Andrea Giuliani

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Giuliani

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Andrea

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0000-0002-6823-2807

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Publications1 - 10 of 70

Kimberlites reveal 2.5-billion-year evolution of a deep, isolated mantle reservoir
Woodhead, Jon; Hergt, Janet; Giuliani, Andrea; et al. (2019)
Nature
Water contents and hydrogen isotope compositions of amphibole in aillikites from the Tarim large igneous province, NW China: Insight into Earth’s deep water cycle
Wang, Changhong; Zhang, Zhaochong; Giuliani, Andrea; et al. (2024)
GSA Bulletin
Water is known to play a crucial role in the generation of many large igneous provinces (LIP) worldwide, but the amount and origin of the water in their sources is still under debate. To address this question, this paper presents in situ major-, trace-element, and Sr isotope data coupled with bulk-mineral O-H-He isotope analyses of amphibole in a suite of aillikites from the Tarim LIP (NW China). The cores of zoned macrocrysts and microcrysts display partially overlapping compositions ranging between edenite and pargasite (75–83 versus 75–80 Mg#), which suggest a common origin from an evolving magma. The rims (Mg# = 75–80) of both macrocrysts and microcrysts are very similar to their cores for many elements, except for higher Sr and Ba contents. All the amphibole zones show similar primitive mantle–normalized trace element patterns, suggesting that they crystallized at different stages during magmatic evolution. This interpretation is confirmed by the homogenous Sr isotope compositions (⁸⁷Sr/⁸⁶Sr₍ᵢ₎ = 0.70298–0.70394) of these amphiboles, which overlap with those of magmatic apatites and perovskites in these aillikites. The hydrogen isotope compositions (δD = −120‰ to −140‰) of the amphiboles are significantly lower than average mantle values. Given the low water contents (<0.66 wt%) of these minerals, the low H isotope signatures of the amphiboles might be caused by variable H₂O loss during magma ascent. However, open-system Rayleigh fractionation modeling suggests that the hydrogen isotope compositions of these amphibole phenocrysts cannot be fully reproduced by crystallization following magmatic degassing. These low δD values require incorporation of recycled altered oceanic crust containing hydrous components in the plume source of these aillikites, which is consistent with the previously published moderately radiogenic He isotope ratios of olivine separates and bulk-rock Os and Pb isotope data for these same samples. We conclude that water in these magmas was derived from a plume source containing recycled water-bearing oceanic crust.
Correlations between olivine composition and groundmass mineralogy in Sierra Leone kimberlites provide constraints on craton-specific melt-lithosphere interactions
Viljoen, Anton; Howarth, Geoffrey H.; Giuliani, Andrea; et al. (2022)
Lithos
Sierra Leone contains two Jurassic-aged diamondiferous kimberlite clusters, namely Koidu and Tongo-Tonguma (hereafter referred to as Tongo), consisting of eruptive pipes and NE-SW trending dikes. In this study, a combination of detailed petrography, and phlogopite, spinel, and olivine compositions in hypabyssal samples is presented to classify and constrain the petrogenesis of these kimberlites. Both the Koidu and Tongo rocks are predominantly macrocrystic and highly micaceous with phlogopite abundances, normalised to olivine-free, of 36–65 vol% in the groundmass. These phlogopite contents are comparable to those of some cratonic lamproites and significantly higher than any other kimberlites. Other groundmass minerals include spinel, perovskite, and apatite set in a base of carbonate and serpentine. Phlogopite and spinel have similar compositions in the Koidu and Tongo samples, displaying evolutionary trends consistent with those observed in worldwide kimberlites. Olivine macrocrysts and microcrysts display complex zoning with distinct cores, internal zones, and rims. The core compositions display a range in Mg# (81–95) and are interpreted to be derived from the disaggregation of lithospheric mantle xenoliths and proto-kimberlite related megacrysts. The Tongo olivine rims, interpreted to be primary magmatic crystallisation products, have similar compositions from various locations within the cluster whereas the Koidu samples display a range in rim compositions (Mg# 87–89) from different locations within the cluster. The average Koidu rim compositions display a strong positive correlation with the average core compositions, consistent with the trend formed by kimberlites worldwide and indicative of a strong control by melt-lithosphere interaction on melt compositions. Previously, it has been shown that the Mg# of olivine rim's negatively correlates with the abundance of groundmass phlogopite (± oxide minerals) in kimberlites. However, the Koidu and Tongo kimberlites are exceptionally phlogopite-rich given their olivine Mg#, fall outside of the worldwide kimberlite array and have olivine compositions and phlogopite abundances like some cratonic lamproites. This leads us to suggest that Koidu and Tongo represent a rare style of highly micaceous kimberlite magmatism, not previously reported in other cratonic regions, and are genetically linked to the assimilation of K2O-rich, metasomatic mantle lithologies. We further suggest that the K2O content, reflected by groundmass phlogopite abundances, of worldwide kimberlite and cratonic lamproite parent magmas ascending to the surface may be related to assimilation of craton-specific styles of metasomatic lithologies in the SCLM.
Major-, trace-element and Sr-Nd-Hf isotope geochemistry of diamondiferous dykes from Tonguma and Koidu, Sierra Leone: Highly micaceous kimberlites formed by assimilation of metasomatised lithospheric mantle rocks
Fitzpayne, Angus; Giuliani, Andrea; Howarth, Geoffrey H.; et al. (2023)
Chemical Geology
The Man Craton in West Africa hosts two clusters of highly micaceous kimberlites at Koidu and Tonguma with mineralogical and geochemical features transitional towards those of cratonic lamproites. To develop a better understanding of the genesis of these peculiar rocks, which have also been documented elsewhere (e.g., South Africa, China, Russia, India), we have undertaken a geochemical and isotopic study of representative fresh samples from Koidu and Tonguma. New mica Rb[sbnd]Sr dating by isotope dilution confirms the Jurassic age of Koidu (∼145 Ma) and establishes that the Tonguma dykes are marginally younger (∼138 Ma). Bulk-rock analyses of samples from both localities show strong enrichment in K2O (up to 4.4 wt%), but also CaO and CO2 (up to 24 wt% and 18 wt%, respectively), which is entirely consistent with the micaceous nature of these rocks and their variable abundance of calcite. Despite their highly potassic compositions, the incompatible trace element systematics (e.g., Ba/Nb) of these rocks are typical of archetypal kimberlites and unlike those of cratonic lamproites. The Sr-Nd-Hf isotopic composition of the bulk samples range between moderately depleted values (age-corrected 87Sr/86Sr ∼ 0.7032; εNd = +3.8; εHf = +3.9) resembling the common depleted (Prevalent Mantle or PREMA-like) kimberlite component and more geochemically enriched compositions (87Sr/86Sr ∼ 0.7058; εNd = −0.4; εHf = +0.3). The direct relationship between bulk-rock Nd isotopes and average Mg# of magmatic olivine, combined with the previously reported correlation between average Mg# of magmatic and xenocrystic olivine of lithospheric mantle derivation in these kimberlites, suggests that the observed spread in radiogenic isotope compositions stems from the following petrogenetic scenario. Primary kimberlite melts of similar asthenospheric origin with unradiogenic Sr and moderately radiogenic Nd and Hf isotopes interacted with variably metasomatised (i.e., containing phlogopite) and hence geochemically enriched lithospheric mantle wall rocks. Mass balance calculations suggest that the transfer of incompatible elements to the asthenospheric melts was mediated by incongruent melting of metasomatic components in the lithospheric mantle rather than their bulk addition. This study reinforces previous suggestions that interaction between carbonate-rich asthenospheric melts and the sub-continental lithospheric mantle can generate the range of potassic magmas observed within cratons.
Mica Rb-Sr dating by laser ablation ICP-MS/MS using an isochronous calibration material and application to West African kimberlites
Giuliani, Andrea; Oesch, Senan; Guillong, Marcel; et al. (2024)
Chemical Geology
In-situ Rb-Sr dating of mica by laser ablation ICP-MS/MS has recently emerged as a new tool to date a range of geological processes including magmatic, metamorphic and hydrothermal events. The majority of age results presented to date are based on Rb/Sr calibration using the MicaMG pressed nano-powder pellet. However, some studies have reported low accuracy associated with mica Rb-Sr ages using this method, which are attributed to the different ablation properties of MicaMG and natural mica. In this work, we document the results of a systematic comparison between isotope-dilution and laser -ablation Rb/Sr ages of micas in Cretaceous diamondiferous kimberlites from Bultfontein (South Africa), Koidu and Tonguma (Sierra Leone), with additional in-situ data for the Mt. Dromedary mica (Australia). We adopt two different analytical strategies. While NIST610 SRM is employed to calibrate 87Sr/86Sr in both cases, the first approach follows the convention to utilise MicaMG as calibration material for Rb/Sr. In the second approach, Rb/Sr quantification is initially undertaken with NIST610, with a subsequent correction for Rb/Sr offset based on the age of isochronous micas (WBLK) from the Cretaceous Wimbledon lamproite (South Africa). We show that, using our instrumental set -up, employment of MicaMG as Rb/Sr calibration material commonly provides inaccurate results (generally 5-10% older than the solution -based ages). Conversely, our new approach returns accurate ages (within 3%) and represents a promising avenue to improve the accuracy of mica Rb-Sr age by laser ablation ICP-MS/MS especially once isochronous mica reference materials will be developed. Application of this methodology to multiple samples from Koidu and Tonguma shows that, contrarily to previous inferences based on age-dating of fewer samples, magmatism occurred at both localities in the 145-151 Ma interval. After a temporal hiatus, magmatism restarted exclusively in the Tonguma cluster between 135 and 139 Ma. The relative volumetric importance of these two magmatic stages in the Tonguma diamond mine awaits further evaluation.
Compositional Variations in Primitive Kimberlite Melts and Entrained Mantle Cargo from a Global Survey of Trace Element Compositions in Kimberlite Olivine
Howarth, Geoffrey H.; Giuliani, Andrea; Soltys, Ashton; et al. (2022)
Journal of Petrology
Olivines are the dominant phase of kimberlites and the majority of grains display distinct compositional zoning with xenocrystic cores surrounded by magmatic rims. Previous work has documented large variations in both core and rim major and minor element compositions in kimberlites globally, which has been related to variable entrainment and assimilation of sub-continental lithospheric mantle (SCLM) material. However, there is limited knowledge of trace element variations in olivine from kimberlites and it is unclear whether mantle assimilation has any effect on the trace element composition of kimberlite melts. To fill this gap, we present a global survey of olivine trace element compositions, along with previously reported major and minor element compositions, for samples representing the full spectrum of olivine compositional variations in kimberlites, including samples from the following: Lac de Gras, Canada (Grizzly, Koala); Kimberley, South Africa (Bultfontein, De Beers, Kimberley Big Hole, Wesselton); Kaalvallei, South Africa (New Robinson, Samada); and Alto Paranaiba, Brazil (Limpeza-18, Tres Ranchos-04). Trace element concentrations of olivine cores can discriminate between those derived from the disaggregation of peridotitic material and those related to the megacryst suite. The megacrystic olivine cores exhibit a negative correlation between Al and Mn, which is absent in peridotite-derived cores, and are characterised by high concentrations of temperature-dependent elements (e.g. Al, Na, V) as well as Zn, Ti, and Mn. Following pre-screening of cores for megacrystic and spinel peridotite-derived grains, we applied the Al-in-olivine thermometer to assess the P–T equilibration conditions of cores in equilibrium with garnet and estimate the sampling depth of kimberlite magmas in the lithospheric mantle. Our results are consistent with predominant entrainment of deep lithosphere xenocrysts in highly diamondiferous compared with diamond-poor kimberlites. Temperature-dependent elements display a gradational increase with depth due to higher T with Ca, Cu and, to a lesser extent, Zn and Ti being higher and Mg# being lower towards the base of the SCLM, which is consistent with melt modification of the lower lithosphere. The Zn, Ti, Co, Mn, Li, Al, Cr, Na, and V concentrations of magmatic olivine rims display systematic variations that have a negative correlation with Mg# (whereas Cr is positively correlated). Lac de Gras olivine feature Mg-rich rims (Mg# >90) and low concentrations of these trace elements, whereas the Fe-rich olivine rims (Mg# ~85) of the Kaalvallei kimberlites have higher concentrations of these elements, with the Kimberley and Alto Paranaiba kimberlites being intermediate. Direct correlations between average Ti, Zn, Co and Li compositions of olivine cores and rims suggests that the olivine rim (i.e. proxy for primitive melt) variations are related to variable assimilation of metasomatised SCLM and can be effectively used to track the composition of the lithospheric column that is traversed by kimberlite magmas. These observations further imply an intimate link between early proto-kimberlite melt, leading to formation of megacrystic olivine at the base of the SCLM, and the composition of kimberlite melts which entrain and assimilate these products. We conclude that lithospheric mantle assimilation has a major and previously overlooked influence on the trace element composition of kimberlite magmas.
In-situ Rb-Sr dating of mica: method refinement and application to mineral resources
Giuliani, Andrea; Guillong, Marcel; Oesch, Senan; et al. (2023)
Proceedings of the 17th SGA Biennial Meeting. Volume 3
In-situ Rb-Sr dating of mica by laser ablation ICP-MS/MS has recently emerged as a new tool to date a range of geological processes including mineralisation events. The majority of age results presented to date are based on Rb/Sr calibration using the widely available MicaMG pressed-powder pellet. However, several studies have reported low accuracy associated with mica Rb-Sr ages using this method or large variations in calculated Rb/Sr for the same mica attributed to the different ablation properties of MicaMG and natural mica. In this work, we first report the results of a systematic comparison between isotope-dilution and in-situ Rb/Sr ages of micas in diamondiferous kimberlites from South Africa and Sierra Leone. We confirm that employment of MicaMG as calibration material may provide inaccurate results and present a new approach to obtain accurate ages, which relies on Rb/Sr calibration using the synthetic glass NIST610 SRM and an in-house mica standard. This updated procedure is then employed to constrain the ages of other diamond deposits and address the formation of Au mineralisation in the Eastern Goldfields (Australia).
Genesis and evolution of kimberlites
Giuliani, Andrea; Schmidt, Max W.; Torsvik, Trond H.; et al. (2023)
Nature Reviews Earth & Environment
Kimberlites are volcanic rocks enriched in CO2 and H2O and derive from the deepest-sourced melts (up to 300 km) that reach Earth's surface. The mantle processes that generate such deep melts and allow them to traverse through thick (>= 150 km), cold lithosphere carrying dense mantle fragments, such as xenoliths and diamonds, are debated. In this Review, we explore the composition, formation and evolution of kimberlite melts and the mechanisms of their ascent. Both deep-mantle plumes and shallower convective motions linked to lithospheric extension could trigger kimberlite melting by bringing upwelling mantle rocks to depths above Fe-metal stability (similar to 160-250 km depth). Despite the CO2 enrichment in kimberlite melts, their sources are peridotites not necessarily enriched in carbon. Kimberlite primary melts are transitional between silicate and carbonate compositions and evolve towards increasing silica and lower CO2 concentrations during ascent, while concurrently interacting with the lithospheric mantle. These ascent processes promote the exsolution of CO2-H2O fluids during decompression, a prerequisite for the fast ascent (up to tens of metres per second) of kimberlite magmas. Key unresolved questions include the volatile and alkali budget of kimberlites and their mantle sources; their relationship with 'superdeep' diamonds; and their potential link to plumes from the core-mantle boundary.
Primordial neon and the deep mantle origin of kimberlites
Giuliani, Andrea; Kurz, Mark D.; Barry, Peter H.; et al. (2025)
Nature Communications
The genesis of kimberlites is unclear despite the economic and scientific interest surrounding these diamond-bearing magmas. One critical question is whether they tap ancient, deep mantle domains or the shallow convecting mantle with partial melting triggered by plumes or plate tectonics. To address this question, we report the He-Ne-Ar isotopic compositions of magmatic fluids trapped in olivine from kimberlites worldwide. The kimberlites which have been least affected by addition of deeply subducted or metasomatic components have Ne isotopes less nucleogenic than the upper mantle, hence requiring a deep-mantle origin. This is corroborated by previous evidence of small negative W isotope anomalies and kimberlite location along age-progressive hot-spot tracks. The lack of strong primordial He isotope signatures indicates overprinting by lithospheric and crustal components, which suggests that Ne isotopes are more robust tracers of deep-mantle contributions in intraplate continental magmas. The most geochemically depleted kimberlites may preserve deep remnants of early-Earth heterogeneities.
Structural Evolution, Exhumation Rates, and Rheology of the European Crust During Alpine Collision: Constraints From the Rotondo Granite—Gotthard Nappe
Ceccato, Alberto; Behr, Whitney M.; Zappone, Alba Simona; et al. (2024)
Tectonics
The rheology of crystalline units controls the large-scale deformation geometry and dynamics of collisional orogens. Defining a time-constrained rheological evolution of such units may help unravel the details of collisional dynamics. Here, we integrate field analysis, pseudosection calculations and in situ garnet U–Pb and mica Rb–Sr geochronology to define the structural and rheological evolution of the Rotondo granite (Gotthard nappe, Central Alps). We identify a sequence of four (D1–D4) deformation stages. Pre-collisional D1 brittle faults developed before Alpine peak metamorphism, which occurred at 34–20 Ma (U–Pb garnet ages) at 590 ± 25°C and 0.9 ± 0.1 GPa. The reactivation of D1 structures controlled the rheological evolution, from D2 reverse mylonitic shearing at amphibolite facies (520 ± 40°C and 0.8 ± 0.1 GPa) at 18–20 Ma (white mica Rb–Sr ages), to strike-slip, brittle-ductile shearing at greenschist-facies D3 (395 ± 25°C and 0.4 ± 0.1 GPa) at 14–15 Ma (white mica and biotite Rb–Sr ages), and then to D4 strike-slip faulting at shallow conditions. Although highly misoriented for the Alpine collisional stress orientation, D1 brittle structures controlled the localization of D2 ductile mylonites accommodating fast (∼3 mm/yr) exhumation rates due to their weak shear strength (<10 MPa). This structural and rheological evolution is common across External Crystalline Massifs (e.g., Aar, Mont Blanc), suggesting that the European upper crust was extremely weak during Alpine collision, its strength controlled by weak ductile shear zones localized on pre-collisional deformation structures, that in turn controlled localized exhumation at the scale of the orogen.

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