For estimations of P - T conditions of igneous and metamorphic rocks, Fe 3+ in coexisting minerals is either assumed to be zero or is calculated from electron microprobe analyses (EMPA) based upon stoichiometry and charge balance. Geothermobarometers that involve Fe 2+ - Mg 2+ exchange can be significantly affected by either neglecting Fe 3+ or using incorrect values. Ratios of Fe 3+ /ΣFe in garnet and clinopyroxene measured by a Mössbauer milliprobe were compared to those calculated from EMPA of garnet and clinopyroxene from eclogite xenoliths from the Udachnaya kimberlite in Yakutia. The effects of Fe 3+ contents in garnet and clinopyroxene on temperature estimations were evaluated. The following Fe 3+ /ΣFe (in at%) values were obtained (EMPA/Mössbauer): Gt = 9.4/6.0; 11.5/7.0; 19.4/16.0; and 24.7/15.0; Cpx = 22.0/22.9; 34.2/22.0. The effects of Fe 3+ in clinopyroxenes on calculated temperatures are illustrated by taking eclogitic clinopyroxene compositions and changing contents of certain elements within the range of standard deviations for EMPA of those particular elements. Increasing Na 2 O contents from 5.67 to 5.74 wt% (< 2.0% relative error) would lead to increasing Fe 3+ /ΣFe from 31.6 to 47.1%, thereby decreasing the calculated temperature from 1026 to 941 °C. Various Fe 3+ /ΣFe values for garnet and clinopyroxene were also tested for their effects on calculated temperatures: for clinopyroxene, T decreases with increasing Fe 3+ /ΣFe whereas for garnet, T increases with increasing Fe 3+ /ΣFe. This compensation effect between garnet and clinopyroxene moderates the variation in temperature estimations of eclogites based on Fe 3+ corrected vs. uncorrected microprobe analyses. Little correlation exists between EMPA-calculated and Mössbauer-measured Fe 3+ /ΣFe values for these mantle-derived garnets and clinopyroxenes. Even a small relative error in Fe 3+ may significantly change calculated temperatures of equilibration, seriously affecting petrologic interpretations. In particular, uncertainty in Fe 3+ calculated from EMPA of silicate minerals leads to serious questions with regard to K D values obtained from natural assemblages.
Xenoliths from the upper mantle have undergone a wide variety of processes at varied temperatures and pressures, as recorded by mineral compositions and textures. Eclogite xenoliths in kimberlites are a unique source from which to obtain information about such processes. Furthermore, eclogites that contain accessory diamond yield important compositional constraints on the deeper upper mantle. Whether eclogites have mantle or crustal origins is still a subject of controversy. Mineralogy and petrography of 29 eclogite xenoliths from the Udachnaya kimberlite in Yakutia, Siberia are presented and combined with previous studies of these eclogites by our group (Jerde et al., 1993, 1994; Snyder et al., 1993a; Sobolev et al., 1994). Five different petrographic groups are defined, based on texture, mineral color, and degree of alteration. Chemical compositions of eclogitic minerals span a complete range from high to low-jadeite content in clinopyroxenes and from pyrope-almandine to grossular in garnets. Eclogites appear to behave systematically in terms of Kd (Fe/Mg) values. Two eclogites (U-25/84 and U-281/84) are classified as Group A eclogites as per Coleman et al. (1965) and Taylor and Neal (1989). These two samples are unusual in chemical composition, being enriched in Cr2O3 (up to 64% of pyrope component in garnet). There is no evidence that points definitively to a crustal origin for the Udachnaya eclogites but there is abundant evidence that is at least consistent with a mantle origin: (a) oxygen isotope (δ18O) values of individual minerals that vary from 4.8 to 7.0 (Jerde et al., 1993); (b) carbon isotope values in diamonds, which are not indicative of a biogenic, crustally derived component; (c) extremely low87Sr/86Sr values, which suggest a long-lived, depleted (low Rb/Sr) component consistent with upper mantle, as well as very old reconstructed whole-rock model ages; and (d) reconstructed whole-rock εNd values (at 389 Ma) for U-24/84 and U-281/84 of +4.3 and +6.7, respectively, indicative of a depleted peridotite protolith. The distinct correlations between δ18O and CaO, FeO, etc. reported by Jacob et al. (1993) do not exist with our much larger data base. We do not agree with their conclusions that all eclogites in Siberia, as well as South Africa, are the result of subducted oceanic crust. Although there may be a crustal component (i.e., δ18O = 7.02) in some of the Udachnaya eclogites, the majority are of mantle origin. It is clear that Yakutian eclogites possess distinctive differences from their South African counterparts (Bellsbank), some of which are thought to be derived from Archean oceanic crust.
During the Early Proterozoic (2.5 to 2.3 Ga), three types of coeval structural provinces developed in the eastern Baltic Shield—(1) the Karelian and Kola granite-greenstone cratons, (2) the relatively high grade Lapland-Umba granulite belt (LUGB), and (3) the Belomorian (White Sea) mobile belt (BMB). The LUGB represents a compensated compressional zone where synkinematic crustal-derived magmatism of the enderbite-charnockite series predominates. The BMB is a transitional nappe-folded zone between these high- and low-grade terranes, which consists mainly of reworked granite-greenstone lithologies of the adjacent cratons. These cratons were vast extensional areas with mantle-derived, siliceous, high-Mg (boninite-like) series (SHMS) magmatism. This SHMS magmatism occurs in volcano-sedimentary sequences, large layered intrusions, and dike swarms within graben-like structures. One of the more interesting types of tectonomagmatic activity occurred within the BMB and is expressed as the unique Drusite Complex. It is represented by thousands of small intrusions of mafic and ultramafic rocks, dispersed among the higher-grade BMB host rocks. Geological features of these intrusions show that their formation was synkinematic with deformations within the belt, although they have undergone later, post-solidification deformation and metamorphism. As a result, intrusions often were transformed into lenticular, boudin-like bodies with primary igneous textures preserved only in their central portions. Compositions of the Drusite Complex intrusions, although forming small, individual bodies with associated chill zones, are similar to large layered intrusions in adjacent cratons (plagioclase harzburgites and lherzolites, pyroxenites, troctolites, olivine norites and norites, gabbronorites, anorthosites, and diorites). The areal distribution of the drusite intrusions and their correlation with large layered mafic intrusions in adjacent cratons suggests a vast magma-generation zone beneath western Russia during the Early Proterozoic. The character and extent of magmatism suggests that during the Early Proterozoic (in Sumian— Sariolian time) the Kola and Karelian cratons were vast extensional areas above spreading plume heads. Within this scenario, the LUGB was an area of intense crustal sagging between these two cratons. The BMB was a transitional zone of tectonic flowage between the LUGB and the cratons, where movements were not as intense; there a nappe-folded structure formed. As a result, the intrusion of new melts occurred under rapidly changing conditions and a specific type of disseminated, intrusive magmatism—The Drusite Complex—emerged instead of the formation of layered intrusions. The petrologic and mineralogic compositions of the Drusite Complex intrusions are indistinguishable from coeval layered mafic intrusions of the adjacent Karelia and Kola cratons, suggesting similar parental magmas and a large zone of magmatism (i.e., large igneous province, or LIP) beneath the eastern Baltic Shield. These magmas were derived either from depleted mantle melts that had assimilated a significant crustal component, or from enriched mantle.
Geochemical data for Ni, Co, Cr, V, and Mn have played an important role in theories for the Moon's origin. It has been argued that the data for these elements strongly support formation of the Moon as ejecta from the Earth, either as a result of one giant or numerous smaller impacts on the proto-Earth. These theories have come to be known as the "Giant Impact" and "Impact-triggered Fission" hypotheses, respectively, and the first of these has been the leading explanation for the origin of the Moon over the past decade. Data for these same "diagnostic" elements also have been used to argue for significant distinctions between the bulk compositions of the Moon and a eucrite (HED) parent body, which otherwise appear to be remarkably similar in their compositions. We review geochemical evidence pertaining to the origin of the Moon, focusing on the diagnostic elements, and find that there is no strong geochemical support for either the Giant Impact or Impact-triggered Fission hypotheses. We show that basalts produced in the Moon and a HED parent body (mare basalts and eucrites, respectively) were derived by the melting of source regions with similar compositions. Mare and eucrite basalts differ in Ni and Co abundances but lie on the same igneous fractionation trend. Chromium, Mg# (Mg/[Fe+Mg]), and V abundance systematics suggest a close similarity between mare and eucrite basalts, and a significant difference from terrestrial volcanic rocks, which are depleted in Cr. Mare and eucrite basalts differ in their Mn abundances and Fe/Mn ratios, but the same can be said for mare and terrestrial basalts. On the whole, the Moon appears to be more chemically similar to the HED parent body than to the Earth. This suggests that either: (1) the HED parent body (probably the asteroid Vesta) formed by an impact mechanism and is an escaped satellite, or (2) the Moon is a captured body that formed independently of Earth. Similar conclusions were reached long ago by Anders and colleagues (e.g., Anders, 1977), long before the Giant Impact Hypothesis attained popularity.
