Ca-Al-rich inclusions (CAIs) are the earliest formed crystalline material in our solar system and they record early Solar System processes. Here we present petrographic and delta Mg-25 data of melilite mantles in a Type B1 CAI that records early solar nebular processes

Andreasen, R.

Jeffcoat, C. R.

Kerekgyarto, A. G.

Lapen, T. J.

Righter, M.

Ross, D. K.

NASA Technical Reports Server

STABLE MAGNESIUM ISOTOPE VARIATION IN MELILITE MANTLE OF ALLENDE TYPE B1 CAI 
EK 459-5-1.  A. G. Kerekgyarto1, C. R. Jeffcoat1, T. J. Lapen1, R. Andreasen1, M. Righter1, D. K. Ross2 
1Department of Earth and Atmospheric Sciences, University of Houston, Houston Tx 77204 
(agkerekgyarto@uh.edu), 2Jacobs, NASA-JSC, Houston Tx 77058 
 
 
Introduction:  Ca-Al-rich inclusions (CAIs) are 
the earliest formed crystalline material in our solar 
system [1] and they record early Solar System process-
es. Here we present petrographic and δ25Mg data of 
melilite mantles in a Type B1 CAI that records early 
solar nebular processes. 
Petrography and Major Element Chemistry: EK 
459-5-1 is a type B1 CAI measuring roughly 13.5 by 
7.5mm, on loan from the Houston Museum of Natural 
Science. It is characterized by a spinel-fassaite-
anorthite-melilite bearing core, with a ~2mm mantle of 
zoned melilite, indicative of the B1 classification [2]. 
Minor occurrence of 1-5 μm grains of fassaite and spi-
nel in the melilite mantle. Distinct spinel-free islands 
(SFI) within the core (Fig. 1a) are Na-enriched, as ob-
served in [3]. Interior fassaite shows subtle sector zon-
ing as well as a sharp ‘chaotic’ [4] zoning. The melilite 
mantle occurs in solid solution between gehlenite 
(Ca2Al2SiO7) and åkermanite (Ca2MgSi2O7). The man-
tle has two concentric textural types. An outer section 
is texturally massive and chemically zoned and an in-
ner section that contains 3 or 4 bands of Mg, Al and Si 
oscillatory chemical zoning in the melilite (Fig. 1c). 
These bands typically contain small fassaite grains in 
alignment with the bands. Between the mantle and 
core, there is a sharp change in melilite composition 
and the occurrence of additional phases (Fig. 1c). 
Along the core boundary and boarders of SFIs, spinel 
exists in linear agglomerations of an almost ‘wall-like’ 
texture. Preliminary 26Al-26Mg data indicates that this 
CAI formed while 26Al was extant.  
Methodology: Initial petrographic work was done 
at JSC through ARES laboratories. X-ray elemental 
maps were produced on a JEOL JSM-7600F SEM and 
major element data was collected with a Cameca SX-
100 electron microprobe. In-Situ isotope studies have 
been done on a Nu Plasma II MC-ICP-MS and Analyte 
193nm excimer laser system. The laser was operated 
with a spot size of 50 µm, 10 Hz rep rate, and a fluence 
of 2.99 J/cm2. Natural gehlenite standards were used to 
correct for instrumental isotope fractionation. Data is 
reported in δ notation relative to DSM3 [5].  
Results: Both line scans show a positive enrich-
ment in δ25Mg, expected from early evaporative pro-
cesses [6], and a sharp rim ward decrease in δ25Mg 
(Fig. 2). No definitive correlation can be made be-
tween Åk# and δ25Mg. Both vary as a function of 
     Figure 1. False colored qualitative 3 element x-ray maps. 
All labels are in RGB order (ex: MgCaAl; Mg-Red, Ca-
Green, Al-Blue). (a) The entire studied grain, EK 459-5-1. 
(b) Both line scan transects across the melilite mantle, white 
line: EMPA transect, white open circles: trace element abla-
tion pits, blue/red open circles: isotope spot locations. All 
open circles are 50 µm in diameter. Line scan localities rela-
tive to the whole grain can be seen in [7]. (c) X-ray map 
showing transitions between massive mantle, oscillatory 
zone and core-mantle boundary. Notice color change in 
melilite (darker, more åkermanitic melilite) as well as trunca-
tion of chlorine (blue) and Ti zoning in fassaite (edge of core 
vs. interior).  
 
rim-core position so there is a net positive, core-ward 
trend to both. The variation seen from rim to core is 
~Åk42 to Åk72 and ~Åk46 to Åk71 for MML1 and 
MML2, respectively. There is a 1‰ increase in δ25Mg 
from rim to core for both line scans. Additionally, both 
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line scans exhibit a notable change in δ25Mg between 
the massive and the oscillatory zones of the mantle 
(Fig. 2, 3) but no measureable difference between the 
core and mantle melilite. Both line scans show a steady 
decrease in δ25Mg from the oscillatory zone to the 
massive zone. The massive section of the mantle 
shows variable major element patterns (seen as Åk# in 
Fig. 2) as well as δ25Mg. Trace element variations also 
exist between oscillatory zoned and massive sections, 
although not consistent between line scans. The trace 
element abundances are discussed in [7].  
 
Figure 2 δ25Mg and Åk# plotted vs. distance from rim. 
This is the complete transverse for both line scans. Cracks 
and obvious sections of alteration are indicated. The transi-
tions between core, oscillatory zone and massive section are 
indicated with a dashed blue line. All error is reported as 2σ, 
much of the error is smaller than the data points.  
 
Discussion: The sharp decrease in δ25Mg from ~7-
8 to 1 ‰ towards the rim is likely a late stage event 
signified by a disturbance in 26Al-26Mg systematics 
(loss of 26Mg*) but this is not well constrained. Our 
data is consistent with previous hypotheses [8, 9] that 
this event seems to mark a period of open system iso-
topic exchange with an adjacent reservoir of chondritic 
value, at least with regards to δ25Mg.  
Crosscutting relationships imply that the core of 
this CAI had to have been in place before the for-
mation of the present mantle as evidenced by trunca-
tion of Cl rich zones in the core lithology (Fig. 1c). A 
normally zoned outward-in crystallization sequence 
due to localized surficial volatile depletion as present-
ed in [10] does not seem to be the likely formation 
mechanism for EK 459-5-1. The core of EK 459-5-1 
persisted through the melting event[s] which formed 
the mantle. The isotopic profile presented here does 
not agree with a complete outside-in crystallization 
model, as initial surficial melilite crystallization would 
render the CAI a closed system yielding no mechanism 
for core-ward enrichment in δ25Mg [9]. 
 
Figure 3. Line scans with rim-ward data removed (and 
the points lying on interior cracks in MML1) to further inter-
pret trends and present the resolution of the data. The core 
melilite δ25Mg values are essentially the same as the first 
onset of the mantle. In MML2 the oscillatory zone and the 
core show a flat δ25Mg signature. The weighted mean of 
these sections was calculated ignoring the two data points 
which fell on cracks. MML1 on the other hand shows an 
almost constant decrease with a definite ‘step’ once the mas-
sive part of the mantle was reached. 
 
δ25Mg of the melilite in the core is essentially the 
same as the inner melilite mantle, indicating the same 
(or similar) source but from a different event as dis-
cussed above.  The δ25Mg profile of MML2 (Fig. 3) 
appears to vary as a step function correlating to the 
separate petrologic zones of the mantle. This could be 
a record of multiple partial remelting events interacting 
with different reservoirs of δ25Mg or entirely different 
material altogether. This profile is not seen in MML1, 
although the variation (and subsequent decrease) be-
tween mantle zones is. It is clear that distinct petro-
logic, chemical and isotopic zones exist within this 
CAI, correlating to a diverse, multi-stage history. 
 References: [1] Bouvier A. and Wadhwa M. (2010) 
Nature Geosci, 3, 637-641. [2] Wark D. A. and Lovering J. 
F. (1982) GCA, 46, 2581-2594. [3] El Goresy A. et al. (1985) 
GCA, 47, 1635-1650. [4] Meeker G. P. (1995) MAPS, 30, 71-
84. [5] Young E. D. and Galy A. (2004) RIMG, 55, 197-230. 
[6] Davis A. M. et al. (1990) Nature, 347, 655-658. [7] 
Jeffcoat C. R. et al. (2014) LPS XLV Abstract #2523. [8] 
Simon J. I. et al. (2005) EPSL, 238, 272-283. [9] Simon J. I. 
and Young E. D. (2011) EPSL, 304, 468-482. [10] 
Mendybaev R. A. (2006) GCA, 70, 2622-2642. [11] Richter 
F. M. et al. (2006) MAPS, 41, 83-93. 


Stable Magnesium Isotope Variation in Melilite Mantle of Allende Type B1 CAI EK 459-5-1

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