The Bamble sector of southern Norway comprises metagabbros and metasediments that were metasomati... more The Bamble sector of southern Norway comprises metagabbros and metasediments that were metasomatically altered to various extents during a late stage of the Sveconorwegian orogeny (~1.06 Ga). The infiltration of highly saline brines along veins led to penetrative scapolitization and albitization on a regional scale and the local deposition of Fe–Ti oxides. Typical secondary mineral assemblages include either scapolite + apatite + amphibole + phlogopite + tourmaline, or albite + epidote + calcite + chlorite + white mica, indicating that the fluids introduced large amounts of Na, Cl, Mg, Ca, K, P, and B to the system. Metasomatic tourmalines associated with different alteration stages as identified by variations in majorelement composition and initial 87Sr/86Sr were analyzed for B isotopic compositions to constrain possible sources and the evolution of the hydrothermal fluid(s). Measured δ11B values range from −5 to +27 ‰ relative to SRM-951, suggesting marine evaporites interlayered with various amounts of continental detritus and pelagic clay as a possible B source reservoir. The influence of a seawater-derived component is clearly indicated by the heavy B isotope signature of tourmaline related to Al–Mg-rich metapelites. In contrast, negative δ11B values can be explained by the influence of pneumatolytic fluids associated with granitic pegmatites. On a regional scale (i.e., several km), δ11B values in tourmaline vary widely, whereas variations within a single outcrop (tens of m) are typically small and can be ascribed to different generations of tourmaline related to several fluid pulses.
The Oetmoed Granite–Migmatite Complex (OGMC), Central Damara Orogen, Namibia, consists mainly of ... more The Oetmoed Granite–Migmatite Complex (OGMC), Central Damara Orogen, Namibia, consists mainly of ∼526 to ∼516 Ma garnet- and cordierite-bearing granite and subordinate ∼488 to ∼494 Ma hornblende- and titanite-bearing granite in the form of planar sheets and dykes. Additionally, a slightly elongated granite body occurs in the center of the complex. The garnet- and cordierite-bearing granite has major- and trace-element characteristics of S-type granite but the hornblende- and titanite-bearing granite has higher HFSE and REE contents similar to A-type granite. Whereas the garnet- and cordierite-bearing granite contains numerous restitic xenoliths, the hornblende- and titanite-bearing granite is xenolith-free. The country rocks are cordierite–sillimanite–K-feldspar–garnet-bearing metasedimentary rocks and migmatite. Cordierite- and garnet-rich xenoliths in the S-type granite do not represent primary restite, their depleted chemical composition is best explained by varying and large degrees of partial melting of incorporated country rocks. Most chemical variations among the garnet- and cordierite-bearing granite can be explained by processes linked with fractional crystallization of plagioclase, biotite and accessory phases, mostly monazite and zircon. Major and trace element data and high δ 18O values suggest that the least evolved members of the garnet- and cordierite-bearing granite were derived from metapelitic rocks at ca. 800°C as inferred from monazite and apatite dissolution thermometry. Higher CaO and Na2O but lower SiO2 contents and lower Rb/Sr ratios as well as lower δ 18O values of the hornblende- and titanite-bearing granite suggest that they are more likely generated by partial melting of non-pelitic sources (metagranitoids?) at temperatures in excess of 900°C. Decreasing TiO2, Na2O, FeOtot., MgO, CaO, total REE content but increasing Al2O3 and K2O indicate fractionation of mainly hornblende and titanite in the case of the hornblende- and titanite-bearing granite. The differing compositions of the garnet- and cordierite-bearing granite and the hornblende- and titanite-bearing granite are attributed to different source rocks (metapelite instead of metagranitoid) and different temperatures during melting as inferred from accessory phase dissolution thermometry. Furthermore, significant entrainment of country rock in some samples played a major role during petrogenesis of the garnet- and cordierite-bearing granite but was not important during the evolution of the hornblende- and titanite-bearing granite. Intrusion of such hot, felsic magmas close to the inferred peak of metamorphism has probably caused, in part, the high temperature metamorphism and anatexis of the country rocks at relatively low pressures.
... The UPb ID-TIMS zircon ages presented here of 407.7 ± 0.7 Ma (Bundenbach 'Hans-Plat... more ... The UPb ID-TIMS zircon ages presented here of 407.7 ± 0.7 Ma (Bundenbach 'Hans-Platte') and 392.2 ± 1.5 Ma (Wetteldorf 'Hercules I') are in reasonable agreement with previous work ... Nesbitt, RW, Pascual, E., Fanning, CM, Toscano, M., Saez, R. & Almodovar, GR 1999. ...
The Lu decay constant recently determined by Scherer et al. 2001 (i.e., 1.865 x 10-11 yr-1) agree... more The Lu decay constant recently determined by Scherer et al. 2001 (i.e., 1.865 x 10-11 yr-1) agrees with the results of the two latest physical counting experiments (1.86 x 10-11 yr-1; Dalmasso et al 1992, Nir-El and Lavi 1998), but is ca. 4 percent lower than the decay constants that have been used throughout the Hf isotope literature (e.g., 1.94
The isotope (146)Sm undergoes alpha-decay to (142)Nd, with a half-life of 103 million years. Meas... more The isotope (146)Sm undergoes alpha-decay to (142)Nd, with a half-life of 103 million years. Measurable variations in the (142)Nd/(144)Nd values of rocks resulting from Sm-Nd fractionation could therefore only have been produced within about 400 million years of the Solar System's formation (that is, when (146)Sm was extant). The (142)Nd/(144)Nd compositions of terrestrial rocks are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High (142)Nd/(144)Nd values measured in some Archaean rocks from Greenland hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low (142)Nd/(144)Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low (142)Nd/(144)Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are 'hidden' in Hadean crust, the D'' layer of the lowermost mantle or, as we propose here, also within the roots of old cratons.
Garnet (Grt), a common product of metamorphic reactions in a wide range of rock types, not only p... more Garnet (Grt), a common product of metamorphic reactions in a wide range of rock types, not only plays a key role in thermobarometery, but also makes a useful geochronometer by fractionating the parent/daughter element ratios of several isotope systems. Grt is thus valuable for reconstructing pressure-temperature-time (P-T-t) paths in metamorphosed terranes. Lu-Hf Grt dating has flourished with the development of the MC-ICP-MS and vastly simplified chemical separation procedures. Lu-Hf has several advantages over other systems for dating Grt, notably better age resolution at lower ages, high closure temperature, and better tolerance for certain types of inclusions. Constraining P-T-t paths often involves comparing the results of several absolute dating methods (e.g., Lu-Hf in Grt, Rb-Sr in mica, U-Pb in sphene, Ar-Ar in hornblende), for which the decay constants and closure temperatures (T_C) are known. The 176Lu decay constant is controversial however, with estimates ranging from 1.70 to 1.98 × 10-11yr-1, a ˜15% spread. Nevertheless, our recent experiments using 7 different terrestrial mineral samples yield a mean lambda 176Lu of [1.869 ± 0.016] × 10-11yr-1 (2 s.d.). Using this recommended value, differences between and Lu-Hf and Sm-Nd garnet ages from slowly cooled granulite facies rocks suggest that the T_C of Lu-Hf in these Grt-whole rock systems is ˜50--150^oC higher than that of Sm-Nd in a given sample. Though absolute T_C values await diffusion data for Hf and Lu in garnet, the T_C is probably high enough to record garnet growth in many amphibolite facies rocks and low-T eclogites. Lu-Hf ages for high-T eclogites and granulites are probably cooling ages. An additional consideration specific to dating metamorphic events is that unlike igneous rocks, which often satisfy the isochron requirement of initial isotopic homogeneity among all phases, metamorphic rocks are often not completely homogenized. Relict matrix minerals (e.g., magmatic Cpx in eclogite) and inherited trace element rich inclusions (e.g., zircon, monazite, clinozoisite) in Grt can cause spurious internal isochron ages. Though mineral separation techniques and careful hand-picking can effectively remove relict matrix minerals, it is practically impossible to produce inclusion-free Grt separates. To improve the quality of Sm-Nd (and Lu-Hf) isochrons, several workers have employed acid "leaching" to remove monazite and clinozoisite inclusions before the final Grt digestion. Unfortunately, zircon and rutile inclusions, which severely affect the Hf budget of Grt, are not removed, leading to imprecise and sometimes inaccurate Lu-Hf ages. To solve this problem, we developed a selective digestion technique that dissolves Grt and achieves spike-sample equilibration, without dissolving zircon or rutile. We will illustrate this new method using eclogite examples from the Variscan fold belt of southern Germany.
The Bamble sector of southern Norway comprises metagabbros and metasediments that were metasomati... more The Bamble sector of southern Norway comprises metagabbros and metasediments that were metasomatically altered to various extents during a late stage of the Sveconorwegian orogeny (~1.06 Ga). The infiltration of highly saline brines along veins led to penetrative scapolitization and albitization on a regional scale and the local deposition of Fe–Ti oxides. Typical secondary mineral assemblages include either scapolite + apatite + amphibole + phlogopite + tourmaline, or albite + epidote + calcite + chlorite + white mica, indicating that the fluids introduced large amounts of Na, Cl, Mg, Ca, K, P, and B to the system. Metasomatic tourmalines associated with different alteration stages as identified by variations in majorelement composition and initial 87Sr/86Sr were analyzed for B isotopic compositions to constrain possible sources and the evolution of the hydrothermal fluid(s). Measured δ11B values range from −5 to +27 ‰ relative to SRM-951, suggesting marine evaporites interlayered with various amounts of continental detritus and pelagic clay as a possible B source reservoir. The influence of a seawater-derived component is clearly indicated by the heavy B isotope signature of tourmaline related to Al–Mg-rich metapelites. In contrast, negative δ11B values can be explained by the influence of pneumatolytic fluids associated with granitic pegmatites. On a regional scale (i.e., several km), δ11B values in tourmaline vary widely, whereas variations within a single outcrop (tens of m) are typically small and can be ascribed to different generations of tourmaline related to several fluid pulses.
The Oetmoed Granite–Migmatite Complex (OGMC), Central Damara Orogen, Namibia, consists mainly of ... more The Oetmoed Granite–Migmatite Complex (OGMC), Central Damara Orogen, Namibia, consists mainly of ∼526 to ∼516 Ma garnet- and cordierite-bearing granite and subordinate ∼488 to ∼494 Ma hornblende- and titanite-bearing granite in the form of planar sheets and dykes. Additionally, a slightly elongated granite body occurs in the center of the complex. The garnet- and cordierite-bearing granite has major- and trace-element characteristics of S-type granite but the hornblende- and titanite-bearing granite has higher HFSE and REE contents similar to A-type granite. Whereas the garnet- and cordierite-bearing granite contains numerous restitic xenoliths, the hornblende- and titanite-bearing granite is xenolith-free. The country rocks are cordierite–sillimanite–K-feldspar–garnet-bearing metasedimentary rocks and migmatite. Cordierite- and garnet-rich xenoliths in the S-type granite do not represent primary restite, their depleted chemical composition is best explained by varying and large degrees of partial melting of incorporated country rocks. Most chemical variations among the garnet- and cordierite-bearing granite can be explained by processes linked with fractional crystallization of plagioclase, biotite and accessory phases, mostly monazite and zircon. Major and trace element data and high δ 18O values suggest that the least evolved members of the garnet- and cordierite-bearing granite were derived from metapelitic rocks at ca. 800°C as inferred from monazite and apatite dissolution thermometry. Higher CaO and Na2O but lower SiO2 contents and lower Rb/Sr ratios as well as lower δ 18O values of the hornblende- and titanite-bearing granite suggest that they are more likely generated by partial melting of non-pelitic sources (metagranitoids?) at temperatures in excess of 900°C. Decreasing TiO2, Na2O, FeOtot., MgO, CaO, total REE content but increasing Al2O3 and K2O indicate fractionation of mainly hornblende and titanite in the case of the hornblende- and titanite-bearing granite. The differing compositions of the garnet- and cordierite-bearing granite and the hornblende- and titanite-bearing granite are attributed to different source rocks (metapelite instead of metagranitoid) and different temperatures during melting as inferred from accessory phase dissolution thermometry. Furthermore, significant entrainment of country rock in some samples played a major role during petrogenesis of the garnet- and cordierite-bearing granite but was not important during the evolution of the hornblende- and titanite-bearing granite. Intrusion of such hot, felsic magmas close to the inferred peak of metamorphism has probably caused, in part, the high temperature metamorphism and anatexis of the country rocks at relatively low pressures.
... The UPb ID-TIMS zircon ages presented here of 407.7 ± 0.7 Ma (Bundenbach 'Hans-Plat... more ... The UPb ID-TIMS zircon ages presented here of 407.7 ± 0.7 Ma (Bundenbach 'Hans-Platte') and 392.2 ± 1.5 Ma (Wetteldorf 'Hercules I') are in reasonable agreement with previous work ... Nesbitt, RW, Pascual, E., Fanning, CM, Toscano, M., Saez, R. & Almodovar, GR 1999. ...
The Lu decay constant recently determined by Scherer et al. 2001 (i.e., 1.865 x 10-11 yr-1) agree... more The Lu decay constant recently determined by Scherer et al. 2001 (i.e., 1.865 x 10-11 yr-1) agrees with the results of the two latest physical counting experiments (1.86 x 10-11 yr-1; Dalmasso et al 1992, Nir-El and Lavi 1998), but is ca. 4 percent lower than the decay constants that have been used throughout the Hf isotope literature (e.g., 1.94
The isotope (146)Sm undergoes alpha-decay to (142)Nd, with a half-life of 103 million years. Meas... more The isotope (146)Sm undergoes alpha-decay to (142)Nd, with a half-life of 103 million years. Measurable variations in the (142)Nd/(144)Nd values of rocks resulting from Sm-Nd fractionation could therefore only have been produced within about 400 million years of the Solar System's formation (that is, when (146)Sm was extant). The (142)Nd/(144)Nd compositions of terrestrial rocks are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High (142)Nd/(144)Nd values measured in some Archaean rocks from Greenland hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low (142)Nd/(144)Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low (142)Nd/(144)Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are 'hidden' in Hadean crust, the D'' layer of the lowermost mantle or, as we propose here, also within the roots of old cratons.
Garnet (Grt), a common product of metamorphic reactions in a wide range of rock types, not only p... more Garnet (Grt), a common product of metamorphic reactions in a wide range of rock types, not only plays a key role in thermobarometery, but also makes a useful geochronometer by fractionating the parent/daughter element ratios of several isotope systems. Grt is thus valuable for reconstructing pressure-temperature-time (P-T-t) paths in metamorphosed terranes. Lu-Hf Grt dating has flourished with the development of the MC-ICP-MS and vastly simplified chemical separation procedures. Lu-Hf has several advantages over other systems for dating Grt, notably better age resolution at lower ages, high closure temperature, and better tolerance for certain types of inclusions. Constraining P-T-t paths often involves comparing the results of several absolute dating methods (e.g., Lu-Hf in Grt, Rb-Sr in mica, U-Pb in sphene, Ar-Ar in hornblende), for which the decay constants and closure temperatures (T_C) are known. The 176Lu decay constant is controversial however, with estimates ranging from 1.70 to 1.98 × 10-11yr-1, a ˜15% spread. Nevertheless, our recent experiments using 7 different terrestrial mineral samples yield a mean lambda 176Lu of [1.869 ± 0.016] × 10-11yr-1 (2 s.d.). Using this recommended value, differences between and Lu-Hf and Sm-Nd garnet ages from slowly cooled granulite facies rocks suggest that the T_C of Lu-Hf in these Grt-whole rock systems is ˜50--150^oC higher than that of Sm-Nd in a given sample. Though absolute T_C values await diffusion data for Hf and Lu in garnet, the T_C is probably high enough to record garnet growth in many amphibolite facies rocks and low-T eclogites. Lu-Hf ages for high-T eclogites and granulites are probably cooling ages. An additional consideration specific to dating metamorphic events is that unlike igneous rocks, which often satisfy the isochron requirement of initial isotopic homogeneity among all phases, metamorphic rocks are often not completely homogenized. Relict matrix minerals (e.g., magmatic Cpx in eclogite) and inherited trace element rich inclusions (e.g., zircon, monazite, clinozoisite) in Grt can cause spurious internal isochron ages. Though mineral separation techniques and careful hand-picking can effectively remove relict matrix minerals, it is practically impossible to produce inclusion-free Grt separates. To improve the quality of Sm-Nd (and Lu-Hf) isochrons, several workers have employed acid "leaching" to remove monazite and clinozoisite inclusions before the final Grt digestion. Unfortunately, zircon and rutile inclusions, which severely affect the Hf budget of Grt, are not removed, leading to imprecise and sometimes inaccurate Lu-Hf ages. To solve this problem, we developed a selective digestion technique that dissolves Grt and achieves spike-sample equilibration, without dissolving zircon or rutile. We will illustrate this new method using eclogite examples from the Variscan fold belt of southern Germany.
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