Journal Of Petrology Research Articles

New Thermodynamic Models for Anhydrous Alkaline-Silicate Magmatic Systems

Abstract A new thermodynamic model for silicate melt in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–Fe2O3–Cr2O3 model system is presented, building on the tholeiitic through to granitic melt model of Holland et al. (2018) [Journal of Petrology, 59, 881–900] but extending for the first time into anhydrous alkaline systems. The new melt model is accompanied by new thermodynamic models for nepheline, kalsilite, leucite, melilite and ilmenite. Collectively these models enable pseudosection modelling of alkaline-silicate magmatic systems, providing a new tool for investigating this geologically- and economically-important compositional space. The models are calibrated with respect to experimental data on phase relations among minerals and melt, and the fit is benchmarked here via detailed comparison with seven experimental datasets, which encompass a range of pressure (0–22 kbar), temperature (680–1350°C), oxygen fugacity (log fO2 ΔFMQ-3 to +1), total alkali (3–16 wt %) and silica (37–70 wt %) conditions. The calculated pseudosections successfully reproduce experimental crystallisation sequences and phase compositions, indicating that the thermodynamic models are well calibrated across this spectrum of conditions. Redox buffered experimental conditions are simulated using oxygen buffered pseudosections. Contouring of oxygen buffered pseudosections with XFe3+ (mol. Fe3+/Fetotal), or pseudosections of varying XFe3+ with ΔFMQ, reveals (i) often complex and non-intuitive relationships between these two representations of oxidation state, and (ii) substantial variation in ferric iron over narrow temperature intervals in some oxygen buffered sets of experiments. An implication is that simulating oxygen buffering is vital when benchmarking thermodynamic models using experimental results. Furthermore, because natural igneous systems likely feature a near-constant XFe3+, it is important to assess experimental results in this framework when making inferences about natural systems, recognising that oxygen fugacity is a consequence not a control of phase equilibria in nature. Overall, our new models provide a novel tool to explore the role of variables, such as pressure, fractional crystallisation and crustal assimilation in the petrogenesis of alkaline-silicate magmatic systems and their associated mineralisation.

Correction to: ‘The Generation of Eocene Mafic dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–118

Correction to: ‘The Generation of Eocene Mafic dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–118

Comment on ‘The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–18

Abstract Azizi, H., Daneshvar, N., Asahara, Y., Minami, M. & Anma, R. (2023). The generation of Eocene mafic dike swarms during the exhumation of a Core complex, Biarjmand area, NE Iran. Journal of Petrology64, 1–18 have attributed the E–W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic–Early Cretaceous age. They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this article provides to shed light on specific aspects of the Late Mesozoic–Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al.'s publication. Specifically, the assignment of an Eocene age to the emplacement of the E–W-oriented dike swarm, even though purportedly supported by U–Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the article contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari, A., Axen, G. J., Heizler, M. T. & Hassanzadeh, J. (2017). Large-magnitude continental extension in the northeastern Iranian plateau: insight from K-feldspar 40Ar/39Ar thermochronology from the Shotor Kuh-Biarjmand metamorphic core complex. Geosphere13, 1207–1233 previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al.'s paper. In this comment, I outline the problems with the structural and regional geology, the zircon U–Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al.'s work.

Corrigendum to: Redox in Magmas: Comment on a Recent Treatment of the Kaiserstuhl Volcanics (Braunger et al., Journal of Petrology, 59, 1731–1762, 2018) and Some Other Misconceptions

Corrigendum to: Redox in Magmas: Comment on a Recent Treatment of the Kaiserstuhl Volcanics (Braunger et al., Journal of Petrology, 59, 1731–1762, 2018) and Some Other Misconceptions

Geochemistry, origin and anatexis temperature of monzogranite formation in Mount Khalaj (Mashhad, Iran)

Geochemistry, origin and anatexis temperature of monzogranite formation in Mount Khalaj (Mashhad, Iran)

Chemical variation, modal composition and classification of granitoids

Abstract Granites ( sensu lato ) come in many types and flavours, defining distinct magmatic series/suites/types. A good classification not only gives generally accepted and understandable names to similar rocks but also links the bulk chemical composition to the stoichiometry of the constituent minerals and, potentially, also to the likely source, magmatic evolution and tectonic setting. The ‘ideal’ granitoid classification should be based on chemical criteria amenable to an objective treatment. Statistical analysis helps to identify the most discriminant variables. The key properties are (1) acidity/maficity, (2) alkalinity (balance of Na + K v. Ca), (3) aluminosity (balance of Al v. Ca, Na and K), (4) Fe/Mg balance and (5) Na/K balance and K contents at the given SiO 2 level. These are used by successful classifications, e.g. the I/S dichotomy is based mainly on aluminosity, while the Frost et al. (2001; ‘A geochemical classification for granitic rocks', Journal of Petrology , 42 , 2033–2048, https://doi.org/10.1093/petrology/42.11.2033 ) classification includes all but Na/K. Even though it is commonplace to use weight percentages of oxides, we suggest that a better strategy is to employ simple atomic parameters (e.g. millications-based) that can be directly linked to modal proportions and compositions/crystal structure of individual rock-forming minerals. This facilitates a petrological interpretation, which, in turn, can be related to petrogenesis and, ultimately, to likely tectonic setting(s).

Redox in Magmas: Comment on a Recent Treatment of the Kaiserstuhl Volcanics (Braunger et al., Journal of Petrology, 59, 1731–1762, 2018) and Some Other Misconceptions

Redox in Magmas: Comment on a Recent Treatment of the Kaiserstuhl Volcanics (Braunger et al., Journal of Petrology, 59, 1731–1762, 2018) and Some Other Misconceptions

The Origin of Salts in Unaltered Kimberlites. Comment on: ‘Was Crustal Contamination Involved in the Formation of the Serpentine-free Udachnaya-East Kimberlite? New Insights into Parental Melts, Liquidus Assemblage and Effects of Alteration’ by Abersteiner et al. (2018), Journal of Petrology 59, 1467–1492

The Origin of Salts in Unaltered Kimberlites. Comment on: ‘Was Crustal Contamination Involved in the Formation of the Serpentine-free Udachnaya-East Kimberlite? New Insights into Parental Melts, Liquidus Assemblage and Effects of Alteration’ by Abersteiner et al. (2018), Journal of Petrology 59, 1467–1492

Rhyolite-MELTS vs DERP—Newer Does not Make it Better: a Comment on ‘The Effect of Anorthite Content and Water on Quartz–Feldspar Cotectic Compositions in the Rhyolitic System and Implications for Geobarometry’ by Wilke et al. (2017; Journal of Petrology, 58, 789–818)

Rhyolite-MELTS vs DERP—Newer Does not Make it Better: a Comment on ‘The Effect of Anorthite Content and Water on Quartz–Feldspar Cotectic Compositions in the Rhyolitic System and Implications for Geobarometry’ by Wilke et al. (2017; Journal of Petrology, 58, 789–818)

Rhyolite-MELTS vs DERP – Reply to Comment by Gualda et al. on ‘The Effect of Anorthite Content and Water on Quartz–Feldspar Cotectic Compositions in the Rhyolitic System and Implications for Geobarometry’ by Wilke et al. (2017), Journal of Petrology, 58, No. 4, 789–818

Rhyolite-MELTS vs DERP – Reply to Comment by Gualda et al. on ‘The Effect of Anorthite Content and Water on Quartz–Feldspar Cotectic Compositions in the Rhyolitic System and Implications for Geobarometry’ by Wilke et al. (2017), Journal of Petrology, 58, No. 4, 789–818

Comment on: ‘Petrography of the Snap Lake Kimberlite Dyke (Northwest Territories, Canada) and its Interaction with Country Rock Granitoids’ by Fulopet al.(2018), Journal of Petrology, doi: 10.1093/petrology/egy025

Comment on: ‘Petrography of the Snap Lake Kimberlite Dyke (Northwest Territories, Canada) and its Interaction with Country Rock Granitoids’ by Fulop<i>et al.</i>(2018), Journal of Petrology, doi: 10.1093/petrology/egy025

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

The Geochemical Complexity of Kimberlite Rocks and their Olivine Populations: a Comment on Cordieret al.(Journal of Petrology, 56, 1775–1796, 2015)

The Geochemical Complexity of Kimberlite Rocks and their Olivine Populations: a Comment on Cordier<i>et al.</i>(<i>Journal of Petrology</i>, 56, 1775–1796, 2015)

‘From Igneous Petrology to Ore Genesis’: an Introduction to this Thematic Issue ofJournal of Petrology

‘From Igneous Petrology to Ore Genesis’: an Introduction to this Thematic Issue of<i>Journal of Petrology</i>

پترولوژی و ژئوشیمی هورنبلند گابروهای پروتروزوئیک جنوب شرق فریمان، استان خراسان رضوی

Introduction Hornblende-bearing gabbroic rocks are quite common in subduction-related magmatic suites and considered to represent magmatic differentiation process in arc magmas (Heliker, 1995; Hickey-Vargas et al., 1995; Mandal and Ray, 2012). The presence of hornblende as an important mineral phase in gabbroic rocks of subduction zone has been considered either as an early crystallizing mineral from water-bearing mafic magmas (Beard and Borgia 1989; Mandal and Ray, 2012) or as a product of reaction of early crystallized minerals (olivine, pyroxene and plagioclase) and water-rich evolved melt/aqueous fluid (Costa et al., 2002; Mandal and Ray, 2012). The careful study of petrology and geochemistry of hornblende-bearing gabbroic rocks from Chahak area, of Neoproterozoic age, can provide important information about their petrogenesis. Because of the special characteristics of Chahak hornblende gabbros according to their age and their situation in the main structural units of Iran, their study can present critical keys for the knowledge of geological history of Iran specially central Iran zone. Material and Methods This study carried out in two parts including field and laboratory works. Sampling and structural studies were carried out during field work. Geological map for the study area was also prepared. 65 thin and polished thin sections for petrographical purpose were studied. Major oxides, rare earth elements and trace elements were analyzed for 4 samples (92P-1, 92P-3, B1and B6) from hornblende gabbros on the basis of 4AB1 method using ICP-MS of ACME Laboratory from Canada. In addition, major oxides of three hornblende gabbro samples (89P-62, 89P-59 and 89P-46) were used from Partovifar (Partovifar, 2012). Results and discussion Fariman metamorphic terrains, of Proterozoic age, consist of metamorphosed sedimentary and igneous (plutonic and volcanic) rocks. Hornblende gabbros of the study area include plagioclase, hornblende, biotite pyroxene and olivine as major minerals and apatite, ilmenite and magnetite as minor minerals. In many examples, hornblende and biotite can be seen as corona textures around plagioclase, pyroxene and olivine, while plagioclase, pyroxene and olivine show obviously corrosion features. This can be considered to be formed by the reaction of early formed crystals with aqueous fluid/evolved melt. In some cases, amphiboles show rhythmic overgrowths. The rhythmic amphibole overgrowths represent deep-seated crystallization in a volatile-rich magma under conditions of high but varying gas pressure. In the study area, the most dominant texture of the hornblende gabbros is hypidiomorphic granular, but intergranular and porphyric textures are common too. Based on geochemical data from major and minor elements, studied rocks belong to tholeiite series with meta–aluminous nature. The geochemical behavior of main elements of the studied rocks reveals the normal trend of differentiation in their magma. Chondrite-normalized REE diagram of hornblende gabbros indicates an obvious enrichment of LREE in compare with HREE. MORB-normalized spider diagrams indicate variable enrichment in LILE and depletion in high field strength elements (HFSE). Primitive mantle-normalized spider diagram show negative anomaly for Nb and Zr. Gabbros from southeast of Fariman have an island arc tholeiite nature and based on trace element diagrams, they formed as a result of 3 to 10% partial melting of a garnet lherzolite source. The mineralogy, texture and geochemistry of the studied rocks show striking similarities with gabbroic rocks of subduction zone developed in supra subduction zone of arc-marginal basin setting. Acknowledgments The Research Foundation of Ferdowsi University of Mashhad, Iran, supported this study (Project 28035.2). I thank the university authorities for funding. Reference Beard, J.S., and Borgia, A., 1989. Temporal variation of mineralogy and petrology in cognate gabbroic enclaves at Arenal volcano, Costa Rica. Contributions to Mineralogy and Petrology, 103(1): 110–122. Costa, F., Dungan, M.A., and Singer, B.S., 2002. Hornblende- and phlogopite-bearing gabbroic xenoliths from Volc´an San Pedro (36◦S), Chilean Andes: Evidence for melt and fluid migration and reactions in subduction-related plutons. Journal of Petrology, 43(3): 219–241. Heliker, C., 1995. Inclusions in Mount St. Helens dacite erupted from 1980 through 1983. Journal of Volcanology and Geothermal Research, 66(1-4): 115–135. Hickey-Vargas, R., Abdollahi, M.J., Parada, M.A., Lopez-Escobar, L. and Frey, F.A., 1995. Crustal xenoliths from Calbuco volcano, Andean southern volcanic zone: Implications for crustal composition and magma-crust interaction. Contributions to Mineralogy and Petrology, 119(4): 331–344. Mandal, A., Ray, A., Debnath, M. and Paul, S.B., 2012. Geochemistry of hornblende gabbro and associated dolerite dyke of Paharpur, Puruliya, West Bengal: Implication for petrogenetic process and tectonic setting. Journal of Earth System Science, 121(3): 793–812. Partovifar, F., 2012. Petrology and geochemistry studies of granitic rocks from Chahak village, Kariz-Now area, southeast of Fariman, Iran. Unpublished M.Sc. thesis, Ferdowsi University of Mashhad, Mashhad, Iran, 145 pp. (in Persian)

Rapid Crystallization of Plagioclase Phenocrysts in Silicic Melts during Fluid-saturated Ascent: Phase Equilibrium and Decompression Experiments

Three crystal-poor obsidian samples (one dacite, 67 wt % SiO2; two rhyolites, 73 and 75 wt % SiO2), which erupted effusively from monogenetic vents, contain sparse (<2%) plagioclase phenocrysts that span a remarkably wide and continuous range in composition ( 30 mol % An). Many, but not all, of the plagioclase crystals display diffusion-limited growth textures (e.g. swallow-tails, skeletal, vermiform). Hypotheses to explain the paradox of a wide compositional range despite a low abundance of plagioclase include (1) incorporation of xenocrysts and/or magma mingling, (2) slow crystallization of plagioclase driven by slow cooling in a magma chamber, (3) slow crystallization of plagioclase followed by a resorption (e.g. heating) event, and (4) crystallization driven by rapid degassing (i.e. loss of melt H2O)6 rapid cooling during ascent. To test these hypotheses, a series of phase equilibrium experiments were conducted under pure-H2O fluid-saturated conditions in a cold-seal pressure vessel between 30 and 300 MPa and 750 and 950 C. The results show that the plagioclase population in each obsidian sample could have grown from their respective melts, with the exception of a single calcic core (An60–63) in one sample. The results additionally rule out slow cooling in a magma chamber, because this would lead to equilibrium abundances of plagioclase ( 20%), which are far higher than what is observed in the samples (<2%). Nor can resorption (i.e. heating) explain the low abundance of plagioclase, because this would eliminate the more sodic plagioclase crystals and hence the wide compositional range of plagioclase that is observed. The most viable hypothesis is that the sparse plagioclase phenocrysts grew relatively rapidly during magma ascent to the surface; this is consistent with the results of isothermal (850 C) continuous decompression experiments (2 9, 1 0, 0 8, and 0 1 MPa h), under pure-H2O fluid-saturated conditions, which were performed on one of the rhyolites (MLV-36; 73 wt % SiO2) and quenched at PH2O1⁄489, 58 and 40 MPa. The four decompression rates correspond to degassing rates of 1 6, 0 56, 0 45 and 0 06 wt % H2O per day. Decompressions 1 0 MPa(PH2O) h, initiated above the liquidus, quenched to 100% glass at all final PH2O. Decompressions at 0 8 MPa(PH2O) h, also initiated above the liquidus, led to plagioclase crystals nearly five times larger than those grown in runs decompressed at the same rate, but initiated just below the plagioclase-in curve. It is the kinetic hindrance to nucleation that permits crystal growth to be concentrated on relatively few crystals, leading to larger crystals. Plagioclase growth rates from these experiments show that the largest phenocrysts ( 1 mm) in the MLV-36 obsidian could have grown in <42 h. A cooling rate of 1 2 C h closely matches both the increase in melt viscosity with time and the effective undercooling with time that occurs during the 0 8 MPa(PH2O) h decompression over the first 50 h. The combined results point to crystallization of sparse plagioclase driven by relatively rapid rates of degassing6 cooling VC The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 981 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2015, Vol. 56, No. 5, 981–1006 doi: 10.1093/petrology/egv025

Petrogenesis and zircon U-Pb dating of skarnified pyroxene-bearing dioritic rocks in Bisheh area, south of Birjand, eastern Iran

در منطقه بیشه واقع در شرق بلوک لوت، هورنبلند دیوریت های پورفیری در سنگهای رسوبی و آذرین (توف‌ها و سنگهای دیوریتی پیروکسن‌دار) نفوذ کرده و به‌ترتیب تشکیل اگزواسکارن و اندواسکارن داده است. بخشهای تغییر نیافته سنگهای دیوریتی پیروکسن‌دار، دارای ماهیت کالک آلکالن پتاسیم بالا و متاآلومین (78/1 A/NK≥ و 95/0(A/CNK≤ هستند. نمودار چند عنصری به‌هنجار شده با گوشته اولیه نشان دهنده غنی شدگی از عناصر LIL مانند Rb، Ba، Cs و تهی شدگی از عناصر HFS نظیر Nb، P، Ti، Y و Yb است. نمودار عناصر نادر خاکی بیانگر مقادیر (La/Yb)N از 75/7 تا 63/8 و بی هنجاری منفی ضعیف Eu می باشد. این خصوصیات به همراه مقادیر بالای Th/Yb و Ta/Yb نشان‌دهنده ماگماتیسم وابسته به مناطق فرورانش حاشیه قاره هاست. تهی‌شدگی مشخص Nb و Ti ، میزان نسبتا بالای Mg#، نسبتهای اولیه 87Sr/86Sr ( 70606/0)، 143Nd/144Nd ( 512424/0) و Nd e (05/3-) نشان می دهد که ماگمای سازنده سنگهای مورد مطالعه از گوشته غنی شده منشأ گرفته و دچار آغشتگی پوسته ای شده است. مقادیر بالای Rb، Th، K و مقدار پایین P و Ti در سنگهای مورد بررسی مؤید آلودگی ماگما در پوسته فوقانی طی تکامل ماگما است. میزان Ba/Rb پایین (4/7-6/4) و Rb/Sr بالا (22/0-17/0)، نشاندهنده حضور فلوگوپیت در گوشته منشأ است. با توجه به میزان بالای CaO+ Al2O3 نمونه های مورد مطالعه (متوسط 3/22 درصد)، میتوان گارنت پیروکسنیت فلوگوپیتدار را به‌عنوان منشأ ماگما در نظر گرفت. سن‌سنجی به روش زیرکن-اورانیم-سرب بر روی پیروکسن دیوریت پورفیری نشان می‌دهد که این ماگماتیسم در 07/44 میلیون سال پیش یعنی در ائوسن میانی(Lutetian) رخ داده است.

Statistical evaluation of tectonomagmatic discrimination diagrams for granitic rocks and proposal of new discriminant-function-based multi-dimensional diagrams for acid rocks

The four tectonic discrimination diagrams of Pearce et al. [Journal of Petrology, v. 25, p. 956–983] for granitic rocks were first evaluated using the literature cited by these authors as well as from our new database. The first diagram (Y−Nb) cannot discriminate volcanic-arc and collision settings. Both Y−Nb and Yb−Ta diagrams have an overlapping field for within-plate and ocean-ridge granitoids. The remaining two diagrams (Y + Nb−Rb and Yb + Ta−Rb) use a mobile element (Rb) in their y-axis. Although these diagrams successfully discriminate volcanic-arc and within-plate granites, they perform less well for collision tectonics. Besides, felsic or acid rocks are scarce in ocean-ridge settings, which limits the usefulness of these diagrams for this geological environment. Therefore, using an extensive database, we proposed a set of five new discriminant-function-based multi-dimensional diagrams for acid magmas from four tectonic settings (island arc, continental arc, continental rift, and collision). The very similar tectonic settings of island and continental arcs are discriminated for the first time. These diagrams are based on correct statistical treatment of compositional data, because they use natural logarithm transformation of major-element ratios and linear discriminant analysis (LDA). The use of discordant outlier-free samples prior to LDA improved the success rates by about 3–5%. Success rates of these diagrams as inferred from a testing set were between 76% and 88% for island arc, 60% and 92% for continental arc, and 72% and 84% for both continental rift and collision settings. Finally, application of these new diagrams to case studies not compiled in our initial database used for constructing these diagrams provided the following results: a collision setting for the Himalayas at about 30 Ma; an island arc setting for Quaternary acid rocks from geothermal boreholes in El Salvador; an island- or continental-arc setting for northern Italy at 35–52 Ma; a continental-arc setting for the Italy–Austria border at about 30 Ma; either a rift or a collision setting for northern Nigeria at about 164 Ma; a collision setting for central Nigeria at about 144 Ma and for the Cretaceous Masirah ophiolites of Oman; and an island arc setting for the Cretaceous Semail ophiolites of Oman. In spite of the relative mobility of major elements, these applications suggest utility of the new discrimination diagrams for all four tectonic settings.

Thermal state of the upper mantle beneath the Northern Cordilleran Volcanic Province (NCVP), British Columbia, Canada

Data are presented for a suite of peridotitic mantle-derived xenoliths collected from basanite lavas within the Llangorse volcanic field, northwest British Columbia. The xenoliths comprise spinel lherzolite and subordinate spinel harzburgite. Two-pyroxene thermometry based on the Brey and Köhler (1990) [Brey, G.P., Köhler, T., 1990. Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. Journal of Petrology, 31, 1353–1378.] calibration was applied to 44 xenolith samples. The resulting geothermometry define minimum (800–850 °C) and maximum (1050–1100 °C) temperatures of equilibration for the xenolith suite which are estimates of thermal conditions in the underlying lithospheric mantle. We take the minimum temperatures as indicative of the maximum MOHO temperature; the maximum xenolith temperatures provide a minimum temperature for the transition from lithospheric to asthenospheric mantle. The geothermometry data are combined with published heat flow data to produce a set of model geotherms for this portion of the northern Cordillera. The model geotherms constrain the thickness of the mantle lithosphere in the northern Canadian Cordillera to between 16 and 30 km, corresponding to depths to the lithosphere/asthenosphere boundary of 52–66 km. We show that this model is consistent with an underlying convecting asthenospheric mantle with an average temperature and viscosity of 1210–1250 °C and 10 19.4 Pa s, respectively. We conclude by exploring the implications this model has for the source regions of alkaline magmas erupted over this portion of the northern Cordilleran volcanic province.

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)

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 ΔFMQ (‘relative oxygen fugacity’, where FMQ is fayalite–magnetite–quartz) values is inherent to the method and therefore of questionable geological significance. Increases in ΔFMQ do not necessarily mirror oxidation processes in the mantle controlled by subduction-related fluids.