Olivine Megacrysts Research Articles

Olivine Megacrysts in Alkaline Basalt Dikes of the Western Sangilen Region, Southeastern Tuva

Olivine Megacrysts in Alkaline Basalt Dikes of the Western Sangilen Region, Southeastern Tuva

The 2023 Litli-Hrútur eruption of the Fagradalsfjall Fires, SW-Iceland: Insights from trace element compositions of olivine

This study provides data on the trace element composition of olivine from olivine tholeiitic basalts sampled during the July–August 2023 Litli-Hrútur eruption of the Fagradalsfjall Fires in the Reykjanes Volcanic Belt. Chemistry of the Litli-Hrútur olivine is characteristic for volcanic olivine crystals that represent products of magmatic crystallisation. The investigated olivine megacrysts show forsterite (Fo) content in the range of 81 (rims) to 85 (cores) mole percent [defined as Fo = Mg/(Mg + Fe)]. Olivine Ni concentrations (1540–1840 ppm) correlate positively with the Fo contents. In addition, Ca contents show a range from 1890 to 2460 ppm at relatively low Ti concentrations. Olivine from the Litli-Hrútur samples shows an equilibrium with peridotitic mantle melts, yet the data show that the 2023 Litli-Hrútur and the 2022 Meradalir olivine populations crystallised from compositionally more evolved magma batches than olivine crystals from the 2021 Geldingadalir eruption of the Fagradalsfjall Fires. These results imply that magmatic differentiation has taken place between the initial 2021 events and the subsequent 2022 and 2023 eruptions as a result of crystal–liquid fractionation, shifting the overall magma chemistry towards more evolved compositions with time. This implies that fractional crystallisation in sub-alkaline magma reservoirs operates on the scale of years, which is a fundamental advance in our understanding of these common magmatic systems.

Constraints on martian depleted shergottite volcanism from the petrogenesis of olivine-phyric shergottites NWA 2046 and NWA 4925

The 1.1Ma ejection-paired depleted shergottites are sourced from a long-lived volcanic centre, likely located on the Tharsis plateau. Here we present bulk-rock geochemistry, mineral major- and trace-element concentrations, and in-situ maskelynite and pyroxene 87Sr/86Sr data for two depleted olivine-phyric shergottites, Northwest Africa (NWA) 2046 and NWA 4925, from this 1.1 Ma ejection event. The earliest formed olivine megacryst cores in NWA 2046 (Fo83) and NWA 4925 (Fo81) are among the most Mg-rich recorded in martian meteorites and are also in Mg–Fe equilibrium with their respective bulk-rock compositions. We infer the olivine megacrysts in NWA 2046 and NWA 4925 are phenocrysts and suggest that both samples represent melt compositions. CI chondrite-normalised rare earth element patterns for melts calculated to be in equilibrium with pyroxene in NWA 2046 and NWA 4925 are parallel to those of melts in equilibrium with maskelynite and merrillite, which suggests that both samples crystallised from continuously fractionating melts unaffected by crustal contamination. Oxygen fugacity estimates for NWA 2046 (QFM -2.7) and NWA 4925 (QFM -2.9), derived from the V-in-olivine oxybarometer, are comparable to other depleted olivine-phyric shergottites, including Tissint, Larkman Nunatak (LAR) 12095/12240, Dar Al Gani (DaG) 476 and Dhofar (Dho) 019.Effects of terrestrial alteration are observed in bulk-rock and mineral compositions from NWA 2046 and NWA 4925. The degree of terrestrial alteration is most pronounced in NWA 4925, consistent with an older reported exposure age relative to NWA 2046. The low Ni-contents of bulk-rock and olivine megacrysts in NWA 2046 and NWA 4925 suggests that both samples are not primary mantle-derived melt, but rather have undergone 8–10% and 10–13% fractional crystallisation, respectively, in a staging chamber at depth. Pyroxenes Ti/Al for NWA 2046 and NWA 4925, suggest initial crystallisation at ∼ 10 kbar, consistent with other ejection-paired olivine-phyric shergottites, indicating either a common or multiple staging chambers near the base of the martian crust. The 87Sr/86Sr compositions for unaltered maskelynite grains in NWA 2046 (0.70130 ± 7) and NWA 4925 (0.70128 ± 10) suggest that both samples originate from a similar mantle source to other depleted olivine-phyric shergottites, particularly DaG 476, and Sayh al Uhaymir (SaU) 005 and Yamato 980459 (Y-980459).

The capturing and decompression of melt inclusions in olivine and ilmenite megacrysts from the Monastery kimberlite, South Africa

Ilmenite and olivine megacrysts from the 89 Ma Monastery kimberlite (Kaapvaal Craton, South Africa) captured abundant and large melt inclusions containing quenched Si-Mg-rich melt, calcite, spinel, perovskite, phlogopite, and serpentine. Textural observations and 3D X-ray tomography of ilmenite and olivine megacrysts show melt inclusion shapes, sizes and distribution patterns indicative of melt capture during primary crystal growth near the base of the subcontinental lithospheric mantle (SCLM). Patterns supporting secondary melt injection along fractures or veins, such as planar arrays of melt inclusions, are absent. Melt inclusions in olivine, in some examples reaching the dimension of centimetres, likely were captured in skeletal voids forming in fast growing, up to decimetre-sized olivine megacrysts. These large melt inclusions commonly decrepitated, forming apophyses, radial fractures, and veins, along which residual volatile- and Si-Mg-rich melt was extracted. We attribute the decrepitation of melt inclusions in olivine to the rapidly increasing difference between the melt pressure in the inclusions, captured at mantle depth, and the decreasing confining stress to which the host olivines were exposed during magma ascent and after emplacement. In ilmenite, melt inclusions up to ∼ 6 mm in diameter remained commonly intact during the kimberlite ascent from its mantle source to the shallow crust. The quenched silicate melt in olivine- and ilmenite-hosted melt inclusions, in some places preserved as unaltered hydrous and CO3-bearing glass, shows systematic major element compositional variations that suggest that this melt formed by similar fractionation and depletion processes, irrespective of the hosting megacryst phase. Apparent modal variations in quenched silicate melt, calcite, and oxide contents suggest that the melt batches captured as inclusions in ilmenite and olivine either record different evolution stages in the megacryst magma, or document compositional heterogeneities in this magma at the time of megacryst growth.

Platinum-Group Element Geochemistry and Magma Evolution of the Mount Hagen (Papua New Guinea) Magmatic System

Abstract Mount Hagen is a mid-Pleistocene stratovolcano located in the Papua New Guinea Highlands. It was selected for this study because of (1) its close location to several giant Cu–Au porphyry deposits in the region; (2) its high-K calc-alkaline character (absarokite–shoshonite) and (3) its wide compositional range (2 to 12 wt % MgO). Whole-rock geochemistry, petrography and QEMSCAN observations are consistent with fractional crystallisation of the evolving magma being controlled by the phases now represented by the megacrysts in the lavas: olivine and clinopyroxene at high MgO and hornblende and plagioclase at low MgO. However, the role of plagioclase is enigmatic. Although it is an abundant megacryst phase in the low-MgO samples, plagioclase has little or no influence on fractional crystallisation, suggesting that it may be an antecryst. The appearance of plagioclase megacrysts does not slow the rate of increase in Al2O3 or Sr with decreasing sample MgO and the low-MgO samples do not have Eu anomalies. At ~5.5 wt % MgO, there are significant changes in the system that are inconsistent with simple fractional crystallisation. These changes include hornblende and plagioclase replacing olivine and clinopyroxene as the principal megacryst phases, a sharp rise in platinum-group elements (PGE) concentrations, the brief reappearance of high Fo olivine megacrysts and reversed zoning in the plagioclase megacrysts. These changes are interpreted to have resulted from a new pulse of magma entering the system, with higher water, MgO, SiO2 and PGE concentrations than the original parent magma. Scatter in the PGE data is attributed to the presence of micronuggets in all samples, including the most mafic samples. We conclude that the magma system became sulphide saturated during an early stage in its fractionation history, probably before leaving the crust to deep crustal magma chamber, and then became undersaturated as a consequence of decreasing pressure as magma ascended into a mid-crustal magma chamber. The early saturation episode had a pronounced influence on PGE concentration but had little affect Cu and Au, due to their lower partition coefficient into sulphides. The magma became sulphide saturated again in the crustal magma chamber at ~8.5 wt % MgO, this time affecting all chalcophile elements. Given the early episodes of sulphide saturation, depletion in Au in the Mount Hagen magma system and the absence of a known porphyry system, it is unlikely that Mount Hagen produced economic porphyry mineralisation.

Nano- and Micrometer-Sized PGM in Ni-Cu-Fe Sulfides from an Olivine Megacryst in the Udachnaya Pipe, Yakutia, Russia

ABSTRACT This paper focuses on a nanoscale study of nano- and micrometer-size Os-rich mineral particles hosted in a Ni-Fe-Cu sulfide globule found in an olivine megacryst from the Udachnaya pipe (Yakutia, Russia). These platinum-group element mineral particles and their host sulfide matrices were investigated using a combination of techniques, including field emission gun electron probe microanalyzer, field emission scanning electron microscopy, and focused ion beam and high-resolution transmission electron microscopy. The sulfide globule is of mantle origin, as it is hosted in primitive olivine (Fo90–93), very likely derived from the crystallization of Ni-Fe-Cu sulfide melt droplets segregated by liquid immiscibility from a basaltic melt in a volume of depleted subcontinental lithospheric mantle. Microscopic observations by means of field emission scanning electron microscopy and single-spot analysis and mapping by field emission gun electron probe microanalyzer reveal that the sulfide globule comprises a core of pyrrhotite with flame-like exsolutions (usually <10 μm thickness) of pentlandite, which is irregularly surrounded by a rim of granular pentlandite and chalcopyrite. Elemental mapping by energy dispersive spectroscopy (acquired using the high-resolution transmission electron microscopy) of the pyrrhotite (+ pentlandite) core reveals that pentlandite exsolution in pyrrhotite is still observable at the nanoscale as fringes of 100 to 500 nm thicknesses. The sulfide matrices of pyrrhotite, pentlandite, and chalcopyrite contain abundant nano- and micrometer-size platinum group element mineral particles. A careful inspection of eight of these platinum group element particles under focused ion beam and high-resolution transmission electron microscopy showed that they are crystalline erlichmanite (OsS2) with well-developed crystal faces that are distinctively oriented relative to their sulfide host matrices. We propose that the core of the Ni-Fe-Cu sulfide globule studied here was derived from a precursor monosulfide solid solution originally crystallized from a sulfide melt at >1100 °C, which later decomposed into pyrrhotite and the pentlandite flame-like exsolutions upon cooling at <600 °C. Once solidified, the solid monosulfide solid solution reacted with non-equilibrium Cu-and Ni-rich sulfide melt(s), giving rise to the granular pentlandite in equilibrium with chalcopyrite now forming the rim of the sulfide globule. Meanwhile, nano- to micron-sized crystals of erlichmanite crystallized directly from or slightly before monosulfide solid solution from the sulfide melt. Thus, Os, and to a lesser extent Ir and Ru, were physically partitioned by preferential uptake via early formation of nanoparticles at high temperature instead of low-temperature exsolution from solid Ni-Fe-Cu sulfides. The new data provided in this paper highlight the necessity of studying platinum group element mineral particles in Ni-Fe-Cu sulfides using analytical techniques that can image nanoscale textural features in order to better understand the mechanisms of platinum group element fractionation in magmatic systems. These processes may play a crucial role in controlling the background geochemical budgets for siderophile and chalcophile elements in a wide range of mantle-derived magmas.

Comment on “Biogeochemical cycling of iron (hydr-)oxides and its impact on organic carbon turnover in coastal wetlands: A global synthesis and perspective” [Earth-Science Reviews (2021) 103658

Models for a xenocryst origin for kimberlite olivines emphasise the similarity between their core compositions and those in mantle peridotites. While this permits a xenocryst origin, it does not provide proof, as magmas generated in equilibrium with mantle olivines could, in principle, crystallize initial olivines matching those in the source region. Further, in several kimberlites, there is a striking disparity between the compositional range of olivine cores and that in associated mantle peridotite xenoliths from the same locality. Olivine-liquid Mg-Fe exchange coefficients and Ni partition coefficients permit equilibrium between Mg-rich mantle olivines (Mg # ~ 94–93) and magmas matching kimberlite bulk rock compositions. Glass inclusions in olivine megacrysts from the Monastery kimberlite, with compositions which overlap the range of archetypal Group I kimberlites, were interpreted to represent original liquids trapped at pressures of 4.5–6 GPa. These glass inclusions provide direct petrographic support for primitive melts matching kimberlite bulk chemistry in the lower SCLM.A majority of kimberlitic olivines show normal (decreasing Mg #) core to rim zonation. Cores of normal-zoned kimberlitic olivines are typically homogeneous, but collectively define a field with a range in Mg # and invariant or slightly decreasing Ni towards more Fe-rich compositions. The most Mg-rich cores of normal-zoned olivines typically have Mg # in the range 94–93, but there are marked differences in the Fe-rich extreme of the normal-zoned population between different kimberlite clusters. Olivine rims typically define a field characterized by steeply decreasing Ni, coupled with invariant or slightly increasing or decreasing Mg #, which invariably overlaps the Fe-extreme of core compositions of the relatively Mg-rich, normal-zoned olivines. Consequently, while there is a sharp inflection in chemical gradient between the respective fields of cores and rims, they nevertheless define a continuous compositional field. Trace element modelling demonstrates that these zonation patterns can be explained in terms of a Raleigh crystallization model.Most, if not all kimberlites are characterized by a subordinate group of olivine macrocrysts with cores that are Fe-rich relative to the field for rims, and thus show reverse zonation, which are interpreted to be linked to the Cr-poor megacryst suite. Rare Mg-rich olivines (relative to rims), have high-pressure inclusions of garnet, clinopyroxene and orthopyroxene. When present, such inclusions often show disequilibrium features such as internal chemical zonation. This points to a very short mantle residence time prior to entrainment by the host kimberlite, indicating a link to the Cr-rich megacryst suite rather than mantle peridotites. In addition to a variable, but generally subordinate proportion of olivines derived from Cr-poor and Cr-rich megacrysts, xenocrysts derived from disaggregated mantle peridotites will undoubtedly be present. While their proportions are difficult to quantify, the collective evidence points to a cognate origin for a majority of kimberlitic olivines. A kimberlite magma ascent model is proposed which provides a framework for understanding both olivine compositional variation and apparently enigmatic internal and external olivine morphology.

Cognate versus xenocrystic olivines in kimberlites – A review

Models for a xenocryst origin for kimberlite olivines emphasise the similarity between their core compositions and those in mantle peridotites. While this permits a xenocryst origin, it does not provide proof, as magmas generated in equilibrium with mantle olivines could, in principle, crystallize initial olivines matching those in the source region. Further, in several kimberlites, there is a striking disparity between the compositional range of olivine cores and that in associated mantle peridotite xenoliths from the same locality. Olivine-liquid Mg-Fe exchange coefficients and Ni partition coefficients permit equilibrium between Mg-rich mantle olivines (Mg # ~ 94–93) and magmas matching kimberlite bulk rock compositions. Glass inclusions in olivine megacrysts from the Monastery kimberlite, with compositions which overlap the range of archetypal Group I kimberlites, were interpreted to represent original liquids trapped at pressures of 4.5–6 GPa. These glass inclusions provide direct petrographic support for primitive melts matching kimberlite bulk chemistry in the lower SCLM. A majority of kimberlitic olivines show normal (decreasing Mg #) core to rim zonation. Cores of normal-zoned kimberlitic olivines are typically homogeneous, but collectively define a field with a range in Mg # and invariant or slightly decreasing Ni towards more Fe-rich compositions. The most Mg-rich cores of normal-zoned olivines typically have Mg # in the range 94–93, but there are marked differences in the Fe-rich extreme of the normal-zoned population between different kimberlite clusters. Olivine rims typically define a field characterized by steeply decreasing Ni, coupled with invariant or slightly increasing or decreasing Mg #, which invariably overlaps the Fe-extreme of core compositions of the relatively Mg-rich, normal-zoned olivines. Consequently, while there is a sharp inflection in chemical gradient between the respective fields of cores and rims, they nevertheless define a continuous compositional field. Trace element modelling demonstrates that these zonation patterns can be explained in terms of a Raleigh crystallization model. Most, if not all kimberlites are characterized by a subordinate group of olivine macrocrysts with cores that are Fe-rich relative to the field for rims, and thus show reverse zonation, which are interpreted to be linked to the Cr-poor megacryst suite. Rare Mg-rich olivines (relative to rims), have high-pressure inclusions of garnet, clinopyroxene and orthopyroxene. When present, such inclusions often show disequilibrium features such as internal chemical zonation. This points to a very short mantle residence time prior to entrainment by the host kimberlite, indicating a link to the Cr-rich megacryst suite rather than mantle peridotites. In addition to a variable, but generally subordinate proportion of olivines derived from Cr-poor and Cr-rich megacrysts, xenocrysts derived from disaggregated mantle peridotites will undoubtedly be present. While their proportions are difficult to quantify, the collective evidence points to a cognate origin for a majority of kimberlitic olivines. A kimberlite magma ascent model is proposed which provides a framework for understanding both olivine compositional variation and apparently enigmatic internal and external olivine morphology.

Leucite basanites of Virunga (East African Rift): some insights into petrogenesis and source composition

The leucite basanites of the Visoke and Muhavura volcanoes from the province of Virunga (Western Branch of the East African Rift) were investigated in detail. The major and trace element contents and Sr, Nd and Pb isotopic composition in the whole rocks were determined. The isotope ratios (87Sr/86Sr: 0.70644–0.70793; 143Nd/144Nd: 0.51233–0.51243) show that these rocks are far from the bulk Earth on the “mantle trend” and enriched in radiogenic Sr. The major element compositions reveal that they are not primary mantle melts as they have Mg# = 0.50–0.59. Moreover the leucite basanites are enriched by LILE and LREE relative to PM which may indicate an enriched mantle source. Pb isotopes of the studied leucite basanites (206Pb/204Pb: 19.112–19.438, 207Pb/204Pb: 15.737–15.783, 208Pb/204Pb: 40.509–40.866) along with the data of Karisimbi volcano form trends on isotope diagrams towards the EM II or ancient continental crust. The position of leucite basanite data on the 87Sr/86Sr - 208Pb*/206Pb* correlation line suggests that they can be a result of melt enrichment/vein metasomatism within the mantle.The phenocryst composition of leucite basanites varies in range: olivine- from Fo55 to Fo89 with NiO content 0.01–0.34% wt.; clinopyroxene - from 0.65 up to 0.86 Mg# with Cr2O3 from 0.02 to 1.5% wt., plagioclase corresponds to labradorite (50–70% An). The ore minerals present are titanomagnetite and ilmenite. The evaluation of T-fO2 based on titanomagnetite and ilmenite composition shows that phenocryst crystallization took place under QFM buffer. The presence of quartz and rutile xenocrysts in one Visoke leucite basanite lava points to the contamination of this melt by crustal material. At the same time, nonequilibrium megacrysts of high-magnesian olivine (Fo89) from another sample of Visoke leucite basanite resulted from leucite-basanite magma mixing with a more primitive melt.We identify melts from two different sources at Visoke edifice: ugandite and its possible derivatives were formed from the mantle source, whereas the leucite basanites are enriched in radiogenic strontium and could be contaminated by deep metamorphic rocks. The origin of the studied leucite basanites is the result of a combination of various processes: melting of the metasomatized mantle, fractional crystallization, magma mixing and crustal contamination.

The age and origin of cratonic lithospheric mantle: Archean dunites vs. Paleoproterozoic harzburgites from the Udachnaya kimberlite, Siberian craton

Cratonic lithospheric mantle is believed to have been formed in the Archean, but kimberlite-hosted coarse peridotites from Udachnaya in the central Siberian craton typically yield Paleoproterozoic Re-depletion Os isotope ages (TRD). By comparison, olivine megacrysts from Udachnaya, sometimes called “megacrystalline peridotites”, often yield Archean TRD ages, but the nature of these rare materials remains enigmatic. We provide whole-rock (WR) Re-Os isotope and PGE analyses for 24 olivine-rich xenoliths from Udachnaya as well as modal and petrographic data, WR and mineral major and trace element compositions. The samples were selected based on (a) high olivine abundances in hand specimens and (b) sufficient freshness and size to yield representative WR powders. They comprise medium- to coarse-grained (olivine < 1 cm) dunites, a megacrystalline (olivine > 1 cm) dunite, olivine megacrysts and low-orthopyroxene (11–21% opx) harzburgites equilibrated at 783–1154 °C and 3.9–6.5 GPa; coarse dunites have not been previously reported from Udachnaya; two xenoliths contain ilmenite. The harzburgites and dunites have similar WR variation ranges of Ca, Al, Fe, Cr and Mg# (0.917–0.934) typical of refractory cratonic peridotites, but the dunites tend to have higher MgO, NiO and Mg/Si. Mineral abundances and those of Ca and Al are not correlated with Mg#WR; they are not due to differences in melting degrees but are linked to metasomatism. Several samples with high 187Re/188Os show a positive linear correlation with 187Os/188Os with an apparent age of 0.37 Ga, same as eruption age of host kimberlite. Robust TRD ages were obtained for 16 xenoliths with low 187Re/188Os (0.02–0.13). TRD ages for low-opx harzburgites (1.9–2.1 Ga; average 2.0 ± 0.1 Ga, 1 σ) are manifestly lower than for dunites and megacrysts (2.4–3.1 Ga); the latter define two subsets with average TRD of 2.6 ± 0.1 Ga and 3.0 ± 0.1 Ga, and TMA of 3.0 ± 0.2 Ga and 3.3 ± 0.1 Ga, respectively. Differences in olivine grain size (coarse vs. megacrystalline) are not related to age. The age relations suggest that the dunites and megacrysts could not be produced by re-melting of harzburgites, e.g. in arc settings, nor be melt channel materials in harzburgites. Instead, they are relict fragments of lithospheric mantle formed in the Archean (likely in two events at or after 2.6 Ga and 3.0 Ga) that were incorporated into cratonic lithosphere during the final assembly of the Siberian craton in the Paleoproterozoic. A multi-stage formation of the Siberian lithospheric mantle is consistent with crustal basement ages from U-Pb dating of zircons from crustal xenoliths at Udachnaya and detrital zircons from the northern Siberian craton (1.8–2.0, 2.4–2.8 and 3.0–3.4 Ga). The new data from the Siberian and other cratons suggest that the formation of strongly melt-depleted cratonic lithosphere (e.g. Mg# ≥0.92) did not stop at the Archean-Proterozoic boundary as is commonly thought, but continued in the Paleoproterozoic. The same may be valid for the transition from the ‘Archean’ (4–2.5 Ga) to modern tectonic regimes.

Terrestrial alteration mineral assemblages in the NWA 10416 olivine phyric shergottite

We report on the alteration history of the olivine-phyric, highly depleted (HD) shergottite, Northwest Africa (NWA) 10416, paying particular attention to the origin of the aqueous alteration seen affecting the meteorite’s olivine megacrysts. The rock’s interior displays 1 mm, zoned, altered olivine megacrysts set in a groundmass of clinopyroxene, unzoned olivine, and interstitial plagioclase and maskelynite. Synchrotron micro X-ray diffraction (µ-XRD) and transmission electron microscopy (TEM) show that plagioclase and maskelynite have been partially replaced by kaolinite. The relict olivine megacryst cores display a unique concentric colouration for martian meteorites, having central amber-coloured zones surrounded by a brown mantle zone, with the rims remaining clear and unaltered. This colouration is a result of fluid alteration and partial replacement, with hydration. TEM analysis revealed the ∼200 nm scale banded and largely amorphous nature of the alteration, but with some (∼20%) relict crystalline olivine patches. Although the coloured olivine zones show cation and anion site vacancies compared to stoichiometric olivine, a relict igneous compositional trend is preserved in the megacrysts, from Mg-rich altered cores (Mg# = 76) to unaltered stoichiometric rims (Fo53). Synchrotron Fe-K X-ray absorption near-edge structure (XANES) analysis revealed that the coloured zones of the megacryst have different Fe oxidation values. High ferric contents are present in the brown mantle zones (Fe3+/ΣFe ≤ 0.92) and the amber zones (Fe3+/ΣFe ≤ 0.30), whereas the clear rims are ferrous. This suggests alteration occurred in an oxidising environment and that the sharp contrast in colour of the megacryst (brown to clear) is a record of a relict fluid reaction front.In order to test the terrestrial or extraterrestrial origin of the alteration, olivine material from a shock-melt vein was analysed by TEM. The analysis revealed 0.952 nm curved d-spacing’s from clay alteration undisturbed by any shock effects, strongly suggesting a terrestrial origin. The d-spacing values most likely represent a collapsed saponite or vermiculite, showing that in some places olivine has been replaced by crystalline clay.Oxygen isotope analysis of bulk (Δ17O = 0.309 ± 0.009 (2σ) ‰) and amber-coloured megacryst material (=0.271 ± 0.002 (2σ) ‰), are also consistent with terrestrial alteration. We propose a model in which, during the meteorite’s time in Northwest Africa, low-temperature, likely acidic, groundwater exploited fractures. The fluid altered the olivine megacrysts in a way that was controlled by the pre-existing, igneous compositional zonation, with Mg-rich olivine being more susceptible to alteration in this fluid environment. The plagioclase and maskelynite were also altered to a high degree. After the alteration event it is likely that NWA 10416 had a significant residence time in Northwest Africa, accounting for terrestrial calcite and the dehydration of some clay phases.

Multiple origins of xenoliths and xenocrysts in the Elephant Moraine 79001 Lithology A olivine‐phyric shergottite

AbstractPetrographic examination of the xenolith and xenocryst populations in the olivine‐phyric shergottite Elephant Moraine 79001 Lithology A shows more chemical heterogeneity than previously documented. Analyses of olivine grains in 18 megacrysts and in 4 lithic fragments show that these two populations either do not have the same source or that this source is heterogeneous in terms of its time–temperature history. Additionally, among the four lithic fragments analyzed, two are distinctive (1) one contains a major‐element‐equilibrated, euhedral olivine grain and (2) the second contains high‐magnesium pyroxene cores. Furthermore, two populations of ferroan olivine were identified (1) one more ferroan than any other reported in EET 79001 and (2) a slightly less ferroan, unequilibrated olivine, but with a restricted range in Mg#. We have also observed an equilibrated pyroxene grain associated with a zoned olivine megacryst. As a result, we recognize that the xenolith/xenocryst population does not represent the incorporation of a single xeno‐lithology into Lithology A, and propose that it be subdivided into a suite of seven identified lithologies, with the understanding that more are likely to be identified with further study. The abundance of Ca in olivine in the xeno‐lithologies suggests a set of crustal, rather than deep mantle, lithologies. Diffusion rates in olivine suggest that the lithologies were incorporated shortly before rapid cooling of the host magma, preserving preexisting mineral chemical zoning. These mineral chemical zones could have been preserved at lower crustal temperatures for up to 10s of Ka. Trace‐element studies of these distinct populations would be required to test whether they are related by igneous processes from a common source magma.

Heterogeneous kimberlite metasomatism revealed from a combined He-Os isotope study of Siberian megacrystalline dunite xenoliths

The Siberian sub-continental lithospheric mantle (SCLM) is regionally heterogeneous due to the complex modification of ancient cratonic material by various metasomatic fluids and interaction with the Siberian plume at ∼250 Ma. Here, we assess the extent and origin of this heterogeneity by analysis of helium isotopes, rhenium-osmium isotopes, trace-element, and highly siderophile element [HSE: Os, Ir, Ru, Pt, Pd, Re] abundances in a suite of mantle-derived megacrystalline dunite xenoliths from the 360 Ma Udachnaya East kimberlite pipe, Siberia. This enables assessment of the style and extent of metasomatism acting to modify the volatile budget of the Siberian SCLM. The olivine megacrysts have 3He/4He values that range from 2.3 to 7.4 RA (where RA = the 3He/4He of air), outside the canonical range for the lithospheric mantle (6.1 ± 2.1 RA; Day et al., 2015). High [He] and low U + Th concentrations in these olivine megacrysts indicate that the Udachnaya megacrystalline dunite xenolith suite has undergone minimal post-eruptive modification of He isotopes by 4He recoil. We instead interpret He isotope variations to reflect pre- or syn-eruptive metasomatic signatures during kimberlite emplacement. The dunites can be divided into two groups, ultimately reflecting modification by two distinct metasomatic styles. Group 1 dunites are characterized by highly unradiogenic Os isotope compositions (γOs360Ma −16 to −13) and ancient melt depletion ages (∼3 Ga) typical of ancient cratonic lithospheric mantle; yet display a range of He isotope ratios (2.3–5.9 RA). This indicates that group 1 dunites have been modified by gas-rich metasomatic fluids acting to change He abundances and isotope ratios, while preserving unradiogenic Os isotope and HSE abundance systematics. Group 2 dunites extend to significantly more radiogenic Os isotope signatures and higher He isotope values (γOs360Ma > −5 to +53; 3He/4He from 3.7 to 7.4 RA). These two groups define a negative hyperbolic array between Os and He isotopes which we interpret to result from metasomatism by variable mixtures between: (1) a gas-rich and HSE-poor fluid (similar to the fluids acting to modify Group 1 dunites) with helium isotope compositions above the MORB range (>8 RA) derived from the asthenosphere, and: (2) strongly radiogenic fluids (∼2 RA; 187Os/188Os > 0.25) likely sourced from within the SCLM. The Udachnaya megacrystalline dunite xenolith suite provides insight into how mantle-plume derived fluids interact with metasomatic fluids within the SCLM.

Polymineralic inclusions in kimberlite-hosted megacrysts: Implications for kimberlite melt evolution

Megacrysts are large (cm to >20 cm in size) mantle-derived crystals, which are commonly entrained by kimberlite magmas, comprising of olivine, orthopyroxene, clinopyroxene, phlogopite, garnet, ilmenite and zircon as common phases. Numerous studies have shown megacrysts to contain polymineralic inclusions, which have been interpreted to represent entrapped kimberlite melt. To constrain the origin of these inclusions in megacrysts and their relationship to kimberlite magmatism, we present a detailed petrographic and geochemical study of clinopyroxene and olivine megacrysts and their hosted inclusions from the Diavik, Jericho, Leslie (Slave Craton, Canada) and Udachnaya-East (Siberian Craton, Russia) kimberlites. The studied megacrysts are between 1 and 3 cm in size and representative of both the Cr-rich and Cr-poor suites. Megacrysts contain two types of inclusions:i.Large (<0.5–5 mm in size) round-to-irregular shaped polymineralic inclusions, which are composed of minerals similar to the host kimberlite groundmass, and consist of olivine, calcite, spinel, perovskite, phlogopite and apatite (± serpentine, alkali-carbonates, alkali-chlorides, barite).ii.Swarms/trails of ‘micro melt inclusions’ (MMI; <1–5 μm in size), which surround polymineralic inclusions, veins and fractures, thereby forming a ‘spongy’ texture. MMIs generally contain multiphase assemblages similar to polymineralic inclusions as well as various additional phases, such as alkali-carbonates or alkali-chlorides, which are typically absent in polymineralic inclusions and the surrounding kimberlite groundmass.Textural and geochemical evidence suggests that polymineralic inclusions in megacrysts crystallised from kimberlite melt, which infiltrated along fracture/vein networks. The polymineralic inclusion assemblages resulted from disequilibria reactions between the host megacryst and infiltrating kimberlite melt, which was likely enhanced by rapidly changing conditions during magmatic ascent. The connectivity of polymineralic inclusions to the kimberlite groundmass via network veins/fractures suggests that they are susceptible to infiltrating post-emplacement fluids. Therefore, the vast majority of polymineralic inclusions are unlikely to represent ‘pristine’ entrapped kimberlite melt. In contrast, MMIs are isolated within megacrysts (i.e. not connected to fractures/veins and therefore shielded from post-magmatic fluids) and probably represent entrapped remnants of the variably differentiated kimberlite melt, which was more enriched in alkalis-Cl-S-CO2 than serpentinised polymineralic inclusions and the host rocks exposed at Earth's surface as kimberlites.

Petrology and geochemistry of olivine-phyric shergottites LAR 12095 and LAR 12240: Implications for their petrogenetic history on Mars.

Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine-phyric shergottite lnds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine-phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low- and high-Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low-Ca pyroxene, which is one of the earliest formed REE-bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last-formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed-system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.

Geochemistry and geodynamic implications on the source of Paraná-Etendeka Large Igneous Province evidenced by the 128 Ma Rosário-6 kimberlite, southern Brazil

The Rosário-6 is a non-diamondiferous hypabyssal kimberlite located above the Rio de la Plata craton and near the south-eastern edge of the Paraná Basin, in southern Brazil.It is petrographically an inequigranular texture, macrocrystal kimberlite, fresh and the groundmass exhibits a microporphyritic texture and round megacrysts of olivine, which are derived from disaggregated mantle xenoliths. Olivine is also present as macrocrysts, microphenocrysts and in the groundmass together with phlogopite and apatite. These microphenocrysts are immersed in a groundmass of olivine, monticellite, phlogopite, CaTiO3-perovskite, apatite, Mg-chromite and Mg-ulvöspinel and melilite. A mesostasis assemblage of phlogopite, melilite, soda melilite, akermanite and calcium carbonate is segregated from the groundmass. Its geochemical signature is similar to those of transitional kimberlites of Kaapvaal Craton, South Africa, and the UPb ages of ~ 128 Ma on perovskite reveal that Rosário-6 kimberlite post-dates the main pulse of volcanism in the Paraná-Etendeka Large Igneous Province (LIP). The high Ti content of some minerals, such as Mg-chromite, Mg-ulvöspinel, phlogopite and melilite, and the presence of perovskite suggest a Ti-rich source. The petrographic, geochemical and isotopic data indicate that the Rosário-6 kimberlite source is a depleted mantle metasomatized by H2O-rich fluids, CO2-rich and silicate melts derived from the recycling of an ancient subducted oceanic plate (eclogite) before the South Atlantic opening. Although several authors indicate the influence of Tristan da Cunha plume for the generation of alkaline magmatism associated to the Paraná-Etendeka flood basalts, our data demonstrates that Tristan da Cunha plume has no chemical contribution to the generation of Rosário-6 kimberlite, except by its thermal influence.

New constraints on archetypal South African kimberlite petrogenesis from quenched glass-rich melt inclusions in olivine megacrysts

The evaluation of primary kimberlite compositions is hindered by significant melt modifications during ascent through the lithosphere by entrainment of xenolithic material, volatile degassing, and near surface alteration. Consequently, hypabyssal kimberlite emplaced in the upper crust may not provide a true reflection of the primary kimberlite magma. This contribution places new constraints on kimberlite melt composition by providing an assessment of quenched glass-rich polymineralic melt inclusions hosted in olivine megacrysts from the Monastery kimberlite, South Africa. Melt inclusions predominantly contain variable proportions of euhedral or skeletal grains of calcite, phlogopite, spinel, perovskite, serpentine, and fresh to devitrified glass. Estimates of the bulk compositions of melt inclusions, and the compositions of crystalline phases present therein, are compatible to those of hypabyssal kimberlites worldwide and show a volatile-rich (CO2 + H2O ~10–17 wt%) carbonated silicate (SiO2 ~27–41 wt%) composition. The glass component has a Si-Mg-Fe-rich and largely CaO-, K2O- and TiO2-free major element composition and is REE-depleted. It also contains approximately 10 wt% H2O but is CO2−free. The glass represents a residual melt that existed after crystallization of the observed mineral assemblage. From some, but not all melt inclusions, apophyses radiate outwards. These fractures typically contain partially devitrified glass that is compositionally identical to the fresh residual glass within the melt inclusions, indicating fracture formation during decompression of the hosting megacryst and at a stage after the melt had evolved. These features are consistent with a trapping of the melt inclusions at high pressure, prior to kimberlite ascent to the surface, in the SCLM at a depth corresponding to 4.5–6 GPa. Textures and compositions of phases within the melt inclusions represent stages of the kimberlite melt and magma evolution. They provide evidence in support of high-pressure experimental studies suggesting a carbonated silicate primary melt rather than a carbonatite. Furthermore, the composition of fresh glass in the melt inclusions, which is compositionally similar to serpentine, suggests that much groundmass serpentine in hypabyssal kimberlites may have formed from similar silicate melt or devitrified glass.

Dunite channels within a harzburgite layer from the Horoman peridotite complex, Japan: Possible pathway for magmas

AbstractTwo discordant dunite channels in a polished slab were collected from a harzburgite layer in the Horoman orogenic lherzolite massif, Hokkaido, Japan. The dunite channels show extreme grain‐size variations, and include very large olivine megacrysts up to 1.6 m long. Spinel‐rich pyroxenite veins or veinlets occasionally cross‐cut the olivine megacrysts. Mineral compositions differ between channels, and the mineral compositions and textures of the dunite channels and the host harzburgite suggest a replacive origin for the dunites. The Sr and Nd isotopic ratios of clinopyroxene separates from the pyroxenite veins in two dunite channels are different, but these ratios are generally similar to those of mid‐ocean ridge basalts. Assuming the channels formed by the through‐flow of melts, then the trace element compositions of the melts estimated from the clinopyroxenes are broadly similar. The melts have similar features such as enriched in incompatible elements and depleted in heavy rare earth elements, but there are differences in the depletions in Ti and Ba, indicating melts that were different but of similar origins. A Rb–Sr crystallization age (~ 52 Ma) was obtained for clinopyroxene–orthopyroxene pairs in a dunite channel and its host harzburgite. The distinctive dunite channels with megacrysts formed as a result of the through‐flow of mafic magma(s), and those magmas had trace and minor element contents that are indicative of contributions of a melt derived from subducted oceanic lithosphere.

Olivine megacryst chemistry, Monastery kimberlite: Constraints on the mineralogy of the HIMU mantle reservoir in southern Africa

Olivine phenocryst chemistry is a useful tracer of mantle source lithology as olivine is typically the first mineral to crystallize in a range of mafic to alkaline magma types and contains geochemical information about the primary parent magma composition. It is particularly useful in discriminating between pyroxenite (i.e.,recycled crustal component) and peridotite mantle source lithologies. Radiogenic isotope studies of HIMU basalts have shown the important role of a recycled crustal component in the source. However, olivine chemistry of HIMU basalts suggests a dominantly peridotitic mantle source with a subduction-derived metasomatic carbonate component. To further investigate how olivine chemistry can be used to understand the source of HIMU magmas, I present major and trace element data for olivine megacrysts related to the 89 Ma Cr-poor megacryst suite from the Monastery kimberlite (South Africa), which have previously been interpreted to crystallize at high P-T conditions of ~1400 °C and ~5 GPa from a magma sourced from a HIMU reservoir. Olivine megacrysts have high-Ni concentrations at a given Fo (forsterite) content, overlapping ocean island basalts (OIB) interpreted to have formed from pyroxenite-dominated sources but are distinct from typical olivine in HIMU basalts. However, they have low 100*Mn/Fe (0.8–1.1) and no correlation is observed between Ni and trace elements indicative of recycled components such as Co, Li, or Zn. The olivine megacryst chemistry is similar to that of aillikite olivine interpreted to be controlled by phlogopite in the source rather than pyroxenite. Comparison with olivine chemistry from orangeites/Group II kimberlites (sourced from phlogopite-rich mantle lithologies) supports a phlogopite controlon low 100*Mn/Fe in olivine. Further comparison with olivine phenocryst chemistry of HIMU melilitites (76–58 Ma) in southern Africa suggests a mineralogically heterogeneous HIMU reservoir formed by metasomatic modification of the lithospheric mantle in southern Africa. Thus, olivine megacryst chemistry supports interpretations for the recycled component of the HIMU reservoir as mineralogically complex metasomatic lithologies formed by the infiltration of subduction-derived melts into the base of the lithospheric mantle. In addition to carbonate-richHIMU mantle lithologies in southern Africa and worldwide, Monastery kimberlite olivine megacrysts and olivine in melilitites suggest that a phlogopite-richHIMU lithology is present in southern Africa. An important question arising from this study is in reconciling the homogenous isotopic ratios of HIMU basalts worldwide with an apparent heterogeneous lithological source indicated by olivine chemistry.

Olivinites in the angrite D’Orbigny: Vestiges of pristine reducing conditions during angrite formation

Olivinites, together with olivine megacrysts, are the most magnesian phases found in angrites. Their chemical composition (mg# 90) is out of equilibrium with the groundmass and far away from that of possible precipitates from angrite parent melts. Therefore olivinites, as well as olivine megacrysts, were considered as xenoliths and xenocrysts. We report here a detailed study of five olivinites from the angrite D’Orbigny. Our results indicate that D’Orbigny experienced metasomatic alteration processes, which led to enrichments in FeO and MnO (relative to the original composition), changing the initial Mg-rich composition of the olivines to the one seen now. As this process took place in equilibrium with a chondritic reservoir (e.g., Fe/Mn ratios spreading around primitive values), the primitive (Mg-rich) olivine chemical composition was changed towards a more fayalitic one while preserving a chondritic signature. This chondritic signature was preserved in the Fe/Mn ratio of the olivinites, olivine megacrysts, augite grains in olivinites and groundmass olivine of D’Orbigny. Therefore the fayalite content of about 35mol.% that characterizes the groundmass olivine of this rock – as well as other angrites-does not correspond to its original composition but may be the result of a late metasomatic process that affected these rocks. If so, olivinites and Mg-rich olivines might not be compositionally exotic phases but are an early constituent phase that retained the pristine more reducing conditions that have been preserved in some angrites, where they form either a small part of the rock (e.g., Asuka 881371 and D’Orbigny) or the majority of it (NWA 8535).