Journal of Petrology

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (2) Request Permissions Google Scholar Articles by LATTARD, D. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

Journal of Petrology Volume 42 Number 4 Pages 847-851 2001
© Oxford University Press 2001

Comment on ‘Mexican Peridotite Xenoliths and Tectonic Terranes: Correlations among Vent Location, Texture, Temperature, Pressure, and Oxygen Fugacity’ by J. F. Luhr & J. J. Aranda-Gomez (1997)

DOMINIQUE LATTARD^,*

MINERALOGISCHES INSTITUT DER UNIVERSITÄT, INF 236, D-69120 HEIDELBERG, GERMANY

Received August 1, 1999; Revised typescript accepted June 26, 2000

	ABSTRACT

TOP ABSTRACT INTRODUCTION SOME REMINDERS ABOUT {Delta}FMQ DIFFERENT ESTIMATES OF... THE PRESSURE DEPENDENCE OF... THE SIGNIFICANCE OF RELATIVE... REFERENCES

 
This comment addresses the interpretation of oxygen fugacity data for spinel peridotite xenoliths from five Mexican volcanic fields presented by Luhr & Aranda-Gomez (Journal of Petrology, 38, 1075–1112, 1997). The postulated east–west increase of the FMQ (‘relative oxygen fugacity’, where FMQ is fayalite–magnetite–quartz) values is inherent to the method and therefore of questionable geological significance. Increases in FMQ do not necessarily mirror oxidation processes in the mantle controlled by subduction-related fluids.

KEY WORDS: mantle metasomatism; Mexico; peridotite xenoliths; relative oxygen fugacity

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SOME REMINDERS ABOUT {Delta}FMQ DIFFERENT ESTIMATES OF... THE PRESSURE DEPENDENCE OF... THE SIGNIFICANCE OF RELATIVE... REFERENCES

 
Luhr & Aranda-Gomez (1997) presented a study of 26 spinel peridotite xenoliths from five Mexican volcanic fields, disposed in a 1700 km SE–NW profile across central Mexico that spans four tectonostratigraphic terranes of the region. This comment addresses the interpretation of the oxygen fugacity data of Luhr & Aranda-Gomez (1997). In comparing the FMQ values of all xenoliths from the five volcanic fields, the authors described a systematic increase from east to west, which they interpreted as ‘a consequence of subduction-related oxidation of the Mexican lithospheric mantle, perhaps a result of eastward descent of the Farallon Plate beneath the Mesozoic and Tertiary’. In this discussion I show that the east–west increase of FMQ values is inherent to the method, and therefore of questionable geological significance. Variations in FMQ among xenoliths do not necessarily mirror oxidation or reduction processes in the mantle, and subduction-related fluids need not increase FMQ. The paper of Luhr & Aranda-Gomez (1997) is a good example of a tendency to overinterpret oxygen fugacity results for mantle xenoliths.

	SOME REMINDERS ABOUT FMQ

TOP ABSTRACT INTRODUCTION SOME REMINDERS ABOUT {Delta}FMQ DIFFERENT ESTIMATES OF... THE PRESSURE DEPENDENCE OF... THE SIGNIFICANCE OF RELATIVE... REFERENCES

 
The olivine–orthopyroxene–spinel oxygen barometer that Luhr & Aranda-Gomez (1997) used to estimate the oxygen fugacity of the Mexican xenoliths they studied is based on the fayalite–ferrosilite–magnetite (‘FFsM’) oxygen buffer equilibrium:

Oxygen fugacity can be calculated using the equation

in which H°_T,FFsM and S°_T,FFsM are the enthalpy and entropy differences of the FFsM reaction at P° = 1 bar and temperature T, and V_T,FFsM refers to the volume difference of the solids at this temperature. In practice, several equations, based on theoretical models and on experimental results, are substituted. Two of these, the versions of Wood et al. (1990) and of Ballhaus et al. (1991) have been utilized by Luhr & Aranda-Gomez (1997). The volume term [equation (2)], however, is identical in all formulations and generally simplified to

assuming zero thermal expansion and compressibility terms.

Because the strong temperature dependence of log fO₂ is similar for most oxygen buffer equilibria, it can be successfully minimized by normalizing to the fayalite–magnetite–quartz (FMQ) equilibrium, to yield the so-called ‘relative oxygen fugacity’ or ‘FMQ’ values (e.g. Ohmoto & Kerrick, 1977; Frost et al., 1988), defined in the case of the FFsM equilibrium as FMQ_FFsM = log fO₂(FFsM) –log fO₂(FMQ).

	DIFFERENT ESTIMATES OF FMQFFsM

TOP ABSTRACT INTRODUCTION SOME REMINDERS ABOUT {Delta}FMQ DIFFERENT ESTIMATES OF... THE PRESSURE DEPENDENCE OF... THE SIGNIFICANCE OF RELATIVE... REFERENCES

 
Luhr & Aranda-Gomez (1997) noted that the FMQ_FFsM values calculated using the method of Ballhaus et al. (1991) are systematically lower than those from the calibration of Wood et al. (1990). In view of the good correlation between the two sets of values (Fig. 1a), the authors considered only those obtained from the Wood et al. (1990) method. However, the slope of the correlation line between the two datasets differs significantly from unity and the deviation between the two sets increases with decreasing FMQ_FFsM (Fig. 1a). The mean difference is 0·8 log units, but at low fO₂ the estimates can be as much as 2·2 log units apart (Fig. 1b). Consequently, the FMQ_FFsM values calculated following Ballhaus et al. (1991) yield a much larger range (Fig. 1a) than those obtained with the Wood et al. (1990) method. The difference between the results of the two estimation methods increases as the Fe³⁺/Fe ratio in spinel decreases (Fig. 1b). This is related to the fact that the Fe³⁺ contents in spinel are estimated differently for the two fO₂ calculation methods. Ballhaus et al. (1991) recommended calculating the Fe³⁺/Fe ratio in spinel assuming stoichiometry. In contrast, the estimates with the Wood et al. (1990) method employed corrected Fe³⁺/Fe [see table 6 of Luhr & Aranda-Gomez (1997)], but not according to the usual method (Wood & Virgo, 1989). Luhr & Aranda-Gomez (1997) give only a vague description of their correction procedure and do not demonstrate that it yields improved values. In any case, there are large uncertainties on the resulting Fe³⁺/Fe values, especially with Fe³⁺-poor spinels. This is critical because it has been shown (Parkinson & Arculus, 1999) that even a small relative error of ±1·5% in the R₂O₃ contents of Fe³⁺-poor spinels propagates to a wide range in error of FMQ_FFsM (up to ±2 log units).

View larger version (19K): [in this window] [in a new window]   Fig. 1. (a) The FMQFFsM values determined using Ballhaus et al. [1991; FMQ(B)] compared with those of Wood et al. [1990; FMQ(W)]. Despite a good correlation (R = 0·94) between the two datasets, the deviation increases with decreasing FMQFFsM. (b) Inverse correlation of the differences between the two estimates and the Fe3+/Fe values in spinel (determined by stoichiometry). The estimates deviate most for spinels with low Fe3+ (symbols surrounded by a shaded curve). Abbreviations for volcanic fields: SQ, San Quintin; MC, Mesa Cacaxta; DGO, Durango; VES, Ventura–Espiritu Santo; SD, Santo Domingo.

 
Consequently, the differences between the FMQ_FFsM values registered in the different volcanic fields may not be as significant as suggested by Luhr & Aranda-Gomez (1997).

	THE PRESSURE DEPENDENCE OF FMQFFsM

TOP ABSTRACT INTRODUCTION SOME REMINDERS ABOUT {Delta}FMQ DIFFERENT ESTIMATES OF... THE PRESSURE DEPENDENCE OF... THE SIGNIFICANCE OF RELATIVE... REFERENCES

 
Although the use of FMQ-normalized log fO₂ values for the FFsM dampens the temperature effect, it does not compensate for pressure dependence, because V_FFsM is about half of V_FMQ [equation (2); Fig. 2a]. Thus FMQ_FFsM decreases by 0·4 log unit per 1 GPa. It would be better to normalize to the nickel–bunsenite equilibrium (NNO_FFsM), because in contrast to FMQ_FFsM, NNO_FFsM displays a negligible pressure dependence (Fig. 2a).

View larger version (21K): [in this window] [in a new window]   Fig. 2. (a) Dependence of log fO2 upon pressure at T = 1000°C of four oxygen buffers: NNO (dashed lines), FMQ (with - or ß-quartz), and FFsM. The numbers plotted along the lines are the Vs of the respective buffer reactions. Molar volumes were assumed to be constant (values at 298·15 K; from Berman, 1988). fO2 calculated with the GE0-CALC software (Brown et al., 1988). (b) FMQFFsM and (c) NNOFFsM values for peridotite xenoliths from five Mexican volcanic fields, plotted as a function of pressure. The FMQFFsM values were retrieved by Luhr & Aranda-Gomez (1997) using the method of Wood et al. (1990); the NNOFFsM data were calculated from log fO2(FFsM) based upon Wood et al. (1990) (see Luhr & Aranda-Gomez, 1997, table 10); log fO2(NNO) values were obtained from the equation of O’Neill (1987).

 
In the literature, the pressure dependence of FMQ_FFsM is typically dismissed in view of the restricted pressure range of the spinel-lherzolite facies (e.g. Wood et al., 1990; Woodland et al., 1992). Pressure estimates are not made because of their large uncertainties and the FMQ_FFsM values are calculated with a fixed pressure (typically 1·5 GPa; e.g. Wood et al., 1990; Woodland et al., 1992; Ballhaus, 1993).

The dataset of Luhr & Aranda-Gomez (1997) provides a good opportunity to test the pressure dependence of FMQ_FFsM, because it includes pressure estimates (after Köhler & Brey, 1990) of spinel peridotites that apparently represent widely different depths of the lithosphere. The FMQ_FFsM values of the Mexican xenoliths obtained with the method of Wood et al. (1990) display a slight inverse correlation with pressure (Fig. 2b), which is not seen for corresponding NNO_FFsM values (Fig. 2c). The strongest pressure effects are shown by the low-pressure xenoliths from the San Quintin field.

The pressure dependence of FMQ_FFsM is responsible for the slight westward increase of the mean values of FMQ_FFsM retrieved with the Wood et al. (1990) method for the four western Mexican volcanic fields (SQ, MC, DGO, VES; Fig. 3a) because this increase is not reproduced using NNO_FFsM values (Fig. 3b). Using the log fO₂ estimates of Ballhaus et al. (1991), neither the mean FMQ_FFsM values nor the mean NNO_FFsM values vary systematically among these volcanic fields (Fig. 3c and d). Only the mean value of relative oxygen fugacity for the easternmost Santo Domingo field (SD) is lower than the other three, a feature that is independent of the estimation and normalization methods. However, the small number of samples and the large uncertainties of low oxygen fugacity values retrieved using spinels of very low Fe³⁺ contents suggest caution in interpreting the results for the Santo Domingo field. In conclusion, the data of Luhr & Aranda-Gomez (1997) do not sustain the systematic increase of the relative oxygen fugacity from east to west that they proposed.

View larger version (32K): [in this window] [in a new window]   Fig. 3. Comparison between the means () and standard deviations (±1) of the FMQFFsM (a, c) and NNOFFsM (b, d) values for the peridotites of each Mexican volcanic field (except the MC field), using either the calibration of Wood et al. (1990) (a, b), or that of Ballhaus et al. (1991) (c, d). In the case of the MC field, the values for the single xenolith are represented by open circles. The normalization to NNO is explained in the caption for Fig. 2. Positions along the x-axis correspond to approximate distances from the proposed Mesozoic–Cenozoic palaeotrench, with Baja California restored to its pre-rift configuration [compare fig. 9 of Luhr & Aranda-Gomez (1997)]. Abbreviations are as in Fig. 1.

 
Given the large uncertainties in pressure estimates for spinel lherzolites, which show an average 1 of ±4·5 kbar with the Ca-exchange olivine–clinopyroxene geobarometer of Köhler & Brey (1990), the oxygen fugacities estimated with the FFsM equlibrium should be normalized to the NNO, not to the FMQ equilibrium.

	THE SIGNIFICANCE OF RELATIVE OXYGEN FUGACITY VALUES REGARDING THE OXIDATION STATE OF THE MANTLE

TOP ABSTRACT INTRODUCTION SOME REMINDERS ABOUT {Delta}FMQ DIFFERENT ESTIMATES OF... THE PRESSURE DEPENDENCE OF... THE SIGNIFICANCE OF RELATIVE... REFERENCES

 
Luhr & Aranda-Gomez (1997) interpret the increase in FMQ_FFsM that they calculated to reflect oxidation related to subduction. However, there is no reason to expect a clear correlation between FMQ_FFsM and the oxidation state of peridotites. The oxygen fugacity (an intensive variable) quantifies the partitioning of Fe³⁺ and Fe²⁺ between coexisting mineral phases [equation (2)] and has no direct relation to the bulk Fe³⁺/Fe (an extensive variable). Canil et al. (1994) and Canil & O’Neill (1996), who have studied the distribution of Fe³⁺ in upper-mantle assemblages using Mössbauer spectra of minerals, have concluded that ‘the complex nature of the partitioning of Fe³⁺ between mantle phases results in complicated patterns of the activities of the Fe³⁺-bearing components, and thus in calculated equilibrium fO₂, which show little correlation with whole-rock Fe³⁺’ (Canil & O’Neill, 1996).

Luhr & Aranda-Gomez (1997) also wonder, as have others (e.g. Ionov & Wood, 1992; Brandon & Draper, 1996), why there is no inverse correlation between indicators of the refractory nature and FMQ_FFsM, although residual peridotites should have lost most of their Fe³⁺ to the melt they produced. To explain this, they invoke an overprint though oxidizing, subduction-related fluids, as has already been proposed by Wood & Virgo (1989), Ionov & Wood (1992) or Brandon & Draper (1996) for other mantle regions.

An oxidizing metasomatic overprint should increase whole-rock Fe³⁺/Fe. However, all available reliable (Mössbauer based) values from metasomatized xenoliths (five samples; Canil et al., 1994; Brandon & Draper, 1996) fall within the normal range for spinel peridotite and garnet peridotite. In contrast, the analyses of small slices of a composite xenolith that contains the remnant of a magmatic dyke reveal high Fe³⁺/Fe values that increase towards the dyke, but only moderately high FMQ_FFsM values, which do not increase with whole-rock Fe³⁺/Fe (McGuire et al., 1991). The Fe³⁺/Fe ratios of spinel, orthopyroxene and olivine, although greater than in comparable unmetasomatized peridotites, do not change with distance from the fluid source. Instead, the increase of whole-rock Fe³⁺/Fe towards the dyke reflects the increasing modal abundance of metasomatic, Fe³⁺-rich amphibole (McGuire et al., 1991).

The detailed study of McGuire et al. (1991) demonstrates how complex the redox effect of metasomatism is, and how little is known about it. Because FMQ_FFsM values alone do not necessarily give a clue to metasomatic effects, it is not surprising that opinions diverge concerning the ‘typical’ values for metasomatized mantle xenoliths. Some researchers observe that modally metasomatized xenoliths plot at the high FMQ_FFsM end of the sample distributions (e.g. Mattioli et al., 1989; Wood & Virgo, 1989; Bryndzia & Wood, 1990; Ballhaus et al., 1991; Woodland et al., 1992), yet others see the opposite relationship (e.g. Chen et al., 1991) or find no correlation beween metasomatism and relative oxygen fugacity (Ionov & Wood, 1992).

In conclusion, Luhr & Aranda-Gomez (1997) present no convincing evidence that the FMQ_FFsM variations in their xenolith samples are related to the interaction with oxidizing fluids.

	ACKNOWLEDGEMENTS

 
I am grateful to Eduard Woermann for clarifying discussions on oxygen fugacity some years ago in Aachen. Discussions and comments by Axel Brunsmann, Gerhard Franz, Enno Zinngrebe (Berlin) and Bernard Evans (Seattle) were much appreciated. The manuscript benefited greatly from constructive reviews by Mike Toplis (Nancy), Laurence Warr (Heidelberg) and two anonymous reviewers, and from the recommendations and accurate editing of Sorena Sorensen (Washington, DC).

	FOOTNOTES

 
^*Telephone: +49 (0) 6221-54-4810. Fax: +49 (0) 6221-54-4805. E-mail: dlattard{at}min.uni-heidelberg.de

	REFERENCES

TOP ABSTRACT INTRODUCTION SOME REMINDERS ABOUT {Delta}FMQ DIFFERENT ESTIMATES OF... THE PRESSURE DEPENDENCE OF... THE SIGNIFICANCE OF RELATIVE... REFERENCES

 
Ballhaus, C. (1993). Redox states of lithospheric and asthenospheric upper mantle. Contributions to Mineralogy and Petrology 114, 331–348.

Ballhaus, C., Berry, R. F. & Green, D. H. (1991). High pressure experimental calibration of the olivine–orthopyroxene–spinel oxygen geobarometer: implications for the oxidation state of the upper mantle. Contributions to Mineralogy and Petrology 107, 24–40.

Berman, R. G. (1988). Internally consistent thermodynamic data for minerals in the system Na₂O–K₂O–CaO–MgO–FeO–Fe₂O₃–Al₂O₃–SiO₂–TiO₂–H₂O–CO₂. Journal of Petrology 29, 445–522.[Abstract/Free Full Text]

Brandon, A. D. & Draper, D. S. (1996). Constraints on the origin of the oxidation state of mantle overlying subduction zones: an example from Simcoe, Washington, USA. Geochimica et Cosmochimica Acta 60, 1739–1749.[Web of Science]

Brown, T. H., Berman, R. G. & Perkins, E. H. (1988). GE0-CALC: software package for calculation and display of pressure–temperature–composition phase diagrams using an IBM or compatible personal computer. Computers and Geosciences 14, 279–289.

Bryndzia, L. T. & Wood, B. J. (1990). Oxygen thermobarometry of abyssal spinel peridotites: the redox state and C–O–H volatile composition of the Earth’s sub-oceanic upper mantle. American Journal of Science 290, 1093–1116.[Abstract/Free Full Text]

Canil, D. & O’Neill, H. S. C. (1996). Distribution of ferric iron in some upper-mantle assemblages. Journal of Petrology 37, 609–635.[Abstract/Free Full Text]

Canil, D., O’Neill, H. S. C., Pearson, D. G., Rudnik, R. L. & McDonough, W. F. (1994). Ferric iron in peridotites and mantle oxidation states. Earth and Planetary Science Letters 123, 205–220.

Chen, Y. D., Pearson, N. J., O’Reilly, S. Y. & Griffin, W. L. (1991). Application of olivine–orthopyroxene–spinel oxygen barometer to the redox state of the upper mantle. Journal of Petrology, Special Issue, 291–306.

Frost, B. R., Lindsley, D. H. & Andersen, D. J. (1988). Fe–Ti oxide–silicate equilibria: assemblages with fayalitic olivine. American Mineralogist 73, 727–740.[Abstract]

Ionov, D. A. & Wood, B. J. (1992). The oxidation state of subcontinental mantle: oxygen thermobarometry of mantle xenoliths from central Asia. Contributions to Mineralogy and Petrology 111, 179–193.

Köhler, T. P. & Brey, G. P. (1990). Calcium exchange between olivine and clinopyroxene calibrated as a geothermobarometer for natural peridotites from 2 to 60 kb with applications. Geochimica et Cosmochimica Acta 54, 2375–2388.

Luhr, J. F. & Aranda-Gomez, J. J. (1997). Mexican peridotite xenoliths and tectonic terranes: correlations among vent, location, texture, temperature, pressure, and oxygen fugacity. Journal of Petrology 38, 1075–1112.

Mattioli, G. S., Baker, M. B., Rutter, M. J. & Stolper, E. M. (1989). Upper mantle oxygen fugacity and its relationship to metasomatism. Journal of Geology 97, 521–536.

McGuire, A. V., Dyar, M. D. & Nielson, J. E. (1991). Metasomatic oxidation of upper mantle peridotite. Contributions to Mineralogy and Petrology 109, 252–264.

Ohmoto, H. & Kerrick, D. (1977). Devolatilization equilibria in graphitic systems. American Journal of Science 277, 1013–1044.

O’Neill, H. S. C. (1987). Free energies of formation of NiO, CoO, Ni₂SiO₄, and Co₂SiO₄. American Mineralogist 72, 280–291.[Abstract]

Parkinson, I. J. & Arculus, R. J., (1999). The redox state of subduction zones: insights from arc-peridotites. Chemical Geology 160, 409–423.

Wood, B. J. & Virgo, D. (1989). Upper mantle oxidation state: ferric iron contents of lherzolite spinels by ⁵⁷Fe Mössbauer spectroscopy and resultant oxygen fugacities. Geochimica et Cosmochimica Acta 53, 1277–1291.

Wood, B. J., Bryndzia, L. T. & Johnson, K. E. (1990). Mantle oxidation state and its relationship to tectonic environment and fluid speciation. Science 248, 337–345.[Abstract/Free Full Text]

Woodland, A. B., Kornprobst, J. & Wood, B. J. (1992). Oxygen thermobarometry of orogenic lherzolite massifs. Journal of Petrology 33, 203–230.[Abstract/Free Full Text]

CiteULike    Connotea    Del.icio.us    What's this?

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (2) Request Permissions Google Scholar Articles by LATTARD, D. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

Online ISSN 1460-2415 - Print ISSN 0022-3530

Copyright © 2009 Oxford University Press

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics