Data from the Clementine and Prospector missions have shown that a simple model of lunar crustal stratigraphy in which an upper crust of ferroan anorthosite is underlain by a noritic lower crust is inadequate. The Moon shows a strong asymmetry in crustal thickness, composition, and distribution of KREEP and mare basalt. Some asymmetry may be primary, some may be a consequence of impacts focused on the nearside that have redistributed the upper layers, and some may be related to mantle overturn. In an effort to reconstruct the primary lunar stratigraphy, we are examining mineral and rock fragments in a subset of relatively unshocked pristine crustal samples to deduce thermal histories from which we can compute depths of burial. In this study we report on the cooling history of a ferroan noritic anorthosite clast from lunar breccia 67215 previously studied by Norman et al. [1], who determined the Sm-Nd age to be 4.4 ± 0.11 Ga with an initial εNd of +0.85 ± 0.53. The positive εNd is consistent with derivation of some ferroan anorthosites from a LREE-depleted source. The 39Ar-40Ar release profile is indicative of a degassing event ~3.9 Ga ago that incompletely degassed high temperature sites of 40Ar.
Sample 67215: an anomalous ferroan anorthosite
CAMARA ARTIGAS, Fernando;
2002-01-01
Abstract
Data from the Clementine and Prospector missions have shown that a simple model of lunar crustal stratigraphy in which an upper crust of ferroan anorthosite is underlain by a noritic lower crust is inadequate. The Moon shows a strong asymmetry in crustal thickness, composition, and distribution of KREEP and mare basalt. Some asymmetry may be primary, some may be a consequence of impacts focused on the nearside that have redistributed the upper layers, and some may be related to mantle overturn. In an effort to reconstruct the primary lunar stratigraphy, we are examining mineral and rock fragments in a subset of relatively unshocked pristine crustal samples to deduce thermal histories from which we can compute depths of burial. In this study we report on the cooling history of a ferroan noritic anorthosite clast from lunar breccia 67215 previously studied by Norman et al. [1], who determined the Sm-Nd age to be 4.4 ± 0.11 Ga with an initial εNd of +0.85 ± 0.53. The positive εNd is consistent with derivation of some ferroan anorthosites from a LREE-depleted source. The 39Ar-40Ar release profile is indicative of a degassing event ~3.9 Ga ago that incompletely degassed high temperature sites of 40Ar.
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