Title: Accretion timescale and impact history of Mars deduced from the isotopic systematics of martian meteorites

Abstract

High precision Sm-Nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on Mars associated with the formation of their mantle sources. Our data is used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic. Ages determined from the Sm-Nd isotopic systematics of individual samples are strongly dependent on the assumed Nd isotopic composition of the bulk planet. This assumption is problematic given differences observed between the Nd isotopic composition of Earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary Nd isotopic compositions. Ages determined from the slope of Sm-146-Nd-142 whole rock isochrons are not dependent on the assumed Nd-142/Nd-144 ratio of the planet, but require the sample suite to be derived from complementary, contemporaneously-formed reservoirs. In this work, we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a Nd-143-Nd-142 whole rock isochron and is also independent of any a priori assumptions regarding the bulk isotopicmore » composition of the planet. This expression is also applicable to mineral isochrons and has been used to successfully calculate Nd-143-Nd-142 model crystallization ages of early refractory solids as well as lunar samples. This permits ages to be obtained using only Nd isotopic measurements without the need for Sm-147/Nd-144 isotope dilution determinations. When used in conjunction with high-precision Nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 +/- 6 Ma. Because the Sm-Nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification, the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the Solar System. Recent thermal models, however, suggest that Mars-sized bodies cool rapidly in less than similar to 5 Ma after accretion ceases, even in the presence of a thick atmosphere. In assuming these models are correct, an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 Ma. Late accretional heating of Mars could either be associated with protracted accretion occurring at a quasi-steady state or alternatively be associated with a late giant impact. If this scenario is correct, then accretion of Mars-sized bodies takes up to 60 Ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation. This further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets, and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed.« less

Authors:

Borg, Lars E.  ^[1];

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Brennecka, Gregory A. ^[2]; Symes, Steven J. K. ^[3]

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1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Chemical Sciences Division
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Chemical Sciences Division; Univ. of Munster (Germany). Inst. of Planetology
3. Univ. of Tennessee, Chattanooga, TN (United States). Dept. of Chemistry

Publication Date:

Sat Dec 12 00:00:00 EST 2015

Research Org.:

Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Sponsoring Org.:

USDOE; National Aeronautics and Space Administration (NASA)

OSTI Identifier:

1395530

Alternate Identifier(s):

OSTI ID: 1397582

Report Number(s):

LLNL-JRNL-700773
Journal ID: ISSN 0016-7037; PII: S0016703715006948

Grant/Contract Number:  

AC52-07NA27344; NNH12AT84I

Resource Type:

Accepted Manuscript

Journal Name:

Geochimica et Cosmochimica Acta

Additional Journal Information:

Journal Volume: 175; Journal Issue: C; Journal ID: ISSN 0016-7037

Publisher:

The Geochemical Society; The Meteoritical Society

Country of Publication:

United States

Language:

English

Subject:

58 GEOSCIENCES; 79 ASTRONOMY AND ASTROPHYSICS; SM-146 HALF-LIFE; MAGMA OCEAN; CORE FORMATION; RB-SR; DIFFERENTIATION HISTORY; SOLAR-SYSTEM; FRACTIONAL CRYSTALLIZATION; MANTLE DIFFERENTIATION; HEMISPHERIC DICHOTOMY; SNC METEORITES