GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 284-9
Presentation Time: 3:55 PM


HOBART, Catherine E., Geology and Geophysics, Texas A&M University, 400 Bizzell St., College Station, TX 77843

Diogenites, which include cumulate orthopyroxenites, harzburgites, and a dunite, come from the howardite, eucrite, diogenite (HED) suite of meteorites, thought to originate from asteroid 4 Vesta [1]. While most are brecciated, a few unbrecciated diogenites exist, including MIL 07001 and GRA 98108 [2]. MIL 07001, the sample analyzed here, is a diogenite with mm- to cm- sized orthopyroxenes enclosing olivine grains. It has an igneous texture and is believed to contain a trapped component of its parent melt [3]. GRA 98108 is a diogenite with a similar texture, but with distinct trace element chemistry, suggesting important differences between the samples.

We previously analyzed minor element abundances in GRA 98108 by electron microprobe (EPMA), including spot analyses and high-current maps over 1-mm square areas [4-5]. Previous work on MIL 07001 characterized the trapped melt component; here, we examine a different sample by EPMA and focus on the melt-free zones to allow textural comparison between the two unbrecciated diogenites.

Our analysis shows some details previously seen in MIL 07001 and several new characteristics. As with preceding work, we found high Al and Cr on one grain boundary, consistent with trapped melt, and a cross-hatched orthopyroxene [6]. However, we also identified a grain boundary that is depleted in both Al and Cr, unexpected if the high Al and Cr boundaries are contaminated by the last crystalized melt. This Al and Cr depleted grain boundary resembles those in GRA 98108, which is not thought to contain a similar trapped melt component.

Instead of a trapped melt, the Al and Cr depleted grain boundaries in GRA 98108 connect to chromite grains that may have taken up these elements via subsolidus diffusion, possibly indicating that this process occurred in MIL 07001 as well. Also, we find that MIL 07001 shows differences in major element composition at pyroxene grain boundaries, while GRA 98108 does not.

We will present trace element measurements of MIL 07001 and GRA 98108 to further this comparison.

[1] McSween et al. (2010) Space Sci. Rev. 163, 141-174. [2] Mittlefehldt et al (2013). [3] Mittlefehldt et al. (2014) [4] Balta et al. (2011), LPSC42, #1107. [5] Milman-Baris et al. (2008), CMP 155. [6] Mittlefehldt (2015) Chem. der Erde. 75, 155-183.