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Temporal scaling of marine respiration
A540
Goldschmidt Conference Abstracts 2006
Ugandan kamafugites: Re-melting of a variable enriched veined subcontinental lithospheric mantle A. ROSENTHAL1, S.F. FOLEY2, G.D. PEARSON3, G. NOWELL3, S. TAPPE2 1
Research School of Earth Sciences, The Australian National University ([email protected]) 2 Department of Earth Sciences, Universitaet of Mainz (foley@ uni-mainz.de) 3 Department of Geological Sciences, University of Durham ([email protected]; [email protected]; g.m. [email protected]) Ugandan kamafugites occur in the Toro-Ankole province along the Western Branch of the East African Rift within a Proterozoic-Archean basement complex of the Tanzanian craton. Our comprehensive geochemical study explains the origin of the potassic-ultrapotassic kamafugites by re-melting of a variably, and episodically enriched veined lithospheric mantle characterized by highly variable supra-chondritic radiogenic Os. Impregnation of the source region by carbonititic and/or highly alkaline silicate melts resulted in the introduction of modal phlogopite and enrichment in Fe and Re. The extreme silica-undersaturation (SiO2 = 31.8–41.8 wt%) of kamafugites, their high MgO (up to 22.5 wt%), low Al2O3 (<8.0 wt%), and high CaO contents (up to 16.6 wt%) are reflected in the presence of modal kalsilite, leucite, melilite and perovskite. Their primitive features such as high Mg# olivines (up to 91.1), high whole-rock Mg# (up to 80.2), and high Ni (up to 1066 ppm), Cr (up to 1560 ppm) and Os (up to 1.45 ppb) are in strong contrast to their extreme enrichment in LILE, HFSE, and LREE. Isotopic signatures (87Sr/86Sr = 0.7046–0.7054, eNd = 0.08 to 4.70, and eHf = 3.64 to 8.84, cOs = 16–290) tend to much higher superchondritic cOs (cOs = 16–290) than oceanic island basalts. Kamafigutes exhibit numerous signs of mixing, both mineralogical and geochemical (e.g., complexly zoned olivines; three generations of clinopyroxene; inverse trends on cOs vs. Mg#, Ni [ppm], Cr [ppm], Os [ppb] and 87Sr/88Sr ratios; linear trends on cOs vs. 1/Os [ppb], 187Re/188Os ratios, eNd and eHf). Changes in kamafugite compositions are controlled by the mineralogy of vein assemblages, by vein-wall-rock melting processes, and by changes in CO2/H2O, alkalinity, fO2 , and fS2 conditions during ascent. Partial melting of alkali clinopyroxenites, xenolithic material as found in the same volcanic field, are thought to represent the source material of katungites, whereas mafuritic to uganditic compositions require the incorporation of peridotite wall-rock material, and hence higher degrees of partial melting.
Temporal scaling of marine respiration DANIEL H. ROTHMAN1, DAVID C. FORNEY2 1
Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA 02139, USA ([email protected]) 2 Department of Mechanical Engineering, MIT, Cambridge, MA 02139, USA ([email protected]) The extent to which respiration balances photosynthesis determines not only environmental concentrations of CO2 but also O2. An understanding of this balance is therefore crucial to understanding coevolving biogeochemical and evolutionary transitions. Respiration occurs at all time (t) scales, but at progressively slower rates, decaying roughly like 1/t as organic matter falls from the sea surface to the sea floor, and as it proceeds through degradation in sediment (Middleburg, 1989). Qualitatively, this slowdown can be at least partially attributed to the decreased accessibility of organic carbon to microbial degradation. In particular, the association of sedimentary organic carbon with clay mineral surface area is thought to preferentially protect some organic detritus from microbial degradation (Hedges and Keil, 1995). These observations motivate a theory for detrital decay. The key assumption is that the process is limited by the diffusion of hydrolyzed organic carbon from solid (i.e., mineral) surfaces to bacteria. The site-specific rate of decay is assumed to be proportional to diffusive fluxes associated with surface sites. Averaging over these microscopic rates one can then obtain the observed macroscopic 1/t decay. This correspondence appears valid for a reasonably wide range of porous microgeometries characterizing assemblages of minerals, microbes, and organic matter. Biogeochemical interest lies not so much with the 1/t slowdown in rates but rather the decay of the concentration of organic carbon. Observations suggest that the latter decays like t a, where a varies between about 0.1 and 1.0. Under certain general assumptions, however, the theory described above predicts a unique value of a. A better understanding of a’s natural variability is relevant to understanding changes in the carbon cycle at both short and long time scales. In particular, the latter may provide an important clue in attempts to understand the timing and mechanisms underlying the long-term production of atmospheric oxygen, especially insofar as they may be related to changes in the sedimentary deposition of clay minerals (Kennedy et al., 2006).
References Hedges, J.I., Keil, R.G., 1995. Marine Chem. 49, 81. Kennedy, M., Droser, M., Mayer, L.M., Pevear, D., Mrofka, D., 2006. Science 311, 1446. Middleburg, J.J., 1989. Geochim. Cosmochim. Acta 53, 1577. doi:10.1016/j.gca.2006.06.997
doi:10.1016/j.gca.2006.06.996
Goldschmidt Conference Abstracts 2006
Ugandan kamafugites: Re-melting of a variable enriched veined subcontinental lithospheric mantle A. ROSENTHAL1, S.F. FOLEY2, G.D. PEARSON3, G. NOWELL3, S. TAPPE2 1
Research School of Earth Sciences, The Australian National University ([email protected]) 2 Department of Earth Sciences, Universitaet of Mainz (foley@ uni-mainz.de) 3 Department of Geological Sciences, University of Durham ([email protected]; [email protected]; g.m. [email protected]) Ugandan kamafugites occur in the Toro-Ankole province along the Western Branch of the East African Rift within a Proterozoic-Archean basement complex of the Tanzanian craton. Our comprehensive geochemical study explains the origin of the potassic-ultrapotassic kamafugites by re-melting of a variably, and episodically enriched veined lithospheric mantle characterized by highly variable supra-chondritic radiogenic Os. Impregnation of the source region by carbonititic and/or highly alkaline silicate melts resulted in the introduction of modal phlogopite and enrichment in Fe and Re. The extreme silica-undersaturation (SiO2 = 31.8–41.8 wt%) of kamafugites, their high MgO (up to 22.5 wt%), low Al2O3 (<8.0 wt%), and high CaO contents (up to 16.6 wt%) are reflected in the presence of modal kalsilite, leucite, melilite and perovskite. Their primitive features such as high Mg# olivines (up to 91.1), high whole-rock Mg# (up to 80.2), and high Ni (up to 1066 ppm), Cr (up to 1560 ppm) and Os (up to 1.45 ppb) are in strong contrast to their extreme enrichment in LILE, HFSE, and LREE. Isotopic signatures (87Sr/86Sr = 0.7046–0.7054, eNd = 0.08 to 4.70, and eHf = 3.64 to 8.84, cOs = 16–290) tend to much higher superchondritic cOs (cOs = 16–290) than oceanic island basalts. Kamafigutes exhibit numerous signs of mixing, both mineralogical and geochemical (e.g., complexly zoned olivines; three generations of clinopyroxene; inverse trends on cOs vs. Mg#, Ni [ppm], Cr [ppm], Os [ppb] and 87Sr/88Sr ratios; linear trends on cOs vs. 1/Os [ppb], 187Re/188Os ratios, eNd and eHf). Changes in kamafugite compositions are controlled by the mineralogy of vein assemblages, by vein-wall-rock melting processes, and by changes in CO2/H2O, alkalinity, fO2 , and fS2 conditions during ascent. Partial melting of alkali clinopyroxenites, xenolithic material as found in the same volcanic field, are thought to represent the source material of katungites, whereas mafuritic to uganditic compositions require the incorporation of peridotite wall-rock material, and hence higher degrees of partial melting.
Temporal scaling of marine respiration DANIEL H. ROTHMAN1, DAVID C. FORNEY2 1
Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA 02139, USA ([email protected]) 2 Department of Mechanical Engineering, MIT, Cambridge, MA 02139, USA ([email protected]) The extent to which respiration balances photosynthesis determines not only environmental concentrations of CO2 but also O2. An understanding of this balance is therefore crucial to understanding coevolving biogeochemical and evolutionary transitions. Respiration occurs at all time (t) scales, but at progressively slower rates, decaying roughly like 1/t as organic matter falls from the sea surface to the sea floor, and as it proceeds through degradation in sediment (Middleburg, 1989). Qualitatively, this slowdown can be at least partially attributed to the decreased accessibility of organic carbon to microbial degradation. In particular, the association of sedimentary organic carbon with clay mineral surface area is thought to preferentially protect some organic detritus from microbial degradation (Hedges and Keil, 1995). These observations motivate a theory for detrital decay. The key assumption is that the process is limited by the diffusion of hydrolyzed organic carbon from solid (i.e., mineral) surfaces to bacteria. The site-specific rate of decay is assumed to be proportional to diffusive fluxes associated with surface sites. Averaging over these microscopic rates one can then obtain the observed macroscopic 1/t decay. This correspondence appears valid for a reasonably wide range of porous microgeometries characterizing assemblages of minerals, microbes, and organic matter. Biogeochemical interest lies not so much with the 1/t slowdown in rates but rather the decay of the concentration of organic carbon. Observations suggest that the latter decays like t a, where a varies between about 0.1 and 1.0. Under certain general assumptions, however, the theory described above predicts a unique value of a. A better understanding of a’s natural variability is relevant to understanding changes in the carbon cycle at both short and long time scales. In particular, the latter may provide an important clue in attempts to understand the timing and mechanisms underlying the long-term production of atmospheric oxygen, especially insofar as they may be related to changes in the sedimentary deposition of clay minerals (Kennedy et al., 2006).
References Hedges, J.I., Keil, R.G., 1995. Marine Chem. 49, 81. Kennedy, M., Droser, M., Mayer, L.M., Pevear, D., Mrofka, D., 2006. Science 311, 1446. Middleburg, J.J., 1989. Geochim. Cosmochim. Acta 53, 1577. doi:10.1016/j.gca.2006.06.997
doi:10.1016/j.gca.2006.06.996