Fwd: SEMINAR: GEOTOPICS - The Moon is a Harsh Mistress

[Andrew Jo <ajo@rsmas.miami.edu>]

The Moon is a Harsh Mistress

Speaker: Dr. James Natland

(RSMAS - MGG) 

 

Monday, November 5th, 2012

3:30, SLAB, Library Map and Chart Room

Refreshments at 3:15 PM

 

ABSTRACT

Initial studies of lunar samples returned from the Apollo and Luna missions took place before rocks of the Earth’s lower ocean crust, chiefly varieties of gabbro cumulates (assemblages of minerals that separated and were concentrated from melts), were widely known or understood. Continuing exploration of the ocean crust invites some new comparisons. When volcanic rocks and glass from Apollo 11 and 17 were discovered to have very high TiO₂ contents (8-14%), nothing comparable was known from Earth. The high-TiO₂ lunar samples were soon described as primary melts, termed pristine, that were derived from considerable depths in the lunar mantle. Other lunar samples have only very low TiO₂ contents (~0.2%) and very low concentrations of highly incompatible elements such as Zr and Sr. Today, dredging and drilling results indicate that oxide gabbros rich in magmatic oxides and sulfides and with up to 12% TiO₂ comprise a significant percentage of the gabbroic portion of the ocean crust especially at slowly spreading ridges. These are very late stage differentiates, and are commonly juxtaposed by high-temperature deformation processes with more primitive olivine gabbro cumulates and troctolites having only ~0.2% TiO₂ and low concentrations of Zr and other incompatible elements. The rocks have very low concentrations of incompatible elements set by proportions of separated (cumulus) minerals, and with little contribution from the liquids that produced them. In addition, some lunar gabbros with highly calcic plagioclase are similar to gabbros and troctolites found in island arcs. All of these similarities suggest that very few lunar basaltic rocks are pristine; instead they all could be nearly complete shock fusion products produced by meteorite impact prior to 3.5 Ga into a diverse assemblage of lunar gabbros that included both low- and high-TiO₂ gabbroic rocks. Although magmatic environments on the ancient Moon and in the modern ocean crust were different in important ways, the general course of basaltic differentiation must have been similar; the chief difference was the importance of deep impact melting on the ancient Moon that rendered gabbros into melts, and its absence in the present-day ocean crust. Aspects of lunar basaltic geochemistry, particularly the origin of both positive and negative Eu anomalies in gabbros and basalts, that I shall do my best to explain, need reconsideration in light of this possibility. 

 

Hope to see you all there!

GEOTOPICS coordinators,

Mandy Mulcan and Andrew Jo