Olivine Mn Research Articles

Nickel–manganese variability in olivine and Al‐in‐olivine thermometry for olivine‐phyric shergottites

AbstractOlivine‐phyric shergottites represent primitive Martian magmas, but they commonly contain excess olivine and rarely represent primary mantle melts. Olivine chemistry, however, tracks magma evolution and preserves information on the original parent magma composition. Here, we investigate the applicability of the Al‐in‐olivine thermometer in tandem with olivine chemistry in a suite of 13 olivine‐phyric shergottites to constrain the compositions and conditions of their mantle sources and parental magmas. We show that the Al‐in‐olivine thermometer is a robust method for constraining crystallization temperatures in shergottites, yielding temperatures in agreement with experimental work. In contrast, we do not recommend olivine–spinel thermometry relying on Mg‐Fe in olivine, which underestimates crystallization temperatures by 160–380 °C. Olivine chemistry reveals distinct differences in Ni concentrations between shergottite groups, with enriched shergottites generally having higher Ni at a given forsterite (Fo) content. Nickel variations in terrestrial olivine are often accredited to contributions from a pyroxenite source; however, the same mechanism is not responsible for Ni variations in Martian olivine. Here, we favor a model for variable olivine modal abundance, caused by multiple melting episodes in the depleted mantle source, affecting Ni partitioning during melting to account for the Ni variations observed. In addition, we show that olivine Ni‐Mn variations in depleted shergottites indicate variable petrogenetic histories and parental magmas. Tissint contains elevated Ni in olivine more similar to the enriched shergottites whereas DaG 1037 has elevated Mn in olivine suggesting an Mn‐enriched parent magma relative to other depleted shergottites.

Petrogenesis of main group pallasite meteorites based on relationships among texture, mineralogy, and geochemistry

AbstractMain group pallasite meteorites are samples of a single early magmatic planetesimal, dominated by metal and olivine but containing accessory chromite, sulfide, phosphide, phosphates, and rare phosphoran olivine. They represent mixtures of core and mantle materials, but the environment of formation is poorly understood, with a quiescent core–mantle boundary, violent core–mantle mixture, or surface mixture all recently suggested. Here, we review main group pallasite data sets and petrologic characteristics, and present new observations on the low‐MnO pallasite Brahin that contains abundant fragmental olivine, but also rounded and angular olivine and potential evidence of sulfide–phosphide liquid immiscibility. A reassessment of the literature shows that low‐MnO and high‐FeO subgroups preferentially host rounded olivine and low‐temperature P2O5‐rich phases such as the Mg‐phosphate farringtonite and phosphoran olivine. These phases form after metal and silicate reservoirs back‐react during decreasing temperature after initial separation, resulting in oxidation of phosphorus and chromium. Farringtonite and phosphoran olivine have not been found in the common subgroup PMG, which are mechanical mixtures of olivine, chromite with moderate Al2O3 contents, primitive solid metal, and evolved liquid metal. Lower concentrations of Mn in olivine of the low‐MnO PMG subgroup, and high concentrations of Mn in low‐Al2O3 chromites, trace the development and escape of sulfide‐rich melt in pallasites and the partially chalcophile behavior for Mn in this environment. Pallasites with rounded olivine indicate that the core–mantle boundary of their planetesimal may not be a simple interface but rather a volume in which interactions between metal, silicate, and other components occur.

Minor elements in olivine inspect the petrogenesis of orogenic peridotites

A large dataset of minor elements (Ni, Mn, Co, Zn, Ca, Ti, Al, Cr and Na) in olivines from orogenic peridotites is reported to investigate the application of these elements as sensitive tracers of mantle processes in subduction zones. Orogenic mantle peridotites can be distinguished from crustal peridotites by overall higher Ni and lower Co and Zn contents in olivine. In particular, the Ni/Co ratio of olivine can successfully discriminate between the two types of peridotites. When mantle peridotites underwent transfer from hot lithospheric mantle to cool subduction zones, olivine Ca, Al and Cr contents dramatically decreased, and Ca in olivine can be a useful geothermometer for orogenic peridotites. The Na contents of olivines from garnet-facies mantle peridotites (mostly >20 ppm) are typically higher than those from spinel-facies mantle peridotites (mostly <11 ppm), indicating that Na in olivine is a potential pressure indicator for orogenic peridotites. The high Ti contents of olivine may be caused by subducted crustal metasomatism, producing the wide occurrence of Ti-clinohumite and rutile in strongly metasomatized garnet lherzolites. Olivines from these high-Ti garnet lherzolites also have Mn contents deviating from the normal mantle olivine range, implying high sensitivity of Mn in olivine to crust–mantle interaction in subduction zones. Our data highlight the broad prospects for olivine geochemistry in the realm of subduction zone geodynamics.

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.

Multiple Metasomatism beneath the Nógrád–Gömör Volcanic Field (Northern Pannonian Basin) Revealed by Upper Mantle Peridotite Xenoliths

Peridotite xenoliths from the Nograd-Gomor Volcanic Field (NGVF) record the geochemical and structural evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. The xenoliths are divided into a lherzolitic and a wehrlitic series based on their modal composition and textural features. This paper focuses on the lherzolite series and presents petrography and major- and trace-element geochemistry for xenoliths 51 selected from all xenolith-bearing localities of the NGVF. The xenoliths consist of olivine, orthopyroxene, clinopyroxene, spinel ± amphibole. Protogranular textures are most frequently found in the northern part, while to porphyroclastic and equigranular textures are more dominant in the central and southern parts. The xenoliths have undergone varying degrees (<5-30 %) of partial melting with overprinting by different metasomatic processes. Based on heir Mg#, the spinel lherzolite xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 92, whereas Group II has Mg# <89, and significant enrichment of Fe and Mn in olivine and pyroxenes, and of Ti in spinel. Trace-element contents of the xenoliths vary widely, allowing a further division based on LREE enrichment or depletion in pyroxenes. REE patterns of amphiboles match those of clinopyroxenes in each xenolith where they appear, and depict different origins based on their Nb (and other HFSE) content. It is inferred that Nb-poor amphiboles record the oldest metasomatic event, caused by volatile-bearing silicate melts, followed by at least two further metasomatic processes: one that resulted in U-Th-(Nb-Ta)- LREE-enrichment and crystallization of Nb-rich amphibole, affecting selective domains under the entire NGVF, and another evidenced by Fe-Mn-Ti-enrichment which overprinted previous geochemical signatures. We suggest that the metasomatic agents in both cases were basaltic silicate melts, compositionally similar to the host basalts. These melts were generated during the Miocene extension of the Pannonian Basin. The effects of heating and subsequent cooling are evident in significantly different equilibration temperatures.

Mobility of nutrients and trace metals during weathering in the late Archean

The evolution of the geosphere and biosphere depends on the availability of bio-essential nutrients and trace metals. Consequently, the chemical and isotopic variability of trace elements in the sedimentary record have been widely used to infer the existence of early life and fluctuations in the near-surface environment on the early Earth, particularly fluctuations in the redox state of the atmosphere. In this study, we applied late Archean weathering models (Hao et al., 2017), developed to estimate the behavior of major elements and the composition of late Archean world average river water, to explore the behavior of nutrient and trace metals and their potential for riverine transport. We focused on P, Mn, Cr, and Cu during the weathering of olivine basalt.In our standard late Archean weathering model (pCO2,g = 10−1.5 bars, pH2,g = 10−5.0 bars), crustal apatite was totally dissolved by the acidic rainwater during weathering. Our model quantitatively links the pCO2,g of the atmosphere to phosphate levels transported by rivers. The development of late Archean river water (pH = 6.4) resulted in riverine phosphate of at least 1.7 μmolar, much higher than at the present-day. At the end of the early Proterozoic snowball Earth event when pCO2,g could be 0.01–0.10 bars, river water may have transported up to 70 μmolar phosphate, depending on the availability of apatite, thereby stimulating high levels of oxygenic photosynthesis in the marine environment.Crustal levels of Mn in olivine dissolved completely during weathering, except at large extents of weathering where Mn was stored as a component of a secondary carbonate mineral. The corresponding Mn content of river water, about 1.2 μmolar, is higher than in modern river water. Whiffs of 10−5 mole O2 gas or HNO3 kg−1 H2O resulted in the formation of pyrolusite (MnO2) and abundant hematite and simultaneous dramatic decreases in the concentration of Mn(II) in the river water.Chromite dissolution resulted in negligible dissolved Cr in Archean river water. However, amorphous Cr(OH)3 representing easily-weatherable Cr-bearing minerals dissolved totally during the weathering simulations, resulting in concentrations of Cr(III) in the river water of up to 0.14 μmolar, higher than at the present-day.Late Archean weathering of accessory chalcopyrite produced chalcocite and bornite, and extremely low concentrations of Cu (<10−15 molar) because of the low solubilities of the copper sulfides. However, pulses of either O2,g or HNO3 produced native copper, chalcocite, and bornite, much more hematite, and river water containing levels of dissolved Cu comparable to the present-day. Copper mineralogy predicted by weathering models might provide a new correlation with evidence from studies of copper mineral evolution.Overall, our results implied that the redox state of the atmosphere, the pH of surface waters, and the availability of easily-weatherable minerals are all important factors controlling the dissolution of trace elements in river water. Interpretation of the sedimentary signatures of trace elements should consider not only the redox state but also the pH and availability of accessory minerals.

Diversity of the initial rocky planetary building materials at the edge of the solar system

AbstractAsteroids and comets are surviving members of the vast planetesimal population that was distributed across the early solar system. They appear to be a diverse set of bodies but we present evidence from comet samples that the body‐to‐body diversity of the initial rocky component mix in planetesimals may have declined with distance from the Sun. Laboratory measurements of the minor element Mn in olivine collected from Comet Wild 2 suggests that the micron‐sized rocky crystalline contents of this comet formed in numerous inner solar system environments. The results are consistent with a scenario where silicates such as olivine form at incandescent temperatures in multiple environments and then mix as they are transported to distant cold regions where silicates could accrete with ice and organics to form comets. Accreting far from silicate formation regions, many ice‐rich planetesimals are likely to have started with similar complex mixtures of diverse rocky components formed in various high‐temperature environments. This contrasts with asteroidal meteorite parent bodies whose silicates retain regional properties that give different chondrite classes their distinctive properties.

Effect of pressure on Fe–Mg, Ni and Mn diffusion in (Fe xMg 1− x) 2SiO 4 olivine

The pressure dependence of Fe–Mg interdiffusion coefficients, as well as of diffusion coefficients of the trace elements Ni and Mn in olivine, have been determined from diffusion anneals carried out using oriented single crystals in multianvil presses. This is the first determination of activation volumes of diffusion of trace elements (Ni, Mn) in olivine. Results from an isothermal polybaric (1673 K, 6–12 GPa) and an isobaric polythermal (1623–1823 K, 12 GPa) set of experiments were combined to constrain an internally consistent set of Arrhenius parameters for the diffusion processes. It is found that within the uncertainty of the data, (a) diffusion rates of Fe–Mg, Mn and Ni are very similar to each other at all conditions, consistent with more detailed observations from 1 atmosphere experiments ( Petry et al., 2004), (b) apparent activation volumes using different fitting protocols lie between 4 and 7 cm 3 mol −1, but a value of 5.3 ± 1.0 cm 3 mol −1 (cf. Misener, 1974; Farber et al., 2000), describes the pressure dependence of diffusivity of these elements in olivine over its entire stability range (in pressure) adequately along the Ni–NiO fO 2 buffer, (c) the activation volume at constant fO 2 is higher, ∼7 cm 3 mol −1 and (d) at depths corresponding to the lowermost stability of olivine (∼12 GPa), activation energies of diffusion are ∼100 kJ mol −1 higher than at near surface conditions, which means that closure temperatures are higher by several hundred degrees. Along geothermal gradients within the Earth, diffusion rates increase with depth in the lithosphere but decrease with depth, in spite of the increase in temperature, in the convective asthenospheric part of the mantle. Consequently, there is a maxima in diffusion rates at the lithosphere–asthenosphere boundary and for the olivine part of the mantle, this is likely to be the region where most geochemical mixing and eradication of heterogeneities occur.

Specific heat, magnetic properties and critical behaviour of Mn 2SiS 4 and Fe 2GeS 4

The olivine Mn 2SiS 4 is antiferromagnetic (AF) up to 83 K, behaves as a weak ferromagnet (WF) from 83 to 86 K, and becomes paramagnetic (P) at higher temperatures. Its counterpart Fe 2GeS 4 undergoes similar transitions in a reversed order: WF up to 69 K, AF up to 144 K and then P. These transitions are characterized by susceptibility measurements and investigated by high resolution specific heat experiments. The transition to the paramagnetic state is of the critical type and belongs to the universality class of the Heisenberg model for Mn 2SiS 4 and the 3D Ising model for Fe 2GeS 4. The WF/AF or AF/WF transition is of first order with a latent heat ΔH = 0.09 J/mol for Mn 2SiS 4 and ΔH = 105 J/mol for Fe 2GeS 4. The lattice is found to be stiffer for the Mn thiosilicate than for the Fe thiogermanate.

Neutron diffraction study of magnetic phases in polycrystalline Mn 2SiS 4

A narrow region of spontaneous magnetization was found to occur in the synthetic olivine Mn 2SiS 4 between 83 and 86 K. Neutron diffraction analysis shows that for the manganese ions there is antiferromagnetic spin alignment along the crystallographic a-axis, with opposing ferromagnetic c-components on two sites of different symmetries, inversion and mirror. Below 83 K the analysis shows canted antiferromagnetism progressing to collinear alignment on the b-axis at 4.2 K.

Experimental determination of Mn-Mg mixing properties in garnet, olivine and oxide

We have measured the mixing properties of Mn-Mg olivine and Mn-Mg garnet at 1300° C from a combination of interphase partitioning experiments involving these phases, Pt-Mn alloys and Mn-Mg oxide solid solutions. Activity coefficients of Mn dilute in Pt-Mn alloys at 1300° C/1 atm were measured by equilibrating the alloy with MnO at known f O 2. Assuming that the log f O 2 of the Mn-MnO equilibrium under these conditions is-17.80 (Robie et al. 1978), we obtain for γMn: logγMn = −5.25 + 3.67 XMn + 11.41X2 Mn Mixing properties of (Mn,Mg)O were determined by reversing the Mn contents of the alloys in equilibrium with oxide at known f O 2. Additional constraints were obtained by measuring the maximum extent of immiscibility in (Mn,Mg)O at 800 and 750° C. The data are adequately described by an asymmetric (Mn,Mg)O solution with the following upper and lower limits on nonideality: (a) WMn = 19.9kj/Mol; WMg = 13.7kj/Mol; (b) WMn = 19.9kj/Mol; WMg = 8.2kj/Mol; Olivine-oxide partitioning was tightly bracketed at 1300° C and oxide properties used to obtain activity-composition relations for Mn-Mg olivine. Despite strong M2 ordering of Mn in olivine, the macroscopic properties are adequately described by a symmetric model with: Wol = 5.5 ± 2.5 kj/mol (1-site basis) Using these values for olivine, garnet-olivine partitioning at 27 kbar/1300° C leads to an Mn-Mg interaction parameter in garnet given by: Wgt = 1.5 ± 2.5kJ/mol (1-site basis) Garnet-olivine partitioning at 9 kbar/1000° C is consistent with the same extent of garnet nonideality and the apparent absence of excess volume on the pyrope-spessartine join indicates that any pressure-dependence of WGt must be small. If olivine and garnet properties are both treated as unknown and the garnet-olivine partitioning data alone used to derive WOl and WGt, by multiple linear regression, best-fit values of 6.16 and 1.44 kJ/mol. are obtained. These are in excellent agreement with the values derived from metal-oxide, oxide-olivine and olivine-garnet equilibria.

Phase relationships in mixed metal germanium sulfide systems (Mn 2GeS 4T 2GeS 4, where T = Co, Ni, Zn)

The influence of substitution of Mn 2+ by Co 2+, Ni 2+, and Zn 2+ in the olivine Mn 2GeS 4 has been investigated by x-ray powder methods on polycrystalline samples. Contrary to similar systems starting from spinel type compounds we could not obtain new layered materials in these systems. A series of olivine type mixed crystals are formed in the system Mn 2−xCo xGeS 4 for 1 ≥ x ≥ 0. In the corresponding Ni system the olivine type region reaches from x = 0 to at least x = 1.7. Zn 2+ cannot replace Mn 2+ in the olivine structure. Substitution of Zn 2+ in Zn 2GeS 4 by Mn 2+ changes the structure from a defect sphalerite to defect wurtzite type. Co 2GeS 4 and Ni 2GeS 4 could not be obtained.