Magnesian Eclogite as a Source for 'Websteritic' Diamonds
Diamonds with mineral inclusions classified as 'websteritic' are rare and have compositions intermediate between those of 'peridotitic' and 'eclogitic'. Here we provide evidence that 'websteritic' diamonds may have been sourced from magnesian eclogites rather than from websterites. We studied diamonds and mantle xenoliths of the Jericho kimberlite (Northern Slave craton) that include a variety of eclogite types and websterites. Eight percent of diamonds with mineral inclusions are classified as 'websteritic', i.e. contain magnesian garnet with 0.4 wt% <Cr2O3< 1 wt%, diopside and enstatite. 'Websteritic' diamond inclusions (DI) form single compositional trends with 'eclogitic' inclusions. On these trends, 'websteritic' DI occupy the most magnesian parts. 'Websteritic' DI in the Jericho diamonds are invariably more chromian than corresponding garnet, clinopyroxene and orthopyroxene found in Jericho websterite xenoliths. Some DI of 'websteritic' garnet are similar in composition to garnet found in the most magnesian Jericho eclogites. We therefore suggest that a parent rock for 'websteritic' diamonds at Jericho and perhaps in some other worldwide locations could be magnesian eclogite. The magnesian character of the Jericho eclogite can be ascribed to an ultramafic picritic protolith or to melt extraction that affected mafic eclogites. Abundant evidence suggests the eclogites are residues of melt extraction. It is indicated by partial recrystallization of primary eclogite minerals to secondary garnet and clinopyroxene enriched in Mg and depleted in relatively low-T components grossular, almandine and jadeite. The recrystallization of the eclogite, however, cannot be ascribed solely to melt extraction, as new grains of garnet and clinopyroxene are enriched in incompatible trace elements Ba, Zr, Hf. Partial melting must have been associated with metasomatism and the influx of hydrous P, Nb and Ti-rich fluid. There is geochemical and textural evidence that melt extraction and associated metasomatism in the Jericho mantle was a repetitive, 1.5 Gy-long process separated in time into several episodes. The most recent episode recorded in eclogite textures and complex mineral zoning may be analogous to older metasomatic processes overprinted and concealed by a long residence in the mantle. A prior melt extraction event that modified compositions of diamondiferous eclogites after formation of diamonds is witnessed by the contrasting chemistry of garnet and clinopyroxene inside and outside diamond. Garnets and clinopyroxenes that crystallized outside of diamond in diamondiferous eclogites are more Mg-rich than the DI garnet and clinopyroxene and compared to them are depleted in incompatible Na, Al, Rb, Ba, La, Ce, Pr and HREEs. One of older episodes of metasomatism may have also been involved in the genesis of diamonds and their DI. It is suggested by the following: 1) clinopyroxene in diamonds is closer to recrystallized secondary clinopyroxene than to primary clinopyroxene in eclogites by low Na2O and high contents of MgO, Ba, HREEs, La and Ce, and 2) Garnet in diamonds is characterized by elevated Zr and Hf, typical of recrystallized eclogitic garnet. Very magnesian eclogites like those at Jericho are uncommon and found only in few locations worldwide. Diamondiferous magnesian eclogites are increasingly rare as most 'eclogitic' diamonds are sourced from more sodic and calcic eclogites. The rarity of diamondiferous Mg-rich eclogites among less magnesian counterparts may explain a low abundance of 'websteritic' DI with respect to 'eclogitic'.
Origin of Diamond Rich, High-MgO Eclogite Xenoliths from the Jericho Kimberlite, Nunavut
A suite of diamond-rich eclogites from the Jericho kimberlite, Nunavut, has an unusual but uniform geochemical and isotopic composition that is unlike any other eclogite suite worldwide. Compared to other suites, garnets from most Jericho diamond-bearing eclogites have high MgO (19.6-21.2 wt.%), Cr2O3 (0.28- 0.60 wt.%), Sc (88-113 ppm) and Zr (32-36 ppm) contents and have highly fractionated chondrite-normalized HREE ([Lu/Gd]=5.1-6.6). Interestingly, a Fe-rich garnet was discovered as an inclusion within diamond in one of these Mg-rich eclogites. Na-poor clinopyroxenes have uniform LREE-enriched chondrite-normalized patterns (La 79-104; Nd 77.1-90.4) and radiogenic 87Sr/86Sr isotopic composition between 0.7057 and 0.7061. Diamonds in these eclogites occur in two modes, as inclusions in garnet and as discrete, larger crystals at grain boundaries. In contrast to the diamond eclogites, the remaining eclogites at Jericho are Fe- and Ca-rich, typical of most other eclogite suites worldwide. Garnets from this suite have lower MgO (5.0 -13.0 wt.%), higher FeO (up to 26.5 wt% FeO) and high CaO (up to 17.5 wt.% CaO). Positive Eu anomalies ([Eu/Eu*] =1.1-2.1) and flat or slightly depleted chondrite-normalized HREE ([Lu/Gd] 0.5-1.6) are common. Oxygen isotopic analyses of a subset of garnets from this suite yielded oxygen isotope values of 6.3 to 7.6 permil. Clinopyroxene from the Fe- and Ca-rich eclogite suites are Na2O-rich (up to 8 wt.%), relatively lower in chondrite-normalized LREE (La 0.2-6.7; Nd 0.7-20.2) and have variable but lower 87Sr/86Sr (0.7036-0.7044) than the diamond eclogites. The geochemical and isotopic characteristics of the Fe- and Ca-rich eclogites are consistent with having an origin as remnants of subducted oceanic crust. The composition of the Jericho diamond eclogites is not compatible with an origin as either remnants of subducted oceanic crust or cumulates from high-pressure mafic melts, and have striking compositional similarities with mantle peridotites. We propose an origin that involves multiple metasomatic events, coupled with hybridization between low-MgO eclogite and mantle peridotite. Emplacement of eclogites within the diamond stability field is followed by an initial carbon-bearing, LREE-enriched metasomatic event recorded by clinopyroxene and diamond inclusions in garnet. Subsequent partial melting of eclogite produces melts that enable diffusional elemental exchange between residual eclogite and local peridotite. Through this stage the eclogites attain their distinct high-Mg and Cr composition. Finally, carbonatite-like modal metasomatism causes the growth of phlogopite, carbonate and apatite and facilitates new growth of diamond, producing the extreme diamond enrichments found in these eclogites.
First Results From Greenland Eclogite Xenoliths: Evidence for an Ultra-depleted Non- peridotitic Component Within the North Atlantic Craton Mantle Lithosphere
Some of Earth's most refractory peridotitic mantle is known to form the keel of the North Atlantic craton (NAC) beneath West Greenland and geochemical and isotopic evidence suggest that this cratonic root stabilized due to extensive 'shallow' melting during the Early Archean. The nature of the eclogitic component, however, was unknown until recent discovery of mantle eclogite xenoliths from Neoproterozoic kimberlite dykes. Here we report first results from these eclogites recovered from locations in the craton interior (Nuuk region) and near the craton margin (Maniitsoq region). The Nuuk region eclogites have high MgO garnet compositions (15-20 weight percent) with slightly fractionated chondrite-normalized HREE ([Lu/Gd] = 1.3-4.7) but strong LREE depletion ([La/Sm] less than 0.1). Clinopyroxene has moderate Na2O contents (3.6-4.4 weight percent) and also shows LREE depletion. Calculated equilibration temperatures at 4 GPa are relatively cool ranging between 700-900°C. Clinopyroxene Pb-Pb isotope systematics suggest that the precursor melt that formed the eclogite protolith(s) was formed during the Early Paleoproterozoic (model ages between 2.2 and 1.9 Ga). The eclogitic clinopyroxene Pb isotope compositions exhibit strong similarity to that of the juvenile Early Paleoproterozoic arc crust of West Greenland, suggesting a possible link between the refractory eclogites and felsic craton-building magmatism. In contrast, eclogites from the Maniitsoq region near the craton margin are typically less refractory, and have garnet with lower MgO (10-15 weight percent), higher Ca and Fe contents, and clinopyroxene with higher Na2O (3-7 weight percent). Garnet has less fractionated chondrite-normalized HREE ([Lu/Gd] = 1.2-2.3) and slightly enriched LREE ([La/Sm] = 0.12-0.4) patterns, and clinopyroxene shows a pronounced LREE enrichment (chondrite-normalized Nd = 11-41) when compared to the Nuuk region eclogites (Nd = 5.7-30). Equilibration temperatures range between 950-1150°C and, thus, the Maniitsoq eclogites appear to be derived from greater depths than the highly refractory Nuuk eclogites. The clinopyroxene Pb isotope compositions of the Maniitsoq eclogites have a wide range and appear to be 'disturbed', most likely due to metasomatism. Our study demonstrates that eclogites from different locations across the NAC are compositionally distinct and that highly refractory eclogite was only preserved beneath the craton interior at shallow depths. This ultra-depleted eclogitic component most likely represents melt residues from subducting oceanic lithosphere, but unlike the ultra-depleted Early Archean NAC peridotites, these eclogites were emplaced into the cratonic mantle during the Early Paleoproterozoic amalgamation of the Laurentia supercraton.
Are diamond-bearing Cretaceous kimberlites related to shallow-angle subduction beneath western North America?
The origin of deep-seated magmatism (in particular, kimberlites and lamproites) within continental plate interiors remains enigmatic in the context of plate tectonic theory. One hypothesis proposes a relationship between kimberlite occurrence and lithospheric subduction, such that a subducting plate releases fluids below a continental craton, triggering melting of the deep lithosphere and magmatism (Sharp, 1974; McCandless, 1999). This study provides a quantitative evaluation of this hypothesis, focusing on the Late Cretaceous- Eocene (105-50 Ma) kimberlites and lamproites of western North America. These magmas were emplaced along a corridor of Archean and Proterozoic lithosphere, 1000-1500 km inboard of the plate margin separating the subducting Farallon Plate and continental North America Plate. Kimberlite-lamproite magmatism coincides with tectonic events, including the Laramide orogeny, shut-down of the Sierra Nevada arc, and eastward migration of volcanism, that are commonly attributed to a change in Farallon Plate geometry to a shallow-angle trajectory (<25° dip). Thermal-mechanical numerical models demonstrate that rapid Cretaceous plate convergence rates and enhanced westward velocity of North America result in shallow-angle subduction that places the Farallon Plate beneath the western edge of the cratonic interior of North America. This geometry is consistent with the observed continental dynamic subsidence that lead to the development of the Western Interior Seaway. The models also show that the subducting plate has a cool thermal structure, and subducted hydrous minerals (serpentine, phengite and phlogopite) remain stable to more than 1200 km from the trench, where they may break down and release fluids that infiltrate the overlying craton lithosphere. This is supported by geochemical studies that indicate metasomatism of the Colorado Plateau and Wyoming craton mantle lithosphere by an aqueous fluid and/or silicate melt with a subduction signature. Through Cretaceous shallow-angle subduction, the Farallon Plate was in a position to mechanically and chemically interact with North American craton lithosphere at the time of kimberlite-lamproite magmatism, making the subduction hypothesis a viable mechanism for the genesis of these magmas. REFERENCES: McCandless, T.E., Proceedings of the 7th International Kimberlite Conference, v.2, pp.545-549, 1999; Sharp, W.E., Earth Planet. Sci. Lett., v.21, pp.351-354, 1974.
Were deep cratonic mantle roots hydrated in Archean oceans?
Using a compilation of mantle peridotites (n = 2153) I statistically evaluate the frequency of Si enrichment in cratonic and other types of mantle lithosphere. I fit arrays of peridotites in Mg - Al - Si space and define an empirical parameter ('delta Mg/Si') describing the degree of Si enrichment or depletion in mantle lithosphere relative to residue trends defined by high pressure-temperature peridotite melting experiments. Most mantle types are normally distributed about deltaMg/Si of zero. Silica enrichment (a strong skew to negative delatMg/Si) occurs in a significant number of cratonic xenoliths from southern Africa and in abyssal peridotites. I assert that the Si-rich composition of some cratonic samples can be linked to the hydration of their protoliths either on the Archean ocean floor, before being subducted or imbricated to form a craton root. Oxygen isotopic shifts that correlate with bulk Mg/Si in mid-Atlantic ridge seafloor rocks (Atlantis Massif) parallel those seen in the few such data for cratonic peridotite xenoliths, in support of our hypothesis. Most chemical variability in the mantle is canonically viewed to have originated from the bottom up by percolating melts. I turn this idea on its head and explain how silica enrichment, one of the defining parameters of cratonic lithosphere, can originate by a top-down chemical exchange during weathering or hydrothermal activity when such peridotites resided on an Archean ocean floor.
Diamond-bearing Mantle of the Kaapvaal Craton: Implications for Exploration Models and Craton Root Petrogenesis
The association of diamond with subcalcic garnet harzburgite (SGH) in the roots of Archean cratons is a powerful empirical tool in diamond exploration, but is not fully understood. A key question is whether or not the unusual subcalcic, high-Mg# character of SGH is created during the diamond-forming event itself. Here I combine geochemical data on diamond-bearing and diamond-free SGH and chromite harzburgites from Kimberley, South Africa with published results on Kimberley diamond inclusions (DIs) to investigate petrogenesis of diamond-bearing mantle and its relationship to craton evolution. Harzburgites and DIs from this region of the Kaapvaal Craton are uniquely refractory, with Mg# up to 96 in olivine and whole rocks (WR), and 97 in opx, correlated with decreasing CaO. Xenolith WR compositions and calculated modes, as well as the compositions of DIs, indicate that the extreme depletion in fertile components was accompanied by an increase modal opx. The relatively opx-rich character and typical coarse grain sizes differ from diamond-bearing peridotites from Udachnaya, Siberia, which are opx-poor megacrystalline dunites. The abundance of opx at high Mg# also appears to conflict with the observation that Mg#=93 is the composition at which pyroxene is exhausted in the progressive partial melting of peridotite. To explain this paradox, a two-stage melting history is proposed for the Kimberley harzburgites: after primary melt depletion leaving a dunite residue, they underwent a second, open system, volatile-fluxed partial melting event, resulting in the growth of opx and extreme Fe depletion. The latter event apparently occurred prior to (rather than synchronous with) diamond formation because (1) abundant highly refractory opx occurs as DIs and (2) harzburgite DI compositions worldwide show that these extreme levels of depletion are not required for diamond formation. The ultra-depleted, opx-rich character is inferred to be a consequence of the unique subduction-zone processing of the Kaapvaal Craton, also reflected in the average composition of Kaapvaal peridotites. It is concluded that diamond-forming processes do not impart the unique subcalcic chemical character to their harzburgite hosts and inclusions. A plausible explanation for the SGH-diamond association is that diamond forms from residual metasomatic fluids at low fluid-rock ratios, with minimal associated impact on the major element composition of the protolith. At higher fluid-rock ratios these events convert subcalcic harzburgite and dunite to calcic harzburgite and lherzolite.
Diamond Ages and Lithosphere Evolution - Applications to Diamond Exploration
Combinations of studies on diamonds, diamond inclusions, diamond bearing mantle and ultra-high pressure (UHP) metamorphic crustal rocks, kimberlites and lamproites have been successful in delivering insights into major processes such as plate tectonics, craton accretion, the effects of large magmatic events as well as contributing to a better understanding of diamond formation and preservation over an extended period of earth history. The crystal structure of diamond ensures that mineral inclusions in natural diamonds, whether fluid or solid, may be maintained as closed systems over extended periods of geological time and provide useful information about key processes in the mantle, as far back as 3.5 Ga and possibly further. Available diamond ages suggest that all macrodiamonds in economically significant kimberlites and lamproites are xenocrystic and have formed in pre-existing upper mantle assemblages, predominantly peridotite, eclogite and websterite in the sub-continental lithospheric mantle (SCLM) and occasionally in higher-pressure equivalents of such rocks, for example majorite. Diamond ages also suggest that conditions favorable for diamond formation in the SCLM have been episodic, can be repeated in the same pre-existing host rocks at significantly different times and that all investigated ore bodies have more than one population of xenocrystic diamonds contributing to run-of-mine production. Evidence is accumulating as well that diamond forming processes are metasomatic, though this is more conclusively demonstrated for young rather than old diamonds. Given that both diamond formation events and transportation into the crust are episodic and span from the Paleoarchean to the Cenozoic, diamond geology, mineralogy, and chemistry provide a unique opportunity to contribute to knowledge about the evolution of the earth's continental lithosphere through a major part of earth history. While the processes of formation and preservation of diamonds in the cratonic roots are a function of Archean and post-Archean craton evolution and have operated worldwide, the timing of individual diamond-forming events and of their transport to the surface are craton specific. The formation of diamondiferous carbonated, G- 10 garnet-bearing harzburgitic domains in Mesoarchean mantle roots represents the earliest lithospheric- diamond-forming event so far discovered in the Kaapvaal, Zimbabwe, Slave, and Siberian diamond fields. Not only is it the earliest, it remains of primary importance as a contributor to the diamond budget in many of the on-craton deposits. Ideally, exploration for kimberlites containing the G-10 bearing harzburgitic diamond component should concentrate on the Mesoarchean nuclei of Archean cratons or tectonically buried parts thereof. The ancient harzburgitic component makes a lesser or no contribution to the diamond budget of craton-margin and off-craton primary deposits (e.g., the Argyle and Ellendale lamproites, the Colorado/Wyoming State Line kimberlites, the Carolina kimberlite in northwestern Brazil, the kimberlites of the North Lesotho area, and Jagersfontein. This is in accord with the low grade of all these deposits, except Argyle. The presentation will stress that modern exploration should make use of the increasing evidence that diamond formation ages can be correlated with Archean and Proterozoic craton evolution events.