Rb-Sr chronology of volatile depletion in differentiated protoplanets: BABI, ADOR and ALL revisited

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• 作者：Ulrik Hans ; Thorsten Kleine ; Bernard Bourdon
• 关键词：Sr initial ; achondrites ; nucleosynthetic anomalies ; volatile depletion ; accretion ; chronology
• 刊名：Earth and Planetary Science Letters
• 出版年：2013
• 出版时间：15 July, 2013
• 年：2013
• 卷：374
• 期：Complete
• 页码：204-214
• 全文大小：851 K

文摘

A strong depletion in moderately volatile elements is a characteristic feature of many planetary bodies in the inner solar system and either reflects the rapid accretion of planetesimals from an incompletely condensed solar nebula, or is the result of energetic collisions during planetary accretion. To better constrain the origin and timescales of this volatile depletion, we have precisely measured Sr isotope compositions in angrites, eucrites and Ca-Al-rich inclusions (CAI). Angrites have an initial (⁸⁷Sr/⁸⁶Sr)_ADOR=0.698978¡À0.000004, which is indistinguishable from (⁸⁷Sr/⁸⁶Sr)_BABI=0.698970¡À0.000028 obtained for eucrites. In agreement with earlier studies we find that angrites and eucrites have higher initial ⁸⁷Sr/⁸⁶Sr ratios than CAI, at face value corresponding to model timescales for volatile loss of several millions of years. However, all the investigated CAI are characterized by elevated ⁸⁴Sr/⁸⁶Sr ratios compared to angrites and eucrites, which we interpret to reflect an excess of r-process Sr in the CAI. If this is correct, then the nucleosynthetic Sr isotope anomalies in the CAI require an upward correction of their measured ⁸⁷Sr/⁸⁶Sr. After this correction CAI have an initial (⁸⁷Sr/⁸⁶Sr)_CAI=0.698975¡À0.000008, which is indistinguishable from ADOR and BABI. This implies volatile loss from the angrite and eucrite parent bodies within less than ~1 Ma after formation of CAI. The volatile-depleted nature of these differentiated protoplanets thus most likely reflects their rapid accretion from volatile-poor dust in an incompletely condensed solar nebula.