Kimberlite-transported nodules from Colorado-Wyoming; A record of enrichment of shallow portions of an infertile lithosphere

Abstract

Nodules of Devonian age in Colorado-Wyoming kimberlites include infertile, enriched, and ilmenite-bearing peridotites; garnet websterites and associated garnet pyroxenites; garnet-spinel websterites; plus megacrysts, eclogites, and granulites. Both enriched and infertile peridotites equilibrated in the spinel, spinel-garnet, and garnet facies, but enriched garnet peridotites were sampled only at depths of less than 100 km, whereas infertile garnet peridotites extend to depths of 200 km. Occurrence of infertile rocks and cool paleogeothermal gradient define the sampled mantle as lithosphere.Infertile peridotites are interpreted to be residua from a Precambrian melting event involving the entire lithosphere; spinel and spinel-garnet peridotites are generally more infertile than garnet peridotites. The infertile lithosphere was subsequently enriched at shallow depths by igneous processes in two events: one produced Fe-and Ti-enriched ilmenite peridotites; the other, more widespread event was associated with websteritic rocks. A few composite nodules as well as geothermobarometry suggest that websterites formed a network of dikes, veins, and layers cutting spinel and spinel-garnet infertile peridotite, and that these were bordered by zones of enriched peridotite. Websterites are Na-, Ca-, and slightly Ti-enriched, but are magnesian, which requires that the protoliths were crystalline assemblages, not melts.The ratio of collected infertile spinel and spinel-garnet peridotites to enriched peridotites and websterites indicates that enriched rocks occupied 80 percent of the lithosphere sampled between the crust and a depth of 100 km. This very high percentage, unusual among xenolith populations, suggests that ancient enriched rocks occurred along conduits subsequently exploited by kimberlite eruptions. D. H. Eggler and OthersTABLE 1. LITHOSPHERIC NODULES FROM COLORADO-WYOMING KIMBERLITE PIPES Group No. Comments Re ference Granulites 20 Eclog ites 158 Infertile peridotites a. spinel b. spinelgarnet c.garnet Enr iched peridotites a. spinel b. spinelgarnet c.garnet Ilmenitegarnet peridotites Spinelgarnet peridotites Garnet pyroxenites and websterites 36 5 74 56 19 23 11 52 Megacrysts 353 Two-px granulites, two-px gar granulites, cpx garnet granulites, and gabbro-norites. Interpreted to represent lower crust. Cpx-gar eclogites with acc. rutile, sanidine, quartz, and sphene; kyanite eclogites with acc. corundum, rutile, and sanidine. Paleotemps. 700-1300°C. Interpreted to represent subducted oceanic crust (Archean?) now at 80-200 km. A suite dominated by harzburgites occurring in 3 facies. Interpreted to represent infertile residua from extensive Archean(?) melting. Some garnet-facies rocks contain chromite. Textures in all facies coarse equant to porphyroclastic. A suite dominated by lherzolites occurring in 3 facies. The few nodules in garnet facies are shallow. In spinel-garnet facies, rimming of spinels by garnets and exsolution in pyroxenes suggest high-temperature protoliths. Textures dominantly coarse equant. Ilmenite-...

Lithospheric roots beneath western Laurentia: the geochemical signal in mantle garnets

“…9d). Temperatures recorded in xenoliths from the Sloan and Iron Mountain kimberlites show a bimodal distribution at 700-800°C and 1000-1300°C (Eggler et al 1987a(Eggler et al , 1987b. The higher T is similar to that observed for garnet suites in this study, but T Ni values < -800°C are not recorded by garnets from any locality.…”

Silica enrichment in the continental upper mantle via melt/rock reaction

“…Such compositions are also found in xenoliths from cratonic North America [19][20][21]24], and in Tertiary alkaline dikes and lavas postdating the flood basalts in central West Greenland [22]. Our East Greenland xenoliths are mostly spinel harzburgites with high Mg# olivines (average 92.7) and low Fig.…”

“…This was consistent with the hypothesis that high Mg# and Opx contents were intrinsically linked, perhaps by high pressures of melting. However, mantle xenoliths from central East Greenland, central West Greenland and northern North America [18,17,[19][20][21][22] have high Mg#, comparable to the Kapvaal suite, but much lower Opx contents (Fig. 1).…”

Lower crustal and upper mantle xenoliths along the Cheyenne belt and vicinity

“…4; Eggler et al, 1987b). The two xenolith types are most clearly distinguished by the fact that the "fertile" types have lower molar Mg/(Mg + Fe), lower molar Cr/(Cr + Al) and higher whole rock Na 2 O and CaO, as well as jadeite-rich clinopyroxenes.…”