Calcium, Aluminum-rich Inclusions (CAIs) were the first formed solids in our Solar System, with mineral assemblages reflecting the first phases predicted to condense out of a hot nebular gas of Solar composition. Geochemical, textural and crystallographic information in CAIs can be used to constrain the temperature, pressure, and composition (e.g., oxygen fugacity) of the gaseous reservoir(s) from which they formed, as well as any secondary (nebular and parent body) processes they underwent. Coordinated geochemical and textural analyses provide information on nebular conditions (i.e., astrophysical environments and dynamics of nebular gas reservoirs) in which these CAIs formed. In order to better understand the evolution of nebular reservoirs at the time of CAI formation, we analyzed a Type A, B and C CAI using Electron Probe Micro-Analyzer (EPMA) and Electron BackScatter Diffraction (EBSD) at NASA Johnson Space Center (JSC)

Armytage, R. M. G.

Comins, M. B.

Erickson, T. M.

Mane, P.

Ross, D. K.

Simon, J. I.

English

NASA Technical Reports Server

CAPTURING AN EVOLVING NEBULAR ENVIRONMENT: A PETROGRAPHIC AND GEOCHEMICAL 
STUDY OF A TYPE A, B & C CAI.  M. B. Comins1,2, P. Mane2,3, J. I. Simon3, R.M.G. Armytage,4, D. K. Ross4,5, 
T. M. Erickson4 1Dept. of Geosciences and Natural Resources, Western Carolina University, Cullowhee, NC 28723 
(mcomins1@catamount.wcu.edu), 2Lunar and Planetary Institute, Universities Space Research Association, Houston, 
TX 77058, 3Center for Isotope Cosmochemistry & Geochronology, NASA Johnson Space Center, Houston, TX 
77058, 4Jacobs/JETS, NASA Johnson Space Center, Houston, TX 77058, 5UTEP-CASSMAR, El Paso TX, USA. 
 
 
Introduction:  Calcium, Aluminum-rich Inclusions 
(CAIs) were the first formed solids in our Solar System 
[1], with mineral assemblages  reflecting the first phases 
predicted to condense out of a hot nebular gas of Solar 
composition [2]. Geochemical, textural and crystallo-
graphic information in CAIs can be used to constrain the 
temperature, pressure, and composition (e.g., oxygen 
fugacity) of the gaseous reservoir(s) from which they 
formed, as well as any secondary (nebular and parent 
body) processes they underwent. Coordinated geochem-
ical and textural analyses provide information on nebu-
lar conditions (i.e., astrophysical environments and dy-
namics of nebular gas reservoirs) in which these CAIs 
formed. In order to better understand the evolution of 
nebular reservoirs at the time of CAI formation, we an-
alyzed a Type A, B and C CAI using Electron Probe 
Micro-Analyzer (EPMA) and Electron BackScatter Dif-
fraction (EBSD) at NASA Johnson Space Center (JSC). 
Samples and Analytical Methods: Our samples 
were selected because of their diverse attributes, includ-
ing grain size, texture and mineralogy (Figure 1). Leo-
ville (CV3red) Senita is a ~2 x 1 mm, fine-grained Type 
A CAI with spinel, melilite, hibonite and perovskite 
(Fig. 1a.). Allende EK5-1-1 (cm-sized) is coarser-
grained, characteristic of an igneous Type B1 CAI [4], 
containing > 50% melilite—some of which exhibits 
zoning (Fig. 1b.). Also present is anorthite, Al, Ti-rich 
pyroxene, spinel, minor perovskite and alteration phases 
including metal-rich sulfides, sodalite and nepheline. 
Allende (CV3ox) EK5-3B is a fine-grained, ~6 x 1.5 mm 
CAI, consisting mainly of spinel, diopside, anorthite (in 
order of decreasing abundance) and minor perovskite 
(Fig. 1c.). Alteration phases (e.g., nepheline, sodalite, 
Fe-rich pyroxene) are also present in EK5-3B. 
At NASA JSC, a JEOL JXA 8530F Hyperprobe 
EPMA with a 15 kV beam energy and 30 nA beam cur-
rent was used to determine chemical compositions. 
Wavelength dispersive spectroscopy (WDS) spot-anal-
yses were performed on individual phases. Energy dis-
persive spectroscopy (EDS) maps were created, also us-
ing EPMA. Quantitative chemical analyses were ex-
tracted from individual EDS map frames and averaged 
for each CAI to determine bulk composition. Valence 
state for Ti in pyroxene, used to determine oxygen fu-
gacity, was calculated by applying a Ti-V overlap cor-
rection to the EPMA data and then determining its struc-
tural formula. The resulting cation deficiency (indicat-
ing the presence of Ti3+) is divided by total Ti to quan-
tify Ti3+, similar to [3].  
An Oxford Symmetry EBSD detector on a JEOL 
JSM 7600F scanning electron microscope at NASA JSC 
was used to quantify crystallographic orientation rela-
tionships (CORs) of the samples in order to determine 
crystallization sequences and genetic relationships 
among grains. A 20 kV beam energy and 9 nA current 
was used to collect rastered maps with step sizes ranging 
from 0.1 to 0.25 µm. 
Results: Leoville Senita has a bulk composition 
that resembles a Type A CAI. Senita has Åk-poor 
(~Åk3) melilite with grain sizes ranging from 10-50 µm. 
Spinel is Mg end-member that varies from 1-100 µm in 
size. Senita contains hibonite-rich areas enclosed by 
perovskite and spinel grains. Perovskite exists as 10-20 
µm-sized grains and hibonite is upwards of 50-100 µm 
in this CAI. Grain orientations in Senita have a random 
distribution and exhibit triple junctions. Hibonite exhib-
its twin lamellae. Individual spinel grains bear evidence 
of crystal-plastic strain  (Figure 2). The spinel grains 
Figure 2. [left] Inverse pole figure (IPF) map of spinel grains 
from Senita, where gradual color transitions represent intragrain 
crystal-plastic strain resulting from a post-crystallization defor-
mation event. [right] IPF key, indicating which of spinel’s axes in 
the IPF map are pointed towards the EBSD camera. 
Figure 1. MgCaAl (RGB) false-color electron backscatter maps 
taken with EPMA. a.) Leoville Senita (Type A) b.) Allende EK5-
1-1 (Type B) c.) Allende EK5-3B (Type C). 
 
https://ntrs.nasa.gov/search.jsp?R=20200001951 2020-05-24T04:14:42+00:00Z
show a maximum grain orientation spread up to 70 and 
major slip along [100] crystallographic plane.   
Allende EK5-1-1 has a bulk composition that is 
fairly representative of a Type B1 CAI. This inclusion 
has a poikilitic texture in which melilite contains 
anorthite, pyroxene and spinel as chadacrysts. Melilite 
is zoned in EK5-1-1, with the strongest zonation 
occuring across its mantle. WDS analyses range from 
Åk55 in the core to Åk5 in the rim. 
EK5-1-1 also contains compositionally zoned Al,Ti-
rich pyroxene, with grains 100s of µm in size. Enrich-
ments in Ti occur towards the core (as evidenced by 
WDS spot analysis and EDS mapping). Scandium and 
V display a similar trend as Ti, as expected given their 
similar chemical behaviors. Trivalent Ti exhibits an in-
verse trend, becoming enriched towards the rim of the 
pyroxene grains. 
Anorthite grains in EK5-1-1 are ~100 µm in size. 
Spinel is Mg end-member with variable grain sizes from 
1-100 µm. Allende EK5-1-1 contains secondary Fe,Ni-
rich sulfides, unique to this inclusion relative to the 
other two studied. Allende EK5-1-1 exhibits epitaxial 
growth among its pyroxene, anorthite and melilite as 
demonstrated by preferred orientations within grains. 
Allende EK5-3B most closely resembles a Type C 
(anorthite-rich, melilite poor) CAI in terms of its miner-
alogy and bulk composition, and has a fine-grained, lacy 
texture, also described in [5]. Pyroxene composition is 
close to end-member diopside and occurs as irregular 
shapes with variable thicknesses in the core (1-10 µm) 
of the inclusion versus the rim (10-20 µm). Plagioclase 
grains are An100 and ~10 µm wide, generally located be-
tween spinel and diopside. Perovskite occurs as ≤ 1 µm 
grains, often in association with spinel (generally 10-20 
µm). Alteration phases rich in Fe, Na and Cl are distrib-
uted throughout the inclusion. Grains in EK5-3B exhib-
ited random crystallographic orientations. 
Discussion: Leoville Senita has a lack of porosity 
based on totals from EDS mapping; they are ~100% in 
Senita, whereas they are << 100% in EK5-3B. Triple 
junctions and deformation suggest annealing and solid-
state recrystallization during a post-formation thermal 
event. Thus, while we observe no compositional zoning, 
it is unclear whether this CAI records primary conden-
sation because of the possibility of re-equilibration. A 
possible explanation for transient heating lies in parent 
body metamorphism or shock resulting from the out-
ward radial transport of grains that formed near the pro-
tosun [6]. Temperatures must have been maintained be-
low spinel’s solidus at 1513 K [2]. 
Allende EK5-1-1: The coarse grain size and zona-
tion within melilite and pyroxene suggest that EK5-1-1 
crystallized from a melt. It has been shown that precur-
sors to Type B1 CAIs may have undergone nebular 
shock, resulting in pressures of at least 10-4 bars (a min-
imum requirement for the formation of melilite mantles) 
and temperatures > 1790 K (the liquidus of Ak5) that 
began to subside within hours [8]. Once the melilite 
mantle in EK5-1-1 crystallized below 1773 K, evapora-
tive loss of Mg and Si would have occurred. As evi-
denced by the Mg-rich core of EK5-1-1, further Mg loss 
would not have occurred after the formation of the man-
tle. In order to preserve compositional zoning, tempera-
tures could not have been maintained above 1273 K for 
more than 104 years [7].  
The increase in Ti3+ towards the rim of the pyroxene 
grain analyzed in EK5-1-1 suggests a change in the re-
dox conditions towards a more reducing environment. It 
is possible that the Ti3+ enrichment observed at the rim 
is due to the breakdown of a Ti,O-rich phase (e.g., per-
ovskite) that became incorporated along the margins of 
pyroxene grains [3]. It would do well to analyze more 
pyroxene grains for Ti3+ within this inclusion to further 
substantiate this hypothesis. 
EK5-3B is thought to be a “layered” condensate, 
given its small grain size (5-20 µm in the interior and 
20-50 µm towards the margin) and higher porosity. 
Random crystallographic orientations and lack of zon-
ing implies that grains in EK5-3B were in equilibrium 
with the gas from which they formed. The mineralogical 
assemblage of this CAI suggests that it formed at the 
lowest temperatures of the three. EK5-3B bears evi-
dence of evolving conditions during formation, given 
the mid-inclusion transition from a fine-grained, spinel-
rich (lower-temperature phase) interior to a slightly 
coarser-grained and pyroxene-rich mantle. It would do 
well to look for phases which may have been replaced 
by thermal processing in the Solar nebula. 
Conclusion: The inclusions presented in this study 
contain evidence of a rapidly changing nebular environ-
ment. We observe more phases with decreasing refrac-
tory compositions in our Type A, B and C CAIs, respec-
tively. Leoville Senita provides evidence of a transient 
heating event occurring below 1700 K, while EK5-1-1 
indicates a separate heating event with temperatures 
maintained above 1790 K. EK5-3B condensed from a 
cooler and likely more evolved nebular gas, and may 
have been transported to another gaseous reservoir. 
Trace element analysis could further address crystalli-
zation sequences and/or nonextant phases. 
References: [1] Amelin Y. et al. (2010) EPSL 
300(3-4), 343-350.  [2] Grossman L. (1972) GCA, 36, 
597-619. [3] Simon J.I. et al. (2005) EPSL, 238, 272-
283. [4] Brearly A.J. and Jones R.H. (1998) Chondritic 
Meteorites in Planetary Materials, ed. Papike J.J., 83-
124.  [5] Wark D.A. (1987) GCA, 51, 221-242. [6] Sears 
D.W.G. (1988) Vistas in Astronomy, 32, 1-21. [7] Rich-
ter F.M. et al (2006) M&PS, 41, 83-93. 


Capturing an Evolving Nebular Environment: A Petrographic and Geochemical Study of a Type A, B & C CAI

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