High Pressure Behavior Of Sulfur And Carbon In The Deep Earth

As typical multivalent elements in the Earth,sulfur and carbon widely exist in the geological activities of various layers of the Earth and directly influence the redox reactions of the mantle rocks,melts,and fluids.Research on the cycle of sulfur and carbon in the earth has been a long-term focus of geoscience.It is of great significance to determine their existing forms and chemical reactions with other minerals for understanding the internal structure and geological processes in the Earth.However,as a result of the complexity of the extreme temperature and pressure conditions,redox environment,and mineral species,determining the existing forms of sulfur and carbon in the Earth is of great challenge.Some scientific problems accompanying the chemical reactions of sulfur-and carbon-containing minerals remain to be solved,such as the“excess sulfur”degassing during the volcanic eruption,the formation mechanism of boron impurities and molecular nitrogen inclusions in diamond from the transition zone and the top of the lower mantle.This work explores the high-pressure behaviors of sulfur and carbon elements deep in the Earth and delves into three key scientific questions.The following innovative results are obtained:1.The amount of sulfur released by volcanoes as they degassed during eruptions(mainly in the form of sulfur dioxide)is several orders of magnitude greater than the amount of sulfur that can be contained in the erupted melt.This is known as the“excess sulfur”degassing problem.The source of the excess sulfur is still unknown.Much previous research has tried to explain the excess sulfur through shallower magmatism,but the recent studies have found that some of the volcanic sulfur comes from the Earth's deep mantle,so the study of deeper sulfur-bearing minerals may have more significance for the source of excess sulfur.In this work,CALYPSO software was used to predict the structure of sulfur-oxygen compounds,and an atypical stoichiometry of sulfur-oxygen compound S₃O₄ is stable at 79-102 GPa,which contains mixed valence states of S(+II)and S(+IV).According to first-principles calculations,sulfur-containing minerals(Ca SO₄,Mg SO₄,Fe S,and Fe S₂)subducted into the earth's interior can react with the existing Fe and Fe OOH minerals in the earth's interior to produce S₃O₄.S₃O₄can exist stable at the temperature and pressure conditions corresponding to the lower mantle,but when the S₃O₄ are transported into the shallow area of the Earth along with the mantle plume,will decompose into SO₂ and elemental sulfur,providing a potential deep Earth source of excess sulfur.This work provides a new mechanism for solving the mystery of“excess sulfur”and understanding the sulfur cycle between the Earth's surface and the deep mantle.2.Blue diamonds are natural diamonds produced at the top of the lower mantle(660-750 km),in which boron atoms replace some of the carbon atoms in the diamond lattice,causing the color centers and making the diamonds appear blue.But the boron is lacking in the deep mantle,the mechanism of the boron atoms appearing in superdeep diamonds remains unknown.This work constructs a series of redox reactions of carbonates and metal borides under high-pressure conditions related to the lower mantle through first-principles calculations.In collaboration with experimental co-authors,we confirmed the reaction between magnesium carbonate and iron boride under high temperature and high pressure related to the lower mantle and found the existence of elemental boron and diamond in the products.This work provides a possible solution for the source of boron impurities in these blue superdeep diamonds through the redox reaction of carbonates with metal borides.3.A little number of diamonds containing inclusions in the form of molecular nitrogen are found in the transition zone of the mantle,but the abundance of nitrogen is low in the deep mantle.So,the source of its molecular nitrogen and the mechanism of its presence in the diamond inclusion are unclear.The presence of Fe-N in lower mantle diamonds in recent studies may be closely related to the diamond formation mechanism that contains the molecular nitrogen inclusions.This work designs a series of redox reactions between carbonates and metal nitrides at high-pressure conditions of the lower mantle through first-principles calculations.In collaboration with experimental co-authors,we also performed experiments at the temperature and pressure conditions related to the lower mantle.The designed reactions between magnesium carbonate and iron nitride were confirmed by high-pressure and high-temperature experiments,in which nitrogen and diamond were found as products.This work provides a possible explanation for the existence of molecular nitrogen inclusions in superdeep diamonds through the redox reactions of carbonates with metal nitrides.