TY - JOUR
T1 - A fresh isotopic look at Greenland kimberlites: Cratonic mantle lithosphere imprint on deep source signal
AU - Tappe, Sebastian
AU - Pearson, D. Graham
AU - Nowell, Geoff
AU - Nielsen, Troels
AU - Milstead, Phil
AU - Muehlenbachs, Karlis
N1 - Funding Information:
This work is published with permission of the Geological Survey of Denmark and Greenland. Analytical work in the Radiogenic Isotope Facility at the University of Alberta is in part funded by a Major Resource Support grant and at Durham University by NERC operation grants. Chris Ottley is thanked for help with the trace element determinations in Durham. Agnete Steenfelt, Karsten Secher, Sven-Monrad Jensen, and Karina Sand are thanked for help during fieldwork and for providing additional samples. The manuscript has benefited through discussions with Dejan Prelevic, Sonja Aulbach, Katie Smart, Agnete Steenfelt, Roger Mitchell, and Bruce Kjarsgaard. Chad Paton, an anonymous reviewer, and editor Rick Carlson provided thoughtful comments on this manuscript, for which we are sincerely grateful.
PY - 2011/5/1
Y1 - 2011/5/1
N2 - The North Atlantic craton
in West Greenland and northern Labrador has been subjected to deep
volatile-rich melting events between ca. 610 and 550 Ma that produced
compositionally diverse diamond-bearing kimberlite and aillikite magmas.
Whereas kimberlite dyke intrusions appear to be restricted to the
Maniitsoq area in the craton interior between 568 and 553 Ma,
aillikite/carbonatite intrusives preferentially occur at
Paleoproterozoic mobile belts such as in the Sarfartoq area (605–550 Ma) of West Greenland.Although
there is an overlap between the major and trace element compositions of
the exceptionally fresh Maniitsoq kimberlites and Sarfartoq aillikites,
the latter typically show higher TiO2, Al2O3, and K2O, as well as higher Zr, Hf, Cs and Rb contents. Furthermore, the Sarfartoq aillikites are displaced toward lower εHf by ~ 3 epsilon units at similar εNd compared with the isotopically depleted Maniitsoq kimberlites and their garnet and ilmenite megacrysts. The generally lower εHf of aillikites corresponds to lower CO2/K2O
and points to the involvement of a K-rich melt component in aillikite
genesis, most likely derived from a cratonic metasome. In contrast, the
Maniitsoq kimberlite compositions, in particular the high CO2 as well as low Al2O3 and K2O
contents, resemble published carbonate-rich melt compositions that were
produced experimentally from carbonated peridotite in excess of 6 GPa,
i.e., under sublithospheric conditions. By utilizing published
high-pressure carbonated peridotite/melt trace element partition
coefficients, we demonstrate that many of the hallmark geochemical
features of kimberlites, such as relative Zr–Hf depletions, can be
produced by low-degree partial melting of carbonated fertile peridotite
within the asthenosphere.For the Greenland-Labrador
Diamond Province, we propose that a common asthenosphere-derived
carbonated silicate melt component must have been present throughout the
North Atlantic craton base at 610-to-550 Ma. This widespread
carbonate-rich melt component variably interacted with old
phlogopite-bearing cratonic metasomes, giving rise to diverse suites of
aillikites, i.e., hybrid carbonated potassic-silicate magmas, that
locally separated out carbonate fractions to form intrusive carbonatites
at crustal levels. The kimberlites, however, appear to be mixtures of
this asthenosphere-derived carbonate-rich melt component and entrainment
of materials from the refractory cratonic mantle lithosphere, with
little or no involvement of readily fusible phlogopite-rich metasomes.
The model developed herein for West Greenland highlights the importance
of cratonic mantle lithosphere in exerting a major control on worldwide
kimberlitic magma compositions. Moreover, the ability to examine
kimberlite magma compositional variability in time and space clearly
shows that decoupled Nd–Hf isotope systematics cannot be taken
unconditionally as a reliable fingerprint of ultra-deep mantle
processes, because this type of signal can also be imparted to
kimberlitic melts by interaction with cratonic metasomes.
AB - The North Atlantic craton
in West Greenland and northern Labrador has been subjected to deep
volatile-rich melting events between ca. 610 and 550 Ma that produced
compositionally diverse diamond-bearing kimberlite and aillikite magmas.
Whereas kimberlite dyke intrusions appear to be restricted to the
Maniitsoq area in the craton interior between 568 and 553 Ma,
aillikite/carbonatite intrusives preferentially occur at
Paleoproterozoic mobile belts such as in the Sarfartoq area (605–550 Ma) of West Greenland.Although
there is an overlap between the major and trace element compositions of
the exceptionally fresh Maniitsoq kimberlites and Sarfartoq aillikites,
the latter typically show higher TiO2, Al2O3, and K2O, as well as higher Zr, Hf, Cs and Rb contents. Furthermore, the Sarfartoq aillikites are displaced toward lower εHf by ~ 3 epsilon units at similar εNd compared with the isotopically depleted Maniitsoq kimberlites and their garnet and ilmenite megacrysts. The generally lower εHf of aillikites corresponds to lower CO2/K2O
and points to the involvement of a K-rich melt component in aillikite
genesis, most likely derived from a cratonic metasome. In contrast, the
Maniitsoq kimberlite compositions, in particular the high CO2 as well as low Al2O3 and K2O
contents, resemble published carbonate-rich melt compositions that were
produced experimentally from carbonated peridotite in excess of 6 GPa,
i.e., under sublithospheric conditions. By utilizing published
high-pressure carbonated peridotite/melt trace element partition
coefficients, we demonstrate that many of the hallmark geochemical
features of kimberlites, such as relative Zr–Hf depletions, can be
produced by low-degree partial melting of carbonated fertile peridotite
within the asthenosphere.For the Greenland-Labrador
Diamond Province, we propose that a common asthenosphere-derived
carbonated silicate melt component must have been present throughout the
North Atlantic craton base at 610-to-550 Ma. This widespread
carbonate-rich melt component variably interacted with old
phlogopite-bearing cratonic metasomes, giving rise to diverse suites of
aillikites, i.e., hybrid carbonated potassic-silicate magmas, that
locally separated out carbonate fractions to form intrusive carbonatites
at crustal levels. The kimberlites, however, appear to be mixtures of
this asthenosphere-derived carbonate-rich melt component and entrainment
of materials from the refractory cratonic mantle lithosphere, with
little or no involvement of readily fusible phlogopite-rich metasomes.
The model developed herein for West Greenland highlights the importance
of cratonic mantle lithosphere in exerting a major control on worldwide
kimberlitic magma compositions. Moreover, the ability to examine
kimberlite magma compositional variability in time and space clearly
shows that decoupled Nd–Hf isotope systematics cannot be taken
unconditionally as a reliable fingerprint of ultra-deep mantle
processes, because this type of signal can also be imparted to
kimberlitic melts by interaction with cratonic metasomes.
KW - Kimberlite petrogenesis
KW - Low-Cr megacrysts
KW - Mantle reservoirs
KW - North Atlantic craton
KW - Sr-Nd-Hf isotope geochemistry
KW - U-Pb perovskite geochronology
UR - http://www.scopus.com/inward/record.url?scp=79953745641&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2011.03.005
DO - 10.1016/j.epsl.2011.03.005
M3 - Article
SN - 0012-821X
VL - 305
SP - 235
EP - 248
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 1-2
ER -