Application of a textural geospeedometer to the late‐stage magmatic history of MIL 03346

We present a geochemical study of recently discovered Martian meteorite Northwest Africa (NWA) 5790 and use our results to constrain its origin and relationship with the other nakhlites. This nakhlite is a clinopyroxene cumulate composed of phenocrysts of augite, olivine, and rare oxides surrounded by a mesostasis composed of vitrophyric glass, feldspars, oxides, phosphates, and fine‐grained olivines and augite. Petrography, and major and trace element compositions of the phases present are consistent with derivation of NWA 5790 from a parental magma common to all the nakhlites. Olivine cores grew from a distinct, incompatible‐element enriched magma and are surrounded by rims containing augite inclusions that grew from the nakhlite parental liquid, supporting previous arguments for xenocrystic olivine cores in nakhlites. Rare earth element microdistributions suggest derivation of NWA 5790 augites from an evolved, relatively oxidized magma, produced by augite fractionation from the common nakhlite parental liquid. Augite grain shapes and CSD patterns are consistent with rapid cooling and derivation near the top of the nakhlite cumulate pile, but patterns are distinct from other nakhlites thought to have formed near the stratigraphic top. The high mesostasis abundance (~44 vol%) indicates solidification near the top of the nakhlite pile close to locations suggested for nakhlites NWA 817 and Miller Range (MIL) 03346. However, the geochemical and petrographic characteristics of these three samples do not permit their placement in a simple stratigraphic order as would occur in a single lava flow. This lack of simple ordering suggests that the nakhlite lava flow split into multiple sections as would occur during breakouts from a single lava flow. Finally we note that NWA 5790 is unique among currently available nakhlites in having phenocryst abundances low enough to allow it to flow.

Section: Discussionmentioning

confidence: 99%

Northwest Africa 5790: A previously unsampled portion of the upper part of the nakhlite pile

Balta

Sanborn

Mayne

et al. 2016

“…The surface area per unit volume (

S_{v}^{P}

) for pyroxene populations was determined using standard stereological image analysis techniques and detailed methodology of Hammer (). Measurements of

S_{v}^{P}

were used partly because they do not require knowledge of nucleation density.…”

Section: Methodsmentioning

confidence: 99%

“…, ), crystal composition (e.g., Mollo et al. ), crystal composition and qualitative sample and crystal texture (e.g., McCoy and Lofgren ), quantitative crystal texture (e.g., Lentz and McSween ; Hammer ), and chemical diffusion rates (e.g., Miyamoto et al. , ).…”

Section: Introductionmentioning

confidence: 99%

Igneous cooling history of olivine‐phyric shergottite Yamato 980459 constrained by dynamic crystallization experiments

First

Hammer

2016

Abstract-Dynamic crystallization experiments were performed on a liquid having the bulk composition of olivine-phyric shergottite Yamato 980459, to constrain the igneous thermal history of this meteorite. Key characteristics of the meteorite's mineralogy and texture, including several morphologically distinct olivine and pyroxene crystal populations and a glassy mesostasis devoid of plagioclase, were replicated upon cooling from 1435 to 909°C at 1 atmosphere under reducing conditions. Three sequential cooling ramps are required to produce synthetic samples with textures and compositions matching Yamato 980459. Olivine phenocrysts formed at <1°C h À1 , presumably at depth in the Martian crust. Pyroxene phenocrysts formed mainly at~10°C h À1 , consistent with crystallization within a lava flow at depths of 25-45 cm. Increased cooling rate (~100°C h À1 ) in a third stage suppressed the formation of plagioclase and produced groundmass crystals, consistent with crystallization at lava flow depths of 5-7 cm. Although Y 980459 is unique among Martian meteorites (i.e., preserving a primary glassy mesostasis), its emplacement did not require unique physical conditions. Rather, the second and third cooling stages may reflect cooling within the core of a p ahoehoe-like flow and subsequent breakout on the surface of Mars.

“…Based on these variations, it has been suggested that the nakhlites solidified over a range of depths in a single shallow intrusion or a series of lava flows that were emplaced nearly simultaneously (Mikouchi and Miyamoto ; Mikouchi et al. , , ; Bridges and Warren ; Hammer ). A single parent magma, or a series of closely related parent magmas, for all nakhlites is also consistent with their common crystallization age of approximately 1.3 Gyr (Nyquist et al.…”

Section: Introductionmentioning

confidence: 99%

K₂O‐rich trapped melt in olivine in the Nakhla meteorite: Implications for petrogenesis of nakhlites and evolution of the Martian mantle

Goodrich

Treiman

Filiberto

et al. 2013

Abstract-We used new analytical and theoretical methods to determine the major and minor element compositions of the primary trapped liquid (PTLs) represented by melt inclusions in olivine and augite in the Martian clinopyroxenite, Nakhla, for comparison with previously proposed compositions for the Nakhla (or nakhlite) parent magma. We particularly focused on obtaining accurate K 2 O contents, and on testing whether high K 2 O contents and K 2 O/ Na 2 O ratios obtained in previous studies of melt inclusions in olivine in Nakhla could have been due to unrepresentative sampling, systematic errors arising from electron microprobe techniques, late alteration of the inclusions, and/or boundary layer effects. Based on analyses of 35 melt inclusions in olivine cores, the PTL in olivine, PTL oliv , contained (by wt) approximately 47% SiO 2 , 6.3% Al 2 O 3 , 9.6% CaO, 1.8% K 2 O, and 0.9% Na 2 O, with K 2 O/ Na 2 O = 2.0. We infer that the high K 2 O content of PTL oliv is not due to boundary layer effects and represents a real property of the melt from which the host olivine crystallized. This melt was cosaturated with olivine and augite. Its mg# is model-dependent and is constrained only to be ≥19 (equilibrium Fo = 40). Based on analyses of 91 melt inclusions in augite cores, the PTL in augite, PTL aug , contained (by wt) 53-54% SiO 2 , 7-8% Al 2 O 3 , 0.8-1.1% K 2 O, and 1.1-1.4% Na 2 O, with K 2 O/Na 2 O = 0.7-0.8. This K 2 O content and K 2 O/Na 2 O ratio are significantly higher than inferred in studies of melt inclusions in augite in Nakhla by experimental rehomogenization. PTL aug was saturated only with augite, and in equilibrium with augite cores of mg# 62. PTL aug represents the Nakhla parent magma, and does not evolve to PTL oliv by fractional crystallization. We therefore conclude that olivine cores in Nakhla (and, by extension, other nakhlites) are xenocrystic. We propose that PTL oliv and PTL aug were generated from the same source region. PTL oliv was generated first and emplaced to form olivine-rich cumulate rocks. Shortly thereafter, PTL aug was generated and ascended through these olivine-rich cumulates, incorporating fragments of wallrock that became the xenocrystic olivine cores in Nakhla. The Nakhla (nakhlite) mantle source region was pyroxenitic with some olivine, and could have become enriched in K relative to Na via metasomatism. A high degree of melting of this source produced the silica-poor, alkali-rich magma PTL oliv . Further ascension and decompression of the source led to generation of the silica-rich, relatively alkali-poor magma PTL aug . Potassium-rich magmas like those involved in the formation of the nakhlites represent an important part of the diversity of Martian igneous rocks.