Combined Hf and Nd isotope microanalysis of co-existing zircon and REE-rich accessory minerals: High resolution insights into crustal processes

The Sm-Nd and Lu-Hf isotope tracer systems, applied to whole rocks or mineral separates, have provided powerful insights into the formation, differentiation, and evolution of the Earth's continental crust. However, some key questions remain, such as how certain igneous rocks form, and how reliable radiogenic isotope tracers are for tracking melt sources. Here, the potential for combining data from the two isotope tracers, Nd and Hf, obtained on a sub-mineral-scale is explored to further understand how crustal rocks receive their sometimes-puzzling geochemical fingerprints. New data, in combination with results from previous studies, reveal one of the key strengths of combining the two independent isotope systems at a micrometre scale, namely identifying open versus closed system metamorphic and igneous processes. Such knowledge is key for understanding how the continental crust formed and stabilized, and for elucidating the role of mantle-derived magmas in the production of granitic rocks, a long-standing issue that is still highly debated. We show how measurement of Nd and Hf isotope ratios in accessory minerals from the same sample has helped to evaluate the fundamentally different models (e.g., magma mixing; crustal assimilation; reactive melt transport vs. incomplete geochemical homogenization of melt at its source) invoked to explain heterogeneous isotope signatures in igneous rocks. Lastly, we discuss how the dual in situ Nd and Hf isotope approach can be used to to evaluate the extent to which metamorphism obscured the primary isotope signature of Archean gneisses derived from felsic igneous protoliths, which has profound implications for our interpretation of early crust formation processes

An Archean Yellowstone? Evidence from extremely low δ¹⁸O in zircons preserved in granulites of the Yilgarn Craton, Western Australia

We report the discovery of Archean (2980-2670 Ma) zircons from the Yilgarn Craton in Western Australia that record unusually low delta O-18 signatures (to -0.5%). These zircons occur in cordierite-orthopyroxene granulites that retain the geochemical signature of intense premetamorphic hydrothermal alteration. We propose a model whereby the low-delta O-18 zircons crystallized within protoliths that record multiple stages of high-temperature interaction and hydrothermal exchange between shallow crustal material and O-18-depleted meteoric fluids, in a setting analogous to that of the Yellowstone caldera. Burial and subsequent granulitefacies metamorphism of this crust led to the crystallization of zircon, which acquired and preserved the extremely O-18-depleted signature of the whole rock. The apparent absence of strongly O-18 depleted Archean zircons has been a puzzling feature of the global zircon record, but we suggest this is an artifact of poor preservation potential. Our findings suggest that long-lived, shallow crustal magmatic-hydrothermal systems similar to those operating in modern caldera complexes were also a feature of Archean Earth

Deciphering the zircon Hf isotope systematics of Eoarchean gneisses from Greenland : Implications for ancient crust-mantle differentiation and Pb isotope controversies

We report a Hf isotope investigation of zircons from four Eoarchaean orthogneisses from the Godthåbsfjord region of southern West Greenland by laser ablation MC-ICPMS, to elucidate crust-mantle differentiation processes in the early Earth. Zircon crystals of all samples record a complex, multi-stage growth and disturbance history, and these discrete growth phases also exhibit disparate Lu-Hf isotope systematics. The oldest (3.84–3.82 Ga) zircon cores have tightly clustered 176Hf/177Hf ratios that are consistent with derivation of their tonalitic precursors from chondritic mantle at this time, with no evidence of input from older crustal or depleted mantle sources. Younger (3.67–3.62 Ga) zircon overgrowths have subchondritic Hf and plausibly grew from small fraction partial melts of the tonalitic host, involving variable dissolution of the older zircon cores. The Neoarchean (ca. 2.7 Ga) zircon component in some samples extends to significantly higher 176Hf/177Hf than the >3.65 Ga zircon, a feature that is interpreted to reflect addition of radiogenic Hf from the rock matrix during metamorphic zircon growth and recrystallisation at 2.7 Ga. The strongly positive εHf (3.82 Ga) values obtained by dissolution of GGU110999 zircons are interpreted to be an artifact of calculating εHf values at ages that are too old, and also from the inclusion of radiogenic younger domains in the analysed multi-grain fractions, rather than to a contribution from depleted Eoarchean mantle. Such data – from zircon grains with multiple age and isotopic components – should not be used to define the evolution of crust-mantle reservoirs. A re-interpretation of the existing Pb isotope data, incorporating the new Hf isotope constraints, posits that the protoliths to the Godthåbsfjörd gneisses were influenced by radiogenic Pb introduced as a fluid mobile component during recycling of a high-μ stagnant basaltic lid at ≥3.8 Ga. The destruction of this mafic protocrust, with attendant fluid release into chondritic mantle, may have been instrumental for the generation of stable Eoarchean tonalitic crust from ca. 3.8 Ga

Accessory minerals as tracers of crustal processes

This Chemical Geology special issue stems from oral presentations given at two symposia held in 2007; Accessory minerals as tracers of crustal processes, at The Goldschmidt Conference in Köln, 19–24 August, 2007, and Internal textures, and trace element and isotope geochemistry in accessory minerals: Advances in imaging and in situ microanalysis, at Frontiers in Mineral Science held in Cambridge, 26–28 June, 2007. Accessory minerals are, by definition, a volumetrically minor component of most crustal rocks, but they comprise a rich and sometimes unique repository of geological information. For example, the oldest Earth materials known are ancient zircon, which provide glimpses into the evolution and environment of the early Earth. A more comprehensive understanding of the links between igneous, hydrothermal, and metamorphic processes, and geochronology and petrology, however, remains one of the major interdisciplinary challenges in modern geosciences. In addition to opening a window to the distant past, emerging techniques, in tandem with new generations of analytical instruments that combine geochronology, trace element geochemistry, and radiogenic tracer isotopes continue to revolutionise the fields of provenance fingerprinting, geochronology, petrology, and petrogenetic processes.\ud \ud In response to these developments, accessory mineral studies have become increasingly sophisticated and wider ranging in recent years, and this is well encapsulated and amplified by the diverse ensemble of papers of this special issue. Progress has been made on several fronts: First, the microanalytical tool box has grown ever more sophisticated, and as such more of the chemical and isotopic information harboured by these mineral archives is being extracted with greater spatial resolution, accuracy, and precision; Second, analytical strategies are increasingly aligned towards integrating different strands of chemical and isotopic data acquired from the same micro-volumes of individual mineral grains. Much greater emphasis has also been placed on extracting in situ chemical information from accessory minerals in their textural context from the nano- to micro-meter scale, which is an important step forward for the interpretation of rocks with complex histories. This allows geochronological and petrological data to be more directly linked with metamorphic and igneous processes; and Third, a better understanding is being gained of the physiochemical conditions that govern the stability and growth/dissolution of accessory phases during crustal processes. This in turn promotes robust interpretations of geochronological data and also provides new insights into trace element and isotopic behaviour during metamorphism and fluid activity and oxygen fugacity during late magmatic evolution.\ud \ud This special issue, which includes eleven papers from several of the talks presented at these two symposia, provides fresh views on different strategies for how to meet these challenges. In brief, the manuscripts highlight a broad range of topics covering a variety of analytical techniques, and a diverse array of accessory minerals. Recent advances in analytical techniques, provenance tracing, and the ability to unravel igneous and metamorphic processes are emphasised

Detrital zircon age, oxygen and hafnium isotope systematics record rigid continents after 2.5 Ga

The Neoarchean-Paleoproterozoic boundary at 2.5 Ga is marked by fundamental changes in the composition of the mantle, crust and atmosphere-hydrosphere. These changes show that the evolution of Earth's deep interior and its exterior are linked, but the causes of the global transitions are cryptic. The isotopic signatures of detrital zircon enable the nature of felsic magma sources before and after 2.5 Ga to be compared, providing insight into the processes driving secular change. For this purpose, we present new oxygen and Hf isotope data from detrital zircon grains hosted by Paleoproterozoic metasedimentary rocks of the North Australian Craton, which record three magmatic events at 2.7 Ga, 2.5 Ga and 1.87 Ga. Scattered zircon εHf (+6 to −10) coupled with mantle-like δ18O at 2.7 Ga indicates both new crustal addition and the reworking of older materials. At 2.5 Ga, a wide range in zircon εHf (+7 to −12) and δ18O (5 to 7‰) reflects reworking of infracrustal and (subordinate) supracrustal components of various crustal residence age. The dominance of subchondritic zircon εHf suggests that depleted mantle inputs were limited. The εHf array contracts markedly (+3 to −8) at 1.87 Ga and is coupled with isotopically heavy oxygen (δ18O from 7 to 9.5‰), indicating a substantial contribution from clay-rich supracrustal sources. We attribute the contraction of the zircon εHf array at ca. 1.87 Ga to the melting of a range of Neoarchean crustal components, where the disparate Hf isotope signatures of these were partially homogenised by sedimentary processes. The shift in felsic magma sources after 2.5 Ga, from dominantly infracrustal to supracrustal, implies a change in the mechanical behaviour of the lithosphere, from soft to rigid. This may have contributed to the transition in the composition of the continents at the Neoarchean-Paleoproterozoic boundary

Large igneous province or long-lived magmatic arc along the eastern margin of Australia during the Cretaceous? Insights from the sedimentary record

U-Pb geochronology and Lu-Hf isotope analysis of detrital zircon from the mid-Cretaceous Winton and Mackunda Formations in the Eromanga Basin were employed to investigate regional provenance patterns in order to better understand the tectonic setting and paleogeography of eastern Australia during the late Mesozoic. A suite of Mesozoic-aged zircon populations recovered from these formations suggests that volcanism along the eastern margin of Australia was relatively continuous from the Triassic (252 Ma) to at least the mid-Cretaceous (ca. 92 Ma). Cretaceous-age zircon populations dominate the provenance record, and a distinct upsection younging trend in Cretaceous grain ages indicates that deposition was largely synchronous with ongoing volcanism to the east. Lu-Hf isotopic data suggest that these zircon populations were sourced from igneous rocks of a mixed juvenile and crustal source, similar to Lu-Hf isotopic systematics for eastern Australian zircons from Pennsylvanian–Permian igneous assemblages (307–252 Ma), for which an active convergent margin association is well established.\ud \ud An extensive Cretaceous volcanic terrain, now limited to the Whitsunday Igneous Association, was once located along the northeastern margin of Australia. Results from this study support the hypothesis that the Whitsunday igneous association was the main source of Cretaceous sediment to the Eromanga Basin, and likely for sediment transported across the continent southward and into the Ceduna Delta system offshore South Australia. The Whitsunday igneous association has been interpreted as a siliceous large igneous province associated with the onset of rifting in the region and linked to opening of the Tasman and Coral Seas. Yet, in this study, we document a relatively continuous Late Triassic to Late Cretaceous (240–92 Ma) age range for detrital zircons from the Mackunda and Winton Formations, consistent with relatively uninterrupted magmatic activity along the continental margin until ca. 92 Ma (earliest Turonian). Furthermore, zircon grains across this age spectrum exhibit dominantly positive to strongly positive εHf(t) values, between +4 and +12, consistent with values known for zircon suites from older magmatic arc rocks of eastern Australia. Although these data do not support a conclusive interpretation, they are consistent with an east Australian magmatic arc related to westward subduction of paleo-Pacific oceanic crust beneath eastern Australia enduring into the Cretaceous, as distinct from extensional siliceous large igneous province magmatism unrelated to subduction and generated by rupture of continental crust

Distinguishing magmatic zircon from hydrothermal zircon: A case study from the Gidginbung high-sulphidation Au–Ag–(Cu) deposit, SE Australia

Zircons within the mineralized, silica-pyrite zone of the Gidginbung (Temora) high-sulphidation Au–Ag–(Cu)\ud deposit in the Lachlan Orogen, Australia have been analyzed by both ion microprobe and laser ablation inductively\ud coupled plasma mass spectrometry for titanium and rare earth element (REE) concentrations, as well\ud as their oxygen and hafnium isotopic compositions. These zircons were previously interpreted to be hydrothermal\ud in origin, but the newdata are indicative of a magmatic origin. The zircons exhibit chondrite-normalized\ud REE patterns that are characterized by a steep positive slope from La to Lu with a large positive Ce-anomaly and\ud a relatively small negative Eu-anomaly, similar to igneous zircons from the nearby Middledale Gabbroic Diorite\ud and the Boggy Plain Zoned Pluton in the Lachlan Orogen and some hydrothermal zircons from the Mole Granite\ud in the New England Orogen. Apparent temperatures calculated using the Ti-in-zircon thermometer for the\ud Gidginbung zircons are 692° to 782 °C, significantly higher than the inferred ore-forming temperature (≤350 °C)\ud but consistent with magmatic crystallization. The Gidginbung zircons have mantle-like δ18O values averaging\ud 5.4±0.9‰ VSMOW (2 standard deviations, n=17), lower than those from the Barmedman granite (7.6±2.0‰,\ud n=17) in the same area. The Gidginbung zircons also have εHf(t=435 Ma) values between 6.4±0.9 and 7.8±0.9\ud (2 standard errors), while the εHf(t=370 Ma) for zircons from the Barmedman granite is more variable, ranging\ud from 5.1±1.1 to 8.4±0.8. Consideration of the data from this and previous studies indicates that the zircons in\ud the Gidginbung deposit are indistinguishable from magmatic zircons with regards to their morphology, cathodoluminescence\ud patterns, and chemical and isotopic signatures. The large measured oxygen isotopic fractionation\ud between quartz and zircon (average: ∼10‰) indicates that the minerals did not form by the same process\ud and/or from the same reservoir at T≥350 °C. While most inclusions in zircon are interpreted to have been\ud trapped after zircon formation or resorption, it is possible that, like quartz, rutile, and anatase, a few pyrite inclusions occurring wholly within zircons have formed at relatively high temperatures during zircon crystallization in magma. Thus, the U–Pb age of these zircons (436 Ma) represents the igneous crystallization age and the Gidginbung zircons investigated by us are best interpreted as magmatic relicts of the earliest Silurian igneous rocks, which survived the processes of hydrothermal alteration and mineralization

A linked evolution for granite-greenstone terranes of the Pilbara Craton from Nd and Hf isotopes, with implications for Archean continental growth

In felsic igneous rocks, the parent and daughter elements in the widely used Sm–Nd and Lu–Hf isotope tracer systems are mainly hosted in accessory phases. Recrystallisation and/or breakdown of these minerals during metamorphism, deformation and weathering potentially compromises the chemical and isotopic composition of the respective whole rocks, impeding the utility of such information for deducing the timing, rates and processes of crust-mantle differentiation in the early Earth. The different abilities of zircon and REE-rich minerals to withstand metamorphism have been suggested as a reason for the decoupling of the Lu–Hf and Sm–Nd isotope systems observed in a number of ancient gneiss terranes. The controls on element mobility and subsequent isotopic disturbance during recrystallisation and breakdown of LREE-rich accessory minerals are, however incompletely understood. Here, we use petrography, element mapping, and microanalysis of accessory minerals, in tandem with whole rock Sm–Nd data, to assess the reliability of the Sm–Nd system in the 3.59–3.58 Ga Mount Webber Gabbros, the oldest rocks in the Pilbara Craton (Western Australia). We show that despite multiple thermal events, which reset the mineral Sm–Nd systematics, and decomposition of the REE-rich mineral allanite, the Mount Webber rocks retained the Sm–Nd isotope signatures of their magmatic protoliths at the whole-rock scale. We show that the allanite breakdown occurred during modern, near-surface weathering processes at low temperature, such that the REE were sequestered into secondary minerals rather than escaping in higher temperature metamorphic fluids. The whole rock Sm–Nd, and zircon O–Hf signatures, together with new 142Nd isotope data, suggest derivation of the Mount Webber rocks from undifferentiated mantle sources that preserve no evidence for Hadean silicate Earth differentiation. This study highlights the benefits of a combined analytical approach using both in-situ and whole-rock isotope analyses to obtain a more complete record of the source and thermal evolution of ancient, highly metamorphosed igneous rocks

Generation of I-type granitic rocks by melting of heterogeneous lower crust

A melt contamination model is presented to explain the formation of "I-type" granitic rocks, based on studies of migmatitic mafic granulites from the Hidaka Metamorphic Belt (Japan). Analysis of apatite and zircon from leucosomes and mesosomes of the migmatites reveals grain-scale Nd and Hf isotope heterogeneities of >10 epsilon units, inconsistent with closedsystem anatexis. We interpret this marked isotopic variability to instead reflect hybridization between metasedimentary-derived partial melts and interlayered mafic granulite horizons during extraction of silicic melt from the lower crust. This open-system melt-rock interaction induces local isotopic modification of the mafic granulites and shifts the Hf isotope signature of the anatectic melt to more radiogenic values, similar to those of hornblende-bearing (I-type) granitic rocks emplaced into the Hidaka sequence at higher crustal levels. This study shows that the generation of broadly I-type granitic magmas does not require extreme temperatures to extensively melt meta-igneous rocks, nor is the direct input of mantle magma essential. Mineral-scale isotopic heterogeneities in such magmas reflect derivation from contrasting crustal sources and the rate and mechanism of magma transfer through the granulitic lower crust