Origin and History of the Earth

Proceedings of the 30th International Geological Congress, Volume 1

Hongzhen Wang, Borming Jahn, Shilong Mei

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351426404
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

This book deals with the different aspects of the symposia, ranging from, in the original order of the sessions, early history of the earth, continental accretion, coremantle differentiation, biological evolution, palaeoclimate, to interaction between the lithosphere and the hydro-atmo-biosphere.

Origin and History of the Earth

Mantle Degassing and Origin of the Atmosphere

YOUXUE ZHANG

Department of Geological Sciences. The University of Michigan, Ann Arbor, MI 48109–1063, USA

Abstract

This paper first reviews noble gas isotopic data and mantle degassing models. The different models for mantle degassing and origin of the atmosphere are then critically evaluated. All noble gas constraints are presented in a model-independent form so that different models can be examined. Common features and differences of most degassing models are discussed. He-Ne systematics are shown to be more consistent with the notion that Ne degassed more rapidly relative to He. That is, the systematics are more consistent with a solubility-controlled mantle degassing model. However, due to relatively large uncertainties in the isotopic and concentration ratios of various reservoirs, the systematics do not rule out other degassing models at present. Whether the atmosphere originated entirely from mantle degassing is more complicated. Radiogenic nuclides of noble gases (⁴⁰Ar and ²¹Ne^*) almost certainly originated from mantle degassing. Nonradiogenic nuclides of noble gases in the atmosphere may be entirely from mantle degassing if Loihi mantle is 71% degassed. They cannot be entirely from mantle degassing if Loihi mantle is truly undegassed. Fractionation of nonradiogenic Xe isotopes between air/mantle Xe and U-Xe most likely occurred before Xe accreted on earth.

Keywords: air, atmosphere origin, earth evolution, mantle degassing, volatiles

INTRODUCTION

Noble gas isotopic systems have provided powerful constraints on understanding mantle degassing and origin/evolution of the atmosphere. Many models have been proposed. Most of these publications only discuss degassing using a set of model assumptions without a critical evaluation of other models. Previous authors often treat mantle degassing and origin of the atmosphere as one topic in which the atmosphere originates entirely from mantle degassing [e.g., 2, 3, 33, 45, 46, 26]. Ozima and Zahnle [26] discussed in detail some of the major noble gas constraints. Marty [19] proposed that the atmosphere did not entirely originate from mantle degassing. To allow the possibility that only part of the atmosphere originated from mantle degassing and the rest from other sources (such as accretion degassing, cometary injection, etc. [5]), mantle degassing and origin of the atmosphere will be discussed separately in this paper. I first review various noble gas constraints on mantle degassing and examine ways to test models for mantle degassing and atmospheric evolution. The constraints from actual data are presented, instead of model calculations as in many previous publications. I then evaluate different degassing models incorporating recent development in noble gas geochemistry. The origin of the atmosphere is discussed by evaluating whether production of radiogenic and nucleogenic nuclides in the solid earth is enough to provide all the radiogenic nuclide in the atmosphere and whether the amount of primordial nonradiogenic noble gas nuclides in the solid earth is enough to account for their abundance in the atmosphere. All noble gas elements have isotopes that receive a radiogenic or nucleogenic (for simplicity, hereafter “radiogenic” will be used to refer to both radiogenic and nucleogenic contribution. Of the two stable helium isotopes (³He and ⁴He), ³He receives only a minor nucleogenic contribution that is often ignored, whereas most ⁴He in the earth is produced by the a-decay of ²³²Th,²³⁵U and ²³⁸U. Of the three stable isotopes of Ne (²⁰Ne, ²¹Ne and ²²Ne), on a global scale, only ²¹Ne receives a significant nucleogenic contribution through reactions ¹⁸O(a,n)²¹Ne and ²⁴Mg(n,a)²¹Ne [44]. Of the three stable Ar isotopes (³⁶Ar, ³⁸Ar, and ⁴⁰Ar), only ⁴⁰Ar is radiogenic. Practically all ⁴⁰Ar in the earth is due to the decay of ⁴⁰K. Kr isotopic system is rarely discussed in depth because it mimics the Xe isotopic system but shows smaller effects that are more difficult to quantify. Xe isotopic system is complex with nine stable isotopes (¹²⁴Xe, ¹²⁶Xe,¹²⁸Xe,¹²⁹Xe,¹²⁹Xe, ¹³⁰Xe, ¹³¹Xe,¹³²Xe,¹³⁴Xe and ¹³⁶Xe). Among these, ¹²⁹Xe, ¹³¹Xe, ¹³²Xe, ¹³⁴Xe and ¹³⁶Xe receive a fission-genic contribution due to fission of ²³⁵U,²³⁸U,²³²Th, and ²⁴⁴Pu (an extinct nuclide with a half life of 80 million years). The fission-genic component contributes mostly to ¹³⁶Xe and & ¹³⁴Xe but only negligibly to ¹²⁹Xe. ¹²⁹Xe receives a significant radiogenic contribution from ¹²⁹I, an extinct nuclide with a half life of 15.7 million years.

All the parental nuclides that decay to produce the radiogenic noble gas nuclides are less volatile than the noble gases. Therefore, as a part of the mantle becomes degassed (see [45, 26] for degassing mechanisms), the concentrations of the noble gas elements decrease and the ratios of the parent to the nonradiogenic noble gas nuclide (such as ²³⁸U/³He, ²³⁸U/²²Ne,⁴⁰K/³⁶Ar,¹²⁹I/¹³⁰Xe, and ²⁴⁴pu/¹³⁰Xe) increase. With the passage of time, the radiogenic nuclides of the noble gas elements are produced by the parental nuclides, leading to a higher ratio of radiogenic isotope to a nonradiogenic isotope (such as ⁴He/³He,²¹Ne/²², ⁴⁰Ar/³⁶Ar,¹²⁹Xe/¹³⁰Xe,¹³⁶Xe/¹³⁰Xe; collectively referred to as the R/N ratio). The more degassed the part of the mantle, the greater the parent/daughter ratio, and the more rapid increase of the R/N isotopic ratios with time. Therefore, isotopic ratios of noble gas elements in a mantle-derived rock indicate the relative degree of degassing of the mantle source when contamination is negligible or can be corrected.

True (uncontaminated and unfractionated) noble gas isotopic ratios and concentration patterns in different mantle reservoirs are essential in interpreting noble gas isotopic systematics. Because isotopic ratio variations in noble gases are large, isotopic fractionations due to melting, crystallization, and degassing are ignored in discussing noble gases in mantle-derived rocks. However, because noble gas concentrations in mantle-derived rocks are low compared to those in air (except for He), measurement of noble gas isotopic ratios is difficult and is often plagued by air contamination, either in nature before the rock cooled down or in the lab. Easiness of contamination is related to the concentration ratio of the nuclide in the atmosphere to that in the rock. Because He in the atmosphere escapes to outer space, atmospheric He content is low compared to He in rocks. Hence, air contamination to measured ⁴He/³He ratios is often negligible. Therefore, mantle-derived rocks with a smaller ⁴He/³He ratio indicates a lower degree of degassing of the mantel source compared to those with greater ⁴He/³He. As shown later, even though Ne isotopic ratios are almost always contaminated by air Ne, it is possible now to correct for air contamination. He isotopic ratios and the contamination-corrected Ne isotopic ratios may provide powerful constraints in understanding mantle degassing.

NOBLE GAS ISOTOPIC RATIOS IN DIFFERENT RESERVOIRS

In using noble gas isotopic systems to model mantle degassing, the mantle is often divided into two reservoirs [2, 3, 33, 45, 46, 26] based on information from He and other noble gases: one is MORB (mid-ocean ridge basalt) mantle (the degassed mantle) and the other is Loihi mantle (the less degassed mantle). The MORB mantle has also been referred to as the upper mantle. However, among others, Zhang and Zindler showed that ⁴⁰Ar in the air requires that at least 43% and probably 70% of the whole mantle is degassed [45]; that is, the volume of degassed mantle is greater than that of the upper mantle. The Loihi mantle has also been referred to as the undegassed mantle. The justification for the undegassed mantle claim is not clear since noble gas isotopic ratios can only show that it is less degassed than MORB mantle. In this paper, the Loihi mantle is not a priori assumed to be undegassed [26]; it is less degassed. The atmosphere and continental crust (including oceans) are often treated as another single reservoir in terms of volatile components. Therefore, there are three volatile reservoirs for the earth, referred to as AC (atmosphere plus continental crust), MM (MORB mantle), and LM (Loihi mantle or less degassed mantle). The following is a review of noble gas isotopic data.

He

⁴He/³ ratio in air is 7.15x10⁵ [25]. The initial ⁴He/³He ratio in the mantle (that is, the ratio at the time when the earth formed) is not well known. The present-day solar ratio is 2190 [4]. The ratio measured in meteorites (the “planetary”component) is ~7000 [25]. Since initial Ne in the mantle is not “planetary” (see below), the planetary ⁴He/³He ratio may not be applicable to the primordial mantle. However, the present-day solar ratio may not represent the primordial ratio either because it may have been affected by nuclear reactions in the sun [25]. Recently, ⁴He/³He ratio in Jupiter’s atmosphere was determined to be ~9000 [22], which may represent the initia...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Foreword
• KEYNOTE PAPERS
• ORIGIN AND HISTORY OF THE EARTH