Olivine Chemistry Research Articles

Crustal controls on apparent mantle pyroxenite signals in ocean-island basalts

Research Article| February 28, 2019 Crustal controls on apparent mantle pyroxenite signals in ocean-island basalts Matthew L.M. Gleeson; Matthew L.M. Gleeson 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar Sally A. Gibson Sally A. Gibson 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Matthew L.M. Gleeson 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Sally A. Gibson 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Publisher: Geological Society of America Received: 29 Oct 2018 Revision Received: 06 Jan 2019 Accepted: 15 Jan 2019 First Online: 28 Feb 2019 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2019 Geological Society of America Geology (2019) 47 (4): 321–324. https://doi.org/10.1130/G45759.1 Article history Received: 29 Oct 2018 Revision Received: 06 Jan 2019 Accepted: 15 Jan 2019 First Online: 28 Feb 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Matthew L.M. Gleeson, Sally A. Gibson; Crustal controls on apparent mantle pyroxenite signals in ocean-island basalts. Geology 2019;; 47 (4): 321–324. doi: https://doi.org/10.1130/G45759.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Ocean-island basalts (OIBs) provide a unique insight into the extent of lithological heterogeneity (peridotite vs. pyroxenite) in Earth’s convecting mantle. However, crustal processing of these mantle melts significantly influences minor-element concentrations in olivine phenocrysts, challenging the suitability of this widely used approach to identify lithological variations in their mantle source. Using a numerical model of magma recharge, mixing, and diffusional reequilibration, we show that this type of crustal processing—which is widely observed in OIBs—results in elevated Ni and lower Ca contents of forsterite-rich olivine, causing erroneously high estimates of the proportion of pyroxenite-derived melt. We applied our model of magma recharge and mixing to several OIBs, including the Galápagos Islands, Canary Islands, and La Réunion. In particular, we critically examined olivine compositional variations in basalts from the eastern Galápagos, which display Sr- and Pb-isotope ratios similar to normal mid-ocean-ridge basalts. While previous interpretations (based on olivine chemistry) argued for a significant contribution from pyroxenite-derived melt, our results indicate that the postulated presence of pyroxenite in the eastern Galápagos mantle is an artifact of processing of magmas and their olivine cargo as they transition through the crust, consistent with major-element and isotopic evidence for a dominantly peridotitic source in this region. This new model for magma recharge and mixing may have important implications for our understanding of lithological heterogeneity beneath OIBs globally, and it highlights the importance of considering crustal processes when attempting to interpret olivine compositions with regard to mantle heterogeneity. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fertile Lithospheric Mantle Beneath the North African Plate: Evidence from Gharyan Lherzolite Xenoliths (Northern West Libya)

The late-stage basanite (∼12–1 Ma) of the NNW-SSE extending Gharyan Cenozoic volcanics (Northern West Libya) contains numerous fresh lherzolite xenoliths. These xenoliths display magmatic protogranular and porphyroclastic textures. Chemistry of olivine (forsterite content –90–91, NiO = 0.26–0.39 wt %), orthopyroxene (Mg# = 0.91–0.92, Cr# = 0.03–0.07, Al2O3 = 3.64–4.43 wt %), clinopyroxene ((Wo45.59–48.61, En45.89–48.80, Fs4.47–5.81), Mg# = 0.82–0.92, Al2O3 = 5.14–6.58 wt % and Cr2O3 = 0.5–0.95 wt %) and spinel (hercynite–picotite–Al-rich chromite spinels with Cr# = 0.08–0.11) refer to the fertile nature of Gharyan peridotites. Mantle source region was close to the primitive composition with low degrees of melting and depletion in melt (1.5% in average). However, it underwent metasomatism as illustrated by formation of secondary clinopyroxene (Wo37.96–44.77, En47.44–54.18, Fs7.59–8.03) with high contents of Na2O, and enrichment of the Al-spinel in TiO2. The estimated pre-eruptive temperature ranges from 1066 to 837°C. Despite the Gharyan peridotite is similar to mantle components in many districts in Saharan belt of North Africa, it represents juvenile mantle source with minor refractory residues. This compositional heterogeneity is mainly attributed to the local effect of the interaction of the Gharyan mantle with the host basanite magma that may be related to the Cenozoic rifting of the Pan-African basement.

Xenokristály-társulások–xenokristályok a kabai szenes kondritban

Az erősen heterogén összetételű kabai meteorit a fehérzárványhalmazok és a kondrumok, továbbá a vegyes összetételű, szervesanyag-tartalmú alapanyag mellett olivin fenokristályokból, valamint különböző spinellszerkezetű oxidokból, szulfidokból álló, halmazszerű társulásokat is tartalmaz. Ez ideig olivinből–krómitból, olivinből–krómitból piroxénből, és fayalitból–szulfidokból álló társulásokat mutattunk ki. E jól körülhatárolt képződmények mellett a meteorit szórtan, szabálytalan eloszlásban „önálló xenokristályokat”, valamint kondrumtöredékeket is tartalmaz. A változatos összetételű, kissé zónás, részben inhomogén olivinekből, és bonyolult spinellszerkezetű oxidok ból álló együttesek a fenokristálytársulások
 legjellemzőbb képviselői. E társulásokon belül többnyire hipidiomorf–xenomorf töredékeket, valamint főleg idiomorf olivinkristályokat tartalmazó változatokat különböztethetünk meg. A fenokris tá lyok erősen repedezettek. A repedések azonban sem az egyes kristályokat övező üvegfázisban, sem az alap anyagban nem folytatódnak, ami arra utal, hogy a kozmikus forrástest térbeli ütközésekor, vagy részben a formálódó kozmikus testbe történt becsapódásakor keletkezhettek. Ezek szerint a társulásokat alkotó ásványok környezetidegenek, azaz xeno kristályok. Az olivinek viszony lag sok vas(II)-oxidot tartalmaznak, de az ásványok belső részei, és a peremek között többnyire jelentős különbségek mutatkoznak. Nagy része hialosziderites (Fa = 36,1–38,2), a külső övek (keskeny sávok) pedig többnyire horto nolitos–ferrohortonolitos összetételűek (Fa = 54,8 vagy 53,4–89,7). Az inhomogén idiomorf oli vineket tartalmazó változatokban az eltérések kisebbek, a fayalit részaránya 22,7–39,4 mólszázalék (krizo lithialosziderit).
 A spinellszerkezetű oxidok a felsorolt társulásokban különböző részarányokban aluminát-, ferrit- és krómitspinellt egyaránt tartalmaznak. A fehérzárványokban észlelt sok alumínium(III)-, magnézium(II)-, valamint viszonylag kevés króm(III)-oxidot tartalmazó közönséges, ill. Al-spinellel ellentétben a xenokristály-társulásokban a Cr2O3 és az FeO (ritkábban az Fe2O3 is) a spinellrácsú oxidok meghatározó összetevői. Ennek megfelelően — a kémiai elemzések átszámítása szerint — az adott oxidásványok 41,2, ill. 56–72 részarányát krómit alkotja. Emellett változó mennyiségű Alspinellt, magneziokrómitot, magnetitet, továbbá kevés ulvitot is tartalmaznak. A zónás ásványokban a peremek felé az összvas-oxid koncentrációja — ennek megfelelően a magnetit részaránya is — növekszik (max. 20%).
 A fayalitból és szulfidokból álló társulásban a xenofenokristályokat sűrűn harántoló repedések sem az alapanyagban, sem pedig a szulfidokban nem folytatódnak, ami eltérő képződési körülményekre utal.
 A 4,9% forsteritet, valamint 0,9% tefroitot tartalmazó (eredeti) fayalit és környezete erőteljesen átalakult. Az eredetileg mintegy 200×500 μm méretű fenokristályokon belül a jellegzetes kiszorítási folyamat eredményeképpen jelentős mennyiségű magnetit keletkezett, ami erős oxidációs folyamatot valószínűsít. A főleg piroxénekből álló összetört szétesett kondrum környezetét sajátos felépítésű „fenokristályok” — változatos összetételű olivin, andradit — és részlegesen átkristályosodott alapanyag-elegyrészek alkotják. A nagy Fe2O3-tartalmú andradit, valamint a szomszédos ásványok (a hedenbergit, hortonolit ferrohortonolit stb.) az adott társulásegyüttes vasban gazdag környezetben történt keletkezését jelzik.

Composite origin of magnetite deposits hosted in Oman peridotites: Evidence for iron mobility during serpentinization

We describe mineralogy of magnetite deposits concentrated within a nonconformity surface between Aniba serpentinites and overlying limestones in the Southern Oman ophiolite and discuss their origin to understand iron mobility during serpentinization, with special reference to factors controlling the magnetite-orebody sizes. The Aniba magnetite deposits occur in several forms such as fibrous crystals, oval or rounded grains, strings and cluster shapes, in fine-grained iron-rich chlorite and calcite matrices. The chromian spinel (Cr# = 0.38–0.56) within the magnetite deposits is similar in texture, mode of occurrence and chemical composition to that in the underlying serpentinized harzburgites (chromian spinel Cr# = 0.37–0.50), and plots in the space of abyssal peridotites. The mineral chemistry of chromian spinel, olivine and clinopyroxene (Cpx) suggested that the Aniba harzburgites are refractory residue after high-degree (15–25%) partial melting and are similar to abyssal harzburgites from a normal ridge segment. This is confirmed by a Cpx trace-element character with high depletion in light rare earth elements (LREE), Ta and Zr relative to heavy REE. The iron of our magnetite deposits was possibly derived from two sources: one is an essentially internal source of iron from the breakdown of Fe-rich serpentines after olivine during low-T serpentinization (<400 °C) and the external source of iron from hydrous fluids. The essential origin of our magnetite deposits is hydrothermal one due to the mobility and leaching of iron from serpentinites and the in-situ selective precipitation of iron within the nonconformity surface. This process was feasible because the nonconformity zone was a weathering surface, a channel for both hydrothermal-fluid flow providing high fluid/rock ratios that enhance iron mobilization in serpentinites and leach iron, besides chromian spinel grains, from the host ultramafic rocks. Chromian spinel grains (up to 10 vol%) in the magnetite deposits were of a residual detrital origin from the underlying serpentinized peridotites on weathering, and acted as a nucleus for magnetite precipitation. The morphology and colloform shape of some magnetite grains reflect their supergene origin. Factors controlling the size of the magnetite deposits include the compositions of the underlying serpentinized harzburgites as a main source of iron, high fluid/rock ratios and high iron mobility within fluid conduits such as a nonconformity surface, abundance of chromian spinel grains as a nucleus for magnetite concentration, low silica activity, pressure of water and oxygen fugacity.

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.

Origin of complex zoning in olivine from diverse, diamondiferous kimberlites and tectonic settings: Ekati (Canada), Alto Paranaiba (Brazil) and Kaalvallei (South Africa)

Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg# = 78–95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism.

Discriminating between pyroxenite and peridotite sources for continental flood basalts (CFB) in southern Africa using olivine chemistry

Continental Flood Basalts (CFB) result from voluminous outpourings of magma that often precede continental break-up. Notwithstanding the petrogenetic importance of CFBs, the nature of the mantle source for such magmas is contentious, particularly with regard to picrites with Ni-rich olivine phenocrysts. Previous studies have suggested that Ni-rich olivines associated with plume volcanism in regions of thickened (>90 km) lithosphere are related to either source mineralogy differences (peridotite versus pyroxenite) or change in olivine-melt partitioning due to pressure increase. In order to evaluate these two hypotheses, we present trace element data for olivines from the Karoo CFB Tuli and Mwenezi picrites and the Etendeka CFB Horingbaai/LTZ-L type picrites, all of which erupted in regions of thickened (>90 km) lithosphere in southern Africa. Karoo picrite olivines are Ni-rich, Ca- and Mn-poor, and have low (<1.4) 100*Mn/Fe. These compositions are consistent with a pyroxenitic source. Etendeka Horingbaai/LTZ-L picrite olivines do not show Ni-enrichment, but are characterized by high Al and Cr, and high (>1.4) 100*Mn/Fe, which is more consistent with high temperature melting of a dominantly peridotitic source. We also show that the Karoo and Etendeka olivines are characterized by distinct Mn/Zn ratios of <13 and >15, respectively.In addition, bulk rock geochemical data compilations and previously reported olivine δ18O for Karoo and Etendeka CFBs are discussed in order to further constrain source components based on previously described pyroxenite melt geochemical indices such as MgO–CaO systematics, FeO/MnO, Zn/Fe, and FC3MS (FeO/CaO–3*MgO/SiO2). These geochemical indices suggest a pyroxenite-dominated source for Karoo CFBs as well as for Etendeka ferropicrites whereas a peridotite-dominated source is indicated for Etendeka Horingbaai/LTZ-L type picrites analyzed in this study. Based on our data, Ni-enrichment of olivine in plume-related magmas in regions of thickened lithosphere in southern Africa is not ubiquitous. We therefore suggest that mineralogical variation of the source is a more likely major control of olivine chemistry and parent melt variations for Karoo and Etendeka CFBs. We also show that olivine Mn–Zn correlations are a useful discriminator for source variation and recommend the use of olivine Mn/Zn<13 for a pyroxenite-dominated source relative to olivine Mn/Zn>15 for a peridotite-dominated source.

Oxygen isotope characteristics of chondrules from the Yamato‐82094 ungrouped carbonaceous chondrite: Further evidence for common O‐isotope environments sampled among carbonaceous chondrites

AbstractHigh‐precision secondary ion mass spectrometry (SIMS) was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the Yamato (Y) 82094 ungrouped 3.2 carbonaceous chondrite. Twenty‐one of 21 chondrules have multiple homogeneous pyroxene data (∆17O 3SD analytical uncertainty: 0.7‰); 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data. Twenty‐one of 25 chondrules have one or more olivine data matching coexisting pyroxene data. Such homogeneous phenocrysts (1) are interpreted to have crystallized from the final chondrule melt, defining host O‐isotope ratios; and (2) suggest efficient O‐isotope exchange between ambient gas and chondrule melt during formation. Host values plot within 0.7‰ of the primitive chondrule mineral (PCM) line. Seventeen chondrules have relict olivine and/or spinel, with some δ17O and δ18O values approaching −40‰, similar to CAI or AOA‐like precursors. Regarding host chondrule data, 22 of 34 have Mg#s of 98.8–99.5 and ∆17O of −3.9‰ to −6.1‰, consistent with most Acfer 094, CO, CR, and CV chondrite chondrules, and suggesting a common reduced O‐isotope reservoir devoid of 16O‐poor H2O. Six Y‐82094 chondrules have ∆17O near −2.5‰, with Mg#s of 64–97, consistent with lower Mg# chondrules from Acfer 094, CO, CR, and CV chondrites; their signatures suggest precursors consisting of those forming Mg# ~99, ∆17O: −5‰ ± 1‰ chondrules plus 16O‐poor H2O, at high dust enrichments. Three type II chondrules plot slightly above the PCM line, near the terrestrial fractionation line (∆17O: ~+0.1‰). Their O‐isotopes and olivine chemistry are like LL3 type II chondrules, suggesting they sampled ordinary chondrite‐like chondrule precursors. Finally, three Mg# &gt;99 chondrules have ∆17O of −6.7‰ to −8.1‰, potentially due to 16O‐rich refractory precursor components. The predominance of Mg# ~99, ∆17O: −5‰ ± 1‰ chondrules and a high chondrule‐to‐matrix ratio suggests bulk Y‐82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules.

Nickel–cobalt contents of olivine record origins of mantle peridotite and related rocks

Olivine is distinguished from all other minerals in providing a remarkable chemical narrative about magmatic processes that occurred in Earth’s crust, mantle, and core over the entire age of Earth history. Olivines in mantle peridotite have Ni contents and Mg numbers that were largely produced by equilibrium crystallization in an early turbulently convecting magma ocean; subsequent stages of partial melting operated to slightly elevate Ni and Mg number in residual olivines. Olivines from Archean komatiites from the Abitibi greenstone belt have Ni contents and Mg numbers that are consistent with an extensively melted peridotite source at great depths in the mantle. Olivines from basaltic oceanic crust, the Icelandic mantle plume and other Phanerozoic occurrences have compositions that record magma chamber crystallization, recharge, mixing, and partial melting. Olivines from the present-day Icelandic mantle plume have compositions that are consistent the melting of a peridotite source; unlike Hawaii, the melting of recycled crust as a distinct pyroxenite lithology is not evident in the olivine chemistry of Iceland. Paleocene picrites from Baffin Island and West Greenland from the ancient Icelandic plume have olivines with Ni contents that are consistent with either Ni-rich peridotite that formed by core-mantle interaction or by low-pressure crystallization of hot and deep magmas. In general, hot magma oceans, mantle plumes, and ambient mantle magmatism form in ways that are captured by the compositions of the olivine crystals that they contain.

Origin and petrogenetic implications of anomalous olivine from a Cascade forearc basalt

Erupted absarokitic-shoshonitic volcanics in subduction zones make up a relatively small proportion of the total erupted volcanic material. However, they are a critical component for quantifying subduction metasomatism and mantle source heterogeneity. This study examines the geochemistry of anomalous olivine within a forearc absarokite lava and associated tephra, a compositional end-member of the Cascadia subduction zone magmatism, to infer mantle heterogeneity and magma petrogenesis. Ni concentrations from the young (42 ka) Quartzville absarokitic basalt lava flow and tephra in the Cascade forearc are up to ~6400 ppm Ni with correspondingly low Ca, averaging ~850 ppm Ca in Fo 91 olivine cores. Decreasing Ni and Ca toward olivine rims cannot be accounted for by simple fractional crystallization and instead necessitates magma mixing ± diffusive re-equilibration between the high-Ni, low-Ca olivine cores and low-Ni, high-Ca rims. δ 18 O of olivine phenocrysts (5.64‰) are elevated compared to other Cascade compositional components and outside of the range typically associated with peridotitic olivine or olivine crystallizing from peridotite-derived basaltic magmas. Trace element contents of whole rock, melt inclusions, and scoria glass have high Sr, high Dy/Yb, and low Y, characteristic of adakitic slab melts; however, major element compositions are of an alkali-rich basalt. In addition, similar trace element compositions between all analyzed glasses indicate a related petrogenesis between all components. We propose that a slab partial melt has reacted with depleted harzburgite in the mantle wedge, underlying the older Western Cascades (~40–10 Ma). Reaction of the siliceous slab-derived melt with depleted harzburgite could produce a metasomatized, zoned pyroxenite-harzburgite mantle source. High-Ni, low-Ca olivine crystallizes from melts of the subarc mantle reaction-pyroxenite. Later melting of the metasomatized harzburgite produces the absarokitic bulk composition, with olivine compositions recording diffusive re-equilibration following incorporation of pyroxenite-derived olivine in a peridotitic magma. Importantly, then, the observed olivine chemistry reflects mineralogical variations in the subarc mantle while melt trace element variations record melts/fluids derived from the subducting plate. High-Fo olivine rims (up to ~Fo 94 ) appear to be the result of late-stage oxidation in the lava flow resulting from SO 2 degassing and are unrelated to the otherwise complex magma petrogenesis.

Multi-stage kimberlite evolution tracked in zoned olivine from the Benfontein sill, South Africa

Olivine is the dominant mineral present in kimberlite magmas; however, due to the volatile-rich nature of most kimberlites, they rarely survive late-stage serpentinisation. Here we present major and trace element data for a rare example of ultra-fresh olivine in a macrocrystic calcite kimberlite from the Benfontein kimberlite sill complex. Olivines are characterised by xenocrystic cores surrounded by multiple growth zones representing melt crystallisation and late-stage equilibration. Two distinct core populations are distinguished: Type 1) low Fo (88–89), Ni-rich, Ca- and Na-rich cores, interpreted here to be the result of carbonate–silicate metasomatism potentially as part of the earliest stages of kimberlite magmatism, and Type 2) high Fo (91–93), Ni-rich, low-Ca cores derived from a typical garnet peridotite mantle source. In both cases, the cores have transitional margins (Fo89–90) representing equilibration with a proto-kimberlite melt. Trace element concentrations, in particular Cr, of these transition zones suggest formation of the proto-kimberlite melt through assimilation of orthopyroxene from the surrounding garnet peridotite lithology. Trace element trends in the surrounding melt-zone olivine (Fo87–90) suggest evolution of the kimberlite through progressive olivine crystallisation. The final stages of olivine growth are represented by Fe-rich (Fo85) and P-rich olivine indicating kimberlite evolution to mafic compositions. Fine (<60μm), Mg-rich olivine rims (Fo94–98) represent equilibration with the final stages of kimberlite evolution back to Fe-poor carbonatitic melts.We present a step-by-step model for kimberlite magma genesis and evolution from mantle to crust tracked by the chemistry of olivines in the Benfontein kimberlite. These steps include early stages of metasomatism and mantle assimilation followed by direct crystallisation of the kimberlite melt and late-stage equilibration with the evolved carbonatitic residual liquids. The Ca contents of the Type 1 xenocrystic olivines are the highest yet measured for mantle olivines, and do not overlap with any known mantle xenolith lithologies. These olivines likely represent an important stage of metasomatism directly related to the early stages of kimberlite melt ponding at the base of the lithospheric mantle.

Melt/rock reaction at oceanic peridotite/gabbro transition as revealed by trace element chemistry of olivine

Several recent studies have documented that reactions between melt and crystal mush in primitive gabbroic rocks (via reactive porous flow) have an important control in the formation of the lower oceanic crust and the evolution of MORBs. In this context, olivine-rich rocks can form either by fractional crystallization of primitive melts or by open system reactive percolation of pre-existing (possibly mantle-derived) olivine matrix. To address this question, we performed in-situ trace element analyses (by LA-ICP-MS) of olivine from the Erro-Tobbio ophiolite Unit (Ligurian Alps), where mantle peridotites show gradational contacts with an hectometer-scale body of troctolites and plagioclase wehrlites, and both are cut by later decameter-wide lenses and dykes of olivine gabbros. Previous studies inferred that troctolites and olivine gabbros represent variably differentiated crystallization products from primitive MORB-type melts. Olivines in the three rock types (mantle peridotites, troctolites, olivine gabbros) exhibit distinct geochemical signature and well-defined elemental correlations. As expected, compatible elements (e.g. Ni) show the highest concentrations in peridotites (2580–2730ppm), intermediate in troctolites (2050–2230ppm) and lowest in gabbros (1355–1420ppm), whereas moderate incompatible elements (e.g. Mn, Zn) show the opposite behaviour. By contrast, highly incompatible elements like Zr, Hf, Ti, HREE are variably enriched in olivines of troctolites, and the enrichment in absolute concentrations is coupled to development of significant HFSE/REE fractionation (ZrN/NdN up to 80). AFC modelling shows that such large ZrN/NdN ratios in olivines are consistent with a process of olivine assimilation and plagioclase crystallization at decreasing melt mass, in agreement with textural observations. In-situ trace element geochemistry of olivine, combined with microstructural investigations, thus appears a powerful tool to investigate reactive percolation and the origin of olivine-rich rocks in the lower oceanic crust.

A story of olivine from the McIvor Hill complex (Tasmania, Australia): Clues to the origin of the Avebury metasomatic Ni sulfide deposit

Magmatic differentiation and/or assimilation and related segregation of immiscible sulfide liquid are generally believed to be critical processes in the formation of the majority of orthomagmatic Ni sulfide deposits. In recent years, a new class of Ni sulfide deposits formed by metasomatic and/or hydrothermal modification of peridotites has been recognized. The serpentinite-hosted Avebury Ni sulfide deposit (Tasmania, Australia), the largest known non-magmatic sulfide deposit, provides an unprecedented opportunity to understand sources of metals and fluids responsible for this style of economic mineralization. Our study shows that serpentinization of the Ni-bearing olivine in the Cambrian peridotites of the McIvor Hill complex was followed by metasomatic transformation assisted by heat and fluids supplied by the nearby Late Devonian granite intrusion. The role of the above in the formation of an economic concentration of Ni sulfides is supported by (1) abundant Ni-Fe alloys and sulfides associated with serpentinization of peridotitic olivine, (2) metasomatic olivine containing inclusions of serpentine and metalliferous brines, and (3) the Late Devonian age of the Ni sulfide deposit. The Avebury metasomatic olivine is Ni-depleted and enriched in Mn relative to olivine of similar Fo content in nearby unmineralized peridotites, and to olivine in subduction-related mafic magmas generally. The unusual minor element chemistry of olivine is matched by a unique set of olivine-hosted multiphase inclusions composed of fibrous Mg-silicates and various Na-, K-, Fe-, Ca-, Mn-, and Ba-bearing chlorides/hydrochlorides, sulfides, arsenides magnetite, REE minerals, and Fe-Ni alloys. Peridotite whole-rock Sr-Nd-Pb isotope data and U-Pb dating of metasomatic titanite support earlier suggestions that Ni mineralization is temporally and genetically related with the intrusion of the nearby 360 Ma Heemskirk Granite. It appears that the multiphase inclusions in metasomatic olivine demonstrate chemical signatures of both in situ serpentinites (entrapped alloys, sulfides, arsenides, and magnetite) and distal fluids (enrichment in Pb, Bi, Sn, Sb, Sr, Ba, Rb, Cs, and Ce). We propose that magmatic olivine in large ultramafic bodies provides almost infinite Ni to replacive serpentinites and constitutes a major reservoir of disseminated Ni mineralization. In the case of Avebury Ni was locally redistributed from olivine in the Cambrian peridotites to mainly Fe-Ni alloys and sulfides during serpentinization in the early Paleozoic. In the Devonian reheating and interaction with a fluid in the contact aureole of the Heemskirk Granite led to de-serpentinization and formation of metasomatic high-Mn, low-Ni olivine with inclusions of serpentine and entrapped alloys, sulfides, arsenides, and magnetite, and metalliferous brines rich in granitic elements. Nickel released from serpentinite in this process was re-deposited near the margins of the peridotite to form the Avebury Ni orebody. Our model of serpentinization-related release of Ni from magmatic olivine, in situ precipitation of metallic, sulfide, and arsenide Ni-minerals, and their redistribution and recrystallization in hydrothermal conditions represents an alternative to Ni remobilization from magmatic sulfides.

Low Ni olivine in silica-undersaturated ultrapotassic igneous rocks as evidence for carbonate metasomatism in the mantle

Subduction drags a large amount of CO2 into the Earth's interior, which is partly returned to the atmosphere by arc volcanism. Processes involved in the recycling of subducted carbon within the upper mantle are mainly related to mineralogical transformation. Subducted CO2 may dramatically affect the equilibria among peridotitic minerals (olivine vs. pyroxenes) changing their stability fields and hence their modal abundances. This process is accompanied by a subduction-induced change in the budget of some incompatible trace and major elements (e.g., K, Ca, HFSE), whereas it has a minimal effect on the mass balance of compatible elements (e.g., Ni). We report trace elements in olivine in subduction-related mafic alkaline ultrapotassic rocks from Italy, which are used as a proxy to define mantle wedge mineralogy and metasomatic processes. Minor element concentrations, and in particular the high Li and low Ti of all the olivines, confirm a major role for recycled sediment in the generation of Italian ultrapotassic magmas. The distinct contents of Ni, Mn, and Ca in olivine reflect the bimodal character of silica-rich and silica-poor ultrapotassic Italian rocks and constrain two distinct mineralogical reactions between metasomatic agents and peridotite. Olivine chemistry from silica-saturated rocks reflects the reaction of silicate melts with the ambient mantle, with consequent consumption of olivine in favour of orthopyroxene. In contrast, the low-Ni, high-Mn/Fe of olivine crystallised from silica-undersaturated leucitites require a mantle source enriched in olivine (and clinopyroxene) compared to orthopyroxene, as a result of the interaction between the ambient peridotitic mantle and CaCO3-rich metasomatic agents. The change from silica-oversaturated lamproites to silica-undersaturated leucitites and thus the difference in the olivine composition is due to a change in composition of the subducting sediment from pelitic to carbonate-rich. The results of this study provide new insights into how CO2 is recycled via subduction processes deep into the mantle.

The Giant Mascot Ni-Cu-PGE Deposit, British Columbia: Mineralized Conduits in a Convergent Margin Tectonic Setting

The Giant Mascot Ni-Cu-PGE deposit is the only past-producing nickel mine (1958–1974) in the Canadian Cordillera of British Columbia, Canada, with ~4.2 Mt of ore grading 0.77% Ni and 0.34% Cu (Ni/Cu = 2.3). The deposit is part of an emerging class of convergent margin Ni-Cu-PGE sulfide deposits where the host rocks are characterized by abundant orthopyroxene and magmatic hornblende. The Giant Mascot intrusion is a crudely elliptical, 3- × 2-km plug composed of ultramafic arc cumulates (olivine-orthopyroxene, hornblende-clinopyroxene) that intruded the Late Cretaceous Spuzzum dioritic pluton. The ores are contained within subvertical pipe-like, lensoid, and tabular bodies ( n = 28), interpreted to represent mineralized conduits and consist of pyrrhotite, pentlandite (+argentopentlandite), chalcopyrite, minor pyrite, troilite, and Pt-Pd-Ni bismuthotellurides. The sulfides have moderate tenors (3–14 wt % Ni, 0.1–17.1 wt % Cu, 84 ppb to 5 ppm total PGE), and variations in PGE concentrations distinguish two geographic mineralized zones (western and eastern). Sulfur isotopes for sulfides span a restricted range of δ 34 S values (−3.4 to −1.3‰) and overlap with analyses from locally pyritiferous Settler schist (−5.4 to −1.2‰). Olivine chemistry reveals a wide range of both Fo (80–89 mol %) and Ni contents (336–3,859 ppm) and provides evidence that Ni contents of olivine were locally upgraded during equilibration with sulfide liquid. Sulfide saturation in the Giant Mascot parental magma(s) was triggered in response to (1) reduction of an oxidized, mantle-derived arc magma, producing ores that formed at an oxidation state of ~ ΔQFM +1; (2) addition of external sulfur and silica by assimilation of Settler schist and Spuzzum diorites; and (3) fractional crystallization of olivine ± orthopyroxene. The presence of high-tenor sulfides in the Giant Mascot deposit indicates that orthomagmatic Ni-Cu-PGE deposits in convergent margin tectonic settings may be of significance to future global exploration.

Cumulates of arc magmas incorporated into the Sulu UHP metamorphic belt, eastern China

ABSTRACTThe Hujialin ultramafic complex in the central region of the Sulu ultra-high pressure (UHP) metamorphic belt consists of discontinuous lenses of garnet-bearing clinopyroxenite and dunite surrounded by marginal serpentinite. The clinopyroxenite shows relatively low concentrations of compatible elements, such as Cr (≤1670 ppm) and Ni (≤514 ppm) and Ir-group platinum group elements (IPGE; Ir, Os, and Ru; ≤4.8 ppb in total). They show varying ratios (0.02–2.50) of IPGE to Pd-group PGE (PPGE). Their chondrite-normalized rare earth elements (REE) patterns are convex and the total REE concentrations range from 18 to 63 times that of Cl chondrite. The bulk rocks show a ‘subduction-related’ geochemical signature, with high concentrations of fluid-mobile elements (i.e. Sr, Ba) relative to high-field strength elements (i.e. Nb, Y, Zr). Clinopyroxene is diopside and contains low Al2O3 (<2.76 wt.%) and high SiO2 (54.6–56.9 wt.%). Olivine grains enclosed by clinopyroxene and in the matrix show relatively low Fo (76.6–80.7) and NiO contents (0.18–0.29 wt.%). The bulk rock compositions and mineral chemistry of olivine and clinopyroxene suggest that the unit was a cumulate of a subduction-related melt. On the other hand, dunite and its hydration product, serpentinite, have a different origin. The bulk rock and mineral chemistry suggest that dunite represents a mantle wedge peridotite in a spinel-stable field. Both clinopyroxenite and spinel-bearing dunite were once located in the mantle wedge below the southern margin of the North China craton (NCC), and were dragged by a mantle flow into the continental subduction channel along the interface between the subducting Yangtze craton (YZC) and the overlying NCC. Although clinopyroxenite and dunite are dense (2.8–3.2 g/cm3), the buoyancy-driven exhumation of voluminous granitic rocks of the YZC likely brought clinopyroxenite and dunite to shallow crustal depths. The lack of the evidence for high pressure to ultra-high pressure (HP-UHP) metamorphism in spinel-bearing dunite may be explained by overall low Al and Ca in the bulk rocks. Alternatively, dunite was not subducted to deep levels, but exhumed together with the deeply subducted clinopyroxenites and granite during their exhumation.

Supercontinental inheritance and its influence on supercontinental breakup: The Central Atlantic Magmatic Province and the breakup of Pangea

AbstractThe Central Atlantic Magmatic Province (CAMP) is the large igneous province (LIP) that coincides with the breakup of the supercontinent Pangea. Major and trace element data, Sr‐Nd‐Pb radiogenic isotopes, and high‐precision olivine chemistry were collected on primitive CAMP dikes from Virginia (VA). These new samples were used in conjunction with a global CAMP data set to elucidate different mechanisms for supercontinent breakup and LIP formation. On the Eastern North American Margin, CAMP flows are found primarily in rift basins that can be divided into northern or southern groups based on differences in tectonic evolution, rifting history, and supercontinental inheritance. Geochemical signatures of CAMP suggest an upper mantle source modified by subduction processes. We propose that the greater number of accretionary events, or metasomatism by sediment melts as opposed to fluids on the northern versus the southern Laurentian margin during the formation of Pangea led to different subduction‐related signatures in the mantle source of the northern versus southern CAMP lavas. CAMP samples have elevated Ni and low Ca in olivine phenocrysts indicating a significant pyroxenite component in the source, interpreted here as a result of subduction metasomatism. Different collisional styles during the Alleghanian orogeny in the North and South may have led to the diachroneity of the rifting of Pangea. Furthermore, due to a low angle of subduction, the Rheic Plate may have underplated the lithosphere then delaminated, triggering both the breakup of Pangea and the formation of CAMP.

Quaternary high-Mg ultrapotassic rocks from the Qal’eh Hasan Ali maars, southeastern Iran: petrogenesis and geodynamic implications

A set of rare, high-Mg ultrapotassic rocks from the Qal’eh Hasan Ali maars, southeastern Iran, was investigated using phlogopite 40Ar/39Ar geochronology, olivine chemistry, and bulk-rock elemental and Sr–Nd–Hf isotopic geochemistry to decipher their petrogenesis and regional tectonic implications. Phlogopite separates yield inverse isochron 40Ar/39Ar ages from ca. 112 to 119 ka, indicating that magma genesis postdated the onset of the Arabia–Eurasia collision at the Late Eocene–Early Oligocene. The studied rocks are characterized by kamafugitic affinity with relatively low SiO2 and Al2O3 and high CaO and Sr. They contain relatively primitive olivine (Fo85–92) that, on the basis of olivine–liquid Fe/Mg exchange equilibrium, suggests the primary melt to be ultrapotassic with ~13 wt% MgO. Other key geochemical features include extreme enrichment in most incompatible trace elements, depletions in Nb, Ta, P and Ti and enrichment in Pb relative to elements of similar incompatibilities. The Sr–Nd–Hf isotopic ratios of the rocks do not deviate significantly from the bulk silicate Earth (87Sr/86Sr = 0.7055–0.7059, 143Nd/144Nd = 0.5125–0.5126 and 176Hf/177Hf = 0.2827–0.2829). Relatively high Zn/Fe, Gd/Yb, Rb, Rb/Sr and P2O5 and low Yb and P/P* for the rocks are consistent with derivation from a mantle source containing clinopyroxene, phlogopite, apatite and garnet that formed in response to modal metasomatism in the lithospheric mantle. Relatively low Hf/Nd and high Sr/Hf of the rocks indicate that the metasomatized lithologies from which the studied rocks formed were derived dominantly from subducted marly sediments. The Qal’eh Hasan Ali magmatism is best explained by small-scale destruction of the continental mantle in a post-collisional setting, presumably driven by localized convective instability as a result of the Arabia–Eurasia collision.

Near infrared spectroscopy of HED meteorites: Effects of viewing geometry and compositional variations

The howardite, eucrite and diogenite (HED) meteorites are genetically related and represent the most voluminous group of achondrites. They are the closest analog materials to Vesta and V-type asteroids. Many of these meteorites were the focus of intense petrologic and visible to near infrared spectral studies. As ground-based and orbital observations of basaltic asteroids have increased, an improved understanding of HEDs is needed. For this study, we investigated 24 HED samples, mainly new finds from Northwest Africa (NWA). Visible to near infrared (up to 2.5μm) spectral measurements under varying illumination and observation geometries were acquired for 4 samples. Phase reddening and bluing (i.e., increase and decrease in spectral slope) is observed for the visible slope as phase angle increase. Monotonic phase reddening can occur for the near infrared slope as phase angle increase. Non-systematic changes with phase angle are found for the Band Area Ratio parameter. At phase angles higher than ∼60°, the decrease of reflectance and decrease of pyroxene bands depth are undistinguishable from admixture of low albedo material to HED samples. To assess the precision of empirical equations relating spectral properties and composition, the pyroxenes, feldspar, and olivine chemistry of the samples was determined. Using previous calibrations, systematic overestimations of the ferrosilite (Fs) and wollastonite (Wo) contents are found, especially in the 15–40 Fs range. To overcome such discrepancies, a new set of empirical equations is proposed. For an application of the new calibration, we selected two compositional end-member areas on Vesta on the basis of their iron content. For the iron-poor terrain we found an average pyroxene composition of Fs30Wo5 and for the iron-rich terrain an average of Fs47Wo14.

Contribution of crustal materials to the mantle sources of Xiaogulihe ultrapotassic volcanic rocks, Northeast China: New constraints from mineral chemistry and oxygen isotopes of olivine

Ultrapotassic igneous rocks can generally be divided into two sub-groups based on the tectonic settings in which they formed. The orogenic sub-group occurs in subduction-related tectonic settings, while the anorogenic sub-group is confined to stable continental regimes. The Pleistocene Xiaogulihe ultrapotassic volcanic rocks outcrop in the western part of Heilongjiang province, northeast China, and are of intraplate origin with respect to its tectonic settings. Previous elemental and isotopic investigations have suggested that the mantle source of these volcanic rocks had been modified by continental-derived sediments resulting from an ancient subduction (at least older than 1.5Ga). In this contribution, we performed in-situ oxygen isotope analysis on olivine grains in these ultrapotassic rocks using secondary ionization mass spectrometry (SIMS). The olivine grains generally have higher δ18O values and CaO contents than those of mantle peridotite xenoliths in the nearby Keluo potassic rocks and show linear correlations between major and trace elements, and Fo, suggesting that they are cognate phenocrysts resulted from fractional crystallization processes. The restricted and non-correlated variations in δ18O with the Fo of these olivine grains imply that the fractional crystallization processes might have negligible influence on their δ18O values. The relatively higher δ18O values of the olivine phenocrysts than the normal mantle imply the addition of an 18O-rich crustal component into their mantle source after ruling out the crustal contamination of the host magmas. We propose that the high-δ18O feature of the olivine phenocrysts was inherited from the subducted crustal component in their mantle source. Given the rapid oxygen isotopic diffusion under high temperature and the long period between mantle metasomatism event and volcanic eruption, it is postulated that the high-δ18O signature could only be preserved in the relatively cold and stable subcontinental lithospheric mantle. Such speculation is consistent with our previous inference that the Xiaogulihe ultrapotassic volcanic rocks were mainly generated from the lower subcontinental lithospheric mantle which had been metasomatized by potassium-rich silicate melts derived from ancient subducted continental-derived sediments.