52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

1095.pdf

CONSTRAINTS ON THE ICE COMPOSITION OF CARBONACEOUS CHONDRITES N. Sridhar1, J. F. J.
Bryson1, A. J. King2, R. J. Harrison3, 1Department of Earth Sciences, University of Oxford, 2Natural History Museum, London, 3Department of Earth Sciences, University of Cambridge.
a)

Grade
1.1

1.0

1.2

EM1

Bu (T)

0.1

CM chondrites
CI chondrites

10.0

EM30.0V

0.0-10140.0-0.1

PC1 (x10 )Bc (T)

EM20.1-1-0.1
0.00.3

Bc (T)0Bu (T)Bu (T)

0.1

SD

IntV/MD4

PC2 (x10 )

-0.1

0.0

0.3

0.3

Bc (T)

Gradeb)

1.4

1.5

1.6

1.7

Bu (T)CM chondrites
Ungrouped C2chondrites

0.0-0.1

SD

0.00

4

PC4 (x10 )0.5

MD-1.0
-1.0

0.00

0.25
Bc (T)-0.5

0.0
4
PC3 (x10 )EM60.1
Bu (T)

IntSD/V

0.0-0.1

0.25
Bc (T)

0.0

EM5 -0.5

0.1

EM40.1

1.0

Bu (T)

Introduction: Carbonaceous chondrites experienced
various extents of aqueous alteration on their parent
asteroids [1-4]. This event was crucial in their history,affecting their textural, mineralogical, and isotopic
signatures. Despite its importance, the mechanisms andconditions of chondrite aqueous alteration are notcompletely understood.
During aqueous alteration, a variety of secondary
minerals were introduced to the carbonaceous chondrites, including phyllosilicates, carbonates, magnetite,and Fe-sulfides [1]. The abundance and morphology ofmagnetite differs among these meteorites, possiblyreflecting changes in the chemistry and/or conditionsof aqueous alteration experienced by their parent asteroids [5-7]. Variation in magnetite abundance and morphology can impact the bulk magnetic properties ofthese meteorites, such that magnetic measurements canprovide a means of exploring their aqueous histories.
A novel and informative set of bulk magnetic measurements are first order reversal curve (FORC) diagrams. These diagrams can be used to recover the size,
morphology and magnetic domain state of the magnetic grains present [8]. As these properties depend on theprocess by which magnetite formed, FORC diagrams
provide a unique tool for investigating the different
pathways of aqueous alteration experienced by carbonaceous chondrites.
Methods: We measured FORC diagrams of: 2 CO
chondrite powders (petrological type 3.0 and 3.1 [7]);2 CI chondrite powders (petrological type 1.0); 23 CM
chondrite powders (petrological type 1.1-1.7 based on
phyllosilicate fraction [4]), and two chips of the ungrouped C2 chondrite WIS 91600 (petrological type~1.4). We also performed bulk magnetic measurementson ~20 mg powder samples of each chondrite to determine the modal volume percent of magnetite
present. These measurements were conducted using analternating gradient magnetometer at the University ofCambridge, and the FORC diagrams were processedusing the VARIFORC approach in the FORCinel software package [9,10]. We also conducted principal
component analysis (PCA) on the measured FORC
diagrams to identify the proportions of ideal end-member (EM) magnetite morphologies present in the CI,
CM and ungrouped C2 chondrites [11].
Results: CI and C2 chondrites: The FORC diagrams of the CI chondrites and WIS 91600 display atriangular signature that spreads vertically with a peakintensity off the origin, indicating the presence ofstrongly interacting magnetic grains (EM3 in Fig 1a).

0.5

1.0

0.0-0.1
0.00

0.25
Bc (T)

Fig 1a) Plot displaying proportions of PC1 and PC2
present in chondrites of grade 1.0-1.2, along with thepositions of the end-members. EM1 is representativeof a SD grain; EM2 is representative of a V grain;
EM3 is representative of interacting V/MD grains.
Fig 1b) Plot displaying proportions of PC3 and PC4
present in chondrites of grade 1.3-1.7, along with thepositions of the end-members. EM4 is representativeof a SD grain; EM5 is representative of MD grains;
EM6 is representative of interacting V/SD grains.
This is characteristic of tightly packed, <1µm magnetite grains [12], consistent with SEM observations ofof framboids and plaquettes of magnetite in thesechondrites [1,5]. Magnetite modal abundances in WIS
91600 and the CI chondrites range between 2-7 vol%.
CM chondrites: The FORC diagrams of the CM
chondrites display a range of patterns. The CM1.3-1.7
chondrites are broadly dominated by a signal that extends along the horizontal axis and displays minimalvertical spreading along the vertical axis (similar toEM1 in Fig 1a and EM4 Fig 1b). This is characteristicof <0.1µm isolated, non-interacting magnetite grains,with uniformly orientated magnetic spins [9]. The
FORC diagrams of the CM1.1-1.2 chondrites tend todisplay a tri-lobe signature similar to EM2 in Fig 1a.
This is characteristic of <0.1–5µm magnetite grains

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

that have their magnetic spins orientated into one ormore vortices [13]. Magnetite comprises ~0.2-3.5vol% of the CM chondrite samples.
CO chondrites: The FORC diagrams of the CO
chondrites show a tri-lobe pattern, characteristic of<0.1–5µm, vortex state (V) magnetite grains. The
FORC diagrams also display slight spreading along thevertical axis, indicative of weak magnetic interaction
between the grains. Magnetite makes up ~6-11 vol% of
the CO chondrites measured.
PCA: We conducted two sets of PCA: one on
chondrites with grades 1.0-1.2, and one on chondrites
with grades 1.3-1.7. Splitting the chondrites into twogroups based on their extent of aqueous alteration allowed for trends between the groups to be easily identified. The PCA shows that FORC diagrams of CI andC2 chondrites plot in distinct principal component
space compared to the CM chondrites (Fig1a andFig1b). This difference indicates that magnetite formedin WIS 91600 and the CI chondrites through a distinctly different process to that in CM and CO chondrites,
and could be the product of aqueous alteration under
different conditions or via different mechanisms.
Discussion: FORC diagrams, PCA, and SEM images indicate that the CI and C2 chondrites exhibit
magnetite with different morphologies to the CM andCO chondrites, arguing for at least two pathways of
aqueous alteration among these two families of meteorite groups. Previous studies have shown that the ungrouped C2 chondrite Tagish Lake shows a similar
FORC diagram to the CI chondrites [14], and contains
plaquettes and framboids of magnetite [15]. Both ofthese morphologies of magnetite are also present in CR
chondrites [16]. The presence of framboids and plaquettes is not due to preservation bias as there is no correlation between their occurrence and the weathering
grade of the chondrites examined. Ruling out terrestrial
processes, the presence of these unusual forms of magnetite must be related to their conditions of formation
during aqueous alteration on their parent bodies.
A number of factors could have played a role in theaqueous alteration pathway experienced by an individual chondrite, including: the extent of aqueous alteration [2,16]; the primitive water:rock ratio of the meteorite [1,17]; the degree of metamorphism [7,18]; theprimitive composition of metal and sulfides [7,15,16];
and/or the presence of organics [6]. By reviewing published data, we have ruled out systematic variation inthese factors as controls on the pathway of aqueousalteration.
The composition of the hydrous fluid in chondrite
parent bodies could also have affected their pathwaysof aqueous alteration. This composition will have
evolved as different phases reacted, however the initial1095.pdf

fluid composition will have been controlled by the
makeup of the ice accreted into the different parent
asteroids. As magnetite appears to have been one of thevery first phases to form during aqueous alteration[7,19,20], the properties of this mineral are more dependent on the composition of the initial ice accreted
rather than the evolved fluid composition. As we canrule out other factors, we argue that the ice accreted
into CO and CM chondrites differed in its composition
compared to CI, C2, and CR chondrites.
As temperatures decreased in the nebula, a numberof molecules are expected to have condensed onto water ice grains [21]. The first molecule that condenses
onto water ice at geochemically relevant quantities, isammonia [22]. We speculate that the presence of ammoniated ice could have affected the pathways of
aqueous alteration due to an initial change in alkalinity.
This could imply that the CI chondrites, Tagish Lake,WIS 91600 and the CR chondrites accreted in a colder
region of the solar system, past the ammonia ice line.
This is supported by recent work that links framboid
formation to alkaline conditions [23], and petrographic
and chemical studies that indicate similar formation
conditions for the CM and CO chondrites [24]. Given
constraints on parent body accretion ages, this argues
that the CI chondrites, Tagish Lake and WIS 91600originate from a more distal region, and that the ammonia ice line had migrated inwards by the time ofaccretion of the CR chondrite parent body.
References: [1] Brearley (2006) Meteorites and theEarly Solar System II. [2] King et al. (2015) GCA,165, 148-160. [3] Rubin et al. (2007) GCA, 71,2361-2382. [4] Howard et al. (2015) GCA, 149,206-222. [5] Hua & Buseck (1998) MAPS, 33, 215 220. [6] Alexander et al. (2017) Geochem., 77,227-256. [7] Rubin & Li (2019) Geochem., 79,125528. [8] Roberts et al. (2014) Revs. of Geophys, 52,557-602. [9] Egli (2013) Global Planet. Change, 110,302-320. [10] Harrison & Feinberg (2008) Geochem.
Geophys. Geosys., 9, Q05016. [11] Harrison et al.
(2018) Geochem. Geophys. Geosys., 19, 1595-1610.
[12] Muxworthy & Dunlop (2002) EPSL, 203,369-382. [13] Lascu et al. (2018) JGR: Solid Earth,123, 7285-7304. [14] Bryson et al. (2020) TAJ, 892.
[15] Greshake et al. (2005) MAPS, 40, 1413-1431. [16]Harju et al. (2014) GCA, 139, 267-292. [17] Marrocchiet al. (2018) EPSL, 482, 23-32. [18] Alexander et al.
(2013) GCA, 123, 244-260. [19] Davidson et al.
(2019a) GCA, 265, 259-278. [20] Davidson et al.
(2019b) GCA, 267, 240-256. [21] Dodson-Robinson etal. (2009) Icarus, 200, 672-693. [22] Collings et al.
(2004) MNRAS, 354, 1133-1140. [23] White et al.
(2020) PNAS, 117, 11217-11219. [24] Schrader &Davidson (2017) GCA, 214, 157-171.