Interaction of small mantle plumes with the spinel-perovskite phase boundary: implications for chemical mixing

Details Interaction of small mantle plumes with the spinel-perovskite phase boundary: implications for chemical mixing Interaction of small mantle plumes with the spinel-perovskite phase boundary: implications for chemical mixing

Apr 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000e%26psl.177..241m&link_type=abstract

Earth and Planetary Science Letters, Volume 177, Issue 3-4, p. 241-254.

Computer Science

2

Scientific paper

Geochemical observations of ocean island basalts show a broad variety in the chemical composition of trace elements. These observations cannot be explained by different temperature and pressure conditions at the melt production region only but also demand differences in the parental material at the base of the lithosphere. Here, we add a new explanation for this finding based on the different interaction of small and large plumes with the spinel-perovskite phase boundary (SPB) in the Earth's mantle. We assume that mantle plumes detaching from the core-mantle boundary (CMB) are of variable size and excess temperature. While rising through the lower mantle, they form an approximately spherical plume head. When arriving at the SPB, they are retained from further rise depending on their size and excess temperature. We have investigated the interaction of a plume with the SPB using a numerical approach for viscous flow. To focus particularly on this interaction, we simply modelled the initial plume as a hot volume of variable cross-section and excess temperature. Volumes with a radius exceeding about 150 km and an excess temperature of more than 100°C rapidly cross the SPB. With decreasing size and increasing excess temperature, volumes are retarded at the SPB and those with a radius smaller than about 80 km stagnate entirely below the SPB. These results are roughly independent of the particular rheology or two-dimensional versus three-dimensional geometry. In the present study, we investigated the degree of thermal entrainment of material at the SPB for strongly retarded plumes. We estimate the amount of chemical mixing by tracking passive markers from the original plume volume and the regions around the SPB to the base of the lithosphere. The amount of entrainment of material from close to the SPB depends on the retardation time and the thermal growth of the plume head at the SPB. Plume heads which reach the base of the lithosphere after a retardation time of more than 50 Ma might entrain (in the numerical model) large amounts of material from the SBP but they may not be characteristic for the Earth, since they might be distorted by large scale mantle flow. However, even if we limit the retardation time to reach the base of the lithosphere to 50 Ma, plumes, originally composed of material from the CMB and the adjacent lower mantle, might easily entrain up to 15% of their volume of material originating from the SPB.

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