A suite of mafic, granulite facies xenoliths from north Queensland possesses petrographic and geo... more A suite of mafic, granulite facies xenoliths from north Queensland possesses petrographic and geochemical features of basaltic cumulates crystallized at lower crustal pressures. Negative correlations between incompatible trace elements and Mg# and positive correlations between compatible trace elements and Mg# suggest the xenoliths are genetically related and crystallized from a continuously evolving melt. Zr, Hf, Y, HREE, Ti and V do not correlate with Mg#, but show excellent negative correlations with Al 2 O 3 ...
Li isotopes are thought to hold great potential for tracing crustal recycling, due to the very la... more Li isotopes are thought to hold great potential for tracing crustal recycling, due to the very large isotopic fractionations (over 700/00) documented at the Earth's surface. If highly fractionated crustal Li makes it past the subduction environment, it should be sampled by mantle rocks, provided it is not efficiently re-homogenized within the mantle by diffusion or stirring. However, data for arc lavas show that, contrary to initial expectations, Li generally does not track with other fluid-mobile elements and most arc lavas have isotopic ...
High precision trace element data are reported for representative samples of the upper continenta... more High precision trace element data are reported for representative samples of the upper continental crust: 11 loess samples and 22 shale samples PAAS previously used by Taylor and McLennan to define the rare earth element REE content of w the upper crust. Our results confirm the REE concentrations of Taylor and McLennan’s [Taylor, S.R., McLennan, S.M., x 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312 pp.] estimate of the upper continental crust but suggest substantial revisions for Nb and Ta, in agreement with recent work of Plank and Langmuir [Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chem. Geol. 145, 325–394.]. From our data, the upper continental crust has average Nb = 11.5 +/- 2.6 ppm (instead of 25 ppm) and Ta = 0.92 +/- 0.12 ppm (instead of 2.2 ppm), which translates into a bulk crust Nb = 8 ppm, Ta = 0.7 ppm, La / Nb = 2.2, and Nb / Ta = 12–13. These revisions confirm the crustal subchondritic Nb / Ta and superchondritic La / Nb ratios and reinforce the observation that the continental crust and the Depleted Mantle are not strictly complementary: an additional Nb- and Ta-rich reservoir having superchondritic Nb / Ta is required to balance the Silicate Earth. Using the continental crust’s La / Nb ratio to estimate the proportions of crustal growth in convergent margin and intraplate settings, we suggest a plume contribution of only between 5 and 20% to the continents, even lower than previous estimates.
ABSTRACT Granulite xenoliths entrained within Quaternary rift basalts in northern Tanzania docume... more ABSTRACT Granulite xenoliths entrained within Quaternary rift basalts in northern Tanzania document the composition, equilibration conditions, age and petrogenesis of the present-day lower crust beneath the eastern margin of the Tanzanian Craton (Labait) and the adjacent Mozambique Belt (Lashaine and Naibor Soito). Mafic to intermediate Archean lithologies dominate throughout the suites (similar to 2 center dot 66 Ga based on U-Pb zircon ages), demonstrating that deep-seated Archean lithosphere extends far to the east of the margin of the Tanzanian Craton within the Mozambique Belt. There is no evidence for significant additions to the crust via magmatic underplating since that time. Many of the lower crustal xenoliths share compositional similarities with lavas from the greenstone belts of the Tanzanian Craton, suggesting that they crystallized from similar magmas. Extreme depletions of highly incompatible elements (e.g. Cs, Rb, Th and U) in the granulites, relative to the lavas, coupled with unradiogenic Nd-143/Nd-144 (0 center dot 5114-0 center dot 5122) and Sr-87/Sr-86 (0 center dot 7040-0 center dot 7051), suggest that these depletions occurred coincident with or shortly after the rocks crystallized, possibly through partial melting associated with metamorphism. These samples may thus represent high-grade lower crustal complements to the greenstone belt lavas. Compared with the craton-margin samples, granulite xenoliths from within the Mozambique Belt record very high peak equilibration pressures at moderate temperatures (> 1 center dot 2 to > 1 center dot 7 GPa, 750-960A degrees C, based on pseudosections), documenting their equilibration deep within thickened continental crust during the East African Orogeny, c. 560 Myr ago. These samples therefore offer an unusual window into the deepest reaches of the crust in a continental orogen. Despite the fact that the Mozambique Belt experienced significant crustal thickening followed by post-orogenic collapse, there is no evidence for loss of the deep lithosphere associated with these processes.
Eos, Transactions American Geophysical Union, 2009
Cin-Ty Lee received the 2008 Hisashi Kuno Award at the 2008 AGU Fall Meeting Honors Ceremony, hel... more Cin-Ty Lee received the 2008 Hisashi Kuno Award at the 2008 AGU Fall Meeting Honors Ceremony, held 17 December 2008 in San Francisco, Calif. The award is for outstanding contributions to the fields of volcanology, geochemistry, or petrology.
In recognition of Stuart Ross Taylor on the occasion of his sixty-fifth birthday and retirement f... more In recognition of Stuart Ross Taylor on the occasion of his sixty-fifth birthday and retirement from the Research School of Earth Science.
A suite of mafic, granulite facies xenoliths from north Queensland possesses petrographic and geo... more A suite of mafic, granulite facies xenoliths from north Queensland possesses petrographic and geochemical features of basaltic cumulates crystallized at lower crustal pressures. Negative correlations between incompatible trace elements and Mg# and positive correlations between compatible trace elements and Mg# suggest the xenoliths are genetically related and crystallized from a continuously evolving melt. Zr, Hf, Y, HREE, Ti and V do not correlate with Mg#, but show excellent negative correlations with Al 2 O 3 ...
Li isotopes are thought to hold great potential for tracing crustal recycling, due to the very la... more Li isotopes are thought to hold great potential for tracing crustal recycling, due to the very large isotopic fractionations (over 700/00) documented at the Earth's surface. If highly fractionated crustal Li makes it past the subduction environment, it should be sampled by mantle rocks, provided it is not efficiently re-homogenized within the mantle by diffusion or stirring. However, data for arc lavas show that, contrary to initial expectations, Li generally does not track with other fluid-mobile elements and most arc lavas have isotopic ...
High precision trace element data are reported for representative samples of the upper continenta... more High precision trace element data are reported for representative samples of the upper continental crust: 11 loess samples and 22 shale samples PAAS previously used by Taylor and McLennan to define the rare earth element REE content of w the upper crust. Our results confirm the REE concentrations of Taylor and McLennan’s [Taylor, S.R., McLennan, S.M., x 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312 pp.] estimate of the upper continental crust but suggest substantial revisions for Nb and Ta, in agreement with recent work of Plank and Langmuir [Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chem. Geol. 145, 325–394.]. From our data, the upper continental crust has average Nb = 11.5 +/- 2.6 ppm (instead of 25 ppm) and Ta = 0.92 +/- 0.12 ppm (instead of 2.2 ppm), which translates into a bulk crust Nb = 8 ppm, Ta = 0.7 ppm, La / Nb = 2.2, and Nb / Ta = 12–13. These revisions confirm the crustal subchondritic Nb / Ta and superchondritic La / Nb ratios and reinforce the observation that the continental crust and the Depleted Mantle are not strictly complementary: an additional Nb- and Ta-rich reservoir having superchondritic Nb / Ta is required to balance the Silicate Earth. Using the continental crust’s La / Nb ratio to estimate the proportions of crustal growth in convergent margin and intraplate settings, we suggest a plume contribution of only between 5 and 20% to the continents, even lower than previous estimates.
ABSTRACT Granulite xenoliths entrained within Quaternary rift basalts in northern Tanzania docume... more ABSTRACT Granulite xenoliths entrained within Quaternary rift basalts in northern Tanzania document the composition, equilibration conditions, age and petrogenesis of the present-day lower crust beneath the eastern margin of the Tanzanian Craton (Labait) and the adjacent Mozambique Belt (Lashaine and Naibor Soito). Mafic to intermediate Archean lithologies dominate throughout the suites (similar to 2 center dot 66 Ga based on U-Pb zircon ages), demonstrating that deep-seated Archean lithosphere extends far to the east of the margin of the Tanzanian Craton within the Mozambique Belt. There is no evidence for significant additions to the crust via magmatic underplating since that time. Many of the lower crustal xenoliths share compositional similarities with lavas from the greenstone belts of the Tanzanian Craton, suggesting that they crystallized from similar magmas. Extreme depletions of highly incompatible elements (e.g. Cs, Rb, Th and U) in the granulites, relative to the lavas, coupled with unradiogenic Nd-143/Nd-144 (0 center dot 5114-0 center dot 5122) and Sr-87/Sr-86 (0 center dot 7040-0 center dot 7051), suggest that these depletions occurred coincident with or shortly after the rocks crystallized, possibly through partial melting associated with metamorphism. These samples may thus represent high-grade lower crustal complements to the greenstone belt lavas. Compared with the craton-margin samples, granulite xenoliths from within the Mozambique Belt record very high peak equilibration pressures at moderate temperatures (> 1 center dot 2 to > 1 center dot 7 GPa, 750-960A degrees C, based on pseudosections), documenting their equilibration deep within thickened continental crust during the East African Orogeny, c. 560 Myr ago. These samples therefore offer an unusual window into the deepest reaches of the crust in a continental orogen. Despite the fact that the Mozambique Belt experienced significant crustal thickening followed by post-orogenic collapse, there is no evidence for loss of the deep lithosphere associated with these processes.
Eos, Transactions American Geophysical Union, 2009
Cin-Ty Lee received the 2008 Hisashi Kuno Award at the 2008 AGU Fall Meeting Honors Ceremony, hel... more Cin-Ty Lee received the 2008 Hisashi Kuno Award at the 2008 AGU Fall Meeting Honors Ceremony, held 17 December 2008 in San Francisco, Calif. The award is for outstanding contributions to the fields of volcanology, geochemistry, or petrology.
In recognition of Stuart Ross Taylor on the occasion of his sixty-fifth birthday and retirement f... more In recognition of Stuart Ross Taylor on the occasion of his sixty-fifth birthday and retirement from the Research School of Earth Science.
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Papers by Roberta Rudnick
w the upper crust. Our results confirm the REE concentrations of Taylor and McLennan’s [Taylor, S.R., McLennan, S.M.,
x 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312 pp.] estimate of the upper continental crust but suggest substantial revisions for Nb and Ta, in agreement with recent work of Plank and Langmuir [Plank, T.,
Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chem. Geol. 145, 325–394.]. From our data, the upper continental crust has average Nb = 11.5 +/- 2.6 ppm (instead of 25 ppm) and Ta = 0.92 +/- 0.12 ppm (instead of 2.2 ppm), which translates into a bulk crust Nb = 8 ppm, Ta = 0.7 ppm,
La / Nb = 2.2, and Nb / Ta = 12–13. These revisions confirm the crustal subchondritic Nb / Ta and superchondritic La / Nb
ratios and reinforce the observation that the continental crust and the Depleted Mantle are not strictly complementary: an
additional Nb- and Ta-rich reservoir having superchondritic Nb / Ta is required to balance the Silicate Earth. Using the
continental crust’s La / Nb ratio to estimate the proportions of crustal growth in convergent margin and intraplate settings,
we suggest a plume contribution of only between 5 and 20% to the continents, even lower than previous estimates.
w the upper crust. Our results confirm the REE concentrations of Taylor and McLennan’s [Taylor, S.R., McLennan, S.M.,
x 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312 pp.] estimate of the upper continental crust but suggest substantial revisions for Nb and Ta, in agreement with recent work of Plank and Langmuir [Plank, T.,
Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chem. Geol. 145, 325–394.]. From our data, the upper continental crust has average Nb = 11.5 +/- 2.6 ppm (instead of 25 ppm) and Ta = 0.92 +/- 0.12 ppm (instead of 2.2 ppm), which translates into a bulk crust Nb = 8 ppm, Ta = 0.7 ppm,
La / Nb = 2.2, and Nb / Ta = 12–13. These revisions confirm the crustal subchondritic Nb / Ta and superchondritic La / Nb
ratios and reinforce the observation that the continental crust and the Depleted Mantle are not strictly complementary: an
additional Nb- and Ta-rich reservoir having superchondritic Nb / Ta is required to balance the Silicate Earth. Using the
continental crust’s La / Nb ratio to estimate the proportions of crustal growth in convergent margin and intraplate settings,
we suggest a plume contribution of only between 5 and 20% to the continents, even lower than previous estimates.