Given that the age of the oldest zircon samples are about 4.4 billion years old, and meteorites and other extraterrestrial samples can also give us some indication about the Earth's composition into the Hadean era (source: Zircon Chronology: Dating the Oldest Material on Earth.
Many resources refer to the earliest atmosphere as being (for example):
As a result of the high temperatures at the center of the Earth, and due to volcanic activity, the crust emitted halogen gasses, ammonia, hydrogen, carbon dioxide, methane, water vapor, and other gasses. In the following 100 million years, these gasses accumulated to form the primordial atmosphere. This atmosphere was quite similar to the atmosphere of Titan, one of the larger moons of Saturn. The primordial atmosphere is believed to have reached a pressure of 250 atmospheres and would have been extremely toxic to life as we now know it
Source: The Hadean Eon
Many mechanisms are suggested in this and many other resources, from impacts to volcanic outgassing etc. But what does measured geochemical data tell us about the Earth's earliest palaeoclimatic conditions?
This question is related to the previous questions:
- What, if any, paleoclimate data can be derived from igneous rocks?
- What, if any, paleoclimate data can be derived from stromatolite fossils?
But, this question is looking at the earliest palaeoclimatic conditions of the Earth.