The significance of metamorphic fluorite in the Adirondacks

Thermodynamic calculations for selected silicate-oxide-fluorite assemblages indicate that several commonly occurring fluorite-bearing assemblages are restricted to relatively narrow fields at constant P?T. The presence of fayalite-ferrohedenbergite-fluorite-quartz ± magnetite and ferrosalite-fluorite-quartz-magnetite assemblages in orthogneisses from Au Sable Forks, Wanakena and Lake Pleasant, New York, buffered fluorine and oxygen fugacities during the granulite facies metamorphism in the Adirondack Highlands. These buffering assemblages restrict$\text{?}{}_{\mathrm{F}}{}_2$ to 10^{?29 ± 1} bar and $\text{?}{}_0{}_2$ to 10^{?16 ± 1} bar at the estimated metamorphic temperature of 1000K and pressure of 7 kbar. The assemblage biotite-magnetite-ilmenite-K-feldspar, found in the same Au Sable Forks outcrop as the fayalite-fluorite-ferrohedenbergite-quartz-magnetitie assemblage, restricts H₂O fugacities to less than 10^3·3 bar. These fugacities limit H₂ and HF fugacities to less than 10¹ bar for the Au Sable outcrop. The data indicate that relative to H₂O, O₂, H₂, F₂ and HF are not major species in the fluid equilibrated with Adirondack orthogneisses. The calculated F₂ fugacilies are similar to the upper limits possible for plagioclase-bearing rocks and probably represent the upper $\text{?}{}_{\mathrm{F}}{}_2$ limit for metamorphism in the Adirondacks and in other granulite facies terranes.     

Genesis of cordierite-gedrite-cummingtonite rocks of the northern portion of the khetri Copper Belt,Rajasthan, India

The cordierite-gedrite-cummingtonite rocks occur in the zone of sulphide mineralization. Field and petrographic evidences suggest formation of these rocks from the $\text{chlorite}\text{±}\text{garnet}$ schists which are associated with them. Time relations between crystallization and deformation, as evident by textural relations, suggest that this transformation constitutes a progressive sequence in time during prograde metamorphism in the area. Bulk chemical compositions of the cordierite-gedrite-cummingtonite rocks plot in the compositional range of the chlorite schists in the AKF, ACF and AFM diagrams. The AFM diagram shows a discontinuity in the topology as revealed by the intersection of the coexisting garnet-chlorite join with the three-phase field of cordierite-gedrite-garnet, suggesting the reaction: $\text{Al-chlorite}\text{+}\text{quartz}\text{(±}\text{garnet}\text{) ?}\text{gedrite}\text{+}\text{cordierite}\text{+}\text{H}{}_2\text{O}$. The reaction took place under conditions of , brought about by dilution of pore fluid by B, Cl, F, S, etc., which reduce the activity of water.     

Volatiles in basalts and andesites from the Galapagos Spreading Center, 85° to 86°W

Glasses from submarine lavas recovered by the ALVIN submersible from the Galapagos Spreading Center (GSC) near 86°W have been analyzed by electron microprobe for major elements and by high-temperature mass spectrometry for volatiles. The samples studied range in composition from basalt to andesite and are more evolved than typical MORBs. Previous studies indicate that they are related to normal MORB by extensive crystal fractionation in small, isolated magma chambers. The H₂O, Cl and F contents of these lavas are substantially higher than any previously reported for MORBs. H₂O, Cl and F abundances increase linearly with P₂O₅ content, which is used as an indicator of the extent of crystal fractionation. The $\text{Fe}{}_2\text{O}{}_3\text{(FeO + Fe}{}_2\text{O}{}_3\text{)}$ ratios measured in the andesite glasses progressively decrease with increasing P₂O₅ content and are probably related to fractionation of Fe-Tioxides. Reduced carbon gas species, principally CH₄ and CO, were discovered in these glasses. The presence of reduced carbon species in GSC glasses may be indicative of a more reduced oxidation state of the upper mantle than is commonly assumed.     

The system pargasite-H2O-CO2: a model for melting of a hydrous mineral with a mixed-volatile fluid—I. Experimental results to 8 kbar

The stability of the amphibole pargasite [NaCa₂Mg₄Al(Al₂Si₆))O₂₂(OH)₂] in the melting range has been determined at total pressures (P) of 1.2 to 8 kbar. The activity of H₂O was controlled independently of P by using mixtures of H₂O + CO₂ in the fluid phase. The mole fraction of H₂O in the fluid ($\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$) ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at constant P. Above P ? 4 kbar stability T increases as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ is decreased below one, passes through a T maximum and then decreases with a further decrease in $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$. This behavior is due to a decrease in the H₂O content of the silicate liquid as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{= 1}$) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.6}$ at P = 5 kbar to $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.4}$ at P = 8 kbar.The H₂O content of liquid coexisting with pargasite has been estimated as a function of at 5 and 8 kbar P, and can be used to estimate the H₂O content of magmas. Because pargasite is stable at low values of $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at high P and T, hornblende can be an important phase in igneous processes even at relatively low H₂O fugacities.     

87Rb-87Sr age of fragments and soils from the lunar sea of fertility

The Luna 16 materials were dated by the Rb-Sr method.An internal isochron age of 3.4 ± 0.2 has been determined for a 6 mg fragment.The Luna 16 total soil is poorer in radiogenic Sr than any other analyzed soil from the Moon. Apollo 14 and 15 soils have also been studied; all of them fall nearly on a 4.65 b.y. isochron with the ADOR initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio.A comparison of the integrated $\text{Rb}\text{Sr}$ of the basalt source region and the $\text{Rb}\text{Sr}$ of the rocks suggests that these basaltic fragments have been generated with only minor $\text{Rb}\text{Sr}$ fractionation.The existence of an old Rb-rich subcrust which contaminated the basalts is also in agreement with the present results.     

Ammonium in biotite from metamorphic and granitic rocks of Japan

Ammonium contents of biotites from metamorphic and granitic rocks of Japan have been determined, and correlated with the ${}^{18}\text{O}{}^{16}\text{O}$ ratios of the rocks.NH₄ contents of biotites averaged 22 ppm in granitic rocks of non-metamorphic terranes, 67 ppm in granitic rocks in the Ryoke metamorphic belt, and 279 ppm in metamorphosed sedimentary rocks of the Ryoke belt. In granitic rocks, enrichment of NH₄ in biotites is a result of the interaction between the granitic magma and surrounding sedimentary rocks. In metasedimentary rocks, the high NH₄ content in biotites is due to inheritance from original organic material in sedimentary rocks.Biotites from migmatites of the Ryoke belt contain more NH₄ (average, 475 ppm) than those from metasedimentary rocks. This suggests the existence of a metamorphic fluid or anatectic magma enriched in NH₄.     

Composition of the Descartes region,lunar highlands

Analytical data for 40 elements are reported for Apollo 16 soils 60601, 61181, 61501, 64801, 67701, 68501, 65701 and breccias 60015, 60017, 60018, 60315, 61016, 61175, 65015 and 66055. The soils are uniform except for the North Ray Crater rim sample which is richer in Al₂O₃.The breccia components show great diversity in composition. Low-K Fra Mauro basalt, Highland basalt (anorthositic gabbro) and plagioclase are important constituents. Medium-K Fra Mauro basalt is an important constituent of breccias 65015 and 60315.The breccias contain many meteorite fragments and high nickel contents, evidence of the early highland bombardment.Most of the refractory elements (REE, Th, U, Zr, Hf, Nb, Ba) show strong positive correlations, interpreted as resulting from mixing. The REE patterns of the breccias show extreme variation relative to chondrites. There is a good inverse correlation between REE and the europium anomaly ($\text{Eu}\text{Eu}{}^{\mathrm{x}}$). The $\text{La}\text{Yb}$ ratio is constant at 3.1 except in plagioclase. Eu depletion or enrichment is interpreted as due to addition or removal of plagioclase.The Cayley and Descartes formations cannot be distinguished chemically and the differences in surface expression are not due to chemical distinctions. They are interpreted as structural differences, related to early highland cratering and mare basin formation.The complex soil and breccia compositions are related to mixing of four components. These are Low-K Fra Mauro basalt, Highland basalt (anorthositic gabbro) and subordinate plagioclase and Medium-K Fra Mauro basalt. These compositions have been used in a computer program (PETMIX III) to provide fits for the analytical data in terms of the end-members.An average highland composition is proposed, based on the Apollo 15 and 16 orbital data for Si, Al, Mg and Th. Abundances for most other elements are derived from the interelement relationships and correlations, and checked by the mixing program.The resulting composition consists of 69 per cent Highland basalt (anorthositic gabbro) and 31 per cent Low-K Fra Mauro basalt. There is no significant Eu anomaly. The abundances are: SiO₂: 45.2 per cent; TiO₂: 0.68 per cent; Al₂O₃: 24.9 per cent; FeO: 6.3 per cent; MgO: 8.5 per cent; CaO: 13.8 per cent; Na₂O: 0.4 per cent; K₂O: 0.11 per cent; Cr₂O₃: 0.11 per cent; Ba: 144 ppm; Th: 1.8 ppm; U: 0.46 ppm; Pb: 1.6 ppm; Zr: 156 ppm; Hf: 3.2 ppm; Nb: 10.8 ppm; Y: 32 ppm; ΣREE: 85 ppm.     

Kiglapait Geochemistry III: Potassium and Rubidium

The regular geometry and completeness of the Kiglapait intrusion permit its bulk composition to be obtained by summation, and the composition of successive liquids to be obtained by subtraction. The summations for K and Rb give 1806 and 1.08 ppm, yielding Rfrsol|K/Rb= 1670 for the intrusion, taken as equal to the parent magma. R increases slightly from this initial value to 2000 at the end of crystallization where MgO approaches zero in the rocks. K and Rb are therefore closely coherent and their distribution coefficients can differ only by a small amount in the Kiglapait system.Apparent feldspar/liquid distribution coefficients (D^F/L) can be estimated from detailed plots of feldspar and liquid compositions against F_L. The Kiglapait data imply that these coefficients are linear 1:1 functions of plagioclase composition within experimental error, having values given by D_KF/L = 1.42? X_AnD_RbF/L = 1.13? X_An with minimum values of 0.75 and 0.49, respectively. The ratio $\text{R}{}^{\mathrm{F}}\text{R}{}^{\mathrm{L}}$ lies in the range of 1.53± 0.03 for the plagioclase composition range X_An= 0.34 to 0.67 showing that high-R rocks such as anorthosite crystallized from high-R liquids.The apparent feldspar distribution coefficients are much closer to 1.0 than common literature values. They can be reduced by assuming that the cumulate pile was continuously recharged by the circulating magma until an advanced stage of differentiation was reached, and assuming that alkalies were exchanged to the feldspars from the magma. When such an ‘aquifer recharge’ model is calibrated using olivine-liquid equilibria as a time marker for the liquid, the inferred minimum equilibrium values of the distribution coefficients are $\text{D}{}_{\mathrm{K}}\text{F}\text{L}$= 0.42, $\text{D}{}_{\mathrm{Rb}}\text{F}\text{L}$ = 0.25 at the base of the intrusion. Their variation is given by $\text{D}{}_{\mathrm{K}}\text{F}\text{L}\text{= 1.66?1.88X}{}_{\mathrm{An}}$, $\text{D}{}_{\mathrm{Rb}}\text{F}\text{L}\text{= 1.17?1.41X}{}_{\mathrm{An}}$, The equilibrium values are considered to be appropriate for deducing liquid compositions in plutonic bodies where alkali exchange can be shown or inferred to have been inhibited, such as in small intrusions. The apparent values are considered to be appropriate, even though they may be artificial, for large intrusions similar to the Kiglapait.The bulk K and Rb concentrations in the Kiglapait intrusion are consistent with a plagioclase-rich abyssal tholeiite magma. Clinopyroxene and olivine fractionation in the mantle may contribute to the production of such high-Rmagmas.     

Pressure sensitive “silica geothermometer” determined from quartz solubility experiments at 250 °C

Experimentally reversed quartz solubilities at 250°C and at 250, 500 and 1000 bars yield values of the logarithm of the molality of aqueous silica of ?2.126, ?2.087 and ?2.038, respectively. Extrapolation of quartz solubility to the saturation pressure of water at 250°C results in a log molality of aqueous silica of-2.168. These solubility determinations and analyses of fluid pressures in geothermal systems indicate that pressure is significant when calculating quartz equilibrium temperatures from silica concentrations in waters of deep thermal reservoirs.The results of this investigation, combined with other reported quartz solubility measurements, yielded a pressure-sensitive “silica geothermometer” for fluids that have undergone adiabatic steam loss of where P is the fluid pressure in bars and m_Si(OH)₄ · 2H₂O represents the molality of aqueous silica measured in surface samples. The geothermometer is applicable to solutions in equilibrium with quartz from 180°C to 340°C and fluid pressures from H₂O saturation to 500 bars.     

Volatiles in submarine volcanic rocks from the Mariana Island arc and trough

High temperature mass spectrometric analyses of glasses from quenched pillow rims of andesites dredged from 1170 m water depth in the northern portion of the Mariana Island arc indicate substantially less H₂O (~ 1 wt.%) and more CO₂ (~ 0.24 wt.%) than previously reported for volcanic arc rocks. Glass-vapor inclusions within plagioclase phenocrysts from quenched rims have $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios of 1:1. These results are similar to analyses of basaltic samples from the Mariana Trough (a back-arc basin). Generally, F and Cl contents are higher and S lower in the arc rocks compared to the samples from the back-arc basin. These results favor models for the production of island arc magmas which involve melting of the subducted slab, rather than just melting of the overlying mantle wedge because of the high volatile content needed to produce island arc magmas from peridotite (10–15 wt.%). The trough samples, although similar in non-volatile composition to mid-ocean ridge rocks, have much higher H₂O. somewhat higher CO₂ and lower S contents. Either near surface addition of voiatiles has enriched the magmas or H₂O must be a more important component in the generation and evolution of back-arc basin lavas than in the genesis of mid-ocean ridge basalts.     

Petrochemical constraints on the models of Cretaceous-Eocene tectonic evolution of the Eastern Pontic chain (Turkey)

Sixteen selected samples from the Upper Cretaceous volcanic belt of the Eastern Pontids have been analysed for major elements, Rb, Sr and Zr. On the basis of the K₂O versus SiO₂ distribution, two groups of rocks have been distinguished, one with calc-alkaline affinity and a second group with shoshonitic character. The calc-alkaline rocks have porphyritic texture with clinopyroxene, plagioclase and orthopyroxene as phenocryst and in the groundmass. The orthopyroxene is lacking in the shoshonites where plagioclase, clinopyroxene and, in the more evolved terms, amphibole and biotite are the main phenocryst minerals. The shoshonitic rocks have higher $\text{K}{}_2\text{O}\text{Na}{}_2\text{O}$ ratio, K₂O, P₂O₅ and Rb, contents with respect to the calc-alkaline samples. The TiO₂ content is invariably low, never exceeding approximately 1%. The occurrence of volcanic rocks ranging in composition from calc-alkaline to shoshonitic in the Upper Cretaceous volcanic belt of the Eastern Pontids suggests that the Upper Cretaceous volcanic cycle reached its mature stage before the onset of the Eocene calc-alkaline volcanism which is believed to be neither genetically nor tectonically related with the Upper Cretaceous volcanism.     

An experimental study of alkali metal distributions in feldspars and micas

K and Rb distributions between aqueous alkali chloride vapour phase (0.7 molar) and coexisting phlogopites and sanidines have been investigated in the range 500 to 800°C at 2000 kg/cm² total pressure.Complete solid solution of RbMg₃AlSi₃O₁₀(OH)₂ in KMg₃AlSi₃O₁₀(OH)₂ exists at and above 700°C. At 500°C a possible miscibility gap between approximately 0.2 and 0.6 mole fraction of the Rb end-member is indicated.Only limited solid solution of Rb AlSi₃O₈ in KAlSi₃O₈ has been found at all temperatures investigated.Distribution coefficients, expressed as $\text{K}{}_{\mathrm{d}}\text{= (}\text{Rb/K}\text{)}$ in solid/(Rb/K) in vapour, are appreciably temperature-dependent but at each temperature are independent of composition for low Rb end-member mole fractions in the solids. The determined $\text{K}{}_{\mathrm{D}}$ values and their approximate Rb end-member mole fraction ($\text{X}{}_{\mathrm{RM}}$) ranges of constancy are summarized as follows: (°C)$\text{T}$$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Phlog/Vap.}}$$\text{X}{}_{\mathrm{RM}}$$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Sandi/Vap.}}$$\text{X}{}_{\mathrm{rm}}$

     

13.

A systematic deviation from Na-K-Ca geothermometer below 75°C and above 10−4 atm Pco2     

Tomáš Pačes 《Geochimica et cosmochimica acta》1975,39(4):541-544

If the temperature of ground water is below 75°C and the partial pressure of CO₂ in the aquifer is above 10^?4 atm, a chemical steady-state between water and felsic rocks (rather than chemical equilibrium) may be maintained. The temperature of water in the aquifer may be estimated using a modified form of the Na-K-Ca geothermometer from, . where the departure of the steady-state from equilibrium, $\text{I}$, is a function of : .     

14.

Luna 20: mineral chemistry of spinel,pleonaste, chromite,ulvöspinel,ilmenite and rutile     

Stephen E Haggerty 《Geochimica et cosmochimica acta》1973,37(4):857-867

Electron microprobe analyses of the spinel mineral group, ilmenite and rutile have been carried out on part of the Luna 20 soil sample. The spinel group shows an almost continuous trend from MgAl₈O₄ to FeCr₂O₄ and a discontinuous trend from FeCr₂O₄ to Fe₂TiO₄. Well defined non-linear relationships exist within the spinel group for Fe-Mg substitution, for divalent ($\text{FeO}\text{FeO + MgO}$) versus trivalent ($\text{Cr}{}_2\text{O}{}_3\text{Cr}{}_2\text{O}{}_3\text{+ A1}{}_2\text{O}{}_3$), and for divalent versus $\text{TiO}{}_2\text{TiO}{}_2\text{+ A1}{}_2\text{O}{}_3\text{+ Cr}{}_2\text{O}{}_3$. For Cr-Al substitution the relationship is linear and is negative for Mg-rich spinel and positive for Fe-Ti rich spinel. In general a combination of aluminous-rich chromite and ulvöspinel in the Luna 16 samples, combined with the chromian-pleonaste in Apollo 14 define comparable major compositional trends to those observed in Luna 20. Ilmenite is present in trace amounts. It is exsolved from pleonaste and pyroxene, is present in subsolidusreduced ulvöspinel and has undergone reequilibration to produce oriented intergrowths of chromite + rutile. Primary ilmenite is among the most magnesian-rieh (6 wt.% MgO) yet found in the lunar samples. The high MgO, inferred high Cr₂O₃ concentrations and the iron content of rutile (2.5 wt.% FeO) suggest crystallization at high temperatures and pressures for some components of the Luna 20 soil.     

15.

Changes in the content and composition of pedogenic iron oxyhydroxides in a chronosequence of soils in southern California     

Leslie D. McFadden  David M. Hendricks 《Quaternary Research》1985,23(2):189-204

Studies of the pedogenic iron oxyhydroxides in suites of latest Holocene to middle Pleistocene soils formed on fluvial deposits of the transverse ranges, southern California, indicate that the content and composition of iron oxyhydroxide change in a systematic manner. Analysis of total secondary free iron oxides (dithionite extractable, Fe₂O₃d) and ferrihydrite (oxalate extractable, Fe₂O₃o) shows that (1) a single-logarithmic model (Y = a + b log X) or double logarithmic model (log Y = a + b log X), where Y is the total mass of pedogenic Fe oxides (g/cm²-soil column) and X is soil age, describes the rate of increase in Fe₂O₃d with time; (2) the Fe₂O₃d content correlates linearly with soil reddening and clay content; (3) the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio, which indicates the degree of Fe oxide crystallinity, is moderately high to very high (0.22–0.58) in middle Holocene to latest Pleistocene soils and progressively decreases to less than 0.10 in older soils; (4) the value of the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio also appears to be infuenced by climate; and (5) temporal changes in Fe oxide content and mineralogy are accompanied by related, systematic changes in clay mineralogy and organic matter content. These relationships are attributed to a soil environment that must initially favor ferrihydrite precipitation and/or organic matter-Fe complexation. Subsequent transformation to hematite causes increasingly intense reddening and a concomitant decrease in the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio. The results demonstrate that iron oxide analysis is useful for numerical age studies of noncalcic soils and shows potential as an indicator of paleoclimates.     

16.

Influences of kinetics and mechanism in metamorphism: a study of albite crystallization     

Alan Matthews 《Geochimica et cosmochimica acta》1980,44(3):387-402

     

17.

Thermodynamic analyses of equilibria involving olivine,orthopyroxene and garnet     

Toshisuke Kawasaki  Yoshito Matsui 《Geochimica et cosmochimica acta》1983,47(10):1661-1679

     

18.

M and Mg values in igneous rocks: proposed usage and a comment on currently employed Fe2O3 corrections     

C.J. Hughes  E.M. Hussey 《Geochimica et cosmochimica acta》1976,40(4):485-486

It is proposed that the ‘M value’ of an igneous rock should be 100Mg/(Mg + ΣFe) and that the ‘Mg value’ should be 100Mg/(Mg + Fe²⁺). A plea is made to standardize any necessary corrections for Fe₂O₃ so that $\text{Fe}{}_2\text{O}{}_3\text{(Fe}{}_2\text{O}{}_3\text{+ FeO)}\text{= 02}$ for basic rocks.     

19.

Uptake of fluoride by aragonite     

M. Ichikuni 《Chemical Geology》1979,27(3):207-214

The uptake of F by aragonite is attributed to the ion-exchange process, in which one CO₃^2? ion in the structure is replaced by two F^? ions. Under the equilibrium condition at 15° C and 1 atm., the partition of F between aragonite and aqueous solution is described by:

$\text{log}\text{(}\text{[}\text{F}\text{]}\text{a}{}_{\mathrm{<mtext>F</mtext>}}\text{)=1.95 + 0.54}\text{log}\text{a}{}_{\mathrm{<mtext>Ca</mtext>}}$

were [F] denotes the F content of aragonite in mol/g, and a_F and a_Ca are the aqueous activities of F^? and Ca²⁺, respectively. The equation was successfully applied to estimating the F content of marine aragonite.     

20.

The role of CO₂ in the chemical modification of deep continental crust     

William E. Glassley 《Geochimica et cosmochimica acta》1983,47(3):597-616

Compositional differences between granulite facies rocks and equivalent amphibolite facies rocks and the observation of CO₂-rich fluid inclusions in granulites, have led to the suggestion that CO₂ must play a role in modifying the composition of deep continental crust. How CO₂ effects this change has remained unclear. Using the thermodynamic properties of aqueous ions in a fluid of evolving $\text{CO}{}_2\text{H}{}_2\text{O}$ ratio, it is possible to model the incongruent dissolution of feldspars under conditions appropriate for granulite facies metamorphism. The results demonstrate that dissolution will be strongly enhanced at high $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios, with ion solubilities being Na⁺ >K⁺ ? Ca⁺⁺. This enhancement is compatible with the reported compositional contrasts between granulite and amphibolite facies rock, but requires large fluid volumes.To test the dissolution model, a detailed field and petrologic study was conducted in a well exposed granulite facies terrane in West Greenland. Strong correlation between fluid composition and bulk rock chemistry can be documented; CO₂-rich regions contain rocks which consistently have low ratios, while H₂O-rich regions consistently have high ratios. Magnetite rims on sulfide grains are ubiquitous in high regions and are absent in high regions, and they provide evidence that CO₂ was introduced into the region. These correlations and observations are predictable from the properties of the dissolution process. These considerations, along with observations regarding graphite petrogenesis, provide strong arguments that the total fluid volume interacting with the rock during metamorphism was very large, in some cases equaling or exceeding total rock volume. Such large fluid volumes can lead to significant compositional modification of the crust, and will mask the original protolith chemistry. Such processes should lead to Ca- and Al-enriched, Na-, K-, S- and Si-depleted residues in the deep crust.     

(°C)T	$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Phlog/Vap.}}$	$\text{X}{}_{\mathrm{RM}}$	$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Sanid/Vap.}}$	$\text{X}{}_{\mathrm{RM}}$
500	$\text{0.64 ± 0.11}$	0–0.2	$\text{0.17 ± 0.04}$	0–0.07
700	$\text{1.11 ± 0.11}$	0–0.2	$\text{0.33 ± 0.04}$	0–0.1
800	$\text{1.28 ± 0.03}$	0–0.2	$\text{0.45 ± 0.06}$	0–0.1

A systematic deviation from Na-K-Ca geothermometer below 75°C and above 10−4 atm Pco2

If the temperature of ground water is below 75°C and the partial pressure of CO₂ in the aquifer is above 10^?4 atm, a chemical steady-state between water and felsic rocks (rather than chemical equilibrium) may be maintained. The temperature of water in the aquifer may be estimated using a modified form of the Na-K-Ca geothermometer from, . where the departure of the steady-state from equilibrium, $\text{I}$, is a function of : .     

Luna 20: mineral chemistry of spinel,pleonaste, chromite,ulvöspinel,ilmenite and rutile

Electron microprobe analyses of the spinel mineral group, ilmenite and rutile have been carried out on part of the Luna 20 soil sample. The spinel group shows an almost continuous trend from MgAl₈O₄ to FeCr₂O₄ and a discontinuous trend from FeCr₂O₄ to Fe₂TiO₄. Well defined non-linear relationships exist within the spinel group for Fe-Mg substitution, for divalent ($\text{FeO}\text{FeO + MgO}$) versus trivalent ($\text{Cr}{}_2\text{O}{}_3\text{Cr}{}_2\text{O}{}_3\text{+ A1}{}_2\text{O}{}_3$), and for divalent versus $\text{TiO}{}_2\text{TiO}{}_2\text{+ A1}{}_2\text{O}{}_3\text{+ Cr}{}_2\text{O}{}_3$. For Cr-Al substitution the relationship is linear and is negative for Mg-rich spinel and positive for Fe-Ti rich spinel. In general a combination of aluminous-rich chromite and ulvöspinel in the Luna 16 samples, combined with the chromian-pleonaste in Apollo 14 define comparable major compositional trends to those observed in Luna 20. Ilmenite is present in trace amounts. It is exsolved from pleonaste and pyroxene, is present in subsolidusreduced ulvöspinel and has undergone reequilibration to produce oriented intergrowths of chromite + rutile. Primary ilmenite is among the most magnesian-rieh (6 wt.% MgO) yet found in the lunar samples. The high MgO, inferred high Cr₂O₃ concentrations and the iron content of rutile (2.5 wt.% FeO) suggest crystallization at high temperatures and pressures for some components of the Luna 20 soil.     

Changes in the content and composition of pedogenic iron oxyhydroxides in a chronosequence of soils in southern California

Studies of the pedogenic iron oxyhydroxides in suites of latest Holocene to middle Pleistocene soils formed on fluvial deposits of the transverse ranges, southern California, indicate that the content and composition of iron oxyhydroxide change in a systematic manner. Analysis of total secondary free iron oxides (dithionite extractable, Fe₂O₃d) and ferrihydrite (oxalate extractable, Fe₂O₃o) shows that (1) a single-logarithmic model (Y = a + b log X) or double logarithmic model (log Y = a + b log X), where Y is the total mass of pedogenic Fe oxides (g/cm²-soil column) and X is soil age, describes the rate of increase in Fe₂O₃d with time; (2) the Fe₂O₃d content correlates linearly with soil reddening and clay content; (3) the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio, which indicates the degree of Fe oxide crystallinity, is moderately high to very high (0.22–0.58) in middle Holocene to latest Pleistocene soils and progressively decreases to less than 0.10 in older soils; (4) the value of the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio also appears to be infuenced by climate; and (5) temporal changes in Fe oxide content and mineralogy are accompanied by related, systematic changes in clay mineralogy and organic matter content. These relationships are attributed to a soil environment that must initially favor ferrihydrite precipitation and/or organic matter-Fe complexation. Subsequent transformation to hematite causes increasingly intense reddening and a concomitant decrease in the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio. The results demonstrate that iron oxide analysis is useful for numerical age studies of noncalcic soils and shows potential as an indicator of paleoclimates.     

M and Mg values in igneous rocks: proposed usage and a comment on currently employed Fe2O3 corrections

It is proposed that the ‘M value’ of an igneous rock should be 100Mg/(Mg + ΣFe) and that the ‘Mg value’ should be 100Mg/(Mg + Fe²⁺). A plea is made to standardize any necessary corrections for Fe₂O₃ so that $\text{Fe}{}_2\text{O}{}_3\text{(Fe}{}_2\text{O}{}_3\text{+ FeO)}\text{= 02}$ for basic rocks.     

Uptake of fluoride by aragonite

The uptake of F by aragonite is attributed to the ion-exchange process, in which one CO₃^2? ion in the structure is replaced by two F^? ions. Under the equilibrium condition at 15° C and 1 atm., the partition of F between aragonite and aqueous solution is described by:

$\text{log}\text{(}\text{[}\text{F}\text{]}\text{a}{}_{\mathrm{<mtext>F</mtext>}}\text{)=1.95 + 0.54}\text{log}\text{a}{}_{\mathrm{<mtext>Ca</mtext>}}$

were [F] denotes the F content of aragonite in mol/g, and a_F and a_Ca are the aqueous activities of F^? and Ca²⁺, respectively. The equation was successfully applied to estimating the F content of marine aragonite.     

The role of CO2 in the chemical modification of deep continental crust

Compositional differences between granulite facies rocks and equivalent amphibolite facies rocks and the observation of CO₂-rich fluid inclusions in granulites, have led to the suggestion that CO₂ must play a role in modifying the composition of deep continental crust. How CO₂ effects this change has remained unclear. Using the thermodynamic properties of aqueous ions in a fluid of evolving $\text{CO}{}_2\text{H}{}_2\text{O}$ ratio, it is possible to model the incongruent dissolution of feldspars under conditions appropriate for granulite facies metamorphism. The results demonstrate that dissolution will be strongly enhanced at high $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios, with ion solubilities being Na⁺ >K⁺ ? Ca⁺⁺. This enhancement is compatible with the reported compositional contrasts between granulite and amphibolite facies rock, but requires large fluid volumes.To test the dissolution model, a detailed field and petrologic study was conducted in a well exposed granulite facies terrane in West Greenland. Strong correlation between fluid composition and bulk rock chemistry can be documented; CO₂-rich regions contain rocks which consistently have low ratios, while H₂O-rich regions consistently have high ratios. Magnetite rims on sulfide grains are ubiquitous in high regions and are absent in high regions, and they provide evidence that CO₂ was introduced into the region. These correlations and observations are predictable from the properties of the dissolution process. These considerations, along with observations regarding graphite petrogenesis, provide strong arguments that the total fluid volume interacting with the rock during metamorphism was very large, in some cases equaling or exceeding total rock volume. Such large fluid volumes can lead to significant compositional modification of the crust, and will mask the original protolith chemistry. Such processes should lead to Ca- and Al-enriched, Na-, K-, S- and Si-depleted residues in the deep crust.