Kouki Kitajima

U-Pb Zircon Geochronology Using LA-ICP-MS in the North Pole Dome, Pilbara Craton, Western Australia: A New Tectonic Growth Model for the Archean Chert/Greenstone Succession

In situ, spot U-Pb ages of zircons from rhyolite, tuffaceous chert, spherule-bearing chert, and granite in the North Pole Dome obtained by laser ablation ICP-MS (LA-ICP-MS) combined with cathode luminescence imaging (CLI) reveal zircon growth patterns that reflect their igneous origins. Felsic lava forming the structural top of the succession, previously dated as 3458 + 9.1/-4.2 Ma by TIMS, has been dated as 3660 ± 52 Ma. These 3.6 Ga zircons from felsic lava, initially considered to be xenocrysts, are interpreted as euhedral igneous crystals as their CL images show no overgrowth rims and/or evidence of melting. Progressive downward in the section, zircons from tuffaceous cherts yield ages of 3463 ± 34 Ma and 3454 ± 46 Ma. This Archean volcaniclastic sequence was intruded by the North Pole Monzogranite at 3391 ± 33 Ma. Our reconstructed stratigraphy shows that the chert-greenstone secession, previously thought to be a single intact stratigraphic unit, actually consists of five units; most are typified by a base of mafic-ultramafic lavas and a capping chert sequence. The zircon ages demonstrate that these five units decrease in age progressively downward; thus the North Pole Dome chert/greenstone succession was formed by layer-parallel thrusting and horizontal shortening, and grew downward by imbricate stacking.

Diversity in early crustal evolution: 4100 Ma zircons in the Cathaysia Block of southern China

Zircons are crucial to understanding the first 500 Myr of crustal evolution of Earth. Very few zircons of this age (>4050 Ma) have been found other than from a ~300 km diameter domain of the Yilgarn Craton, Western Australia. Here we report SIMS U-Pb and O isotope ratios and trace element analyses for two ~4100 Ma detrital zircons from a Paleozoic quartzite at the Longquan area of the Cathaysia Block. One zircon (207Pb/206Pb age of 4127 ± 4 Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (δ18O = 5.8 to 6.0‰). The other zircon grain has a ~4100 Ma magmatic core surrounded by a ~4070 Ma metamorphic mantle. The magmatic core has elevated δ18O (7.2 ± 0.2‰), high titanium concentration (53 ± 3.4 ppm) and a positive cerium anomaly, yielding anomalously high calculated oxygen fugacity (FMQ + 5) and a high crystallization temperature (910°C). These results are unique among Hadean zircons and suggest a granitoid source generated from dry remelting of partly oxidizing supracrustal sediments altered by surface waters. The ~4100 Ma dry melting and subsequent ~4070 Ma metamorphism provide new evidence for the diversity of the Earths earliest crust.

SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions

Significance Although the existence of the Archaea (one of three all-encompassing domains of life) in the Archean Eon (4,000 to 2,500 million years ago) has been inferred from carbon isotopes in bulk samples of ancient rocks, their cellular fossils have been unknown. We here present carbon isotope analyses of 11 microbial fossils from the ∼3,465-million-year-old Western Australian Apex chert from which we infer that two of the five species studied were primitive photosynthesizers, one was an Archaeal methane producer, and two others were methane consumers. This discovery of Archaea in the Archean is consistent with the rRNA “tree of life,” confirms the earlier disputed biogenicity of the Apex fossils, and suggests that methane-cycling methanogen−methanotroph communities were a significant component of Earth’s early biosphere. Analyses by secondary ion mass spectroscopy (SIMS) of 11 specimens of five taxa of prokaryotic filamentous kerogenous cellular microfossils permineralized in a petrographic thin section of the ∼3,465 Ma Apex chert of northwestern Western Australia, prepared from the same rock sample from which this earliest known assemblage of cellular fossils was described more than two decades ago, show their δ13C compositions to vary systematically taxon to taxon from −31‰ to −39‰. These morphospecies-correlated carbon isotope compositions confirm the biogenicity of the Apex fossils and validate their morphology-based taxonomic assignments. Perhaps most significantly, the δ13C values of each of the five taxa are lower than those of bulk samples of Apex kerogen (−27‰), those of SIMS-measured fossil-associated dispersed particulate kerogen (−27.6‰), and those typical of modern prokaryotic phototrophs (−25 ± 10‰). The SIMS data for the two highest δ13C Apex taxa are consistent with those of extant phototrophic bacteria; those for a somewhat lower δ13C taxon, with nonbacterial methane-producing Archaea; and those for the two lowest δ13C taxa, with methane-metabolizing γ-proteobacteria. Although the existence of both methanogens and methanotrophs has been inferred from bulk analyses of the carbon isotopic compositions of pre-2,500 Ma kerogens, these in situ SIMS analyses of individual microfossils present data interpretable as evidencing the cellular preservation of such microorganisms and are consistent with the near-basal position of the Archaea in rRNA phylogenies.

Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: Petrogenesis of composite granite-quartz veins in the Sulu belt, China

State Key Laboratory of Geological Processes and Mineral Resources and Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan, China Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD, USA Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, Perth, WA, Australia The WiscSIMS Laboratory, Department of Geoscience, University of WisconsinMadison, Madison, WI, USA

Microanalysis of carbonate cement δ18O in a CO2-storage system seal: Insights into the diagenetic history of the Eau Claire Formation (Upper Cambrian), Illinois Basin

Oxygen isotope (δ18O) zonation in carbonate mineral cements is often employed as a proxy record (typically with millimeter-scale resolution) of changing temperature regimes during different stages of sediment diagenesis. Recent advances in secondary ion mass spectrometry allow for highly precise and accurate determinations of cement δ18O values to be made in situ on a micrometer scale, thus significantly increasing the spatial resolution available to studies of diagenesis in sandstone–shale and carbonate systems. Chemo-isotopically zoned dolomite–ankerite cements within shaly sandstone beds of the predominantly silty–shaly Eau Claire Formation (Cambrian, Illinois Basin) were investigated, revealing the following: with increasing depth of burial (from <0.5 to ∼2 km [<1500 to 6500 ft]), cement δ18O values decrease from a high of approximately 24‰ down to approximately 14‰ (on the Vienna standard mean ocean water [VSMOW] scale, equivalent to −6.5‰ to −16.5‰ on the Vienna Peedee belemnite [VPDB] scale). The observed cross-basin trend is largely consistent with cements having formed in response to progressive sediment burial and heating. Within the context of independent burial and thermal history models for the Illinois Basin, cementation began soon after deposition and continued intermittently into the mid-Permian. However, temperatures in excess of burial model predictions are inferred at the time of latest ankerite cement precipitation, which we propose overlapped in time with conductive heating of the Eau Claire Formation (a closed system) from under- and overlying sandstone aquifers that channeled the flow of hot, Mississippi Valley–type mineralizing brines during the mid-Permian (ca. 270 Ma).

Slab-Triggered Arc Flare-up in the Cretaceous Median Batholith and the Growth of Lower Arc Crust, Fiordland, New Zealand

The Mesozoic continental arc in Fiordland, New Zealand, records a c. 110 Myr history of episodic, subduction-related magmatism that culminated in a terminal surge of mafic to intermediate, high-Sr/Y, calc-alkalic to alkali-calcic magmas. During this brief, 10–15 Myr event, more than 90% of the Cretaceous plutonic arc root was emplaced; however, the source of these rocks and the degree to which they represent lower crustal mafic and/or metasedimentary recycling versus the addition of new lower arc crust remain uncertain. We report whole-rock geochemistry and zircon trace element, O-isotope and Hf-isotope analyses from 18 samples emplaced into lower arc crust (30–60 km depth) of the Median Batholith with the goals of (1) evaluating the processes that triggered the Cretaceous arc flare-up event and (2) determining the extent to which the Cretaceous arc flare-up resulted in net addition of lower arc crust. We find that dO (Zrn) values from the Western Fiordland Orthogneiss range from 5 2 to 6 3& and yield an error-weighted average value of 5 74 6 0 04& (2SE, 95% confidence limit). Laser ablation multicollector inductively coupled plasma mass spectrometry results yield initial eHf (Zrn) values ranging from –2 0 toþ 11 2 and an error-weighted average value ofþ 4 2 6 0 2. We explore the apparent decoupling of Oand Hf-isotope systems through a variety of mass-balance mixing and assimilation–fractional crystallization models involving depletedand enriched-mantle sources mixed with supra-crustal contributions. We find that the best fit to our isotope data involves mixing between an enriched, mantle-like source and up to 15% subducted, metasedimentary material. These results together with the homogeneity of dO (Zrn) values, the high-Sr/Y signature, and the mafic character of Western Fiordland Orthogneiss magmas indicate that the Cretaceous flare-up was triggered by partial melting and hybridization of subducted oceanic crust and enriched subcontinental lithospheric mantle. We argue that the driving mechanism for the terminal magmatic surge was the propagation of a discontinuous slab tear beneath the arc, or a ridge–trench collision event, at c. 136–128 Ma. Our results from the Early Cretaceous Zealandia arc contrast with the strong crustal signatures that characterize high-flux magmatic events in most shallow to mid-crustal, circum-Pacific orogenic belts in the North and South American Cordillera and the Australia Tasmanides; instead, our results document the rapid addition of new lower arc crust in <<15 Myr with lower crustal growth rates averaging 40–50 km Ma 1 arc-km 1 from 128 to 114 Ma, and peaking at 150–210 km Ma arc-km 1 from 118 to 114 Ma when 70% of the arc root was VC The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 1145 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 6, 1145–1172 doi: 10.1093/petrology/egx049 Advance Access Publication Date: 24 July 2017

Oxygen isotope thermometry using quartz inclusions in garnet

Oxygen isotope ratios of quartz inclusions within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analyzed and used to determine metamorphic temperatures. Primary quartz inclusions for eight of 12 samples have δ18O values significantly lower than matrix quartz. The primary quartz inclusions retain δ18O values representative of thermal conditions during garnet crystallization whereas the δ18O values of matrix quartz were raised by diffusive exchange with other matrix minerals (e.g., mica and feldspar) during cooling. The δ18O differences between quartz inclusions and matrix quartz show that garnet (a mineral with slow diffusion of oxygen) can armour quartz inclusions from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ18O in matrix quartz and quartz inclusions can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying quartz inclusions that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ18O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in quartz inclusions, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of quartz inclusions preserve primary δ18O and 39% were associated with features that were linked to reset δ18O values. If δ18O in garnet is homogeneous and inclusions are removed, laser fluorination δ18O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ18O(Grt) profiles measured in situ by ion microprobe show no δ18O zonation. Almandine–rich garnet (Alm 60–75) from each sample was measured by laser–fluorination mass–spectrometry (LF–MS) for δ18O and compared with ion microprobe measurements of δ18O in quartz inclusions for thermometry. The Δ18O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640 to 740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625 to 800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the Anorthosite–Mangerite–Charnockite–Granite suite in the central Adirondacks and yield temperatures of 640 to 665 °C, approximately 100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post-peak) crystallization of garnet on the retrograde path. This article is protected by copyright. All rights reserved.

Melt Origin across a Rifted Continental Margin: a Case for Subduction-related Metasomatic Agents in the Lithospheric Source of Alkaline Basalt, NW Ross Sea, Antarctica

&NA; Alkaline magmatism associated with the West Antarctic rift system in the NW Ross Sea (NWRS) includes a north‐south chain of shield volcano complexes extending 260 km along the coast of Northern Victoria Land (NVL), numerous small volcanic seamounts located on the continental shelf and hundreds more within an ˜35 000 km2 area of the oceanic Adare Basin. New 40Ar/39Ar age dating and geochemistry confirm that the seamounts are of Pliocene‐Pleistocene age and petrogenetically akin to the mostly middle to late Miocene volcanism on the continent, as well as to a much broader region of diffuse alkaline volcanism that encompasses areas of West Antarctica, Zealandia and eastern Australia. All of these continental regions were contiguous prior to the late‐stage breakup of Gondwana at ˜100 Ma, suggesting that the magmatism is interrelated, yet the mantle source and cause of melting remain controversial. The NWRS provides a rare opportunity to study cogenetic volcanism across the transition from continent to ocean and consequently offers a unique perspective from which to evaluate mantle processes and the roles of lithospheric and sub‐lithospheric sources for mafic alkaline magmas. Mafic alkaline magmas with > 6 wt % MgO (alkali basalt, basanite, hawaiite, and tephrite) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in silica‐undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, as well as LREE/HREE (heavy REE) and Nb/Y ratios. Radiogenic isotopes also vary, with Nd and Pb isotopic compositions increasing and Sr isotopic compositions decreasing oceanward. These variations cannot be explained by shallow‐level crustal contamination or by changes in the degree of mantle partial melting, but are considered to be a function of the thickness and age of the mantle lithosphere. We propose that the isotopic signature of the most silica‐undersaturated and incompatible element enriched basalts best represent the composition of the sub‐lithospheric magma source with low 87Sr/86Sr (≤0·7030) and &dgr;18Oolivine (≤5·0‰), and high 143Nd/144Nd (˜0·5130) and 206Pb/204Pb (≥20). The isotopic ‘endmember’ signature of the sub‐lithospheric source is derived from recycled subducted materials and was transferred to the lithospheric mantle by small‐degree melts (carbonate‐rich silicate liquids) to form amphibole‐rich metasomes. Later melting of the metasomes produced silica‐undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continentward. Ancient and/or more recent (˜550–100 Ma) subduction along the Pan‐Pacific margin of Gondwana supplied the recycled subduction‐related material to the asthenosphere. Melting and carbonate metasomatism were triggered during major episodes of extension beginning in the Late Cretaceous, but alkaline magmatism was very limited in its extent. A significant delay of ˜30 to 20 Myr between extension and magmatism was probably controlled by conductive heating and the rate of thermal migration at the base of the lithosphere. Heating was facilitated by regional mantle upwelling, possibly driven by slab detachment and sinking into the lower mantle and/or by edge‐driven mantle flow established at the boundary between the thinned lithosphere of the West Antarctic rift and the thick East Antarctic craton.

Ar–Ar dating for hydrothermal quartz from the 2.4 Ga Ongeluk Formation, South Africa: implications for seafloor hydrothermal circulation

Fluid inclusions in hydrothermal quartz in the 2.4 Ga Ongeluk Formation, South Africa, are expected to partially retain a component of the ancient seawater. To constrain the origin of the fluid and the quartz precipitation age, we conducted Ar–Ar dating for the quartz via a stepwise crushing method. The obtained argon isotopes show two or three endmembers with one or two binary mixing lines as the crushing proceeds, suggesting that the isotopic compositions of these endmembers correspond to fluid inclusions of each generation, earlier generated smaller 40Ar- and K-rich inclusions, moderate 40Ar- and 38ArCl (neutron-induced 38Ar from Cl)-rich inclusions and later generated larger atmospheric-rich inclusions. The K-rich inclusions show significantly different 40Ar/38ArCl values compared to the 38ArCl-rich inclusions, indicating that it is difficult to constrain the quartz formation age using only fluid inclusions containing excess 40Ar. The highest obtained 40Ar/36Ar value from the fluid inclusions is consistent with an expected value of the Ongeluk plume source, suggesting that the quartz precipitation was driven by Ongeluk volcanism. Considering the fluid inclusion generations and their compositions, the hydrothermal system was composed of crustal fluid and magmatic fluid without seawater before the beginning of a small amount of seawater input to the hydrothermal system.

Ion microprobe–measured stable isotope evidence for ammonite habitat and life mode during early ontogeny

Abstract. Ammonites have disparate adult morphologies indicative of diverse ecological niches, but ammonite hatchlings are small (~1mm diameter), which raises questions about the similarity of egg incubation and hatchling life mode in ammonites. Modern Nautilus is sometimes used as a model organism for understanding ammonites, but despite their outward similarities, the groups are only distantly related. Trends in ammonite diversity and extinction vulnerability in the fossil record contrast starkly with those of nautilids, and embryonic shells from Late Cretaceous ammonites are two orders of magnitude smaller than nautilid embryonic shells. To investigate possible environmental changes experienced by ammonite hatchlings, we used secondary ion mass spectrometry to analyze the oxygen and carbon isotope composition of the embryonic shells and early postembryonic whorls of five juveniles of Hoploscaphites comprimus obtained from a single concretion in the Fox Hills Formation of South Dakota. Co-occurring bivalves and diagenetic calcite were also analyzed to provide a benthic baseline for comparison. The oxygen isotope ratios of embryonic shells are more like those of benthic bivalves, suggesting that ammonite eggs were laid on the bottom. Ammonite shell immediately after hatching has more negative δ18O, suggesting movement to more shallow water that is potentially warmer and/or fresher. After approximately one whorl of postembryonic growth, the values of δ18O become more positive in three of the five individuals, suggesting that these animals transitioned to a more demersal mode of life. Two other individuals transition to even lower δ18O values that could suggest movement to nearshore brackish water. These data suggest that ammonites, like many modern coleoids, may have spawned at different times of the year. Because scaphites were one of the short-term Cretaceous–Paleogene extinction survivors, it is possible that this characteristic allowed them to develop a broader geographic range and, consequently, a greater resistance to extinction.