Experimental study of carbonated mid ocean ridge basalt at 3.5-21 GPa : implications for the Earth's deep carbon cycle
Creators: name: Kiseeva, Ekaterina
Publisher: name: The Australian National University
Year: 2018
Subjects: subject: QE462.B3 K57 2011
subject: Basalt Analysis
subject: Carbon cycle (Biogeochemistry)
subject: Chemical oceanography.

description: The carbon cycle is one of the most important chemical fluxes of the Earth. It involves the entire planet from the atmosphere, hydrosphere and biosphere (the exosphere) to its deep interior. Carbon is released from the deep earth to the exosphere during volcanism as a result of CO{u2082} degassing from magmas, or during emplacement of carbonate-rich magmas such as carbonatites into the crust. The major carbon return cycle, from the exosphere back into the deep mantle, is via subduction of oceanic lithosphere at convergent margins. Significant amounts of carbonated rocks (including hydrothermally altered basaltic oceanic crust) occur at the top of the subducting slab and may be transported into the Earth's deep interior. In order to estimate the magnitude of the carbon return flux into the deep earth, detailed understanding of the phase relations of carbonate bearing oceanic crust from P-T conditions corresponding to the sub-arc region into the very deep earth, are required. One effective means of gaining this understanding of the deep carbon cycle involves high-pressure experimental petrology. The current investigation is purely experimental and it addresses deep roots of carbon cycle from about 100 km (~3.5 GPa) down to about 700 km (~23 GPa) below the Earth's surface. This study describes high pressure experiments on melting and phase relations of a representative composition of altered (or carbonated) mid-ocean ridge basalts (MORB+10% calcite), inferred to comprise a major component within the upper part of subducted mafic, oceanic crust. The key components of the rock triggering its melting at lower temperatures are H{u2082}O and alkalis, such as Na{u2082}O and K{u2082}O. Even small amounts of water present in the carbonated basalt, can induce silicate melting at temperatures up to 250{u00B0}C lower than at dry conditions. Therefore the first melts to appear on H{u2082}O-undersaturated carbonated MORB melting are silicate- and K-rich, and they flux carbonate melting and removal of most of the carbonate from the system. However, the solidus of H{u2082}O-undersaturated carbonated MORB remains above most of subduction geotherms, allowing its subduction during the prograde path without melting. Although some carbonate may be removed from the subducted slab at relatively shallow depths because of dissolution in hydrous partial melts of altered oceanic crust, the results of this study support the conclusion that the majority of the carbonate may be preserved within the mafic part of the slab to ultra-deep levels, at least to transition zone pressures (410-660 km). The results of this study indicate that solidus temperatures of water-free carbonated eclogite remain similar around 1200-1300{u00B0}C despite progressively increasing pressure. These solidus temperatures, which are significantly lower than estimated mantle adiabats, are likely to be governed by presence of such alkali components as Na{u2082}O and K{u2082}O or/and their combination with carbonate. Thus melts produced by MORB+10% calcite at those depths are carbonatitic and very alkali-rich, however, some carbonate may still be preserved within the rock after partial melting. This study suggests that with the increased solid solution of clinopyroxene into garnet with increasing pressure and the transformation of the rock into garnetite at about transition zone depths, alkali components and especially Na become extremely incompatible and partition either into Na-rich carbonate (at lower temperatures &lt; 1200{u00B0}C) or form Na-rich carbonatitic melts. If these melts segregate from the garnetite residue, they may metasomatise overlying peridotite or undergo reduction to diamond. These melts may play a significant role in generation of enriched magma sources above or within the mantle transition zone. At depths of 300-700 km the oxygen fugacity of the subducting slab and surrounding mantle is likely to be highly reduced and carbonate may also re-crystallize in the form of diamond, as observed in the experiments. If so, C may in some circumstances be subducted into the lower mantle. Key words: Carbonated eclogite; experimental petrology; pressures of 3.5-21 GPa; subduction; kimberlite and carbonatite magmas; metasomatism in the cratonic mantle; trace elements partitioning between major eclogite minerals and carbonate/silicate melts; trace element modelling