Recent work suggests that solidification of the 100 km radius core of a mostly mantle-stripped body may progress from the outside-in mediated by the growth of iron-nickel dendrites. Such growth is now supported by paleomagnetic measurement of iron meteorites and is suggested even for bodies with intact mantles. The late stages of this dendritic growth results in pockets of isolated sulfur-enriched iron-nickel melts surrounded by solid iron-nickel. Isolated low density melt pockets could collapse, causing sulfur-enriched iron-nickel melts to propagate in dikes. Thus, core material of the cooling planetesimal may be intruded into the rocky mantle of a planetesimal or even erupted onto its surface. We refer to these processes collectively as ferrovolcanism and suggest it as a possible origin for pallasite meteorites.
Image by James Tuttle Keane (CalTech)
For a 100-km core radius, the region of iron-nickel dendrites and unsolidified pockets of sulfur enriched melt is ~10–30 km in scale. Individual kilometer-scale dendrites would be stable for ~10–105 yr depending on assumed viscosity and densities; this timescale is inversely proportional to the size of the dendrites. For melt pockets larger than 10 km in scale, overpressures capable of causing magmatism occur.
Pallasites are an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of iron-nickel metal. Pallasites have a low iridium content implying their metal matrix is sourced from a highly evolved up to ~80% crystalized metallic melt. Ferrovolcanism offers a legitimate mechanism to explain pallasites as intrusions of evolved core magmas into the olivine-rich mantle of a body. Main group pallasites have an average sulfur content of 2.3 wt% suggesting intruded core material would have a sulfur content of ~5 wt%. This sulfur content is inconsistent with a highly evolved melt leading some to consider the possibility that sulfur-rich pallasites are simply underrepresented in the meteorite record. Although the amount of sulfur in pallasite source material is debated, even at typical ~5 wt% sulfur, core material could have been intruded a few km into the mantle of the pallasite parent body and possibly farther if the magma chamber is further pressurized.
Paleomagnetic studies show that pallasites record the magnetic field of pallasite parent body. To record such a field, pallasite source material must have been cooled to below ~360 K while a liquid core was still convecting. Thermal models assuming conventional outward core growth suggest that pallasites originated from less than 40 km deep in the mantle of a 200-km diameter body to achieve these low temperatures. Thermal models with inward core growth, however, suggest even outer core material may reach these low temperatures while the inner portions of the core remain liquid. Although further thermal evolution models with inward core solidification are warranted, it is likely that a ferrovolcanic origin for pallasites is consistent with paleomagnetic measurements and thermal constraints, even if the intruded core material does not reach the shallow mantle. Additionally, formation by intrusion of molten impactor core material would require more serendipitous timing and may not be consistent with size and morphology of observed olivine crystals in pallasites.
Psyche: Our findings have implications for Psyche, an asteroid that is the target of an upcoming NASA spacecraft mission. Psyche was argued to be an intact planetary core on the basis of high density and radar albedo measurements, but more recent and precise density estimates have made this interpretation more uncertain. The most recent estimate of bulk density is 4160 ± 640 kg/m3, a number that likely implies an important compositional role for metal but is much lower than the density of iron meteorites. One possibility for Psyche’s structure is that it represents the core of an ancient, differentiated planetesimal that was exposed by hit-and-run collisions, and contains high macroporosity. A second plausible structure is that it represents an undifferentiated mix of rock and metal, and perhaps is the parent body of the mesosiderite class of meteorites.
Ferrovolcanism offers a third possible structure for Psyche consistent with density measurements and observations of both metal and orthopyroxene on the surface. Assuming an average diameter of 226 km, a core density of 7000 kg/m3, and a mantle density ranging between 2500–3500 kg/m3, an average mantle thickness as low as 22 km is consistent with bulk density estimates. This silicate thickness is compatible with ferrovolcanism if sulfur content and pocket size are sufficiently high. Ferrovolcanism may have transported core material to the surface, causing the radar detections of metal. Testing this hypothesis of Psyche as a differentiated rock-metal body with ferrovolcanic surface units can be achieved by the upcoming Psyche mission. Furthermore, such a structure would be consistent with the exciting possibility that Psyche is the pallasite parent body.
Collaborators: This work was led by Brandon. C. Johnson in collaboration with Michael. M. Sori and Alexander J. Evans.
