Preservation of Re‐Os isotope signatures in pyrite throughout low‐T,high‐P eclogite facies metamorphism

Pyrite in LT–HP eclogites from the western Tianshan orogenic belt yields a Re‐Os age of 378.1 ± 8.9 Ma, which is 30–70 Ma older than ages previously obtained for the same rocks using the Rb–Sr, Sm–Nd, Ar–Ar, U–Pb, and Lu–Hf isotope systems. The Tianshan LT–HP eclogite experienced temperatures of up to ~570 °C combined with pressures of up to 2.1 GPa during metamorphism. These conditions are below the transition of pyrite to pyrrhotite, which defines both pyrite stability and possibly its closure temperature for Re‐Os. Pyrite can preserve Re‐Os signatures through eclogite facies peak metamorphic conditions, and thus allow determination of the formation age of pyrite in the protolith.     

Norwegian sea‐surface palaeoenvironments of marine oxygen‐isotope stage 3: the paradoxical response of dinoflagellate cysts

High‐resolution marine palynological data have been obtained from two very long sediment cores (MD952009 and MD952010) retrieved from the southern Norwegian Sea. The dinoflagellate cyst assemblages show pronounced fluctuations in composition, which correlate strongly with magnetic susceptibility records and also mimic the δ¹⁸O signal of the GISP2 Greenland ice‐core. If focusing on the period from 48 to 30 cal. kyr BP, this correlation suggests a paradoxical response of the sea‐surface environments to the atmospheric conditions over Greenland: when the Greenland δ¹⁸O signal reflects warm interstadial conditions, the Norwegian Sea depicts cold sea‐surface temperatures with quasi‐perennial sea‐ice cover (based on dinoflagellate cysts). In contrast, when the Greenland δ¹⁸O records cold stadial periods, the Norwegian Sea‐surface temperatures are warm (based on dinoflagellate cysts), probably linked to inflow of the North Atlantic Drift. These results, similar in both cores, are contrary to those of previous studies and shed light on a possible decoupling of Norwegian sea surface‐water conditions and atmospheric conditions over Greenland. This decoupling could be linked to an atmosphere–ocean system behaving similar to that which the Northern Hemisphere is experiencing at present, i.e. strongly variable owing to the North Atlantic Oscillation. Copyright © 2002 John Wiley & Sons, Ltd.     

The crustal evolution of South America from a zircon Hf‐isotope perspective

Hf‐isotope data of >1100 detrital zircon grains from the Palaeozoic, south‐central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8–4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust‐addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville‐age igneous belt can be postulated along the palaeo‐margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircon was mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc.     

Stable carbon‐isotope record of shallow‐marine evaporative epicratonic basin carbonates,Ordovician Williston Basin,North America

Secular variations in stable carbon‐isotope values of marine carbonates are used widely to correlate successions that lack high‐resolution index fossils. Various environmental processes, however, commonly may affect and alter the primary marine carbon‐isotope signal in shallow epicratonic basins. This study focuses on the marine carbon‐isotope record from the carbonate–evaporite succession of the upper Katian (Upper Ordovician) Red River Formation of the shallow epicratonic Williston Basin, USA. It documents the carbon‐isotope signal between the two major Ordovician positive shifts in δ¹³C, the early Katian Guttenberg and the Hirnantian excursions. Eight δ¹³C stages are identified based on positive excursions, shifts from positive to negative values and relatively uniform δ¹³C_carb values. A correlation between carbon‐isotope trends and the relative sea‐level changes based on gross facies stacking patterns shows no clear relation. Based on the available biostratigraphy and δ¹³C trends, the studied Williston Basin curves are tied to the isotope curves from the North American Midcontinent, Québec (Anticosti Island) and Estonia, which confirm the Late Katian age (Aphelognathus divergens Conodont Zone) of the upper Red River Formation. The differences in the δ¹³C overall trend and absolute values, coupled with the petrographic and cathodoluminescence evidence, suggest that the carbon‐isotope record has been affected by the syndepositional environmental processes in the shallow and periodically isolated Williston Basin, and stabilized by later burial diagenesis under reducing conditions and the presence of isotopically more negative fluids.     

A reinterpretation of the crustal N‐isotope record: evidence for a 15N‐enriched Archean atmosphere?

A new compilation of N‐isotope and abundance data for metasedimentary rocks, and hyrdothermal micas that proxy for bulk crust, show systematic patterns. (1) δ¹⁵N values of kerogen in Precambrian cherts are more negative relative to siliciclastic counterparts, probably due to a mantle hydrothermal component. (2) There is a secular trend from average δ¹⁵N 15.3 ± 1.8‰ in Archean shales, through intermediate values in the Proterozoic, to Phanerozoic counterparts where δ¹⁵N averages +3.5‰. (3) Hydrothermal micas in metamorphic hydrothermal systems of Palaeozoic and Mesozoic age that proxy for crust have δ¹⁵N within the range of contemporaneous sedimentary rocks. (4) Hydrothermal micas track the secular trend of δ¹⁵N for kerogen from 2.7 Ga to the Phanerozoic. (5) Within Precambrian datasets δ¹⁵N does not increase with decreasing N content; accordingly, high δ¹⁵N values cannot stem either from metamorphism or form Rayleigh fractionation. (6) Previous studies show isotopic shifts during metamorphism are only +1 to +3‰ up to amphibolite facies. Values of 10–24‰ are attributed to a high δ¹⁵N Archean atmosphere, a residual signature of CI carbonaceous chondrites where δ¹⁵N is +30‰ to + 42‰.     

New insights into Holocene atmospheric circulation dynamics in central Scandinavia inferred from oxygen‐isotope records of lake‐sediment cellulose

St. Amour, N. A., Hammarlund, D., Edwards, T. W. D. & Wolfe, B. B. 2010: New insights into Holocene atmospheric circulation dynamics in central Scandinavia inferred from oxygen‐isotope records of lake‐sediment cellulose. Boreas, Vol. 39, pp. 770–782. 10.1111/j.1502‐3885.2010.00169.x. ISSN 0300‐9843 Cellulose‐inferred lakewater oxygen‐isotope records have been obtained from two hydrologically open basins (Lake Spåime and Lake Svartkälstjärn), located on a west–east transect across central Sweden, to investigate changes in atmospheric circulation patterns during the Holocene. The Lake Spåime δ¹⁸O record is sensitive to changes in the seasonal distribution of precipitation in the Scandes Mountains of west‐central Sweden, and thus generally portrays variations in δ¹⁸O of precipitation (δ¹⁸O_P) that are governed predominantly by the influence of air masses originating from the North Atlantic. In contrast, the Lake Svartkälstjärn δ¹⁸O record appears to reflect the varying influence of air masses delivering moisture from the North Atlantic and the Baltic Sea. A comparison of inferred changes in δ¹⁸O_P over the Holocene between the two sites reveals systematic patterns of variability over widely different time scales. These include: (1) a previously recognized long‐term regional decline in δ¹⁸O_P, possibly in response to the declining vigour of Northern Hemisphere atmospheric circulation related to decreasing summer solar insolation; (2) newly identified inverse centennial‐ to millennial‐scale δ¹⁸O_P fluctuations at the two sites that may be linked to changes in modes of atmospheric circulation analogous to those described at interannual to multidecadal time scales by the North Atlantic Oscillation (NAO) index; and (3) a prolonged interval of apparent climatic stability in the mid‐Holocene (c. 6300–4200 cal. yr BP) characterized by persistently negative NAO‐like circulation.     

K‐Ar age determination,whole‐rock and oxygen isotope geochemistry of the post‐collisional Bizmişen and Çaltı plutons,SW Erzincan,eastern Central Anatolia,Turkey

Post‐collisional granitoid plutons intrude obducted Neo‐Tethyan ophiolitic rocks in central and eastern Central Anatolia. The Bizmişen and Çaltı plutons and the ophiolitic rocks that they intrude are overlain by fossiliferous and flyschoidal sedimentary rocks of the early Miocene Kemah Formation. These sedimentary rocks were deposited in basins that developed at the same time as tectonic unroofing of the plutons along E–W and NW–SE trending faults in Oligo‐Miocene time. Mineral separates from the Bizmişen and Çaltı plutons yield K‐Ar ages ranging from 42 to 46 Ma, and from 40 to 49 Ma, respectively. Major, trace, and rare‐earth element geochemistry as well as mineralogical and textural evidence reveals that the Bizmişen pluton crystallized first, followed at shallower depth by the Çaltı pluton from a medium‐K calcalkaline, I‐type hybrid magma which was generated by magma mixing of coeval mafic and felsic magmas. Delta ¹⁸O values of both plutons fall in the field of I‐type granitoids, although those of the Çaltı pluton are consistently higher than those of the Bizmişen pluton. This is in agreement with field observations, petrographic and whole‐rock geochemical data, which indicate that the Bizmişen pluton represents relatively uncontaminated mantle material, whereas the Çaltı pluton has a significant crustal component. Structural data indicating the middle Eocene emplacement age and intrusion into already obducted ophiolitic rocks, suggest a post‐collisional extensional origin. However, the pure geochemical discrimination diagrams indicate an arc origin which can be inherited either from the source material or from an upper mantle material modified by an early subduction process during the evolution of the Neo‐Tethyan ocean. Copyright © 2005 John Wiley & Sons, Ltd.     

Stable carbon isotope composition of terrestrial leaves: inter‐ and intraspecies variability,cellulose and whole‐leaf tissue difference,and potential for climate reconstruction

Stable carbon isotope analysis of terrestrial plant leaves preserved in Quaternary lake sediments has the potential to provide high‐resolution reconstructions of past climatic conditions. Preferably, δ¹³C measurements should be made on a single leaf component, e.g. cellulose, but this approach is often precluded by limited leaf availability. Previous work suggests that reliable palaeoclimatic information also may be derived from δ¹³C measurements on whole‐leaf tissue, given a similar degree of leaf decomposition between samples. Leaf δ¹³C data for 12 Scandinavian species of dwarf‐shrubs, shrubs and trees, and a comparison of δ¹³C data on recent and late Holocene Salix herbacea leaves, revealed that the δ¹³C signal registered by holocellulose is largely reflected by measurements on whole‐leaf tissue. Holocellulose was found to be consistently enriched in ¹³C, although this δ¹³C offset was smaller for subfossil leaves. This supports the use of δ¹³C measurements on whole‐leaf tissue for climate reconstruction, at least for leaves preserved in soft, late Holocene sediments with minimal diagenetic effects. Leaf carbon and nitrogen data on fresh leaves of the same 12 Scandinavian species, and corresponding data on late Holocene Salix herbacea leaves, suggest that the leaf C:N ratio is a suitable indicator of the degree of leaf decomposition. Copyright © 2003 John Wiley & Sons, Ltd.     

High‐resolution geochemical record of fluid–rock interaction in a mid‐crustal shear zone: a comparative study of major element and oxygen isotope transport in garnet

Garnet grains from an intensely metasomatized mid‐crustal shear zone in the Reynolds Range, central Australia, exhibit a diverse assortment of textural and compositional characteristics that provide important insights into the geochemical effects of fluid–rock interaction. Electron microprobe X‐ray maps and major element profiles, in situ secondary ion mass spectrometry oxygen isotope analyses, and U–Pb and Sm–Nd geochronology are used to reconstruct their thermal, temporal and fluid evolution. These techniques reveal a detailed sequence of garnet growth, re‐equilibration and dissolution during intracontinental reworking associated with the Ordovician–Carboniferous (450–300 Ma) Alice Springs Orogeny. A euhedral garnet porphyroblast displays bell‐shaped major element profiles diagnostic of prograde growth zoning during shear zone burial. Coexisting granulitic garnet porphyroclasts inherited from precursor wall rocks show extensive cation re‐equilibration assisted by fracturing and fragmentation. Oxygen isotope variations in the former are inversely correlated with the molar proportion of grossular, suggesting that isotopic fractionation is linked to Ca substitution. The latter generally show close correspondence to the isotopic composition of their precursor, indicating slow intergranular diffusion of O relative to Fe²⁺, Mg and Mn. Peak metamorphism associated with shearing (～550 °C; 5.0–6.5 kbar) occurred at c. 360 Ma, followed by rapid exhumation and cooling. Progressive Mn enrichment in rim domains indicates that the retrograde evolution caused partial garnet dissolution. Accompanying intra‐mineral porosity production then stimulated limited oxygen isotope exchange between relict granulitic garnet grains and adjacent metasomatic biotite, resulting in increased garnet δ¹⁸O values over length scales <200 μm. Spatially restricted oxygen interdiffusion was thus facilitated by increased fluid access to reaction interfaces. The concentration of Ca in channelled fracture networks suggests that its mobility was enhanced by a similar mechanism. In contrast, the intergranular diffusion of Fe²⁺, Mg and Mn was rock‐wide under the same P–T regime, as demonstrated by a lack of local spatial variations in the re‐equilibration of these components. The extraction of detailed reaction histories from garnet must therefore take into account the variable length‐ and time‐scales of elemental and isotopic exchange, particularly where the involvement of a fluid phase enhances the possibility of measureable resetting profiles being generated for slowly diffusing components such as Ca and O, even at low ambient temperatures and relatively fast cooling rates.     

Setting,sulfur isotope variations,and metamorphism of Jurassic massive Zn‐Pb‐Ag sulfide mineralization associated with arc‐type volcanism (Skra,Vardar zone, Νorthern Greece)

Massive Zn‐Pb‐Ag sulfide mineralization appears conformable with felsic volcanism, developed in an Upper Jurassic volcanic arc to the Southwest (SW) of the Serbo‐Macedonian continent in Northern Greece. The host volcanic sequence of the mineralization comprises mylonitized rhyolitic to rhyodacitic lavas, pyroclastics, quartz‐feldspar porphyries, and cherty tuffs. A “white mica—quartz—pyrite” mineral assemblage characterizes the volcanic rocks in the footwall and hanging‐wall of massive sulfide ore layers, formed as a result of greenschist‐grade regional metamorphism on “clay‐quartz‐pyrite” hydrothermal alteration haloes. Massive ore lenses are usually underlain by deformed Cu‐pyrite and quartz‐pyrite stockworks. Most of the sulfide ore bodies have proximal‐type features. Ductile deformation and regional metamorphism have transformed many of the stockwork structures. The mineralization is characterized by high Zn, Pb, and Ag contents, while Cu and critical metals are low. Primary depositional textures, for example, layering, clastic pyrite, colloform, and atoll textures were identified. The overall textural features of the mineralization indicate it has undergone mechanical deformation. The most prominent features of the effects of metamorphism, folding and shearing, are modification of the ore body morphology toward flattened and boudinage structures and transformation of the ore textures toward the dominance of planar fabrics. Sulfur isotope analyses of sulfides along with textural observations are consistent with a dual source of sulfide sulfur. Sulfur isotope values for sphalerite, non‐colloform pyrite, galena, and chalcopyrite fall in a limited range from ?1.6 to +4.8‰ (mean δ³⁴S + 2‰), indicating a hydrothermal source derived from the reduction of coeval seawater sulfate in the convective system. Pyrites with colloform and atoll textures are characterized by a ³⁴S depletion, indicating a bacterial reduction of coeval seawater sulfate. The morphology of ore beds, the mineralogy, sulfide textures, and ore chemistry along with the petrology and tectonic setting of the host rocks can be attributed to typical of a bimodal‐felsic metallogenesis. Although similar in many respects to classic Kuroko‐type volcanogenic massive sulfide mineralization, it has some atypical features, like the absence of barite ore, which is possibly a result of significant temporal depletion in sulfate due to bacterial reduction, a conclusion supported by the widespread occurrence of colloidal and atoll textures of pyrite.     

Latest Devonian to earliest Carboniferous conodont and carbon isotope stratigraphy of a shallow‐water sequence in South China

To improve regional and intercontinental correlation of the uppermost Devonian–lowermost Carboniferous, we examined the conodont faunas and carbon isotopic records of the Tangbagou Formation in the Qilinzhai section, southern Guizhou, South China. The Tangbagou Formation is a succession of mixed carbonate–siliciclastic rocks that accumulated on a shallow‐water platform under normal marine conditions. Seven conodont zones for shallow‐water biofacies in South China, the Cl. gilwernensis–Cl. unicornis Zone, the Po. spicatus Zone, the Si. homosimplex Zone, the Si. sinensis Zone, the Si. eurylobata Zone, the Ps. multistriatus Zone and the Po. co. porcatus Zone in ascending order, are recognized in the Tangbagou Formation. Although apparently limited in its value for global correlation, this conodont zonation is more applicable to shallow‐water biofacies in South China. Carbonate samples have yielded carbon isotopic signatures consistent with those recorded in Euroamerica sections, in particular showing four distinct characteristics: (1) the peak values of Hangenberg Carbon Isotope Excursion (HICE) during the latest Devonian, (2) a minor positive shift (P1) in the Si. homosimplex Zone during the early Tournaisian, (3) a second minor positive shift (P2) in the Si. sinensis Zone and (4) the middle Tournaisian Carbon Isotope Excursion (TICE) in the middle part of the Tangbagou Formation. The similarity in peak values (~5.5‰) and magnitude of TICE for the Qilinzhai and Belgian sections indicates that the Euro‐asia δ¹³C_carb trends may reflect the changes in global mean ocean δ¹³C_DIC, rather than having been overprinted by local carbon cycling. Integration of conodont biostratigraphy and δ¹³C stratigraphy provides a powerful tool for stratigraphic correlation. Copyright © 2015 John Wiley & Sons, Ltd.     

A reassessment of late glacial – Holocene diatom oxygen isotope record from Lake Baikal using a geochemical mass‐balance approach

We present a new palaeoenvironmental record of hydrological variability in Lake Baikal, based on re‐modelled δ¹⁸O_diatom values of diatom silica (δ¹⁸O_modelled), where the residual contaminants are identified and compensated for using electron optical imaging and whole‐sample geochemistry. δ¹⁸O_modelled interpretations are based on the balance between rivers with high δ¹⁸O values and rivers with low δ¹⁸O values. Isotopic variability is related to latitudinal differences in precipitation which feed these rivers. The δ¹⁸O_modelled record suggests that rather moist conditions prevailed in the Lake Baikal region during the latter stages of the Younger Dryas. Throughout the Holocene, episodes of low δ¹⁸O_modelled values are, in general, in good agreement with increases in percentage haematite‐stained grains in North Atlantic sediments (indicative of ice‐rafted debris events). Rivers with southerly catchments dominate fluvial input especially between c. 3.3 and 2 cal ka BP, concurrent with high precipitation in the Lake Baikal region. Copyright © 2011 John Wiley & Sons, Ltd.     

Techniques for assessing spatial heterogeneity of carbonate δ13C values: Implications for craton‐wide isotope gradients

The sedimentary record of carbonate carbon isotopes (δ¹³C_carb) provides one of the best methods for correlating marine strata and understanding the long‐term evolution of the global carbon cycle. This work focuses on the Late Ordovician Guttenberg isotopic carbon excursion, a ca 2·5‰ positive δ¹³C_carb excursion that is found in strata globally. Substantial variability in the apparent magnitude and stratigraphic morphology of the Guttenberg excursion at different localities has hampered high‐resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle. This work investigates the magnitude, spatial scale and sources of isotopic variability of the Guttenberg excursion in two sections from Missouri, USA. Centimetre‐scale isotope transects revealed variations in δ¹³C_carb and δ¹⁸O_carb greater than 2‰ across individual beds. Linear δ¹³C_carb to δ¹⁸O_carb mixing lines, together with petrographic and elemental abundance data, demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components. These patterns facilitated objective sample screening to determine the ‘least‐altered’ data. A δ¹⁸O_carb filter based on empirical δ¹⁸O_carb values of well‐preserved carbonate mudstones allowed further sample discrimination. The resulting ‘least‐altered’ δ¹³C_carb profile improves the understanding of regional as well as continental‐scale stratigraphic relations in this interval. Correlations with other Laurentian sections strongly suggest that: (i) small‐scale variability in Guttenberg excursion δ¹³C_carb values may result in part from local diagenetic overprinting; (ii) peak‐Guttenberg excursion δ¹³C_carb values of the Midcontinent are not distinct from their Taconic equivalents; and (iii) no primary continental‐scale spatial gradient in δ¹³C_carb (for example, arising from chemically distinct ‘aquafacies’) is required during Guttenberg excursion‐time. This study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for δ¹³C_carb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry.     

Garnet re‐equilibration by coupled dissolution–reprecipitation: evidence from textural,major element and oxygen isotope zoning of ‘cloudy’ garnet

The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re‐equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell‐shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ¹⁸O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion‐rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ¹⁸O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four‐stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re‐equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re‐equilibration process for garnet in metamorphic rocks and that intra‐mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.     

Micro‐scale silicon isotope heterogeneity observed in hydrothermal quartz precipitates from the >3.7 Ga Isua Greenstone Belt,SW Greenland

Pillow basalt and chert form integral lithologies comprising many Archean greenstone belt packages. To investigate details of these lithologies in the >3.7 Ga Isua Greenstone Belt, SW Greenland, we measured silicon isotope compositions of quartz crystals, by secondary ion mass spectrometry, from a quartz‐cemented, quartz‐amygdaloidal basaltic pillow breccia, recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions. The recrystallized chert, chert clasts and quartz cement have overlapping δ³⁰Si values, while the δ³⁰Si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age, despite amphibolite facies metamorphism. We suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting, consistent with the pillow breccia origin. On the basis of the present data, we conclude that the geological context of each sample must be carefully evaluated when interpreting δ³⁰Si values of quartz.     

Re‐interpretation of the existence of a primitive plume under Australia based on neon isotope fractionation during step heating

ABSTRACT The origin of the volcanism in south‐eastern Australia is poorly understood. Matsumoto and collaborators found near solar neon isotope values in measurements obtained by step heating apatites in ultramafic xenoliths from Bullenmerri (SE Australia), and proposed that these values result from the presence of a primitive plume. However, the results of simple diffusion models indicate that the neon isotopic signature observed by Matsumoto et al. can be generated by mass fractionation during step heating. If such mass fractionation does occur during step heating, then neither neon nor helium isotopes require the presence of primitive mantle, and the rare gas observations are consistent with the involvement of MORB‐source mantle in the generation of south‐eastern Australian volcanics. These results show that both the acquisition and the interpretation of step heating data should be carefully controlled and that a mineral crushing technique may yield more reliable analyses.