Effects of Spin Transition and Cation Substitution on the Optical Properties and Iron Partitioning in Carbonate Minerals

The high-pressure behavior of deep carbonate dictates the state and dynamics of oxidized carbon in the Earth's mantle,playing a vital role in the global carbon cycle and potentially influencing long-term climate change.Optical absorption and Raman spectroscopic measurements were carried out on two natural carbonate samples in diamond-anvil cells up to 60 GPa.Mg-substitution in high-spin siderite FeCO_(3)increases the crystal field absorption band position by approximately 1000 cm^(-1),but such an effect is marginal at>40 GPa when entering the low-spin state.The crystal field absorption band of dolomite cannot be recognized upon compression to 45.8 GPa at room temperature but,in contrast,the high-pressure polymorph of dolomite exhibits a strong absorption band at frequencies higher than(Mg,Fe)CO_(3)in the lowspin state by 2000–2500 cm^(-1).Additionally,these carbonate minerals show more complicated features for the absorption edge,decreasing with pressure and undergoing a dramatic change through the spin crossover.The optical and vibrational properties of carbonate minerals are highly correlated with iron content and spin transition,indicating that iron is preferentially partitioned into low-spin carbonates.These results shed new light on how carbonate minerals evolve in the mantle,which is crucial to decode the deep carbon cycle.

The Mg-carbonate-Fe interaction:Implication for the fate of subducted carbonates and formation of diamond in the lower mantle

The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe^0 system at 70-150 GPa and 800-2600 Kin a laser-heated diamond anvil cell.Using in situ synchrotro n X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by:6 MgCO3+19 Fe=8 FeO+10(Mg0.6Fe^0.4)O+Fe7 C3+3 C.The presented results suggest that the interaction of carbonates with Fe^0 or Fe^0-bearing rocks can produce Fe-carbide and diamond,which can accumulate in the D"region,depending on its carbon to Fe ratio.Due to the sluggish kinetics of the transformation,diamond can remain metastable at the core-mantle boundary(CMB)unless it is in a direct contact with Fe-metal.In addition,it can be remobilized by redox melting accompanying the generation of mantle plumes.

Deciphering the Origin of Abiotic Organic Compounds on Earth:Review and Future Prospects

The geologic production of abiotic organic compounds has been the subject of increasing scientific attention due to their use in the global carbon flux balance,by chemosynthetic biological communities,and for energy resources.Extensive analysis of methane(CH_(4))and other organics in diverse geologic settings,combined with thermodynamic modelings and laboratory simulations,have yielded insights into the distribution of specific abiotic organic molecules on Earth and the favorable conditions and pathways under which they form.This updated and comprehensive review summarizes published results of petrological,thermodynamic,and experimental investigations of possible pathways for the formation of particular species of abiotic simple hydrocarbon molecules such as CH_(4),and of complex hydrocarbon systems,e.g.,long-chain hydrocarbons and even solid carbonaceous matters,in various geologic processes,distinguished into three classes:(1)pre-to early planetary processes;(2)mantle and magmatic processes;and(3)the gas/water-rock reaction processes in low-pressure ultramafic rock and high-pressure subduction zone systems.We not only emphasize how organics are abiotically synthesized but also explore the role or changes of organics in evolutionary geological environments after synthesis,such as phase transitions or organic-mineral interactions.Correspondingly,there is an urgent need to explore the diversity of abiotic organic compounds prevailing on Earth.

Effect of Iron on the Stability of Rhodochrosite at the Topmost Lower Mantle Conditions

Carbonates are viewed as the principal oxidized carbon carriers during subduction,and thus the stability of subducted carbonates has significant implications for the deep carbon cycle.Here we investigate the high pressure-temperature behaviors of rhodochrosite in the presence of iron up to∼34 GPa by in-situ X-ray diffraction and ex-situ Raman spectroscopy.At relatively low temperature below∼1500 K,MnCO_(3)breaks down into MnO and CO_(2).Upon heating to∼1800 K,however,the MnCO_(3)-Fe^(0)reactions occur with the formation of Mn_(3)O_(4),Fe^(O)and reduced carbon.A'three-stage'reaction mechanism is proposed to understand the kinetics of the carbon-iron-manganese redox coupling.The results suggest that Fe^(0)can serve as a reductant to greatly affect the stability of rhodochrosite,which implies that the effect of Fe-metal should be seriously considered for the high pressure-temperature behaviors of other predominant carbonates at Earth's mantle conditions,particularly at depths greater than∼250 km.

Recycling of Paleo-Asian Ocean carbonates and its influence on the lithospheric composition of the North China Craton

The deep carbon cycle,which plays a critical role in mantle evolution and Earth habitability,is closely linked to the recycling of carbonate-bearing rocks through subduction.Marine carbonates are subducted to different depths during the closure of oceanic basins,thus carry important signatures of the disappeared oceanic basins.Petrological and geochemical features of the Hannuoba carbonatites in the northern North China Craton indicate that they were formed by melting of limestone subducted to mantle depths.Here,we show that detrital zircons carried by these carbonatites have a broad spectrum of U-Pb ages from Precambrian to Phanerozoic.Precambrian age peaks are at~2.5 Ga,2.1–2.3 Ga,1.8–2.0 Ga,~1.65 Ga,1.3–1.4 Ga,~1.1 Ga,0.91–0.94 Ga,0.74–0.81 Ga,and 0.62–0.63 Ga,respectively.The recorded age peaks are different from those in the northern North China Craton and thus preclude an origin of crustal contamination.Nevertheless,the Precambrian age spectra are compatible with those of the Xingmeng Orogen in the southeastern Central Asian Orogenic Belt.Furthermore,the significantly positiveεHf(t)values of 7.7–13.5 for the 300–373 Ma zircons are similar to those in the Xingmeng Orogen but different from those in the northern North China Craton.All these features suggest that the limestone precursor for the Hannuoba carbonatites was originated from the Paleo-Asian Ocean,and its deposition time was not earlier than 300 Ma.This indicates that the PaleoAsian Ocean still existed in the late Carboniferous to early Permian.The widespread distribution of metamorphic carbonates in the Central Asian Orogenic Belt indicates that there may have been widespread sedimentary carbonates in the Paleo-Asian Ocean.A large amount of sedimentary carbonates was probably carried into mantle during subduction of the Paleo-Asian oceanic slab,which significantly modified the chemical and physical properties of the lithospheric mantle.

Carbonated Big Mantle Wedge Extending to the NE Edge of the Stagnant Pacific Slab:Constraints from Late Mesozoic-Cenozoic Basalts from Far Eastern Russia

It has been suggested that the carbonated mantle reflected by Mg-Zn isotopic anomalies of Cenozoic intraplate basalts from East Asia coincides with the stagnant West Pacific slab in the mantle transition zone.However,the northern boundary of such carbonated domain beneath East Asia is uncertain.Late Mesozoic-Cenozoic intraplate basalts are widespread in far eastern Russia and thus provide an opportunity to examine this issue.Here we report major-trace element contents and Sr-NdMg-Zn isotopic compositions for 9 Late Mesozoic-Cenozoic basaltic samples from the Khanka Block and Sikhote-Alin accretionary complex.They are characterized by large variations in SiO_(2)contents(41 wt.%to 50 wt.%)and CaO/Al_(2)O_(3)(0.50 to 0.97),enrichments of large-ion lithophile elements(LILE),positive Nb-Ta anomalies and strongly negative K,Pb,Zr,Hf,Ti,Y anomalies in primitive mantle-normalized trace element spider diagram.Furthermore,the rocks show good correlations of Ti/Ti^(*)with Hf/Hf^(*),La/Yb,Fe/Mn and trace element contents(e.g.,Nb).In addition,they have lighter Mg and heavier Zn isotope compositions than the BSE estimates,coupled with depleted Sr-Nd isotope compositions.These elemental and isotopic characteristics cannot be explained by alteration,magma differentiation or diffusion,but are consistent with the partial melting of carbonated peridotite.By and large,the Late Mesozoic-Cenozoic basalts from far eastern Russia bear very similar geochemical characteristics as those Na-series Cenozoic basalts from eastern China.The extended region of Mg-Zn isotopic anomalies is roughly coincident with the stagnant West Pacific slab beneath East Asia,and all of these alkali basalts can be generated from mantle sources hybridized by recycled Mg-carbonates from the Pacific slab stagnant in the mantle transition zone.We infer that(1)the carbonated big mantle wedge extends to the NE edge of the West Pacific slab and may have also appeared in the Late Mesozoic due to the effect of the Paleo-Pacific slab beneath this region,and(2)decarbonation of stagnant slabs in the mantle transition zone is a key mechanism for carbon outgassing from deep mantle to surface via intraplate alkali melts.

Redox-Induced Destabilization of Dolomite at Earth’s Mantle Transition Zone

Carbonates are considered to be important hosts of oxidized carbon during subduction processes.Here we investigate the redox interactions between dolomite and metallic iron in laser-heated diamond anvil cells up to~20 GPa.The identification of recovered samples via in-situ synchrotron X-ray diffraction and ex-situ Raman spectroscopy shows that the reaction occurs with the formation of ferropericlase,graphite and hexagonal diamond,while CaCO_(3) remains stable.The experimental results indicate dolomite and metallic iron phases cannot coexist and demonstrate a possible formation mechanism of ultradeep diamonds via redox reaction between dolomite and iron under the mantle transition zone conditions.The results are significant for understanding carbon transportation during subduction processes and have further implications to the processes in the more complex systems regarding to carbonate-silicate-metal phase relations.