Michael J. Spicuzza

UWG-2, a garnet standard for oxygen isotope ratios: Strategies for high precision and accuracy with laser heating

UWG-2 is a new garnet standard for oxygen isotope analysis prepared from a single large porphyroblast that was homogeneous (±0.21‰) at the millimeter-scale before grinding. The δ 18O value of UWG-2 has been determined in seven laboratories using either a laser probe system or externally heated Ni reaction vessels. The raw laser probe value is 5.74‰ at the University of Wisconsin. If all data are normalized to NBS-28 = 9.59‰, then the UW value (5.89‰) is in good agreement with the average of all labs (5.78‰). There is no significant difference between garnet analyses made with the two techniques, nor among labs using different wavelengths of IR laser. UWG-2 is available for interlaboratory comparison, and for assessing the performance of microanalytical techniques including laser probes and ion microprobes with a recommended value of δ 18O = 5.8‰ SMOW. Multiple, daily analyses of UWG-2 at the University of Wisconsin provide an accurate evaluation of all components in our laser-probe, mass-spectrometer system, and allow analytical problems to be rapidly identified. With this standardization, the accuracy of a single laser probe analysis is better than ±0.10‰. Over 1000 analyses of UWG-2 have been made. The average of all uncorrected δ 18O values is 5.74 ± 0.15‰ (1 sd). The precision on a single day averages ±0.07‰ and is frequently better than 0.05‰. The uncertainty in the mean for all analyses is ±0.005‰ (1σ). A small drift of the daily average over time results from inevitable changes in the vacuum line which require careful attention and maintenance.

Geochemistry of xenolithic eclogites from West Africa, Part I: A link between low MgO eclogites and Archean crust formation

Oxygen isotope, mineral trace element, and measured and reconstructed whole rock compositions are reported for low MgO (6 -13 wt.% MgO in the whole rock) eclogite xenoliths from the Koidu kimberlite complex, Sierra Leone. The d 18 O values of garnet (4.7- 6.8‰), determined by laser fluorination on clean mineral separates, extend beyond the range for mantle peridotites. All low MgO eclogites have reconstructed trace element patterns that are depleted in Ba, Th, U, and light rare earth element (LREE), with jadeite-rich samples having more variable trace element patterns than jadeite-poor samples. These observations, coupled with low SiO2 contents, and Nb-rich but LREE-depleted reconstructed whole rock compositions, suggest the low MgO eclogites are remnants of altered oceanic crust that was partially melted during subduction. Partial melting of a mafic protolith at high pressure (leaving a garnet-bearing residue) is the preferred model to explain the origin of Archean tonalite-trondhjemite-granodiorite (TTG) suites, which make up large portions of the crust in Archean cratons. We therefore suggest that the Koidu low MgO eclogites may be residues from Archean continental crust formation. Copyright

Geochemistry of xenolithic eclogites from West Africa, part 2: origins of the high MgO eclogites

Abstract Oxygen isotope, mineral trace element, and measured and reconstructed whole-rock compositions are reported for the high MgO eclogite xenolith suite (16 to 20 wt% MgO in the whole rock) from the Koidu Kimberlite complex, Sierra Leone. In contrast to the previously published data for low MgO eclogites (6 to 13 wt% MgO) from this area, high MgO eclogites equilibrated at higher temperatures (1080 to 1130°C vs. 890 to 930°C) have only mantlelike δ18O and show variable degrees of light rare earth element (REE) enrichment. Analyses of multiple mineral generations suggest that the heterogeneous REE patterns of the high MgO eclogites reflect variable degrees of metasomatic overprinting. High MgO and Al2O3 contents of the eclogites suggest a cumulate origin, either as high-pressure (2 to 3 GPa) garnet–pyroxene cumulates or low-pressure ( 4 GPa).

The rapid heating, defocused beam technique: a CO2-laser-based method for highly precise and accurate determination of δ18O values of quartz

Abstract We describe a laser fluorination technique specifically for quartz that allows both high-accuracy and high-precision determination of oxygen isotope ratios, regardless of grain size. The rapid heating, defocused beam technique (RHD) utilizes a defocused 32 W CO 2 laser at full power which delivers 17 W (∼20 W/mm 2 ) to the sample to quickly react quartz. In contrast, other recent laser fluorination studies use a tightly focused laser beam and slow heating (i.e. Sharp and Kirschner, 1995 ; Fouillac and Girard, 1996 ; Kirschner and Sharp, 1997 ), and find laser analysis of fine-grained quartz to be inaccurate. Three quartz standards, NBS-28, QZ-BRA, and QZ-CWRU were analyzed using the RHD technique at the University of Wisconsin to test for grain size effects. RHD analyses show no correlation between grain size and δ 18 O values and are in excellent agreement with δ 18 O values obtained using conventional fluorination techniques. Additional analyses of the same quartz standards performed at University of Wisconsin using slow heating and a focused beam yield δ 18 O values that are both less precise and significantly (up to 0.8‰) lower than the accepted values. Attempts to use the RHD technique with a 20 W CO 2 laser which delivers 8 W (∼10 W/mm 2 ) to the sample at BRGM were unsuccessful, probably due to insufficient power density of the defocused beam. We conclude that the RHD technique yields excellent accuracy and precision, but that power densities of >15–20 W/mm 2 may be necessary across a large part of the sample surface. We recommend the use of lasers with at least 30 W of power.

Coesite eclogites from the Roberts Victor kimberlite, South Africa

Abstract Coesite (or pseudomorphic quartz) is more common than previously recognized in the eclogite xenolith suite from the Roberts Victor Mine, South Africa. All coesite eclogites in this study (18 samples) are classified as group I, and have mineral compositions that span virtually the entire compositional range of group I eclogites described from this locality. Most of the Roberts Victor group I eclogite suite may be in equilibrium with free silica, thus weakening the case for these rocks representing residues of partial melting. Oxygen isotope compositions of garnets from 15 samples are in the range 5.32–6.95‰, similar to other Roberts Victor group I eclogites. These observations are consistent with a model in which protoliths of these rocks were oceanic basalts and intrusive rocks that underwent exchange with seawater at relatively low temperatures, to variable extents, prior to subduction and metamorphism to eclogite facies, without suffering significant partial melting during or following subduction.

Oxygen isotope composition of carbonates, silicates, and oxides in selected carbonatites: constraints on crystallization temperatures of carbonatite magmas

Oxygen isotope compositions and fractionations between calcite (Cc) and magnetite (Mt), diopside-rich clinopyroxene (Di), monticellite (Mnt), kimzeyite-rich garnet (Gt), and biotite (Bt) were measured for carbonatites from Oka (Canada), Magnet Cove (USA), Jacupiranga (Brazil), and Essonville (Canada), to obtain crystallization temperatures and explore the crystallization history of carbonatites. The highest isotopic temperatures are obtained from Cc–Mt fractionations from Oka (745–770 °C) and Cc–Mnt fractionations from Magnet Cove (700 and 760 °C). Cc–Mt temperatures for very coarse-grained, euhedral magnetite phenocrysts and calcite from Jacupiranga are 700 °C. In samples that contain diopside and magnetite, the Cc–Mt temperatures are always higher than Cc–Di temperatures. This difference is consistent with crystallization of magnetite before diopside, minor retrograde resetting of magnetite isotopic compositions, and the order of crystallization inferred from inclusions of Mt in Di. Cc–Mt, Cc–Di, and Cc–Mnt fractionations are thus interpreted to represent those established during crystallization at rapid cooling rates (103–104 °C/my). Diffusion model calculations indicate that at slower post-crystallization cooling rates (10–102 °C/my), magnetite compositions should experience significant isotopic resetting by diffusional exchange with Cc, Bt, and apatite, and yield lower temperatures than Cc–Di. Cc–Bt fractionations correspond to the lowest temperatures (440–560 °C). Although some of these are relatively high isotopic temperatures for biotite, they most likely represent those established during subsolidus retrograde exchange between biotite and calcite during rapid subsolidus cooling.

Oxygen isotope variations in Cr-poor megacrysts from kimberlite

Abstract As radiogenic isotope compositions of Cr-poor megacrysts from kimberlite demonstrate that the megacrysts and kimberlite hosts are genetically related (megacrysts are likely deep-seated liquidus phases), the oxygen isotope compositions of Cr-poor megacrysts are indicative of the δ18O of the host kimberlites and their mantle source regions. Oxygen isotope ratios of garnet megacrysts from Group I kimberlites (from slightly depleted mantle sources) worldwide (North America, southern Africa, Australia) are restricted (δ18OVSMOW = 5.24‰, SD = 0.15, SE = 0.01, n = 121) and typical of phenocrysts in magmas derived from upper mantle with “normal” oxygen isotope compositions. No significant involvement of subducted oceanic crust is indicated in the genesis of Group I kimberlites. Garnet megacrysts from Group II kimberlites (from “enriched” mantle, and restricted to southern Africa) have anomalously high 18O/16O ratios (δ18OVSMOW = 5.59‰, SD = 0.18, SE = 0.02, n = 55). Similarities in equilibration temperature and major element composition between garnet megacrysts from Group I and II kimberlites rule out roles for these parameters in producing the marked difference in 18O/16O between these two suites. Instead, the high 18O/16O values of the Group II garnets suggests incorporation of anomalously heavy oxygen from subducted ocean floor material in the source region of Group II kimberlites (as much as 30% if eclogite [with δ18O approximately 6.5‰] with an ocean floor protolith is the contaminant, less if the source of anomalous oxygen is pelagic or terrestrial sediment with higher δ18O). Actual samples of potential Group II kimberlite source rocks (e.g., mica/amphibole veined peridotites) or plutonic crystallization products of such kimberlites (e.g., mica-amphibole-rutile-ilmenite-diopside rocks) should have elevated δ18O values, but no such material has been previously described in southern African xenolith suites.

Oxygen Isotope Composition of Eclogitic and Peridotitic Garnet Xenocrysts from the La Ceniza Kimberlite, Guaniamo, Venezuela

Although diamonds from kimberlites in the Guaniamo region of Venezuela overwhelmingly belong to the eclogite suite, the mantle xenocryst suite is dominated by peridotitic garnets, with a significant harzburgite component. In the 0.5 to 1.0 mm size fraction of heavy mineral concentrate from the La Ceniza kimberlite, approximately 23% of the garnet xenocrysts are classified as G10 (harzburgitic), 75% are G9 (lherzolitic) and only 2% are eclogite-derived. In the peridotite group, both lherzolitic and harzburgitic garnet xenocrysts have oxygen isotope ratios typical of mantle peridotites (δ18O = +5.25 to +5.47‰ VSMOW). Eclogitic garnet xenocrysts overlap the major-element compositional range of eclogite-suite garnets included in diamond from Guaniamo. The oxygen isotope ratios of the eclogitic xenocrysts are in the range of +5.32 to +9.26‰. The higher values represent the most extreme oxygen isotope compositions yet reported for mantle eclogites. These oxygen isotope data support earlier suggestions, based on carbon isotope analyses of diamonds and oxygen isotope analysis of their mineral inclusions, that subduction of altered oceanic lithosphere beneath the Guyana craton played a major role in formation of Guaniamo eclogite-suite diamonds.

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.

The origin, cooling and alteration of A-type granites in southern Israel (northernmost Arabian-Nubian shield): a multi-mineral oxygen isotope study

A multi-mineral oxygen isotope study sheds light on the origin, cooling and alteration of Late Neoproterozoic A-type granites in the Arabian–Nubian shield of southern Israel. The oxygen isotope ratio of zircon of the Timna monzodiorite, quartz syenite and alkaline granite are within the range of mantle zircon (δ 18 O(Zrn) = 5.3 ± 0.6‰, 2σ), supporting the co-genetic mantle-derived origin previously suggested based on geochemical data and similar ɛNd(T) values and U–Pb ages (610 Ma). Likewise, olivine norite xenoliths within the monzodiorite (δ 18 O(Ol) = 5.41 ± 0.07‰) may have formed as cumulate in a parent mantle-derived magma. Within the Timna igneous complex, the latest and most evolved intrusion, an alkaline granite, has the least contaminated isotope ratio (δ 18 O(Zrn) = 5.50 ± 0.02‰), whereas its inferred parental monzodiorite magma has slightly higher and more variable δ 18 O(Zrn) values (5.60 to 5.93‰). The small isotope variation may be accounted for either by small differences in the temperature of zircon crystallization or by minor contamination of the parent magma followed by shallow emplacement and intrusion by the Timna alkaline granite. The Timna alkaline granite evolved, however, from a non-contaminated batch of mantle-derived magma. The formation of Yehoshafat granite (605 Ma; δ 18 O(Zrn) = 6.63 ± 0.10‰), exposed ~30 km to the south of the mineralogically comparable Timna alkaline granite, involved significant contribution from supracrustal rocks. A-type granites in southern Israel thus formed by differentiation of mantle-derived magma and upper crustal melting coevally. Fast grain boundary diffusion modelling and measured quartz-zircon fractionations demonstrate that the Timna and Yehoshafat alkaline granites cooled very rapidly below 600 °C in accordance with being epizonal. One to three orders of magnitude slower cooling is calculated for 30 Ma older calc-alkaline granites of the host batholith, indicating a transition from thick orogenic to extended crust. Significant elevation of the δ 18 O of feldspars occurred through water–rock interaction at moderate temperatures (100–250 °C), most probably during a thermal event in Early Carboniferous times.