首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Fossil ~(26)Al and ~(53)Mn in the Asuka 881394 eucrite: evidence of the earliest crust on asteroid 4 Vesta
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Fossil ~(26)Al and ~(53)Mn in the Asuka 881394 eucrite: evidence of the earliest crust on asteroid 4 Vesta

机译:Asuka 881394辉石中的化石〜(26)Al和〜(53)Mn:最早出现在小行星4 Vesta上的地壳的证据

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摘要

Asuka 991394 is a unique magnesian eucrite with pyroxenes that are Mg-rich like those of cumulate eucrites, but with a granulitic texture unlike the textures of cumulate eucrites. Plagioclase compositions are ~An_(98), and are even more calcic than those in cumulate eucrites. Pyroxene does not show pigeonite-to-orthopyroxene inversion texturees, suggesting different crystallization conditions than those of cumulate eucrites. Mn-Cr isotopic analyses determined initial ~(53)Mn/~(55)Mn = (4.6±1.7) * 10~(-6) and initial ε(~(53)Cr)_I = 0.25±0.17 in A881394. This initial ~(53)Mn abundance corresponds to a formation interval Δt_(LEW) = -6±2 Ma relative to the LEW 86010 angrite, implying an 'absolute' age of 4564±2 Ma. Both the invital ~(53)Mn abundance and the initial ε(~(53)Cr)_I value for A881394 are identical to those previously determined for the HED parent body at the time of its differentiation. Al-Mg isotopic ananlyses determined initial ~(26)Al/~(27)Al = (1.18±0.14) * 10(-6), from which a formation interval Δt_(CAI) = 3.95±0.13 Ma is calculated relative to the canonical value ~(26)Al/~(27)Al=5*10~(-5) for CAI. Combining this formation interval with a recently reported Pb-Pb age of 4567.2±0.6 Ma for CAI gives 4563.2±0.6Ma as the age of A881394, in excellent agreement with the age based on the Mn-Cr formation interval. Algernatively, the ~(53)Mn and ~(26)Al formation intervals of A881394 allow the Mn-Cr and Al-Mg timescales to be intercalibrated, suggesting that an 'absolute' CAI age of 4568 Ma is most consistent with the 4558 Ma Pb-Pb age of LEW 86010. The initial ~(26)Al abundance existing in A881394 would have been insufficient to cause global melting in the HED parent body (probably asteroid 4 Vesta). nevertheless, it could have been derived by radioactive decay over only ~2 Ma from an abundance that would have been sufficient to cause global meriting. The higher value of molar Mg/(Mg+Fe) = 0.57 for A881394 than those of the ordinary (basaltic) eucrites (Mg/(Mg+Fe) = 0.30-0.42) suggests additional factors may have been important for magma genesis on the parent body. If ~(26)Al were the only heat source, partial melting would have been the major process in the interior of the parent body, and Mg/(Mg+Fe) would be lower in the melts than in the primordial source material. late=stage accretion could have supplied relatively magnesian primordial materials to the surface of the parent body, thereby increasing Mg/(Mg+Fe) in a shallow magma ocean from which A881394 crystallized, and also may have augmented ~(26)Al heating. The granulitic texture of A881394 may have been produced during residence in the thin, earliest, crust, kept hot by the magma beneath it. If ~(26)Al was, nevertheless, the major heat source for asteroidal melting, it may account for declining post-accretion heating of main belt asteroids with increasing heliocentric distance.
机译:明日香991394是一种独特的镁质真石,其辉石富含镁,类似于块状真石,但颗粒状结构却不同于块状真石。斜长石的组成为〜An_(98),甚至比累积的真石中的钙长得多。辉石没有显示出钙铝石到邻位辉石的倒置织构,这表明其结晶条件不同于累积的真玉质。 Mn-Cr同位素分析确定了A881394中的初始〜(53)Mn /〜(55)Mn =(4.6±1.7)* 10〜(-6)和初始ε(〜(53)Cr)_I = 0.25±0.17。初始〜(53)Mn丰度对应于相对于LEW 86010角铁的形成间隔Δt_(LEW)= -6±2 Ma,意味着“绝对”年龄为4564±2 Ma。 A881394的〜(53)Mn富集性和初始ε(〜(53)Cr)_I初始值与HED母体分化时先前确定的值相同。 Al-Mg同位素分析确定的初始〜(26)Al /〜(27)Al =(1.18±0.14)* 10(-6),由此计算出形成间隔Δt_(CAI)= 3.95±0.13 Ma CAI的典型值〜(26)Al /〜(27)Al = 5 * 10〜(-5)将这一形成间隔与最近报道的CAI的4567.2±0.6 Ma的Pb-Pb年龄相结合,得出A881394的年龄为4563.2±0.6Ma,与基于Mn-Cr形成间隔的年龄高度吻合。同样,A881394的〜(53)Mn和〜(26)Al形成间隔允许对Mn-Cr和Al-Mg时标进行相互校准,这表明4568 Ma的“绝对” CAI年龄与4558 Ma最一致LEW 86010的Pb-Pb年龄。A881394中最初的〜(26)Al丰度不足以导致HED母体(可能是小行星4 Vesta)整体熔化。然而,它可能是由仅足以引起整体功绩的丰度引起的,仅约2 Ma的放射性衰变而得出的。 A881394的Mg /(Mg + Fe)摩尔Mg /(Mg + Fe)较高(Mg /(Mg + Fe)= 0.30-0.42),表明其他因素可能对岩浆成因很重要。母体。如果〜(26)Al是唯一的热源,则部分熔融将是母体内部的主要过程,并且熔体中的Mg /(Mg + Fe)会低于原始材料。后期积聚可能已经向母体表面提供了相对镁质的原始物质,从而增加了A881394结晶的浅岩浆海中的Mg /(Mg + Fe),也可能增加了〜(26)Al的加热。 A881394的颗粒状质地可能是在停留在稀薄,最早的地壳中时产生的,该地壳被其下方的岩浆加热。但是,如果〜(26)Al是小行星融化的主要热源,则它可能解释了随着日心距增加,主带小行星的吸积后加热下降。

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