Detrital chromites reveal Slave craton’s missing komatiite

Komatiitic magmatism is a characteristic feature of Archean cratons, diagnostic of the addition of juvenile crust, and a clue to the thermal evolution of early Earth lithosphere. The Slave craton in northwest Canada contains >20 greenstone belts but no identified komatiite. The reason for this dearth of komatiite, when compared to other Archean cratons, remains enigmatic. The Central Slave Cover Group (ca. 2.85 Ga) includes fuchsitic quartzite with relict detrital chromite grains in heavy-mineral laminations. Major and platinum group element systematics indicate that the chromites were derived from Al-undepleted komatiitic dunites. The chromites have low 187Os/188Os ratios relative to chondrite with a narrow range of rhenium depletion ages at 3.19 ± 0.12 Ga. While these ages overlap a documented crust formation event, they identify an unrecognized addition of juvenile crust that is not preserved in the bedrock exposures or the zircon isotopic data. The documentation of komatiitic magmatism via detrital chromites indicates a region of thin lithospheric mantle at ca. 3.2 Ga, either within or at the edge of the protocratonic nucleus. This study demonstrates the applicability of detrital chromites in provenance studies, augmenting the record supplied by detrital zircons.

Timing and provenance of Paleoproterozoic supracrustal rocks in the central Thelon tectonic zone, Canada: implications for the tectonic evolution of western Laurentia from ca. 2.1 to 1.9 Ga

Depositional ages and provenance of metasedimentary rocks provide constraints on the architecture of the interface between the Slave and Rae cratons and processes related to the Thelon Orogen. Clastic rocks analysed from the central Thelon tectonic zone are Paleoproterozoic in age and not remnants of the Archean Yellowknife Supergroup (Slave Province), as originally considered. Two assemblages are recognized. An older clastic assemblage deposited after 2.09 Ga contains detrital zircon age modes of 2.3 and 2.17 Ga, with subordinate Neoarchean and Paleoarchean detritus. Its deposition is interpreted to predate Thelon magmatic activity given that (1) it lacks ca. 2.01–1.97 Ga detritus of Thelon magmatic origin, and (2) correlative clastic rocks occur as inclusions in Thelon plutons and contain ca. 2.0 Ga metamorphic monazite. This assemblage is correlative with both the Mary Frances and Rutledge River groups, establishing a >800 km long basin at ca. 2.1 Ga that received detritus from the western Rae and (or) Buffalo Head terrane(s). Separation from the Slave craton at this time is consistent with the absence of any Slave-affinity detritus. A younger assemblage deposited after 1.95 Ga and prior to 1.91 Ga contains mainly 2.02–1.95 Ga detrital zircon, age modes comparable with adjacent Thelon convergent-margin plutonic rocks. The younger assemblage records deposition of the uplifted and eroded Thelon magmatic arc in an intermontane or foreland basin setting during the later stages of post-collisional convergence. These U–Pb zircon data support a tectonic model for western Laurentia that reconciles differences between the Thelon and Taltson magmatic zones involving ca. 2.1 Ga rifting, ca. 2.01–1.97 Ga convergence, followed by <1.95 Ga thrust-driven exhumation.

Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex

Plate subduction greatly influences the physical and chemical characteristics of Earth’s surface and deep interior, yet the timing of its initiation is debated because of the paucity of exposed rocks from Earth’s early history. We show that the titanium isotopic composition of orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition falls on two distinct magmatic differentiation trends. Hadean tonalitic gneisses show titanium isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean granitoid gneisses have titanium isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. Our data therefore document a shift from tholeiitic- to calc-alkaline–style magmatism between 4.02 and 3.75 billion years (Ga) in the Slave craton.

Localisation of the brittle Bathurst fault on pre-existing fabrics: a case for structural inheritance in the northeastern Slave craton, western Nunavut, Canada

The north–northwest-striking Bathurst fault in the northeastern Slave craton displaced the 1.9 Ga Kilohigok basin and the ca. 2.02–1.96 Ga Thelon tectonic zone, and projects beneath the 1.7 Ga Thelon basin where unconformity-associated uranium deposits are spatially associated with basement faults. Here we investigate the deformation–temperature–time history of the Bathurst fault rocks using structural and microstructural observations paired with U–(Th–)Pb and 40Ar/39Ar geochronology. Highly strained hornblende-bearing granitoid rocks, the predominant rock type on the northeastern side of the Bathurst fault in the study area, show ambiguous sense of shear suggesting flattening by coaxial deformation. Quartz and feldspar microstructures suggest ductile deformation occurred at ≥500 °C. Along the main fault trace, brittle features and hydrothermal alteration overprint the pervasive ductile flattening fabric. In situ U–Th–Pb dating of synkinematic monazite suggests ductile fabric formation at ca. 1933 ± 4 Ma and ca. 1895 ± 11 Ma, and zircon from a cross-cutting dyke constrains the brittle deformation to ≤1839 ± 14 Ma. 40Ar/39Ar dating of fabric-defining minerals yield cooling ages of ca. 1920–1900 Ma and ca. 1900–1850 Ma for hornblende and muscovite, respectively, and a maximum cooling age of ca. 1840 Ma for biotite. We suggest the ca. 1933–1895 Ma ductile flattening fabric developed during orthogonal collision and indentation of the Slave craton into the Thelon tectonic zone and Rae craton. Brittle deformation on the Bathurst fault was localised parallel to the ductile flattening fabric after ca. 1840 Ma and preceded Thelon basin deposition. Brittle deformation features in Bathurst fault rocks preserve evidence for fluid–rock interaction and enhanced basement permeability, suggesting the fault is a possible conduit structure for mineralising fluids.

The oxygen content of sulphide inclusions in diamonds and its use as a mantle geothermometer

&lt;p&gt;Sulphide inclusions in diamonds are commonly used for determining both the timing and lithology of diamond formation. Most sulphide inclusions were trapped as melts which then crystallized as Fe-Ni rich monosulphide solid solutions (MSS). Upon cooling below ~1000&amp;#176;C the inclusions recrystallize to phases such as pyrrhotite, Fe&lt;sub&gt;(1-x)&lt;/sub&gt;S (x = 0 to 0.2), and pentlandite, (Fe,Ni)&lt;sub&gt;9&lt;/sub&gt;S&lt;sub&gt;8&lt;/sub&gt;, and sometimes pyrite (FeS&lt;sub&gt;2&lt;/sub&gt;) depending on the bulk composition. Previous experimental studies have shown that oxygen can also partition into sulphide melts. Moreover, measurements of natural sulphide inclusions in diamonds show measurable oxygen concentrations. A systematic parameterization of factors that control the oxygen concentration of sulphide melts in the mantle could be potentially used to understand formation conditions of diamonds.&lt;/p&gt;&lt;p&gt;We performed a series of high pressure (3-15 GPa) and high temperature (1373 - 2000 K) multi anvil experiments to equilibrate a fertile peridotite (KLB-1) mixture with molten sulphide (FeS). The effects of pressure, temperature, oxygen fugacity and composition (both silicate and sulphide) on oxygen content in sulphide melt have been investigated. We also examined the effect of Ni content in sulphide on the oxygen concentration. Iridium was also added in some experiments in sufficient quantities to saturate the sulphides and produce Fe-Ir alloy, which was used to determine the oxygen fugacity of the experiments. Run products consisted of mantle silicate minerals and quenched sulphide melts. Chemical compositions were analyzed using the electron microprobe.&lt;/p&gt;&lt;p&gt;Our experiments show up to 16 mole% of FeO in the sulphide melts at relevant mantle conditions. Moreover, the oxygen content of the sulphides was found to be relatively independent of changes in fO&lt;sub&gt;2&lt;/sub&gt; or fS&lt;sub&gt;2&lt;/sub&gt;, which is in contrast with experimental studies conducted at ambient pressures. Results indicate that the oxygen concentration is primarily controlled by the FeO activity in coexisting silicate phases and the temperature.&lt;/p&gt;&lt;p&gt;By fitting the experimental data, we have developed a thermodynamic model using an end-member equilibrium between olivine, pyroxene and FeO in the sulphide melt. The standard state Gibbs free energy change (&amp;#916;G&lt;sup&gt;0&lt;/sup&gt;) of the equilibrium is calculated using known activity composition relations for the silicates and by refining non-ideal interaction parameters for the sulphide melt in the system FeO-FeS-NiS system. The &amp;#916;G&lt;sup&gt;0&lt;/sup&gt; is well determined as a function of temperature and shows no discernible dependence on pressure. The resulting relationship was used to calculate equilibrium temperatures of natural sulphide inclusions in diamonds. Using our new geo-thermometer, previously measured oxygen concentrations in natural sulphide inclusions in diamonds from the Slave craton reveal temperatures for lithospheric diamond formation generally in the range of 1200 &amp;#8211; 1300&amp;#176;C&lt;/p&gt;