ABSTRACT The %Ca (molar Ca/(Ca + Mg) 100) in sedimentary dolomite, which ranges from 48 %Ca to 62.5 %Ca, can be determined by atomic absorption spectrometry, electron microprobe analysis, or powder X-ray diffraction analysis. The latter procedure, which relies on the fact that the position of the d104 reflection depends linearly on the Ca content, offers an easy and relatively inexpensive method of determining %Ca in dolomite. To date, such analyses have assumed that the dolostone is homogeneous with respect to the %Ca content of its constituent dolomite crystals. Herein, the XRD technique is extended to compositionally heterogeneous dolostones. A considerable amount of important information can be derived from X-ray diffractograms of dolostones if they are analyzed with peak-fitting techniques (PF-XRD). Specifically, this technique allows (1) determination of the %Ca with an accuracy of ± 0.5 %Ca, (2) identification of heterogeneous dolostone samples formed of more than one population of dolomite crystals as defined by their %Ca content, and (3) derivation of the proportion of each population of dolomite crystals with an accuracy of ± 10%. The PF-XRD technique is a powerful analytical tool that provides valuable information on the major-element composition of low-Fe dolomite. Compared to other techniques, the PF-XRD technique is accurate, rapid, relatively inexpensive, and easy to use.
Water in Earth's upper mantle is a minor and yet critically important component that dictates mantle properties such as strength and melting behavior. Minerals with stoichiometric water, such as those of the humite group, are important yet poorly characterized potential reservoirs for volatiles in the upper mantle. Here, we report observation of hydroxyl members of the humite group as inclusions in mantle-derived diamond. Hydroxylchondrodite and hydroxylclinohumite were found coexisting with olivine, magnesiochromite, Mg-bearing calcite, dolomite, quartz, mica, and a djerfisherite-group mineral in a diamond from Brazil. The olivine is highly forsteritic (Mg# 97), with non-mantle-like oxygen isotope composition (δ
Alkaline igneous rocks of the Crowsnest Formation in southwestern Alberta and in the Howell Creek area in southeastern British Columbia have been suggested previously to be cogenetic. To test this hypothesis, samples of both suites were characterized petrographically and their major and trace element geochemistry was determined. A subset of the samples was analyzed for whole-rock Sr and Nd isotope geochemistry. The samples of the two suites are latites, trachytes, and phonolites based on the International Union of Geological Sciences (IUGS) total alkalis versus silica (TAS) diagram. Samples from both suites show similar patterns on mantle-normalized trace element diagrams, being enriched relative to mantle values but depleted in the high field-strength elements Nb, Ta, and Ti relative to the large-ion lithophile elements. The chondrite-normalized rare-earth element (REE) patterns for both suites are light REE enriched, with no Eu anomaly and flat heavy REE. The isotope geochemistry of both suites is characterized by low initial 87 Sr/ 86 Sr (Sr T = 0.704 to 0.706) and low εNd T (–7 to –16). The Howell Creek samples have lower εNd T and higher Sr T than do the Crowsnest samples. Based on the intra- and intersuite differences in the isotope geochemistry, we conclude that these samples are not cogenetic, but rather represent samples that have experienced similar evolutionary histories from a heterogeneous source region in the subcontinental lithospheric mantle.
The Torrie, Sputnik, and Eddie kimberlite rocks, located near Yamba Lake, central Slave province, N.W.T., are volcaniclastic, macrocrystic, heterolithic, olivine-rich tuff, and olivine-rich tuff breccia. Torrie and Sputnik kimberlite rocks contain pyroxene and garnet xenocrysts and megacrysts with major-element compositions consistent with derivation mostly from disaggregated garnet lherzolite, with subordinate contributions from eclogite, spinel lherzolite, garnet harzburgite, and websterite. The presence of primary groundmass phlogopite and compositionally evolved spinel, and the absence of mantle xenocrysts, xenoliths, and megacrystic ilmenite distinguish the Eddie kimberlite pipe from the other two kimberlite pipes. Large variations in δ 18 O of garnet and clinopyroxene in xenocrysts and xenoliths (+3.98 to +6.36), nonequilibrium intermineral isotopic fractionation, and major-element heterogeneity are interpreted as resulting from infiltration of fluids or melts produced by dehydration or melting of subducted oceanic crust into overlying peridotite. Although the timing is unconstrained for the xenocysts, the xenolith must have experienced this metasomatic interaction shortly before entrainment in the kimberlite. Variable δ 18 O values for magnesian ilmenite are also interpreted to result indirectly from such metasomatic activity in the mantle as well. The Torrie and Sputnik kimberlite rocks have low concentrations of diamond indicator minerals consistent with their low-diamond grades. These kimberlite rocks did not sample a significant amount of garnet harzburgite, the rock type commonly associated with high-diamond grades in other kimberlite rocks. Furthermore, metasomatism just prior to kimberlite eruption may have caused the resorption of any diamond present.
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