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Ferrovolcanism on metal worlds and the origin of pallasites

机译：金属世界上的铁钒铁和方石的起源

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Pockets of iron-rich melt within asteroids get progressively enriched in sulfur while the asteroid is cooling, generating excess pressure that can push the melt up to the surface. This 'ferrovolcanism' can be the origin of the stony-iron pallasite meteorites and can explain the contradicting results from asteroid 16 Psyche.As differentiated planetesimals cool, their cores can solidify from the outside in(1), as evidenced by palaeomagnetic measurements and cooling-rate estimates of iron meteorites(2,3). The details of outside-in solidification and fate of residual core melt are poorly understood. For a core primarily composed of iron and nickel alloyed with lighter constituent elements such as sulfur, this inward core growth would probably be achieved by growth of solid iron-nickel dendrites(4). Growth of iron-nickel dendrites results in interconnected pockets of residual melt that become progressively enriched in sulfur up to a eutectic composition of 31 wt% sulfur as iron-nickel continues to solidify(4). Here, we show that regions of residual sulfur-enriched iron-nickel melt in the core attain sufficient excess pressures to propagate via dykes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the planetesimal's surface. We refer to these processes collectively as ferrovolcanism. Our calculations show that ferrovolcanic surface eruptions are more likely on bodies with mantles less than 50 km thick. We show that intrusive ferromagmatism can produce pallasites, an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of iron-nickel metal(4). Ferrovolcanic eruptions may explain the observations that asteroid 16 Psyche has a bulk density inconsistent with iron meteorites(5) yet shows evidence of a metallic surface composition(6).
机译：当小行星冷却时，小行星中的富含铁的熔体囊会逐渐富集硫，从而产生过高的压力，可将熔体推至表面。这种``铁硼烷主义''可能是石铁古陨石的起源，并且可以解释小行星16 Psyche产生的矛盾结果。陨石的高速率估计（2,3）。从外而内凝固的细节和残余芯熔体的结局知之甚少。对于主要由铁和镍与较轻的构成元素（例如硫）合金化的铁心，这种向内的铁心增长可能是通过固态铁-镍枝晶的生长来实现的（4）。铁镍树枝状晶体的生长会导致相互连接的残余熔体袋，随着铁镍继续凝固，残余熔体逐渐富集至硫含量高达31 wt％的低共熔成分（4）。在这里，我们表明岩心中残留富硫铁镍熔体的区域达到足够的过大压力，可通过堤坝传播到地幔中。因此，核心材料将侵入上覆的岩石地幔，甚至可能喷发到小行星的表面。我们将这些过程统称为铁硼烷主义。我们的计算结果表明，地幔厚度小于50 km的地体更有可能发生铁钒铁表面喷发。我们证明了侵入性铁磁作用会产生白云石，这是由铁镍金属基质中夹带的橄榄石晶体组成的一类神秘的陨石（4）。 Ferrovolcanic火山喷发可能解释了以下观察结果，即小行星16 Psyche的堆积密度与铁陨石不一致（5），但显示出金属表面成分的证据（6）。

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  • 来源
    《Nature Astronomy》 |2020年第1期|41-44|共4页
  • 作者

    Johnson Brandon C.; Sori Michael M.; Evans Alexander J.;

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    Purdue Univ Dept Earth Atmospher & Planetary Sci W Lafayette IN 47907 USA;

    Univ Arizona Lunar & Planetary Lab Tucson AZ 85721 USA;

    Brown Univ Dept Earth Environm & Planetary Sci Providence RI 02912 USA;

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