Duluth Complex Research Articles

Evaluating reference materials and common-Pb corrections for high-resolution apatite U[sbnd]Pb geochronology

We report isotope dilution thermal ionization mass spectrometry (ID-TIMS) and laser ablation split stream inductively coupled plasma mass spectrometry (LASS) UPb data for a suite of widely available reference apatites: Fish Canyon Tuff, Mount Dromedary, TEMORA 2, and Duluth Complex anorthosite. We apply different common-Pb correction strategies to the UPb data sets: (1) anchoring to a Stacey and Kramers (1975) model Pb composition; (2) unanchored 2-D 238U/206Pb-207Pb/206Pb isochron regressions; and (3) unanchored 3-D 238U/206Pb-207Pb/206Pb-204Pb/206Pb isochron regressions. The different common-Pb corrections yield consistent dates within each ID-TIMS and LASS data set, with 3-D regression method producing the highest precision isochrons. FCT apatite produces an ID-TIMS 3-D isochron age of 28.8 ± 3.7 Ma with (207Pb/206Pb)i = 0.851 ± 0.021. Mount Dromedary apatite yields an ID-TIMS 3-D isochron age of 98.4 ± 0.5 Ma with (207Pb/206Pb)i = 0.839 ± 0.003. TEMORA 2 apatite has an ID-TIMS 3-D isochron age of 402 ± 7 Ma and (207Pb/206Pb)i = 0.839 ± 0.008. Duluth Complex anorthosite apatite yields an ID-TIMS 3-D isochron age of 1077 ± 9 Ma with (207Pb/206Pb)i = 0.849 ± 0.046. The MSWDs associated with isochrons calculated from both the ID-TIMS and LASS data sets are larger than expected for a single age population, revealing complexities that are otherwise not captured by 2-D isochron methods. In the case of FCT apatite, the ID-TIMS data indicate significant heterogeneity in the initial Pb ratio ((207Pb/206Pb)i = 0.845–0.856), invalidating this sample as a viable reference apatite for high-precision geochronology. Additionally, the common-Pb compositions of TEMORA 2 and Duluth Complex anorthosite apatites calculated using the ID-TIMS data deviate from bulk Earth Pb evolution models beyond 2σ uncertainty. The data emphasize the utility of unanchored age regressions in generating the highest fidelity apatite UPb dates. Further, TEMORA 2 and Duluth Complex apatite ages are both younger than their corresponding zircon UPb ages, highlighting the need to independently verify the ages of prospective reference apatites.

Sedimentary and metamorphic processes priming black shale for magmatic assimilation of sulfur: an example from the Virginia Formation, Minnesota, United States

AbstractThe copper-nickel(-platinum-group element) sulfide resources of the Duluth Complex, Minnesota, USA, formed by assimilation of sulfur from the Virginia Formation black shale. In the normal black shale of the Virginia Formation, sulfur is mainly hosted in disseminated pyrite, whereas mm-scale pyrrhotite laminae dominate in the sulfur-rich Bedded Pyrrhotite Unit. The Bedded Pyrrhotite Unit was the main supply of sulfur in some of the magmatic sulfide deposits but its origin has not been studied in detail. Using Raman spectroscopy, we show that the carbonaceous material within the regionally metamorphosed normal black shale is graphitized biogenic material. The Bedded Pyrrhotite Unit contains pyrobitumen that represents residues of oil that accumulated to porous horizons, which formed due to dissolution of precursor sedimentary clasts. Replacement of the clasts by quartz and sulfides facilitated the formation of the pyrrhotite laminae of the Bedded Pyrrhotite Unit, which likely occurred during regional metamorphism.The pyrite-bearing normal black shale experienced loss of H2O, Corg, and sulfur during devolatilization caused by the Duluth Complex. The contact-metamorphosed Bedded Pyrrhotite Unit shows no systematic depletion of volatiles and is the most Corg and sulfur-rich part of the Virginia Formation. During devolatilization, sulfur was preserved because unlike pyrite, pyrrhotite was stable. Consequently, magmatic assimilation of sulfur from the Bedded Pyrrhotite Unit required partial melting. Retrograde hydration introduced H2O, and possibly Corg, and sulfur, to the contact-metamorphosed Bedded Pyrrhotite Unit, which further affected the volatile budget. Our findings highlight why constraining diagenetic and regional metamorphic processes is important to understand magma-sediment interaction processes.

Genesis of Fe–Ti Oxide-Bearing Ultramafic Intrusions in the Duluth Complex, Minnesota, USA

Abstract The Duluth Complex is a large mafic intrusive system located in northeastern Minnesota emplaced as part of the 1.1-Ga Midcontinent Rift. Several Fe–Ti oxide-bearing ultramafic intrusions are hosted along the Western Margin of the Duluth Complex, and are discordant bodies present in a variety of geometries, hosted in multiple rock types, and dominated by peridotite, pyroxenite, and semi-massive to massive Fe–Ti oxide rock types. Their origin has been debated, and here we present geochemical evidence and modeling that supports a purely magmatic origin for the Titac and Longnose Fe–Ti oxide-bearing ultramafic intrusions. Ilmenite and titanomagnetite textures indicate a protracted cooling process, and δ34S values of sulfides reveal little assimilation of the footwall Virginia Formation, a fine-grained pelitic unit that contains sulfide-rich bands. We model the crystallization of a hypothetical parental magma composition to the host intrusion of Longnose using Rhyolite-MELTS and demonstrate that the accumulation of Fe–Ti oxides in the discordant intrusions cannot be explained by density-driven segregation of crystallized Fe–Ti oxides. Instead, we show that the development of silicate liquid immiscibility, occurring by the unmixing of the silicate melt into conjugate Si- and Fe-rich melts, can result in the effective segregation and transportation of the Fe-rich melt. The Fe-rich melt is ~2 orders of magnitude less viscous than the Si-rich melt, allowing the Fe-rich melt to be more effectively segregated and transported in the mush regime (crystallinities >50%). This suggests that viscosity, in addition to density, plays a significant role in forming the discordant Fe–Ti oxide-bearing ultramafic intrusions. We propose a genetic model that could also be responsible for the Fe–Ti oxide-rich layers or bands that are hosted within the igneous stratigraphy of mafic intrusions of the Duluth Complex.

High-precision U-Pb geochronology links magmatism in the Southwestern Laurentia large igneous province and Midcontinent Rift

Abstract The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25−0.24+0.67 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ~2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ~2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift.

Microbial communities from weathered outcrops of a sulfide-rich ultramafic intrusion, and implications for mine waste management.

The Duluth Complex (DC) contains sulfide-rich magmatic intrusions that represent one of the largest known economic deposits of copper, nickel, and platinum group elements. Previous work showed that microbial communities associated with experimentally-weathered DC waste rock and tailings were dominated by uncultivated taxa and organisms not typically associated with mine waste. However, those experiments were designed for kinetic testing and do not necessarily represent the conditions expected for long-term environmental weathering. We used 16S rRNA gene methods to characterize the microbial communities present on the surfaces of naturally-weathered and historically disturbed outcrops of DC material. Rock surfaces were dominated by diverse uncultured Ktedonobacteria, Acetobacteria, and Actinobacteria, with abundant algae and other phototrophs. These communities were distinct from microbial assemblages from experimentally-weathered DC rocks, suggesting different energy and nutrient resources in environmental samples. Sulfide mineral incubations performed with and without algae showed that photosynthetic microorganisms could have an inhibitory effect on autotrophic populations, resulting in slightly lower sulfate release and differences in dominant microorganisms. The microbial assemblages from these weathered outcrops show how communities develop during weathering of sulfide-rich DC rocks and represent baseline data that could evaluate the effectiveness of future reclamation of waste produced by large-scale mining operations.

Duluth Complex FC1 Apatite and Zircon: Reference Materials for (U‐Th)/He Dating?

The accuracy and validation of geo‐ and thermochronological dating hinges on the availability of well‐characterised age reference materials. The Mesoproterozoic gabbroic anorthosite FC1 from the Duluth Complex, Minnesota is a reference material for zircon U‐Pb and a suggested reference material for apatite fission‐track dating. We evaluate FC1 as (U‐Th)/He reference material, and determine its apatite U‐Pb, and zircon and apatite (U‐Th)/He age. Our dating results constrain the thermal history of FC1, showing that fast cooling occurred between ~ 1099 and 1040 Ma from ≥ 600 °C to ~ 200 °C. The zircon (U‐Th)/He data from air‐abraded grains give a robust isochron age of 1037 ± 25 Ma (2s) without overdispersion. The within‐grain homogeneity of U and Th, the availability of FC1 zircon, and the absence of radiation‐damage effects on the (U‐Th)/He age support its use as reference material. Unabraded zircon grains give lower and more dispersed ages, highlighting the usefulness of air abrasion to control for α‐ejection in (U‐Th)/He dating. Our apatite (U‐Th‐Sm)/He single‐grain ages vary between 180 and 300 Ma. Their wide dispersion argues against the use of FC1 apatite as (U‐Th‐Sm)/He reference material and makes the interpretation of their low‐temperature thermal history complicated.

Observational constraints on the process and products of Martian serpentinization.

The alteration of olivine-rich rocks to serpentine minerals, (hydr)oxides, and aqueous hydrogen through serpentinization is long thought to have influenced the distribution of habitable environments on early Mars and the evolution of the early Martian hydrosphere and atmosphere. Nevertheless, the planetary importance of Martian serpentinization has remained a matter of debate. To constrain the process and products of Martian serpentinization, we studied serpentinized iron-rich olivines from the 1.1-billion-year Duluth Complex. These data indicate that serpentinized iron-rich olivine would have been accompanied by a fivefold increase in hydrogen production relative to serpentinized terrestrial mantle peridotites. In contrast to previous expectations, this style of serpentinization yields hisingerite as the dominant iron serpentine mineral at comparatively low temperature and pH, consistent with meteorite mineralogy and in situ rover data. The widespread occurrence of oxidized iron-bearing phyllosilicates in highly magnetized regions of the Martian crust supports the hypothesis that serpentinization was more pervasive on early Mars than currently estimated.

Sulfur Content and Speciation in Dacitic Silicate Melts as a Function of Oxygen Fugacity and the Genesis of Fe-Ti Oxide-Bearing Ultramafic Intrusions of the Duluth Complex, MN

Sulfur Content and Speciation in Dacitic Silicate Melts as a Function of Oxygen Fugacity and the Genesis of Fe-Ti Oxide-Bearing Ultramafic Intrusions of the Duluth Complex, MN

Complex Effects of Assimilation on Sulfide Saturation Revealed by Modeling with the Magma Chamber Simulator: A Case Study on the Duluth Complex, Minnesota, USA

Abstract Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.

Fluids as primary carriers of sulphur and copper in magmatic assimilation

Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic–ultramafic layered intrusion-hosted Cu-Ni sulphide deposits.

Posteruptive Thermal History of the Proterozoic Basaltic North Shore Volcanic Group of the Midcontinent Rift: Evidence from K/Ar Data of Celadonite

Abstract This study reports three K/Ar ages on celadonite, a dioctahedral K-Fe mica, in the Proterozoic North Shore Volcanic Group (NSVG) of the Midcontinent Rift in northeastern Minnesota. Celadonite formed during beginning posteruptive, low-temperature conditions at temperatures<100°C and with input of meteoric water. K/Ar ages between 1062±16 Ma and 955.0±12 Ma document a remarkably long posteruptive thermal history of >100 myrs in a thick continental basaltic sequence. In the stratigraphically lower part of the NSVG, celadonite formation occurred at 1062±16 Ma in an amygdule or a vesicle filled with celadonite, while another celadonite amygdule in a stratigraphically higher flow was dated at 1039.4±14 Ma. Both flows are overprinted by a later multistage lower zeolite-phyllosilicate facies assemblage (laumontite-albite-corrensite±chlorite±smectite±prehnite±pumpellyite). In the stratigraphically higher part of the sequence, celadonite crystallization at an amygdule rim is followed by upper zeolite facies conditions (stilbite-heulandite-smectite assemblage) and was dated at 955.0±12.4 Ma. The constrained time frame of 107 myrs indicates a long-lived, probably not continuous and locally occurring, posteruptive thermal alteration process. The data suggest that alteration was depth-controlled and temporally and spatially inhomogeneous and implies the progression of the sequence from a close-to-the-surface alteration mode with input of meteoric water to a burial metamorphic mode and with locally occurring hydrothermal activity due to continuous magmatic activity. Volcanism in the Midcontinent Rift system is supposed to have lasted between 1109 Ma and 1083 Ma based on U/Pb zircon ages. The first crystallization of celadonite is recorded in the lower part of the NSVG and occurred ca. 30 myrs after the emplacement of the Silver Bay aplite intrusion in the upper part of the NSVG. Burial rates are determined to be 0.04 km·Ma-1and 0.10 km·Ma-1. The hydrothermal alteration under low-temperature burial conditions clearly postdates the rift-related alkaline and tholeiitic magmatism of the Midcontinent Rift and overlaps with the depositional window of the sedimentary rocks that overlie the Midcontinent Rift volcanics, as well as crustal-scale fault systems that were active during Grenvillian tectonic uplift after the cessation of magmatic activity.

Balancing Socio-Ecological Risks, Politics, and Identity: Sustainability in Minnesota’s Copper-Nickel-Precious Metal Mining Debate

In the northeastern corner of Minnesota, two of the state’s most iconic symbols, mining and pristine wilderness, are on a collision course. The Duluth Complex, considered by many to be the world’s largest undeveloped deposit of copper-nickel and precious metals, is the site of mining proposals for several controversial mines. Proponents suggest that mining can be accomplished in an environmentally benign manner, and in the process create nearly 1000 jobs and $500 million in economic benefits annually. Opponents counter that the tourism and recreation industries already provide nearly 18,000 jobs and bring over $900 million in economic benefits annually, and that mining will permanently impair the regions environment. Thus, the copper-nickel and precious metal mining debate has become highly polarized, and serves as an ideal example of how people address national and global sustainability issues at local and regional scales. This study examines this polarization through a Q-sort analysis of subjectivities of residents of the state of Minnesota. Results suggest that partisanship is a strong predictor of attitudes towards mining, and that the strongest differences between respondents were not based on perceptions comparing jobs and the environment, the typical partisan divide, but rather on respondents’ perceived identity with relation to the mining industry or water resources.

Rapid emplacement of massive Duluth Complex intrusions within the North American Midcontinent Rift

Abstract The Duluth Complex (Minnesota, USA) is one of the largest mafic intrusive complexes on Earth. It was emplaced as the Midcontinent Rift developed in Laurentia’s interior during an interval of magmatism and extension from ca. 1109 to 1084 Ma. This duration of magmatic activity is more protracted than is typical for large igneous provinces interpreted to have formed from decompression melting of upwelling mantle plumes. While the overall duration was protracted, there were intervals of more voluminous magmatism. New 206Pb/238U zircon dates for the anorthositic and layered series of the Duluth Complex constrain these units to have been emplaced ca. 1096 Ma in <1 m.y. (duration of 500 ± 260 k.y.). Comparison of paleomagnetic data from these units with Laurentia’s apparent polar wander path supports this interpretation. This rapid emplacement bears similarities to the geologically short duration of well-dated large igneous provinces. These data support hypotheses that call upon the co-location of lithospheric extension and anomalously hot upwelling mantle. This rapid magmatic pulse occurred >10 m.y. after initial magmatism following >20° of latitudinal plate motion. A likely scenario is one in which upwelling mantle encountered the base of Laurentian lithosphere and flowed via “upside-down drainage” to locally thinned lithosphere of the Midcontinent Rift.

How American Mineralogist and the Mineralogical Society of America influenced a career in mineralogy, petrology, and plate pushing, and thoughts on mineralogy's future role

Abstract My geologic research began at Carleton College. I studied heavy minerals in some midcontinent orthoquartzites, publishing my very first paper in American Mineralogist in 1954. As a master's candidate at the University of Minnesota, I investigated igneous differentiation in a diabase-granophyre sill of the Duluth Gabbro Complex. Later, in a Ph.D. program at Johns Hopkins University, I became Joe Boyd's apprentice at the Geophysical Laboratory (GL), and for a time was phase-equilibrium god of the Na-amphiboles. Doctoral research earned me an offer of a UCLA assistant professorship as a mineralogist in 1960. There, I continued pursuing amphibole P-T stability relations in lab and field. My glaucophane phase equilibrium research would later be found to have instead crystallized Na-magnesiorichterite. However, amphibole research led me to map field occurrences of HP-LT (high P-low T) blueschists of the Franciscan Complex. Thus, when plate tectonics emerged in the late 1960s, I was deep in the subduction zone. My recent studies focused on the petrology and geochemistry of oceanic crustal rocks, Californian calc-alkaline arcs, and coesite ± microdiamond-bearing crustal margin rocks in various parts of Eurasia. Other works treated global mineral resources and population, mineralogy and human health, and early Earth petrotectonic evolution. I tried to work on important problems, but mainly studied topics that fired my interest. For the future, I see the existential challenge facing humanity and the biosphere as the imperative to stop our overdrafting of mineral resources. This will require reaching a dynamic equilibrium between the use and replenishment of near-surface resources (i.e., nutrients) essential for life. Earth scientists are planetary stewards, so we must lead the way forward in life-supporting mineral usage, recycling, substitution, and dematerialization. In any event, sustainable development will soon return to the Earth's Critical Zone of life because Mother Nature—the ruling terrestrial economist—abhors long-term overdrafting of resources1.

Evidence for a concealed Midcontinent Rift-related northeast Iowa intrusive complex

Large amplitude aeromagnetic and gravity anomalies over a ~9500 km2 area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC), a buried intrusive igneous complex composed of mafic/ultramafic rocks in the Yavapai Province (1.8–1.7 Ga). Hundreds of meters of Paleozoic sedimentary cover and a paucity of basement drilling have prevented detailed studies of the NEIIC. Long considered, but not proven, to be related to the ~1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent to the richly mineralized Duluth Complex and is similarly located near the margin of the MRS. New geochronological and geophysical data together support an MRS affinity for the NEIIC. A dike swarm imaged in aeromagnetic data is cut by intrusions of the NEIIC, and a new apatite U-Pb date of ~1170 Ma on one of the dikes thus represents a maximum age for the NEIIC. A minimum age constraint is suggested by (1) large-volume magmatism associated with the MRS that was the last such event to affect the region; and (2) the presence of reversely magnetized dikes, similar in character to MRS-related dikes elsewhere, that cut several intrusions of the NEIIC. The NEIIC is largely characterized by the presence of multiple zoned intrusions, many of which contain large volumes of mafic-ultramafic rocks and have strong geophysical similarities to alkaline intrusive complexes elsewhere, including the MRS-related Coldwell Complex of Ontario. The largest of the zoned intrusions are ~40 km in diameter and are interpreted to have thicknesses of many kilometers. Suspected faults, alignments of intrusions, and intrusive margins tend to be aligned along northwest and northeast trends that match the trends of the Belle Plaine fault zone and Fayette structural zone, both previously interpreted as pre-MRS, possibly lithospheric-scale discontinuities that may have controlled NEIIC emplacement. These interpretations collectively imply notable potential for the NEIIC to host several different types of undiscovered base metal and critical mineral deposits.

Dating a Martian meteorite with 20 Myr precision using a prototype in-situ dating instrument

We are continuing the development of a compact resonance ionization mass spectrometer for geochronology of planetary specimens, ultimately for in-situ use on the Moon or Mars. We previously demonstrated 87Rb–87Sr isochrons for the Martian meteorite Zagami and the Duluth Gabbro with a best precision for Zagami of 100 Myr, using 10-ns, 250-μJ laser ablation pulses of 213-nm radiation. In that work, we suggested that our precision had been limited by instabilities in the nanosecond ablation process, which strongly and variably fractionated Rb from Sr. To overcome this limitation, we have replaced the ablation laser with one generating 150-fs, 300-μJ pulses. Furthermore, by modifying the geometry of the sample to reduce plasma penetration into the mass spectrometer, we have improved our instrumental sensitivity, which in turn improves the dating precision we are able to achieve. Our age determination of 150 ​± ​20 ​Ma is 5 times more precise than our best previous reported age, and is no longer limited by the precision we can achieve on the Rb/Sr ratios of individual spots. Instead, we are now limited by the precision we can achieve in the 87Sr/86Sr ratios of the relatively few spots with the highest Rb/Sr ratios. We discuss prospects for even further improvements in the dating precision achievable with resonance ionization mass spectrometry.

High‐Precision, High‐Accuracy Oxygen Isotope Measurements of Zircon Reference Materials with the SHRIMP‐SI

Analytical protocols for SHRIMP‐SI oxygen isotope analysis (δ18O) of a suite of zircon reference materials (RMs) are presented. Data reduction involved a robust estimate of uncertainties associated with the individual spot as well as for groups where the spot data are combined. The repeatability of δ18O measurements is dependent on both the analytical conditions and the choice of the primary reference material. Under optimised conditions, repeatability was often better than 0.4‰ (2s) allowing sample uncertainties to be obtained to better than 0.2‰ (at 95% confidence limit). Single spot uncertainty combined the within‐spot precision with the scatter associated with repeated measurements of the primary zircon reference material during a measurement session. The uncertainty for individual spots measured under optimised conditions was between 0.3 and 0.4‰ (at 95% confidence). The analytical protocols described were used to assess a variety of zircon RMs that have been used for geochronology and for which laser fluorination oxygen isotope data are available (Temora 2, FC1, R33, QGNG and Plešovice), as well as zircons that have been used as RMs for trace element or other types of determination (Mud Tank, Monastery, 91500, AS57, AS3, KIM‐5, OG1, SL13, CZ3 and several other Sri Lankan zircons). Repeated analyses over nine sessions and seven different mounts show agreement within analytical uncertainty for Temora 2, FC1, R33, QGNG, Plešovice and 91500, when normalised to Mud Tank. For existing ion microprobe mounts with these materials, an appropriate δ18O can be determined. However, care should be taken when using zircons from the Duluth Complex (i.e., FC1, AS57 and AS3) as reference materials as our data indicated an excess scatter on δ18O values associated with low‐U zircon grains.

Possible sources for CH4-rich fluid inclusions in olivine from troctolitic rocks of the Bathtub intrusion, Duluth Complex, U.S.A.

Cumulus olivine from the Babbitt deposit in the troctolitic Bathtub intrusion (Duluth Complex) contains abundant CH4-rich fluid inclusions with common hydrous silicate and carbonate precipitates at their inclusion walls. Given the active rifting environment in which magma of the Duluth Complex (1.1 Ga) has intruded as repeated pulses, fluid inclusions in olivine are traceable throughout an entire stratigraphic drill core section with comparable textural features that enable a classification of three generations: (1) primary CH4-rich fluid inclusions oriented parallel to growth planes of olivine or arranged in clusters; (2) intragranular fluid inclusion planes of CH4-rich fluid inclusions extending towards grain boundaries and (3) transgranular fluid inclusion planes of CH4-rich fluid inclusions, closely associated with serpentinization. The hydrous minerals and carbonates contained in fluid inclusions of generations (1) and (2) indicate that original constituents of the early homogenous magmatic fluid must have been H2O and CO2. Upon cooling (below 700 °C), at fixed Pfluid (2 kbar) and fO2fluid at or below QFM conditions, an increase of XCH4 at the expense of CO2 results in the CH4-rich chemistry of the fluid. The formation of late secondary fluid inclusion planes of generation (3) is related to extensive magma-country rock interaction. These inclusions contain no solids and their composition reflects the reduction of CO2 to CH4 during serpentinization in an upper stability limit of 620 °C. This study emphasizes fluid inclusions as powerful indicators in remodeling chemical changes of the magmatic fluid during crystallization of cumulus phases, magma-country rock interaction and subsequent alteration.

Duluth Complex apatites: Age reference material for LA–ICP–MS‐based fission‐track dating

AbstractAnorthositic series apatites of the Duluth Complex, Minnesota, USA, have high spontaneous fission‐track densities of up to ~107 cm–2 and a homogeneous age of ~900 Ma, allowing high‐precision fission‐track dating based on LA–ICP–MS U analysis. Absolute fission‐track dating, track‐length measurement and chemical composition analysis were performed to evaluate a cooling history, which is essential for age reference materials. Preliminary inverse modelling for a sample with a shortened track‐length distribution yielded a monotonic cooling history from ~100°C at 925 Ma. The apatites incur an over‐etching problem when employing the commonly used etching protocol involving 5.5 M HNO3.

Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis

Author(s): Swanson-Hysell, NL; Ramezani, J; Fairchild, LM; Rose, IR | Abstract: The late Mesoproterozoic was a time of large-scale tectonic activity both in the interior and on the margins of Laurentia-most notably the development of the Midcontinent Rift and the Grenvillian orogeny. Volcanism within the North American Midcontinent Rift between ca. 1109 and 1083 Ma, as well as other contemporaneous volcanism within Laurentia, has provided an opportunity to develop extensive paleomagnetic data sets spanning this time period. These data result in an apparent polar wander path (APWP) for Laurentia that goes from a high-latitude apex known as the Logan Loop into a swath known as the Keweenawan Track. A longstanding challenge of these data was the appearance of asymmetry between relatively steep reversed polarity directions from older rift rocks and relatively shallow normal polarity directions from younger rift rocks. This asymmetry was used to support an interpretation that there were large non-dipolar components to the geomagnetic field at the time. Recent data sets support the interpretation that this directional change was progressive and therefore a result of very rapid motion of Laurentia from high to low latitudes rather than a stepwise change across non-dipolar reversals. We present high-precision U-Pb dates from Midcontinent Rift volcanics that result in an improved chronostratigraphic framework for rift volcanics and unconformities that improves correlations as well as constraints on rift development. We use these dates in volcanostratigraphic context to temporally constrain a new compilation of Midcontinent Rift paleomagnetic poles. These paleomagnetic poles include new data from the North Shore Volcanic Group, Minnesota, USA and the Osler Volcanic Group, Ontario, Canada. The U-Pb dates constrain the rate of implied plate motion more precisely than has previously been possible. We apply a novel Bayesian approach to assess the rate of implied plate motion through inverting for paleomagnetic Euler poles. If the path is to be explained by a single Euler pole these inversions reveal that motion of the continent exceeded 27 cm/yr. The path is particularly well-explained by a model wherein there is continuous true polar wander in addition to rapid plate motion that changes direction and slows ca. 1096 Ma. Laurentia's movement from high to low latitudes resulted in collisional tectonics on its leading margin which could be associated with such a change in plate motion. We propose that upwelling of the Keweenawan mantle plume was associated with an avalanche of subducted slab material and associated downwelling that drove fast plate motion. This fast plate motion was followed by the Grenvillian orogeny from ca. 1090 to 980 Ma. Prolonged collisional orogenesis could have been sustained due to this strong convective cell that therefore played an integral role in the assembly of the supercontinent Rodinia.