Bruce A. Kjarsgaard

Timing of eastern North American kimberlite magmatism: continental extension of the Great Meteor hotspot track?

Abstract Twenty-nine new high precision U–Pb perovskite ages for kimberlite and other CO2-rich ultrabasic rocks, primarily from five fields or clusters in eastern North America (Rankin Inlet, Attawapiskat, Kirkland Lake, Timiskaming and Finger Lakes), indicate that at least five periods of Mesozoic kimberlite magmatism can be distinguished. These new data document four periods of kimberlite magmatism previously unrecognized in eastern North America: at 196, 180–176, 148–146 and 142–134 Ma. In addition, the detailed emplacement history of Jurassic kimberlites in the Kirkland Lake field indicates magmatism spanned a period of 13 Myr from 165 to 152 Ma with approximately half of the kimberlites in this field emplaced in less than 2 Myr between 156.9–155.3 Ma. These U–Pb results demonstrate for the first time that there is a NW–SE Triassic to Cretaceous age progression in kimberlite magmatism, which is consistent with the relative plate motions of North America during this interval. This age progression in kimberlite magmatism extends for more than 2000 km from Rankin Inlet through to the Attawapiskat, Kirkland Lake and Timiskaming fields and is interpreted, in part, to be the continental expression of magmatism linked to the Great Meteor mantle plume hotspot track. If correct, then the timing and location of this magmatism provides a more rigid constraint for both the exact position of the hotspot and the relative direction and rate of North American plate motion during the Mesozoic opening of the North Atlantic Ocean.

A Re-Os isotope and PGE study of kimberlite-derived peridotite xenoliths from Somerset Island and a comparison to the Slave and Kaapvaal cratons

The concentrations of platinum-group elements (PGE; Os, Ir, Ru, Pd and Pt) and Re, and the Os isotopic compositions were determined for 33 lithospheric mantle peridotite xenoliths from the Somerset Island kimberlite field. The Os isotopic compositions are exclusively less radiogenic than estimates of bulk-earth (187Os/188Os as low as 0.1084) and require a long-term evolution in a low Re–Os environment. Re depletion model ages (TRD) indicate that the cratonic lithosphere of Somerset Island stabilised by at least 2.8 Ga, i.e. in the Neoarchean and survived into the Mesozoic to be sampled by Cretaceous kimberlite magmatism. An Archean origin also is supported by thermobarometry (Archean lithospheric keels are characterised by >150 km thick lithosphere), modal mineralogy and mineral chemistry observations. The oldest ages recorded in the lithospheric mantle beneath Somerset Island are younger than the Mesoarchean (>3 Ga) ages recorded in the Slave craton lithospheric mantle to the southwest [Irvine, G.J., et al., 1999. Age of the lithospheric mantle beneath and around the Slave craton: a Rhenium–Osmium isotopic study of peridotite xenoliths from the Jericho and Somerset Island kimberlites. Ninth Annual V.M. Goldschmidt Conf., LPI Cont., 971: 134–135; Irvine, G.J., et al., 2001. The age of two cratons: a PGE and Os-Isotopic study of peridotite xenoliths from the Jericho kimberlite (Slave craton) and the Somerset Island kimberlite field (Churchill Province). The Slave–Kaapvaal Workshop, Merrickville, Ontario, Canada]. Younger, Paleoproterozoic, TRD model ages for Somerset Island samples are generally interpreted as the result of open system behaviour during metasomatic and/or magmatic processes, with possibly the addition of new lithospheric material during tectono-thermal events related to the Taltson–Thelon orogen. PGE patterns highly depleted in Pt and Pd generally correspond to older Archean TRD model ages indicating closed system behaviour since the time of initial melt extraction. Younger Proterozoic TRD model ages generally correspond to more complex PGE patterns, indicating open system behaviour with possible sulfide or melt addition. There is no correlation between the age of the lithosphere and depth, at Somerset Island.

Emplacement and reworking of Cretaceous, diamond-bearing, crater facies kimberlite of central Saskatchewan, Canada

In central Saskatchewan, Canada, kimberlites were emplaced into Cretaceous marine and nonmarine clastic sediments. Core recovered from one drill hole that intersects kimberlite (Smeaton FAC/UK core 169/8) was selected for an integrated study involving sedimentology, volcanology, mineralogy, geochemistry, palynology, micropaleontology, organic petrology, and radiometric age determination. Only crater facies kimberlite has been observed; there is no indication of the locations of feeder dikes. Four varieties of kimberlite occur, all originating from subaerial volcanism: (1) fluvial-reworked kimberlite; (2) diamondiferous kimberlite lapillistone air-fall deposits; (3) kimberlite olivine crystal-tuff air-fall deposits; and (4) diamondiferous marine wave-reworked kimberlite. Within the multiple primary eruptive phases of the kimberlite air-fall deposits, the volcanic style changed upward with time, from violent Strombolian to more explosive volcanism. The bulk of the volcanism formed conformable, air-fall deposits on terrestrial sediments of the Cantuar Formation, resulting in the development of positive-relief tephra cones. Subsequent marine transgression associated with the Westgate Formation partially beveled the top of the cone. The kimberlite air-fall deposits contain microdiamonds, 5 to 25 μm in diameter. The maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits, although originally derived from magma at high temperatures, did not thermally affect entrained surficially derived clasts or the country rock during emplacement. The chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile. At least two episodes of kimberlite volcanism occurred in the middle and late Albian (paleontologically assigned). A U-Pb perovskite radiometric age of 101.1 ± 2.2 Ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post–middle Albian and pre–late Albian to late Albian.

Absence of a high time-integrated 3He/(U+Th) source in the mantle beneath continents

Volcanic rocks from ocean island and continental flood basalt provinces can exhibit 3He/4He ratios greatly in excess of those of mid-oceanic-ridge basalts (MORB). High 3He/4He ratios must indicate derivation from a mantle source with high time-integrated 3He/(U+Th) relative to depleted MORB-source mantle. The location of the high 3He/4He mantle reservoir is a poorly resolved but important issue because of the constraints it places upon the structure and convective style of Earths mantle. It has been proposed that the high 3He/4He reservoir resides in the upper mantle, rather than the lower mantle, because Earth should be volatile poor and highly differentiated, with incompatible elements (such as He) concentrated in the upper mantle and crust. This hypothesis can be tested using continental intraplate alkaline volcanics (CIAV) that are generated at or near the boundary between the conducting lithospheric and convecting asthenospheric mantle. Olivine and clinopyroxene phenocrysts from Cretaceous to Miocene CIAV from Canada, South Africa, and Uganda have 3He/4He ratios more radiogenic than MORB, strongly arguing against a widespread high 3He/4He source in the continental lithosphere or the underlying convecting upper mantle. Combined with a global data set of CIAV and continental lithosphere mantle xenoliths, these results provide no evidence for high 3He/4He in any samples known to originate from this environment. Therefore, volcanic rocks with 3He/4He greater than MORB 3He/4He are likely to sample a mantle source with high time-integrated 3He/(U+Th) that cannot exist within or below the continents. This reservoir is also unlikely to exist within the upper mantle as defined by the 3He/4He distribution in MORB.

Spatter-dike reveals subterranean magma diversions: Consequences for small multivent basaltic eruptions

Small-volume, short-lived basaltic volcanoes, Earth9s most common subaerially, are abundant and tractable in size, thus offering opportunities for learning how magma behaves in the critical transition from coherent magma in dikes to pyroclastic dispersions. Spatter-dikes at the Castle Butte Trading Post volcanic complex (Hopi Buttes volcanic field, Arizona, USA), exposed ∼150 m below the paleosurface, present an outstanding record of this transition. They record pulsatory magma rise and eruption, fissure extension, vent stepping, and conduit wall-rock failure, providing a critical link to activity witnessed and geophysically monitored during historic eruptions of well-observed and -monitored large-volume basaltic volcanoes. Castle Butte comprises four linked and closely spaced spatter-dikes and maar-diatremes. The spatter-dikes consist of bedded, variably welded deposits plus wall-rock debris in multiple sequences demarcated by truncation surfaces that indicate progressive southwest-northeast emplacement. They reveal a shallow plumbing cycle of pulsating, weak, hot spatter fragmentation, concurrent wall-rock failure and periodic slips that truncated and downdropped bedded deposits. In addition to vertical pulsation, we infer from the northeastward-younging spatter succession that repeated magma diversions facilitated episodes of magma withdrawal that caused the slips during northeastward extension of the complex toward the large maar-diatreme.

Quantification of dissolved CO2 in silicate glasses using micro-Raman spectroscopy

Abstract This study investigates the potential use of confocal micro-Raman spectroscopy for the quantification of CO2 in geologically relevant glass compositions. A calibration is developed using a wide range of both natural and synthetic glasses that have CO2 dissolved as carbonate (CO32-) in the concentration range from 0.2 to 16 wt%. Spectra were acquired in the 200 and 1350 cm-1 frequency region that includes the ν1 Raman active vibration for carbonate at 1062-1092 cm-1 and the intensity of this peak is compared to various other peaks representing the aluminosilicate glass structure. The most precise and accurate calibration is found when carbonate peaks are compared to aluminosilicate spectral features in the high-frequency region (HF: 700-1200 cm-1), which can be simulated with several Gaussian peaks, directly related to different structural species in the glass. In some samples the “dissolved” CO32- appears to have two different Raman bands, one sharper than the other. This may be consistent with previous suggestions that CO32- has several structural environments in the glass, and is not related to any precipitation of crystalline carbonate from the melt during quenching. The calibration derived using the HF peaks appears linear for both the full range of glass composition considered and the range of CO2 concentrations, even when multiple carbonate peaks are involved. We propose the following, compositionally independent linear equation to quantify the CO2 content in glass with micro-Raman spectroscopy wt% CO2 = 15.17 × CO3/HF where CO3/HF is the area ratio of the fitted ν1 carbonate peak(s) at 1062-1092 cm-1 to the remaining area of the fitted aluminosilicate envelope from 700-1200 cm-1. This is similar to the Raman calibration developed for water, but is complicated by the overlapping of these two fitted components. Using error propagation, we propose the calibration accuracy is better than ±0.4 wt% CO2 for our data set. The ν1 Raman peak position for carbonate is not constant and appears to be correlated with the density of the melt (or glass) in some way rather than the chemical composition.

Lithospheric architecture of the Slave craton, northwest Canada, as determined from an interdisciplinary 3-D model

Regional-scale geologic structures characteristic of mantle lithosphere within cratons found in continent interiors are interpreted using geo-registered diverse data sets from the Slave craton of northwest Canada. We developed and applied a new method for mapping seismic discontinuities in three dimensions using multiyear observations at sparse, individual broadband receivers. New, fully 3-D conductivity models used all available magnetotelluric data. Discontinuity surfaces and conductivity models were geo-registered with previously published P-wave and surface-wave velocity models to confirm first-order structures such as a midlithosphere discontinuity. Our 3-D model to 400 km depth was calibrated by “drill hole” observations derived from xenolith suites extracted from kimberlites. A number of new structural discontinuities emerge from direct comparison of coregistered data sets and models. Importantly, we distinguish primary mantle layers from secondary features related to younger metasomatism. Subhorizontal Slave craton layers with tapered, wedge-shaped margins indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. Mapping of conductivity and metasomatism in 3-D, the latter inferred via mineral recrystallization and resetting of isotopic ages in xenoliths, indicates overprinting of the primary layered structures. The observed distribution of relatively conductive mantle at 100–200 km depths is consistent with pervasive metasomatism; vertical “chimneys” reaching to crustal depths in locations where kimberlites erupted or where Au mineralization is known.

The North America mid-Cretaceous kimberlite corridor: Wet, edge-driven decompression melting of an OIB-type deep mantle source

Thirty new high precision U-Pb perovskite and zircon ages from kimberlites in central North America delineate a corridor of mid-Cretaceous (115 to 92 Ma) magmatism that extends ∼4000 km from Somerset Island in Arctic Canada through central Saskatchewan to Kansas, U.S.A. The least contaminated whole rock Sr, Nd and Hf isotopic data, coupled with Sr isotopic data from groundmass perovskite indicates an exceptionally limited range in Sr-Nd-Hf isotopic compositions, clustering at the low eNd end of the OIB array. These isotopic compositions are distinct from other studied North American kimberlites and point to a sub-lithospheric source region. This mid-Cretaceous kimberlite magmatism cannot be related to mantle plumes associated with the African or Pacific large low-shearwave velocity province (LLSVP). All three kimberlite fields are adjacent to strongly attenuated lithosphere at the edge of the North American craton. This facilitated edge-driven convection, a top-down driven processes that caused decompression melting of the transition zone or overlying asthenosphere. The inversion of ringwoodite and/or wadsleyite and release of H2O, with subsequent metasomatism and synchronous wet partial melting generates a hot CO2- and H2O-rich proto-kimberlite melt. Emplacement in the crust is controlled by local lithospheric factors; all three kimberlite fields have mid-Cretaceous age, re-activated major deep-seated structures that facilitated kimberlite melt transit through the lithosphere.

Tracking the Late Jurassic apparent (or true) polar shift in U-Pb-dated kimberlites from cratonic North America (Superior Province of Canada)

Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (∼30°) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high-precision U-Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9 ± 1.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5 ± 1.2 Ma Triple B kimberlite. The combined ∼156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95 = 2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (∼1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria-Africa and Iran-Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander.