Geochemistry of basaltic rocks from ODP Hole 183-1137A on the Kerguelen Plateau

Cretaceous basalts recovered during Ocean Drilling Program Leg 183 at Site 1137 on the Kerguelen Plateau show remarkable geochemical similarities to Cretaceous continental tholeiites located on the continental margins of eastern India (Rajmahal Traps) and southwestern Australia (Bunbury basalt). Major and trace element and Sr-Nd-Pb isotopic compositions of the Site 1137 basalts are consistent with assimilation of Gondwanan continental crust (from 5 to 7%) by Kerguelen plume-derived magmas. In light of the requirement for crustal contamination of the Kerguelen Plateau basalts, we re-examine the early tectonic environment of the initial Kerguelen plume head. Although a causal role of the Kerguelen plume in the breakup of Eastern Gondwana cannot be ascertained, we demonstrate the need for the presence of the Kerguelen plume early during continental rifting. Activity resulting from interactions by the newly formed Indian and Australian continental margins and the Kerguelen plume may have resulted in stranded fragments of continental crust, isolated at shallow levels in the Indian Ocean lithosphere

Emergence of the Loa Mantle Component in the Hawaiian Islands Based on the Geochemistry of Kauaʻi Shield‐Stage Basalts

Abstract Kauaʻi shield‐stage lavas are central to understanding the origin of the distinct Kea and Loa Hawaiian geochemical trends in Hawaiian basalts. These trends reflect two geochemically distinct sides in the Hawaiian plume, with Loa to the southwest and Kea to the northeast. The geochemistry and Sr‐Nd‐Hf isotopic compositions of shield‐stage lavas from Kauaʻi show a transition from Kea to Loa across the island with the Loa mantle source becoming dominant as the volcano grew. This geochemical transition is gradual from west to east Kauaʻi and supports the hypothesis that the Kauaʻi volcano sampled both sides of the bilateral Hawaiian plume, a phenomenon that is unusual for a Hawaiian volcano. Notably, Kauaʻi marks the arrival of progressively larger volumes of Loa compositions within the Hawaiian mantle plume. The new data from Kauaʻi, combined with an updated and comprehensive database of Hawaiian shield‐stage major element oxides, trace element concentrations, and isotopic compositions normalized to the same standard values, allows for the Pb‐Sr‐Nd‐Hf isotopic compositions of the Average Loa (‘ALOA’) common geochemical component to be estimated. Despite the bilateral Loa‐Kea geochemical trend beginning at Molokaʻi, Loa compositions dominate the erupted volume of Hawaiian volcanoes younger than 3 Ma, validating the volumetric importance of the Loa source in the lower mantle portion of the Hawaiian plume

Primitive neon and helium isotopic compositions of high-MgO basalts from the Kerguelen Archipelago, Indian Ocean

International audienceThe geochemical characteristics of mildly alkalic basalts (24-25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6-17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable 4He / 3He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R / Ra ∼8 to 18) and they show primitive neon isotopic ratios (average 21Ne / 21Neext ∼0.044). The neon isotopic systematics and the 4He / 3He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar 3He / 22Ne and with a higher (U, Th) / 3He ratio compared to typically high R / Ra hotspot basalts such as those from Iceland and Hawaii

Evaluating Downhole Fractionation Corrections in LA-ICP-MS U-Pb Zircon Geochronology

Among the most significant challenges in maximizing the precision and accuracy of U-Pb zircon geochronology by LA-ICP-MS is minimizing the impact of downhole fractionation, the time-dependent evolution of Pb/U ratios caused mainly by complex differences in the volatility and chemical properties of elements as they are excavated from the ablation site. To produce meaningful dates for unknown materials, downhole fractionation is typically quantified in a reference zircon and a time-based correction factor subsequently employed to yield constant Pb/U in both standard and unknown zircons. This assumes that both the reference and unknown zircon exhibit similar downhole behaviour. As a test of this assumption, downhole fractionation trends were characterized and quantified in three common zircon reference materials (Plešovice, 337 Ma; Temora-2, 417 Ma; 91500; 1065 Ma) and in three low-U (\u3c 300 ppm) zircon samples with coherent U-Pb systematics from mafic intrusions (Laramie, 1436 Ma; Bushveld, 2057 Ma; Stillwater, 2710 Ma). Using an exponential downhole correction model based on each of the untreated zircon reference materials, the corrections were applied to each of the “unknowns” and the resulting time-dependent Pb/U ratios and final ages were compared. The effectiveness of pre-treatment protocols was also evaluated by comparing downhole fractionation trends for untreated grains, annealed grains, and grains that were annealed and leached (i.e., chemical abrasion). Each of the three zircon reference materials exhibited distinct downhole fractionation and their calculated correction factors had variable influence in correcting each of the unknowns. Application of the fit parameters either over-corrected or under-corrected the Pb/U ratios of the unknowns due to differences in slope for the different downhole rates at a given time. In most cases, annealing zircon prior to LA-ICP-MS analysis lessened the magnitude of U-Pb mass fractionation during laser ablation, whereas chemical abrasion did not significantly change ablation behaviour beyond simply annealing the grains. Based on the relative effectiveness of the downhole correction that was applied, the resulting U-Pb dates of zircon from the Precambrian mafic intrusions can vary significantly when compared to the CA-ID-TIMS ages established for these samples. The results of this study indicate that a robust downhole correction method for LA-ICP-MS U-Pb geochronology of zircon, particularly for application to magmatic zircon and igneous crystallization ages, involves characterization of downhole fractionation in different reference materials and then applying a correction to the unknown zircon based on a reference zircon that behaves similarly during ablation

A Temperature-Composition Framework for Crystallization of Fractionated Interstitial Melt in the Bushveld Complex from Trace Element Systematics of Zircon and Rutile

The near-solidus crystallization history of the Paleoproterozoic Bushveld Complex, the world’s largest layered intrusion, has been investigated using the in situ trace element geochemistry (LA-ICP-MS) of accessory minerals that crystallized from late, highly fractionated pockets of interstitial melt in layered cumulates and from granitic magmas in felsic roof rocks. Zircon with simple to complex sector zoning occurs in mafic–ultramafic rocks in interstitial pockets that contain quartz–biotite–plagioclase and local granophyric intergrowths. Chondrite-normalized rare earth element patterns are typical of igneous zircon and Ti is negatively correlated with Hf in most samples. Ti-in-zircon thermometry of the cumulates (T= 950–730°C) records the onset of zircon saturation through to the solidus, with notably cooler temperatures determined for Upper Zone and roof rock zircon (T= 875–690°C). Forward modelling of proposed Bushveld parental magmas using rhyolite-MELTS consistently yields similar temperatures for zircon saturation (800–740°C) from highly fractionated melts (~5–20% remaining melt) with late-stage, near-solidus mineral assemblages similar to those observed in the rocks. Anomalously high and variable Th/U (2–77) in zircon from orthopyroxenites in the Critical Zone, including those associated with the PGE-rich UG2 chromitite and Merensky Reef in the Upper Critical Zone, can be related to U loss from the fractionated interstitial melt during exsolution of late, oxidized Cl-rich fluids. In addition to zircon, rutile occurs throughout the Critical Zone of the Bushveld Complex in two different textural settings, as interstitial grains with quartz and zircon and with chromite, each with distinctive chemistry. Euhedral rutile needles found in interstitial melt pockets have relatively high HFSE concentrations (Nb= 1000–20 000 ppm; Ta= 100–1760 ppm), high Zr-in-rutile temperatures (1000–800°C), and are magmatic in origin. Rutile associated with chromite, either as rims or inclusions, is strongly depleted in HFSE (Nb \u3c1000 ppm; Ta \u3c100 ppm) and in Cr and Sc relative to magmatic rutile, and represents a sub-solidus exsolution product of Ti from chromite. Exploring the near-solidus evolution of mafic layered intrusions such as the Bushveld Complex using the trace element chemistry of accessory minerals provides a novel approach to constraining the late stages of crystallization from highly fractionated interstitial melts in these petrologically significant intrusions

The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the Age, Emplacement History and Crystallization of a Large, Open-System Layered Intrusion

The Neoarchean Stillwater Complex, one of the world’s largest known layered intrusions and host to a rich platinum-group element deposit known as the J-M Reef, represents one of the cornerstones for the study of magmatic processes in the Earth’s crust. A complete framework for crystallization of the Stillwater Complex is presented based on the trace element geochemistry of zircon and comprehensive U–Pb zircon–baddeleyite–titanite–rutile geochronology of 22 samples through the magmatic stratigraphy. Trace element concentrations and ratios in zircon are highly variable and support crystallization of zircon from fractionated interstitial melt at near-solidus temperatures in the ultramafic and mafic cumulates (Ti-in-zircon thermometry=980–720℃). U–Pb geochronological results indicate that the Stillwater Complex crystallized over a ~3 million-year interval from 2712 Ma (Basal series) to 2709 Ma (Banded series); late-stage granophyres and at least one phase of post-emplacement mafic dikes also crystallized at 2709 Ma. The dates reveal that the intrusion was not constructed in a strictly sequential stratigraphic order from the base (oldest) to the top (youngest) such that the cumulate succession in the complex does not follow the stratigraphic law of superposition. Two distinct age groups are recognized in the Ultramafic series. The lowermost Peridotite zone, up to and including the G chromitite, crystallized at 2710 Ma from magmas emplaced below the overlying uppermost Peridotite and Bronzitite zones that crystallized earlier at 2711 Ma. Based on the age and locally discordant nature of the J-M Reef, the base of this sequence likely represents an intrusion-wide magmatic unconformity that formed during the onset of renewed and voluminous magmatism at 2709 Ma. The thick anorthosite units in the Middle Banded series are older (2710 Ma) than the rest of the Banded series, a feature consistent with a flotation cumulate or ‘rockberg’ model. The anorthosites are related to crystallization of mafic and ultramafic rocks now preserved in the Ultramafic series and in the lower part of the Lower Banded series below the J-M Reef. The Stillwater Complex was constructed by repeated injections of magma that crystallized to produce a stack of amalgamated sills, some out-of-sequence, consequently it does not constitute the crystallized products of a progressively filled and cooled magma chamber. This calls into question current concepts regarding the intrusive and crystallization histories of major open-system layered intrusions and challenges us to rethink our understanding of the timescales of magma processes and emplacement in these large and petrologically significant and remarkable complexes

Dating the Bushveld Complex: Timing of Crystallization, Duration of Magmatism, and Cooling of the World’s Largest Layered Intrusion and Related Rocks

The Paleoproterozoic Bushveld Complex, including the world’s largest layered intrusion and host to world-class stratiform chromium, platinum group element, and vanadium deposits, is a remarkable natural laboratory for investigating the timescales of magmatic processes in the Earth’s crust. A framework for the emplacement, crystallization, and cooling of the Bushveld Complex based on integrated U–Pb zircon–baddeleyite–titanite–rutile geochronology is presented for samples of different rock types from the Bushveld Complex, including ultramafic and mafic cumulates, mineralized horizons, granitic rocks from the roof, and a carbonatite from the nearby alkaline Phalaborwa Complex. The results indicate that (1) the Bushveld Complex was built incrementally over an ∼5 Myr interval from 2060 to 2055 Ma with a peak in magma flux at c. 2055–2056 Ma, (2) U–Pb zircon crystallization ages do not decrease in an uninterrupted systematic manner from the base to the top of the intrusion, indicating that the Bushveld Complex does not represent the crystallized products of a single progressively filled and cooled magma chamber, and (3) U–Pb rutile dates constrain cooling of the intrusion at the level of the Critical Zone through ∼500 °C by 2053 Ma. The c. 2060 Ma Phalaborwa Complex (pyroxenite, syenite, carbonatite + Cu–Fe-phosphate–vermiculite deposits) represents one of the earliest manifestations of widespread Bushveld-related magmatism in the northern Kaapvaal craton. The extended range and out-of-sequence U–Pb zircon dates determined for a harzburgite from the Lower Zone (c. 2056 Ma), an orthopyroxenite from the Lower Critical Zone (c. 2057 Ma), and orthopyroxenites from the Upper Critical Zone (c. 2057–2060 Ma) are interpreted to indicate that the lower part of the Bushveld Complex developed through successive intrusions and accretion of sheet-like intrusions (sills), some intruded at different stratigraphic levels. Crystallization of the main volume of the Bushveld Complex, as represented by the thick gabbroic sequences of the Main Zone and Upper Zone, is constrained to a relatively narrow interval of time (∼1 Myr) at c. 2055–2056 Ma. Granites and granophyres in the roof, and a diorite in the uppermost Upper Zone, constitute the youngest igneous activity in the Bushveld Complex at c. 2055 Ma. Collectively, these results contribute to an emerging paradigm shift for the assembly of some ultramafic–mafic magmatic systems from the conventional ‘big tank’ model to an ‘amalgamated sill’ model. The volume–duration relationship determined for magmatism in the Bushveld Complex, when compared with timescales established for the assembly of other layered intrusions and more silica-rich plutonic–volcanic systems worldwide, is distinct and equivalent to those determined for Phanerozoic continental and oceanic flood basalts that constitute large igneous provinces. Emplacement of the 2055–2060 Ma Bushveld Complex corresponds to the end of the Lomagundi–Jatuli Event, the largest magnitude positive carbon isotope excursion in Earth history, and this temporal correlation suggests that there may have been a contribution from voluminous Bushveld ultramafic–mafic–silicic magmatism to disruptions in the global paleoenvironment