilevanti per la comprensione petrogenetica delle rocce metamorfiche.

Chemical composition, statistical analysis of the unit cell, and electrostatic modeling of the structure of Al-saturated chlorite from metamorphosed rocks

Abstract Natural Al-saturated chlorite having a wide range of Mg:Fe ratios [Mg/(Mg + Fe) = 0.357 to 0.943] was studied to determine the effect of Mg-Fe substitution on the unit-cell parameters. With a nearly constant Al content [AlT/(AlT + Mg + Fe) = 0.362 to 0.416, where AlT represents total Al contents in both tetrahedral and octahedral sites], the only major variable affecting the unit-cell dimensions of natural chlorite is the Mg:Fe ratio. The value of about 0.4 for the Al content represents apparent Al saturation for chlorites in metamorphosed rocks. Unit-cell parameters were determined by least-squares refinement from 39 chlorite samples obtained over a large range in metamorphic grades, from X-ray data derived from Gandolfi pattern simulations (via single crystals and a single-crystal diffractometer) from this study and from traditional powder diffractometer data from the literature. Nine of the samples were analyzed for their Fe3+ contents (= 0.147 to 0.304 atoms) by Mössbauer spectroscopy; chemical compositions were generally obtained by electron-microprobe analysis. Statistical tests show that the change of the crystallographic a and b axes are linear over the range of Mg:Fe ratios studied, whereas the crystallographic c axis and the β angle have no significant relationship to these Mg:Fe ratios. Equations relating the effect of Mg:Fe ratio on a and b are: a (Å) = −0.092 x + 5.408; x = Mg/(Mg + Fe) and b (Å) = − 0.162 x + 9.370, respectively. Electrostatic models involving Pauling.s electrostatic valency principle for Al-saturated chlorite were determined assuming: (1) Al-Al avoidance for Al-substituted tetrahedra, (2) no vacancies, and (3) no Al3+ substitution in the M1 sites. Electrostatic restrictions occur for R3+ → R2+ substitutions in the M2 sites of the 2:1 layer. These restrictions produce an Al saturation in chlorite at AlT/(AlT + Mg + Fe) ratios of near 0.4. This saturation limit requires that 37.5% of the tetrahedral sites (1.5 out of 4 sites) are occupied by Al, and the electrostatic charge (electrostatic valency units, e.v.u.) close to +1 e.v.u. is favored for the interlayer to offset the net negative charge on the 2:1 layer.