Presentation Information
[PPS12-P02]Evolution of the mesosiderite parent body
*naoji sugiura1, Tomoko Arai (1.none)
Keywords:
mesosiderite
There are many unsolved problems on the evolution of mesosiderites : source of metal, heat source for reheating/metamorphism, paucity of olivine if metal was derived from a core, etc. Here we address the problem of metal accretion. Often, it is assumed that the metal was hot and derived from a molten core (Haba+, 2019). We observed metal nodules in a couple of mesosiderites and also metal aggregates in a primitive mesosiderite. Based on these observations, we argue that the metal was solid at the time of accretion.
A metal nodule in the Bondoc mesosiderite is about 1 inch across in which small silicate grains are evenly distributed. Similar Bondoc nodules have been reported (e.g. Hassanzadeh,1990 : Axon+, 1977) and also available on the web. If the metal was molten, due to high surface energy, it would produce a solid metallic lump without pores, and it would be impossible for silicate particles to penetrate into the metal. This suggests that the nodule was a porous aggregate of solid metal grains at the time of the nodule formation. Silicates filled the pore space during gardening on the parent body and the remaining pore spaces were lost during subsequent reheating events. A nodule in the NWA2924 mesosiderite is ~3 cm across. Here, silicates are more abundant than in the Bondoc nodule, and they are not grainy but tend to be of wormy shape. If look closely, the silicates seem to be separated by a certain distance. Our interpretation is that it (separation) represents the size of metal grains that constituted the porous aggregate. NWA1878 is the most primitive mesosiderite (Kimura+, 2020). Aggregates of spheroidal metal grains are seen. Although at certain locations, metal is larger due to sintering, spheroidal metal grains (~250 microns in diameter) are recognized at other locations. Similarly to the cases of nodules, if the metal were molten, such porous aggregates are not easily produced. Altogether, we suggest that mesosiderite metal accreted as small spheroidal solid grains.
If the metal was not molten at the time of accretion, it cannot be the heat source for reheating of mesosiderites. Then, what could be the heat source? Shock heating is often proposed, but texture for severe shock is not seen in most mesosiderites (Haack, 1996). Recently, we found a mesosiderite that has sulfides containing metal grains and vesicles (Sugiura+, 2025). This is explained by sulfur evaporation from sulfide and resultant excess metal; this is very likely produced by shock heating. But such sulfide is found in only one mesosiderite out of 18 mesosiderites that we examined.
If shock heating is not the heat source for metemorphism, induction heating may be the heat source, by default. A problem with the induction heating is that quantitative estimates of the degrees of heating have not been made. Superflares (due to magnetic reconnection) as high as 2x1038 erg are known for young stellar objects. But this is not sufficient for heating mesosiderites (>tens of meters thick on the parent body, located at 1 AU) by ~1000 C. Highly energetic events that outshine superflares seem to be required. If mesosiderites were heated on the parent body by such events, there is an added bonus that the great dunite shortage problem (Greenwood, 2015) is removed.
References
Axon+ 1977, Mineral. Mag., 41, M1-M3. Greenwood+, 2015, GCA, 169, 115-136. Haack+, 1996, GCA, 60, 2609-2619. Haba+, 2019, Nature Geo., 12, 510-515. Hassanzadeh,1990, GCA, 54, 3197-3208. Kimura+, 2020, MAPS, 55, 1116-1127. Sugiura+, 2025, NIPR16th, OAp2.
A metal nodule in the Bondoc mesosiderite is about 1 inch across in which small silicate grains are evenly distributed. Similar Bondoc nodules have been reported (e.g. Hassanzadeh,1990 : Axon+, 1977) and also available on the web. If the metal was molten, due to high surface energy, it would produce a solid metallic lump without pores, and it would be impossible for silicate particles to penetrate into the metal. This suggests that the nodule was a porous aggregate of solid metal grains at the time of the nodule formation. Silicates filled the pore space during gardening on the parent body and the remaining pore spaces were lost during subsequent reheating events. A nodule in the NWA2924 mesosiderite is ~3 cm across. Here, silicates are more abundant than in the Bondoc nodule, and they are not grainy but tend to be of wormy shape. If look closely, the silicates seem to be separated by a certain distance. Our interpretation is that it (separation) represents the size of metal grains that constituted the porous aggregate. NWA1878 is the most primitive mesosiderite (Kimura+, 2020). Aggregates of spheroidal metal grains are seen. Although at certain locations, metal is larger due to sintering, spheroidal metal grains (~250 microns in diameter) are recognized at other locations. Similarly to the cases of nodules, if the metal were molten, such porous aggregates are not easily produced. Altogether, we suggest that mesosiderite metal accreted as small spheroidal solid grains.
If the metal was not molten at the time of accretion, it cannot be the heat source for reheating of mesosiderites. Then, what could be the heat source? Shock heating is often proposed, but texture for severe shock is not seen in most mesosiderites (Haack, 1996). Recently, we found a mesosiderite that has sulfides containing metal grains and vesicles (Sugiura+, 2025). This is explained by sulfur evaporation from sulfide and resultant excess metal; this is very likely produced by shock heating. But such sulfide is found in only one mesosiderite out of 18 mesosiderites that we examined.
If shock heating is not the heat source for metemorphism, induction heating may be the heat source, by default. A problem with the induction heating is that quantitative estimates of the degrees of heating have not been made. Superflares (due to magnetic reconnection) as high as 2x1038 erg are known for young stellar objects. But this is not sufficient for heating mesosiderites (>tens of meters thick on the parent body, located at 1 AU) by ~1000 C. Highly energetic events that outshine superflares seem to be required. If mesosiderites were heated on the parent body by such events, there is an added bonus that the great dunite shortage problem (Greenwood, 2015) is removed.
References
Axon+ 1977, Mineral. Mag., 41, M1-M3. Greenwood+, 2015, GCA, 169, 115-136. Haack+, 1996, GCA, 60, 2609-2619. Haba+, 2019, Nature Geo., 12, 510-515. Hassanzadeh,1990, GCA, 54, 3197-3208. Kimura+, 2020, MAPS, 55, 1116-1127. Sugiura+, 2025, NIPR16th, OAp2.