The confines of triple oxygen isotope exponents in elemental and complex mass-dependent processes

	The confines of triple oxygen isotope exponents in elemental and complex mass-dependent processes
	Huiming Bao; Xiaobin Cao; Justin A. Hayles
	2015
发表期刊	Geochimica et Cosmochimica Acta
卷号	170 期号:1 页码:39-50
摘要	Small differences in triple isotope relationships, or Delta O-17 in the case of oxygen, have been increasingly used to study a range of problems including hydrological cycles, stratosphere-troposphere exchange, biogeochemical pathways and fluxes, and the Moon's origin in the geochemical and cosmochemical communities. A Delta O-17 value depends on the triple isotope exponent theta of involved reaction steps. However, the probabilistic distribution of the intrinsic and apparent theta values has not been examined for elemental processes and for processes that are out of equilibrium or bearing reservoir-transport complexities. A lack of knowledge on the confines of theta may hamper our understanding of the subtle differences among mass-dependent processes and may result in mischaracterization of a set of mass-dependent processes as being in violation of mass-dependent rules. Here we advocate a reductionist approach and explore theta confines starting from kinetic isotope effects (KIEs) within the framework of transition state theory (TST). The advantage of our KIE approach is that any elemental or composite, equilibrium or non-equilibrium process can be reduced to a set of KIEs with corresponding theta(KIE). We establish that the KIE between a reactant and a transition state (TS) is intrinsic. Given a range of KIEs known for Earth processes involving oxygen, we use a Monte Carlo calculation method and a range of oxygen-bonded molecular masses to obtain a distribution of theta(KIE) values and subsequently that of theta(eq). Next, complexities are examined by looking into expected effects due to reaction progress, unbalanced fluxes, and reference frame. Finally, compounded reservoir-transport effects are examined using two simple processes - Rayleigh Distillation (RD) and Fractional Distillation (FD). Our results show that the apparent theta values between two species or two states of the same evolving species have much broader confines than the commonly used ''canonical" confines of 0.51-0.53, particularly when the overall fractionation factors are close to 1.000. Equilibrium processes exhibit the narrowest alpha-theta value distribution. More complex processes or non-equilibrium further broaden the confines of the apparent theta values for reaction systems. The compounded reservoir-transport effects of RD and FD demonstrate that non-canonical apparent theta and large non-zero Delta O-17 values are achievable even when all involved elemental steps are strictly mass-dependent. This study calls for a research effort to determine KIE and theta(KIE) for important natural processes, and for cautions in interpreting a slope value drawn in a delta'O-17-delta'O-18 space, as the slope is only a superficial manifestation of a set of complex reaction pathways and dynamics.
学科领域	矿床地球化学
收录类别	SCI
语种	英语
文献类型	期刊论文
条目标识符	http://ir.gyig.ac.cn/handle/352002/5757
专题	矿床地球化学国家重点实验室_矿床地球化学国家重点实验室_期刊论文
推荐引用方式 GB/T 7714	Huiming Bao,Xiaobin Cao,Justin A. Hayles. The confines of triple oxygen isotope exponents in elemental and complex mass-dependent processes[J]. Geochimica et Cosmochimica Acta,2015,170(1):39-50.
APA	Huiming Bao,Xiaobin Cao,&Justin A. Hayles.(2015).The confines of triple oxygen isotope exponents in elemental and complex mass-dependent processes.Geochimica et Cosmochimica Acta,170(1),39-50.
MLA	Huiming Bao,et al."The confines of triple oxygen isotope exponents in elemental and complex mass-dependent processes".Geochimica et Cosmochimica Acta 170.1(2015):39-50.

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