B. Ronald Frost

Chrome-spinel in progressive metamorphism—a preliminary analysis

Progressive metamorphism of serpentinite and allied rocks causes systematic changes in the composition of the chrome-spinel phase, when the latter is in equilibrium with chlorite and two magnesium silicate minerals. The stable spinel in antigorite-serpentinites is Al-poor magnetite, Cr-magnetite, or ferrit-chromite, depending on the local CrFe3+ ratio in the rock. With increasing metamorphic grade up to middle amphibolite facies conditions (talc + olivine, or Ca-poor amphibole + olivine stable) more chromiferous spinels (chromites) are encountered, containing modest amounts of Al. Further increase in grade (enstatite + olivine stable) extends the range of possible spinel compositions to green MgAl2O4-rich spinel. The Al-content is governed by P-, T-, and ƒH2O-sensitive equilibria involving chlorite. MgFe2+ ratios in the spinel are a function of Cr, Al and Fe3+ in the spinel, and the ratio MgFe2+ in coexisting silicates. Solid solution in natural chromites on the magnetite-chromite join is complete at ≈500°C, and close to the join chromite-Al spinel (with variable Fe/Mg) it is complete at ≈700°C. The temperature dependence of KD, the olivine-spinel Fe-Mg partition coefficient, is greater than implied by the jackson (1969) geothermometer. A tentative, revised, graphical calibration is offered, based on microprobe-analyzed high-grade metamorphic pairs and pairs from basaltic pumice. This new plot gives results which are broadly consistent with relative temperatures of equilibration inferred from other geothermometers, for alpine peridotites, peridotite nodules and meteorites.

Reduced rapakivi-type granites: The tholeiite connection

Reduced rapakivi-type granites are the most iron enriched and reduced (i.e., least oxidized) of the “anorogenic” granite association. The low oxygen fugacity and chemical composition of these granites severely limit their sources. In this paper we argue that reduced rapakivi-type granites and their eruptive equivalents, high-potassium fayalite rhyolites, are derived from mafic sources, because tholeiitic magmas and their derivatives have the required low oxygen fugacity. Reduced, rapakivi-type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents. They form in extensional environments where the asthenosphere is present at shallow depths. We envision three stages in the origin of these rocks: (1) tholeiitic melts are emplaced at the base of the crust, (2) continued introduction of heat partially remelts these tholeiitic rocks, and (3) the hot, dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites. Partial melting of felsic continental crust may accompany the intrusion of rapakivi-type magmas, thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite.

Sphene (titanite): phase relations and role as a geochronometer

Abstract Useful U–Pb isotopic data may be obtained from sphene (or titanite, CaTiSiO 5 ) because: (1) it is a widespread accessory mineral, (2) it can incorporate uranium in its structure, and (3) it has a high closure temperature. In igneous rocks, sphene is most abundant in relatively oxidized rocks, such as metaluminous rocks of intermediate composition. These rocks have the high Ca/Al ratios wherein sphene is stabilized relative to ilmenite+quartz or ilmenite+anorthite. In metamorphic rocks, sphene is stable to the highest temperatures in mafic and calc-silicate rocks. It is found mostly in greenschist, blueschist, and amphibolite facies, although in calcic rocks its stability may extend into granulite facies. Recent studies show that the closure temperature for sphene lies at the upper limit of amphibolite facies. Because sphene reacts readily during metamorphism, U–Pb sphene ages are likely to yield the age of metamorphic crystallization, rather than resetting by simple diffusion. For this reason, metamorphic sphene may yield complex U–Pb systematics that contain information on the whole metamorphic history of the rock. Sphene from igneous rocks and orthogneisses has initial U contents ranging from 10 to over 100 ppm and ratios of initial U to common Pb ranging from 10 to 1000, ratios that may potentially yield high precision U–Pb ages. Sphene in marbles, calc-silicates, and metagraywackes has a similar range in composition to that from igneous rocks, but sphene from metabasites may have initial U contents of less than 1 ppm and ratios of initial U to common Pb lower than 1, making them unsuitable for geochronology. These low-U sphenes are most commonly found in weakly metamorphosed metabasites. Strategies to extract age information from sphene with moderate initial U/common Pb ratios include estimation of common Pb isotopic composition of sphene from coexisting low-U phases, use of U–Pb and Pb–Pb isochron plots, and step-wise leaching methods to improve 206 Pb/ 204 Pb spread. By correlating sphene compositions to metamorphic or hydrothermal reactions, age determinations on sphene can be used to directly date metamorphism, deformation, and hydrothermal alteration.

The Late Archean history of the Wyoming province as recorded by granitic magmatism in the Wind River Range, Wyoming

Abstract The Wyoming province, a small, ca. 500 000 km 2 Archean craton, is the most southwestern of the Archean provinces in North America. It is composed primarily of Late Archean potassium-rich granitic rocks. In contrast to many other Archean provinces, rocks of tonalite-trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2.8 Ga. Field, petrologic, geochemical and isotopic study of the Late Archean granites exposed in the Wind River Range have allowed us to identify at least four periods of potassic calc-alkalic magmatism at ∼2.8, 2.67, 2.63 and 2.55 Ga. Granitic rocks of these ages appear to be widespread across the Wyoming province. The oldest calc-alkalic granites of the Wind River Range, emplaced at ca. 2.8 Ga, appear to be derived predominantly from pre-existing crust. However, Nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses. During at least two other periods of plutonism, at 2.67 and 2.63 Ga, generation of the Wind River Range batholiths involved the incorporation of substantial amounts of isotopically juvenile material, either from depleted mantle or young continental crust. The information presented below, as well as data available from elsewhere in the Wyoming province, is interpreted to suggest that the Wyoming province, unlike other Archean cratons, is not composed of a tectonic amalgamation of smaller, exotic terranes. Although the Wyoming province did experience crustal addition in Archean time, it was not by lateral accretion, but by incorporation of mantle-derived melts into large granitic batholiths.

Geologic implications of seawater circulation through peridotite exposed at slow-spreading mid-ocean ridges

Peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow-spreading centers hosts hydrothermal circulation of seawater. The reaction of seawater with peridotite causes serpentinization, which generates a high-pH, strongly reducing fluid rich in methane and hydrogen, and is accompanied by as much as 40% volume expansion. Complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion. Diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk-like limestone. This may be the main mechanism of deposition of ophicarbonates common in ophiolites. The degree of induration is a function of the fluid flux through the sediment column. Calcareous ooze infiltrates faults and fractures and can be deformed following lithification. Focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite, aragonite, and brucite, such as those in the recently documented Lost City vent field (30°N, Mid-Atlantic Ridge). Geophysical implications of serpentinization include (1) creation of magnetic anomalies due to growth of magnetite in serpentinite and (2) lowered seismic velocity. Integrated studies of geologic and geophysical effects of serpentinization may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor.

Evidence for extensive Proterozoic remobilization of the Aldan Shield and implications for Proterozoic plate tectonic reconstructions of Siberia and Laurentia

Abstract A geological traverse across the Aldan shield along the Aldan River shows that the area is underlain by two distinct rock associations. The Middle Aldan association consists of metasedimentary rocks, mainly quartzite, that have been intruded by a potassic biotite granite. Downstream, north of the Middle Aldan association, lies the Lower Aldan association, which consists mostly of charnockite with rafts of older granulite gneiss that contains abundant metasedimentary layers. UPb dating of zircons from the Middle Aldan association granite (1900 Ma) and Lower Aldan association charnockite (maximum age 1918 Ma) and granitic gneiss (maximum age 2230 Ma) shows that the majority of the rocks exposed along the Aldan River are Proterozoic in age. Although they have Proterozic crystallization ages, the granite, granite gneiss and charnockite yield Archean Nd model ages, suggesting that they formed by remobilization—that is, partial or complete remelting—of earlier Archean crust. In contrast, pelitic gneisses included within the charnockites of the Lower Aldan association give Proterozoic Nd model ages, indicating that a substantial amount of Proterozoic rock is incorporated within the Lower Aldan association. The present results show that the rocks along the Aldan River, which are part of the Aldan block, display a significantly different history from those in the Olekma block to the west. The Olekma block contains ca 3.0 Ga greenstone belts Late Archean amphibolite-grade granitic gneisses; the Proterozoic remobilization that typifies the Aldan terrane is absent. The Olekma block was thrust under the Aldan block during the ca 1.9 Ga orogeny contemporaneous with, or slightly after the 1.9 Ga charnockitic event in the Aldan. The 1.9 Ga magmatic and granulite event seen in the Aldan is similar in age and character to the Thelon magmatic zone of northern Canada. This correlation allows the development of a preferred construction for the Precambrian Laurentia-Siberia connection in which the present day southern portion of the Siberia platform was connected to the northern margin of Laurentia.

The generation of oxidized CO2-bearing basaltic melts from reduced CH4-bearing upper mantle sources

Redox states of asthenospheric basaltic melts suggest that the asthenosphere is more oxidized than the lithosphere. Theoretical considerations, on the other hand, require the opposite, i.e., that the asthenosphere is more reduced. The implication is that the ƒO2 range of basaltic melts at the Earths surface cannot reflect the ƒO2 range of their mantle sources at depth. We present evidence to show that the asthenosphere is significantly more reduced than the lithosphere. Mantle whose oxygen fugacity is buffered predominantly by ferric-ferrous iron equilibria will experience reduction with increasing depth, due to 1. (1) the negative contributions to free energy from molar volume changes. 2. (2) effects of phase transitions on the chemical potentials of the ferric iron components in solid solution. The extent of reduction per GPa unit pressure increase is modeled to be on the order of 0.6–0.8 log10-bar units in ƒO2 relative to the FMQ buffer. Basaltic melts experience oxidation during melt segregation and decompression relative to their mantle sources. The redox state of a basaltic melt at the Earths surface is roughly proportional to the depth of first mantle melting.

Is water responsible for geophysical anomalies in the deep continental crust? A petrological perspective

Abstract It is common to ascribe conductivity and seismic anomalies in the lower crust to the presence of fluids. We note that fluids cannot be stored long in the lower crust for both mechanical reasons and because at temperatures above ca. 250°C reaction rates are so fast that fluids must be rapidly consumed by hydration reactions. To maintain a “wet” lower crust, therefore, fluid must be continually supplied. There are five sources for such fluids. Gravity-fed meteoric water may percolate many kilometers into the crust, but it cannot move to regions that are hotter than lowermost greenschist conditions because at these temperatures reactions consume water faster that water can be transported. Passive mantle degassing can only take place in areas of active magmatism, because in localities where the lower crust and upper mantle are cold, fluids are incompatible with known mantle mineralogy. Metamorphic devolatilization, evolution of igneous fluids, and tectonic introduction of fluids into the crust are all restricted to areas of active tectonism and are likely to proceed episodically. The fluid flow in the lower crust will vary according to the tectonic environment. In stable cratons fluids would be gravitationally driven but would be able to gain access only to the upper 10 km or so of the crust. Below this the crust would be “dry”. In extensional regimes the fluids would be thermally driven. Carbonic fluids of mantle origin may be present in the lower crust near underplated mantle melts, whereas fluids of meteoric or igneous origin will occur at shallow depths. In compressional regimes fluids may be driven by tectonic or thermal processes and may be of mantle, metamorphic, igneous, or meteoric origin. When considering the causes for lower crustal geophysical anomalies, therefore, one must consider the tectonic regime. Only in areas of active metamorphism or magmatism are fluids likely to play an important role. In cratonal regions the lower crust is “dry”. In such regions enhanced conductivity is likely to be caused by mineral films (graphite, magnetite, or sulfides) and reflectivity by lithologic variations due to either mylonitization or magmatic underplating.

Residual-liquid origin for a monzonitic intrusion in a mid-Proterozoic anorthosite complex: The Sybille intrusion, Laramie anorthosite complex, Wyoming

The Sybille intrusion (≈100 km 2) is one of three large monzonitic intrusions in the 1.43 Ga Laramie anorthosite complex of southeastern Wyoming. The petrographic, geochemical, isotopic, and geophysical characteristics of Sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates (ferrodiorites) and contamination by Archean wall rocks. The exposed part of the intrusion is composed mainly of coarse-grained monzosyenites with abundant alkali feldspar phenocrysts. The monzosyenites preserve mineralogical evidence for high crystallization temperatures (>1000 °C), mid-crustal emplacement pressures (≈3 kbar), relatively reduced crystallization conditions (2 log units below the fayalite + magnetite + quartz [FMQ] oxygen buffer), and they crystallized in the presence of a CO2-rich fluid phase (Fuhrman et al., 1988; Frost and Touret, 1989). The eastern monzosyenites, those adjacent to contemporaneous anorthosite, are distinguished by an anhydrous mineral assemblage (Fo16-Fo8 olivine, high-Ca pyroxene) lacking modal quartz, silica contents of 60 wt%, and smaller Eu anomalies (Eu/Eu* = 1.2 to 1.3). Abundant xenoliths of Archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber. We propose that the monzosyenites represent a relatively thin, 0.5-1.0-km-thick, roof to a magma chamber dominated by dense ferrodioritic cumulates at depth. Extensive, open-system fractionation of a ferrodioritic parent magma, residual after crystallization of anorthosite, produced Fe-enriched monzodioritic and/or monzonitic magma in the upper part of the chamber and complementary Fe- and Ti-rich cumulates in the lower levels. We have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium-crystallization experiments. The presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the Sybille intrusion (Hodge et al., 1973). In the upper parts of the chamber, the fractionated monzodioritic and/or monzonitic magmas eventually became saturated in alkali feldspar. Owing to density contrasts, the alkali feldspar phenocrysts floated to the roof of the chamber, thus producing the exposed porphyritic monzosyenites. In addition, the roof of the chamber was the site of significant melting of Archean gneiss and, locally, metapelite. The Sr and Nd isotopic compositions of the monzosyenites, with Sr isotopic ratios becoming increasingly radiogenic from east ( I Sr = 0.7059 and initial ϵNd = −2.5) to west ( I Sr = 0.7092 and initial ϵNd = −2.6), are consistent with a 5% to 15% addition of Archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks. The stratigraphic and compositional similarity of the Sybille monzosyenites to mangerites in the Bjerkreim-Sokndal intrusion of the Rogaland anorthosite complex, southern Norway, indicates that similar open-system magmatic processes are capable of having produced high-temperature, K-rich monzonitic rocks in other Proterozoic anorthosite complexes.

Highly reducing conditions during Alpine metamorphism of the Malenco peridotite (Sondrio, northern Italy) indicated by mineral paragenesis and H2 in fluid inclusions

During regional metamorphism of the Malenco serpentinized peridotite (Sondrio, northern Italy), the mineral assemblage pentlandite-awaruite-magnetite-native copper-antigorite-brucite-olivine-diopside is formed. The opaque assemblage indicates very reduced fluids with fO2 values 4 log units below QFM. Primary fluid inclusions were trapped in diopside overgrowth, contemporaneous with the opaque assemblage. These metamorphic fluids are saline aqueous solutions (about 10.4 mol% NaCl equivalent) and contain molecular H2 of approximately 1 mol%, as shown by micro-Raman analysis and microthermometry. The fluids are interpreted to have been formed during deserpentinization at the olivine-in isograd under strong reducing conditions.