Key question:
Is the later-veneer neccessary for Earth to reach its water inventory?
Later-delivery v.s. later-veneer
Water, an essential precondition of life, characterizes Earth as a habitable planet. However, due to its position within the "snow-line", the accreted material of Earth is supposed to be depleted in water (O'Brien et al., 2006; O'Brien et al., 2014). To explain the abundance of water on Earth, two end-member scenarios of water delivery by water-rich materials like meteorites or comets are proposed:
Later delivery: Earth reached ~60-90% of its current mass and then started to acquire water when the core was still forming (Schoenbaechler et al., 2010; O'Brien et al., 2014).
Later-veneer: Earth Earth’s water is added after the last giant impact forming the Moon and the core formation(Albarede et al., 2013; Ballhaus et al., 2013).
Angrite: a record of water delivery
We cannot prove these two scenarios directly based on earth rocks because the active geological processes erased the primitive records. However, the Angrite Sahara 99555, one of the oldest meteorites (4564.58 ± 0.14 Ma), recorded water distributions in the accretional process of early inner bodies.
Here we first utilized polarized Fourier transform infrared spectroscopy (FTIR) to measure the hydrogen contents, and electron microprobe analyzer (EPMA) for major element compositions in nominally anhydrous minerals (NAMs) of Sahara 99555: olivine, clinopyroxene, and plagioclase.
Studied clinopyroxene in Angrite
FTIR
FTIR results show H contents:
olivines: dehydrated
clinopyroxene: 22±2 ppm
plagioclases: 17±5 ppm
EPMA
EPMA shows the major elements in these minerals.
Water in magma and Parent body
The Fe# of 48.5~56.8 of our clinopyroxenes indicates it is safe to use them to retrieve primitive melt, given that water contents in the early crystallization of clinopyroxene (Fe# 40~60) are suggested as nearly constant. Fo of our olivines (54.9~62.7) is within Petrolog3 model-predicated Fo (51.3~65.2) of firstly-crystallized olivine. These prove that our selected minerals are valid to calculate the water content in the primitive melt.
We calculated the partition coefficient of water between clinopyroxene and melt (0.0209~0.0613) using major element compositions. The water content in the melt (326~1148 ppm) in equilibrium with clinopyroxene overlaps the water content (267~442 ppm) of melt in equilibrium with plagioclase. Assuming 15% batch melting of mantle and water partition coefficient between peridotite and melt as 0.007, the water content in the mantle of Angrite parent body (APB) is below 200 ppm, revealing that planetesimals in the inner solar system are relatively dry, and therefore, it is likely necessary for Earth to have a later-veneer.
