Jericho Kimberlite Research Articles

Garnet from Cr- and Ca-saturated mantle: implications for diamond exploration

An exotic trend of positively correlated Cr and Ca occurs in garnet from xenoliths in the Jericho kimberlite, northern Canada. The trend shows significantly less enrichment in Cr with increasing Ca than the common lherzolitic trend characteristic of kimberlite concentrate. The trend is found only in spinel–garnet peridotite xenoliths and is attributed to equilibration of garnet with spinel. The compositional variation of garnet within the trend is calculated to require buffering by equilibria involving Cr-spinel and clinopyroxene. Representation of garnet chemistry in terms of additive and exchange components shows that the chromite–clinopyroxene–garnet equilibrium (CCGE) is controlled by the exchange Mg 2Al 3↔Ca 2Cr 3. The equilibration of garnet with chromite and clinopyroxene reduces the extent of cation exchange in octahedral sites and eliminates Fe from active substitution in garnet relative to the common lherzolitic trend. A trend similar to the CCGE trend was described for spinel–garnet xenoliths from South Africa and Colorado Plateau and was produced experimentally for natural lherzolite compositions at high P–T conditions. The fact that the trend is extremely rare is explained by its low visibility and/or the lack of Ca-saturation in most spinel–garnet cratonic peridotite. The CCGE trend is more pronounced in garnet from peridotite xenoliths than in garnet from kimberlite concentrate where it is less visible due to sampling biases and dilution. The paucity of Ca-saturation results from harzburgitic composition of peridotitic mantle at depths of 120–180 km (spinel–garnet facies) under most Archean cratons. The Jericho and Colorado Plateau mantle segments are unique in having lherzolitic composition of the spinel–garnet mantle, where there is sufficient clinopyroxene to ensure the development of the CCGE trend. A CCGE trend in garnet composition reflects the presence of relatively fertile rocks at relatively shallow depths. In the Ca-saturated mantle, the CCGE trend can be used to constrain the depth interval of the mantle sampled by kimberlite. Coupled with the data on kimberlite spinel chemistry and on the local geotherm, the presence of the trend can reveal sampling of non-diamondiferous or diamondiferous mantle horizons.

A steady state conductive geotherm for the north central Slave, Canada: Inversion of petrological data from the Jericho Kimberlite pipe

Mantle xenoliths carried by kimberlite provide direct evidence for the rock types that constitute the roots to ancient cratons and provide critical constraints on the thermal state of the lithosphere. We derive a steady state conductive geotherm for the north central Slave craton based on published data for xenoliths from the Middle Jurassic Jericho kimberlite. The preferred model geotherm assumes a two‐layer lithosphere. The upper layer is D km thick, has constant thermal conductivity, and an exponential decrease in heat production with depth (Ao e−z/D); Ao is a fixed value based on the bedrock geology. Below D, the lithosphere has a higher, constant thermal conductivity and a constant value of radiogenic heat production (Am). Inverting the thermobarometric data produces model estimates of surface heat flow (qo) and the depth‐scale parameter (D) of 54.1 mWm−2, and 25.8 km, respectively. The value of D corresponds to the thickness of upper to middle crust as defined by seismology for the southern Slave craton. On the basis of our analysis the Slave craton is characterized by high surface heat flow, relative to other Archean cratons, which is consistent with a single field measurement from the southern Slave craton. The high surface heat flow results from the high concentrations of radiogenic heat producing elements found in Slave crustal rocks. Despite the relatively high surface flow, the underlying lithosphere is cool relative to mantle beneath the rest of North America and other Archean cratons. The model geotherm predicts a thermal gradient in the mantle (100–200 km) of between 5.0 and 3.7 (°C/km) and a mantle heat flow of 15.9–11.9 mWm−2 that is consistent with other estimates of mantle heat flow beneath Precambrian continental crust.

Petrology of Peridotite and Pyroxenite Xenoliths from the Jericho Kimberlite: Implications for the Thermal State of the Mantle beneath the Slave Craton, Northern Canada

We report comprehensive petrological and thermobarometric data for the following typesof mantle xenoliths from the Jericho kimberlite in the Slave craton: (1) coarse peridotite; (2) porphyroclastic peridotite; (3) megacrystalline pyroxenite; (4) ilmenite–garnet wehrlite and clinopyroxenite. The varieties of the upper-mantle xenoliths, and their proportions, petrography and mineralogy, mainly resemble those from kimberlites of other cratons. Unique characteristics of the north–central Slave mantle include the presence of high-temperature megacrystalline pyroxenite and an unusually high proportion of pyroxenitic magmatic rocks related in origin to megacrysts. Other unique aspects of Jerichoperidotitic mantle are: (1) a pronounced Cr enrichment in mineral chemistry within high-temperature peridotite; (2) an anomalously high proportion of chemically unequilibrated samples; (3) an uncommon pattern in silicate mineral chemistry in spinel-bearing peridotite that reflects equilibration with spinel. The central Slave mantle is colder than the mantle beneath the rest of the North American craton and the mantle beneath Kaapvaal and Siberian cratons. Thermobarometric data are fitted to a model steady-state geotherm with an exponential decrease in heat production with depth using a geologically constrained valueof surface heat production at Jericho. The estimated model values of surface heat flow (Q0 = 52–53 mW/m2) show an excellent agreement with two heat flow measurements available for the Slave craton (53–55 mW/m2). Thus, the cool upper mantle here coexists with a highly radiogenic crust.

Petrology of Peridotite and Pyroxenite Xenoliths from the Jericho Kimberlite: Implications for the Thermal State of the Mantle beneath the Slave Craton, Northern Canada

We report comprehensive petrological and thermobarometric data for the following typesof mantle xenoliths from the Jericho kimberlite in the Slave craton: (1) coarse peridotite; (2) porphyroclastic peridotite; (3) megacrystalline pyroxenite; (4) ilmenite–garnet wehrlite and clinopyroxenite. The varieties of the upper-mantle xenoliths, and their proportions, petrography and mineralogy, mainly resemble those from kimberlites of other cratons. Unique characteristics of the north–central Slave mantle include the presence of high-temperature megacrystalline pyroxenite and an unusually high proportion of pyroxenitic magmatic rocks related in origin to megacrysts. Other unique aspects of Jerichoperidotitic mantle are: (1) a pronounced Cr enrichment in mineral chemistry within high-temperature peridotite; (2) an anomalously high proportion of chemically unequilibrated samples; (3) an uncommon pattern in silicate mineral chemistry in spinel-bearing peridotite that reflects equilibration with spinel. The central Slave mantle is colder than the mantle beneath the rest of the North American craton and the mantle beneath Kaapvaal and Siberian cratons. Thermobarometric data are fitted to a model steady-state geotherm with an exponential decrease in heat production with depth using a geologically constrained valueof surface heat production at Jericho. The estimated model values of surface heat flow (Q0 = 52–53 mW/m2) show an excellent agreement with two heat flow measurements available for the Slave craton (53–55 mW/m2). Thus, the cool upper mantle here coexists with a highly radiogenic crust.