Meteorite paleomagnetism - From magnetic domains to planetary fields and core dynamos
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Geofísica Internacional (2014) 53-3: 343-363
Review
Meteorite paleomagnetism - From magnetic domains to planetary
fields and core dynamos
Jaime Urrutia-Fucugauchi*, Ligia Pérez-Cruz and Daniel Flores-Gutiérrez
Received: April 02, 2014; accepted: April 29, 2014; published on line: July 01, 2014
Resumen interacciones a escalas de dominio magnético.
En este trabajo revisamos los estudios en
Los meteoritos condríticos representan los cóndrulos del meteorito condrítico Allende,
registros más tempranos de la evolución del que revelan relaciones entre los parámetros
Sistema Solar, proveyendo información sobre magnéticos de histéresis y propiedades físicas.
las condiciones, procesos y cronología de la Los parámetros y cocientes de coercitividad,
formación de los primeros materiales sólidos, magnetización remanente y magnetización
planetesimales y cuerpos diferenciados. La de saturación muestran correlaciones con la
evidencia sobre los campos magnéticos en las densidad y tamaño de los cóndrulos, los cuales
etapas tempranas de evolución del sistema están relacionados a la estructura interna,
solar se ha obtenido a partir de estudios en mineralogía, composición y morfología. Los
meteoritos condríticos. Estos meteoritos se cóndrulos compuestos, fragmentados y con
caracterizan por la abundancia de cóndrulos, anillos de recubrimiento se caracterizan
que constituyen pequeñas esferas de silicatos por propiedades de histéresis magnética
de tamaño milimétrico, formadas a partir del distintas, asociadas a la composición y arreglos
polvo en la nebulosa y que fueron calentadas y mineralógicos y microestructuras. Los registros
enfriadas rápidamente. Los condrulos retienen de magnetización remanente y las estimacio-
un registro de magnetización remanente, que nes de paleointensidades derivadas en estudios
data del tiempo de calentamiento y enfriamiento del Allende y otras condrítas carbonáceas
durante la formación de cóndrulos y su acreción apoyan adquisición de la magnetización bajo
en planetesimales. Los estudios sobre las la influencia de campos magnéticos internos
diferentes clases de meteoritos, incluyendo dentro de planetesimales. Los resultados
a los meteoritos condríticos ordinarios y apoyan una rápida diferenciación, siguiendo la
los meteoritos condriticos carbonáceos han formación de las inclusiones de calcio y aluminio
documentado resultados contrastantes con una y condrulos para formar los planetesimales.
rango amplio de magnitudes de los campos Los planetesimales se caracterizarían por una
magnéticos en el disco protoplanetario. Ello estructura diferenciada con núcleos metálicos
ha dificultado definir la naturaleza de los con capacidad de dinamo autosustentable
campos magnéticos en las etapas iniciales por periodos de varios millones de años. El
de evolución. Los desarrollos recientes en meteorito condrítico Allende se formó y derivó
instrumentación y técnicas de análisis de de un planetesimal parcialmente diferenciado,
magnetismo de rocas y paleointensidades con un núcleo de hierro capaz de sostener un
permiten una mayor precisión. Los análisis de campo magnético interno.
micromagnetismo, geoquímica, petrografía y
microscopía electrónica proveen de una alta Palabras clave: Paleomagnetismo, meteoritos,
resolución, previamente no disponible, para campos magnéticos, dínamos, Sistema Solar.
caracterizar las propiedades magnéticas e
J. Urrutia-Fucugauchi* D. Flores-Gutiérrez
L. Pérez-Cruz Instituto de Astronomía
Programa Universitario de Perforaciones en Océanos Universidad Nacional Autónoma de México
y Continentes Coyoacán 04510
Instituto de Geofísica México D.F., México
Universidad Nacional Autónoma de México
Coyoacán 04510
México D.F., México
343
J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
Abstract the Allende meteorite that reveal relationships
among hysteresis parameters and physical
Meteorites represent the earliest records of the properties. Coercivity, remanent and saturation
evolution of the solar system, providing information remanence parameters correlate with chondrule
on the conditions, processes and chronology size and density; in turn related to internal
for formation of first solids, planetesimals and chondrule structure, mineralogy and morphology.
differentiated bodies. Evidence on the nature Compound, fragmented and rimmed chondrules
of magnetic fields in the early solar system show distinct hysteresis properties, related
has been derived from chondritic meteorites. to mineral composition and microstructures.
Chondrules, which are millimeter sized silicate The remanent magnetization record and
spherules formed by rapid melting and paleointensity estimates derived from the
cooling, have been shown to retain remanent Allende and other chondrites support remanent
magnetization records dating from the time of acquisition under influence of internal magnetic
chondrule formation and accretion of planetesimals. fields within parent planetesimals. Results
Studies on different meteorite classes, support that rapid differentiation following
including ordinary and carbonaceous chondrites, formation of calcium-aluminum inclusions
have however provided contrasting results with and chondrules gave rise to differentiated
wide ranges for protoplanetary disk magnetic planetesimals with iron cores, capable of
fields. Developments on instrumentation and generating and sustaining dynamo action for
techniques for rock magnetic and paleointensity million year periods. The Allende chondrite
analyses are allowing increased precision. may have derived from a partly differentiated
Micromagnetic and an array of geochemical, planetesimal which sustained an internal
petrographic and electronic microscopy analyses magnetic field.
provide unprecedented resolution, characterizing
rock magnetic properties at magnetic domain Key words: Paleomagnetism, meteorites,
scales. We review studies on chondrules from magnetic fields, dynamos, Solar System.
Introduction and differentiated classes, which include
ordinary and carbonaceous chondrites and
Studies of meteorites provide the earliest achondrites and iron meteorites, respectively
records concerning the conditions and evolution (Weisberg et al., 2006). Chondrules represent
of the planetary system (Wood, 1988; Cameron, molted droplets of aggregated fine silicate dust,
1988; Lauretta and McSween, 2006). Meteorites recording transient heating events in the solar
constitute the oldest rocks preserved from the nebula (Hewins, 1997; Zanda, 2004; Scott,
initial stages of planetary accretion. Information 2007). They show different textures and mineral
on the age of the solar system, conditions assemblages, with some containing fragments
and chronology for formation of first solids, of other chondrules and rare CAIs. Preservation
planetesimals and early differentiated bodies of chondrules indicates that chondrites were
is based on studies of chondrites and other not melted through their subsequent history
primitive meteorites (Wood, 1988; Hewins et after formation. In contrast, differentiated
al., 1996; Scott, 2007; Connelly et al., 2012). meteorites, which include iron and iron-silicate
Chondrites are composed by millimeter sized meteorites, were subject to various degrees of
silicate spherules named chondrules and calcium- melting and metamorphism, and interpreted as
aluminum rich inclusions (CAIs) embedded in derived from planetesimals later fragmented by
a fine grained silicate matrix. In recent years collisions.
research on the origin and evolution of the solar
system has expanded, with developments from Understanding the early stages of planetary
planetary missions, astronomical observations, system evolution has remained a complex
discovery of exoplanets and theoretical and difficult task. Considerable effort is spent in
numerical simulation models. The multi-and characterizing the distinct classes of meteorites,
interdisciplinary approaches and new data which are investigated with a wide range of
are providing fresh insights on the origin analytical techniques. Studies have documented
of planetary systems. Major questions on new minerals and unraveled evidence on the
fundamental aspects still remain open, including formation and alteration histories of meteorites
conditions and chronology in the early phases. and parent planetesimals with increasing detail
(Taylor et al., 1987; Nyquist et al., 2009; Weiss
Many studies on meteorites focused on the and Elkins-Tanton, 2013). The conditions and
petrography and geochemistry, which permitted nature of processes involved in the formation of
to distinguish and characterize distinct classes of the proto-Sun, chondrules, CAIs, and early accreted
meteorites. Meteorites are divided into primitive planetesimals remain poorly constrained.
344 Volume 53 Number 3
Geofísica Internacional
We review micromagnetic and microstructural range of field magnitudes with relatively
studies on chondrites with particular emphasis large uncertainties, which prompted search of
on the Allende chondrite (Figure 1), focusing on alternative paleointensity determination methods.
the early magnetic fields in the solar system and In the past years, the field of meteoritics has
how they relate to the evolution of meteorite significantly evolved and expanded. In this
parent planetesimals. Developments of review we address recent developments in
analytical techniques and better understanding using micromagnetic analyses and studies on
of rock magnetic properties and magnetization early differentiated bodies with potential for
mechanisms are allowing improved resolution sustained internal magnetic field generation.
in characterizing the magnetic records of Results from individual chondrules provide
meteorites. more consistent data with improved analytical
resolution (Acton et al., 2007; Flores-Gutierrez
The nature and role of magnetic fields in et al., 2010a,b; Emmerton et al., 2011). Formation
the protoplanetary nebula, with formation of of differentiated bodies with iron cores and
the proto-stars and accretionary disk had long capacity for generating internal magnetic fields
been examined (Kulsrud, 1999; Ossendrijver, have added to the interest in documenting the
2003; Vallée, 2011). Interest in magnetic fields characteristics and presence of magnetic fields
and their role in the evolution of protoplanetary in the protoplanetary nebula (Carporzen et al.,
nebulas have been sparked by recent studies 2010; Fu et al., 2012; Tarduno et al., 2012;
with detection of magnetic fields in accretion Sterenborg and Crowley, 2013).
disks (Donati et al., 2005). Studies on meteorite
records of the solar protoplanetary nebula Chondritic meteorites
have focused on characterizing magnetic
properties and mineralogy and in paleointensity Chondrites are part of the primitive meteorites,
determin a t i o n s . T h e m a g n e t i c f i e l d characterized by the abundance of chondrules.
magnitude is estimated from remanent Several classes have been distinguished
magnetization records in chondrites and in petrographic and geochemical studies.
individual chondrules (e.g., Butler, 1972; Chondrites are classified into ordinary (H, L
Nagata, 1979). Early estimates gave a wide and LL), carbonaceous (CB, CH, CI, CK, CM,
Figure 1. (A) Image of a slab fragment of the Allende CV3 chondritic meteorite showing the chondrules, refractory
calcium-aluminum rich inclusions and fine grained silicate matrix. (B) Thin-section microphotographs of Allende
chondrules, showing the diversity of textures, morphologies and mineral compositions.
July-September 2014 345
J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
CO, CR and CV), enstatites (EH and EL) and R the different meteorite classes, where the wide
and K chondrites. Ordinary chondrites represent ranges in magnetic susceptibility and other
the most abundant, making up to 80-85 % of physical properties are used to characterize
falls. The three major groups H, L and LL ordinary chondrites, enstatites and achondrites
are characterized by the distinct petrologic (Rochette et al., 2003, 2009; Macke et al., 2010,
types 3 to 6, interpreted as derived from three 2011).
separate parent planetesimals (Weisberg et al.,
2006). Carbonaceous chondrites are relatively Another aspect to consider in characterizing
rare, representing a small percentage of meteorites is the effects of hydrothermal and
falls. In general, the different groups have aqueous metamorphic alterations (Rubin,
been interpreted as derived from distinct 2000). Studies have shown that most chondrites
parent planetesimals (Weisberg et al., 2006). were affected by various degrees of metal/
Genetic relationships are being investigated for silicate and refractory/volatile fractionation.
different chondrite, iron and other meteorites, The fractionation has been associated with
relating differentiated and undifferentiated types chondrule formation processes; although
(Greenwood et al., 2010). some studies have suggested fractionation
mechanisms triggered by nebular condensation.
Other meteorites include the achondrites and Chondrule formation modified the composition
the primitive achondrites (Weisberg et al., 2006; of the nebula. CI chondrites, characterized by
Rochette et al., 2009; Macke et al., 2011). The olivine and pyroxene minerals and rare to
achondrites represent the group of differentiated absent chondrules, preserve the chemical
meteorites, characterized by variable degrees and isotopic composition of the solar nebula.
of melting and metamorphism, divided into CI chondrites, including the CV Allende
angrites, aubrites, howardites-diogenites- carbonaceous chondrite, were affected by
eucrites (HED meteorites), mesosiderites, aqueous metamorphism, highlighting further
pallasites and iron meteorites. These groups study of alteration and metamorphic processes.
include several subgroups (e.g., iron meteorites
and pallasites), with several ungrouped Chondrules are relatively abundant in
meteorites with distinct mineralogical and the different chondrite classes, constituting
chemical assemblages. Primitive achondrites volumetrically from 15 vol % up to 80 vol %
include acapulcoites, lodrinites, ureilites and (Zanda, 2004; Scott, 2007). The abundance of
winonaites. Silicate inclusions in IAB and IIICD chondrules supports that chondrule formation
iron meteorites are part of this group, probably was a major widespread process in the solar
derived from partial melting of chondrites in a nebula. The rapid melting and cooling of the
parent planetesimal. H chondrites and iron IIE silicate droplets operated over an extended
meteorites are probably coming from a partly period, overlapping with CAI formation (Itoh
differentiated parent planetesimal. Differentiated and Yurimoto, 2003; Bizarro et al., 2004).
meteorites include those derived from known Chondrule sizes are dominantly sub-millimeter
planetary sources such as the lunar and martian and show characteristic ranges in the different
meteorites. chondrite classes. Chondrules show distinct
internal structures, textures and mineralogical
Meteorite classification schemes have assemblages (e.g., Figure 1B). Rimmed,
evolved over time, reflecting development fragmented and composite chondrules indicate
of new analytical techniques. Classification a dynamic complex formation history, involving
schemes are based on petrologic, mineralogic, several heating events and extended formation
geochemical and isotopic analyses. Van Shamus period in the nebula. Several contrasting
and Wood (1967) proposed a widely used mechanisms have been proposed to account for the
classification system for chondrites based on transient heating and chondrule formation and
compositional groups and six petrologic types. for their later incorporation within fine-grained
Petrologic types 1 and 2 correspond to silicate matrix, and eventually into planetesimals
unmetamorphosed chondrites affected by (Hewins et al., 1996; Rubin, 2000; Connelly et
varying degrees of aqueous alteration and al., 2012).
types 3–6 represent to distinct levels of thermal
metamorphism (McSween, 1987, 1979). For Magnetic studies of meteorites
several groups of carbonaceous chondrites,
there have no petrologic type 4 (e.g., CI, CH, Study of rock magnetic properties and
CM, CO, CR and CV). The classification has been remanent magnetization records are the
modified over the years (Weisberg et al., 2006), field of paleomagnetism and rock magnetism
including inclusion of type 7 chondrites (Hus et (Dunlop and Ozdemir, 1997). Early recognition
al., 2006). Differences in magnetic properties that magnetic minerals with domain states
have been used to characterize and distinguish corresponding to single (SD) and pseudo-single
346 Volume 53 Number 3
Geofísica Internacional (PSD) domains are able to preserve records 110 mT from Thellier experiments. Lenoix et on time scales of the order of the solar system al. (1978) reported values from 100 to 700 age provided a tool for studies of ancient fields mT, and Wasilewski (1981) reported estimates (Weiss et al., 2010). Studies on chondritic of 1.2 to 15 mT. Complexities of paleomagnetic meteorites suggested that magnetizations could records and technique limitations hampered the have been acquired at their time of formation, studies resolution. Recent studies on individual opening the possibility of investigating the chondrules using the non-heating REM, REMc presence and nature of magnetic fields in and Preisach methods have provided new the solar nebula. Measurements of magnetic paleointensity estimates. Acton et al. (2006) properties on meteorites documented the summarized paleointensity estimates using the presence of remanent magnetizations, with REM methods on bulk samples and chondrules different coercivity and unblocking temperature from different meteorites with ranged between characteristics (Butler, 1972; Lanoix et al., 3 and 147 mT (Figure 2A). Results using the 1978; Nagata, 1979; Sugiura et al., 1979; Fuller REMc method yielded consistently lower values, and Cisowski, 1987). between
J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
Figure 2. (A) Summary of paleointensity estimates on chondrules from the Allende and Mokoia meteorites. The
graph shows the comparison of estimates obtained using the Preisach and REMc methods (after Emmerton et al.,
2011). Mean and associated error bars are indicated; for Preisach method it is the weighted mean for individual
demagnetization steps and for REMc method it is the arithmetic mean for selected AF demagnetization steps. (B)
Vector plots for AF and thermal demagnetization of Allende samples, showing the characteristic univectorial and
multi-component magnetizations, marked by the vector demagnetization trajectories in different colors. Plots (c)
and (d) after thermal demagnetization reveal the dominant lower unblocking temperature component and the
intermediate middle unblocking temperature component unblocked between 190o C and 290o C. In the insets,
plots depict the high temperature components (taken from Carporzen et al., 2011).
into earth´s atmosphere, characterizing the they have not been affected by melting since
fusion thin crusts. Stepwise demagnetization they were first accreted, but have also been
shows remanence is characterized by low, carried out on other meteorite classes.
intermediate and high coercivity and unblocking
temperature components (Figure 2B; Carporzen Magnetic hysteresis measurements have
et al., 2011). been carried out in bulk samples and in
individual chondrules. Data show occurrence
The contrasting paleomagnetic records, of ferrimagnetic minerals with high saturation
analytical problems and complex magnetic values and intermediate and low coercivities,
mineralogy have stimulated studying the indicating magnetite and low-Ti titanomagnetites.
magnetic properties of meteorites at increasingly Hysteresis parameter ratio plots show
finer resolution using micromagnetic techniques chondrules are characterized by dominantly
(Flores-Gutierrez and Urrutia-Fucugauchi, 2002; PSD and multidomain (MD) states (e.g., Weiss
Acton et al., 2007; Emmerton et al., 2011). et al., 2010; Acton et al., 2007; Emmerton
Studies focused on chondritic meteorites, where et al., 2011; Flores-Gutierrez and Urrutia-
presence of CAIs and chondrules supports that Fucugauchi, 2002). Emmerton et al. (2011)
348 Volume 53 Number 3Geofísica Internacional
presented magnetic data for 28 and 17 MD particles, contrasting with the scattered
chondrules from the Mokoia and Allende pattern shown in the Allende chondrules and
chondrites, respectively. The dominant magnetic matrix (Figure 3).
phase in the chondrites was interpreted as
an FeNi phase (taenite, kamacite or awarite), Coercivity distribution in chondritic meteorites
contributing about 48 % in Mokoia and about suggest they may be affected by overprinting,
42 % in Allende. The Mokoia chondrules showed during metamorphism or collisions, alteration
higher magnetic mineral concentrations, wide in planetesimal bodies, and also exposed to
range of magnetic properties and dominant SD ambient conditions on Earth after atmospheric
domain states. Allende chondrules showed low entry including high magnetic fields, lighting
coercivity distributions and limited magnetic strikes, etc (Weiss et al., 2010). This has
interactions. Alternating field (AF) and thermal suggested that reliable paleointensity estimates
demagnetization of chondrules and bulk are more likely to be obtained from the high
samples reveal univectorial to multicomponent coercivity spectra in chondrules. Sugiura et al.
magnetizations. Some chondrules carry complex (1979) and Sugiura and Strangway (1983)
magnetizations, which are not resolved using reported data supporting that chondrules retain
AF or thermal demagnetization. Acton et al. a remanent magnetization acquired prior to their
(2006) report magnetic hysteresis ratio plots incorporation into the chondritic meteorite
for chondrules and matrix for the Allende, using a form of conglomerate test (Weiss et
Karoonda and Bjurbole chondrites showing wide al., 2010). The relationships uncovered for the
variation ranges, with Mr/Ms ratios between magnetization ratio and coercivity in the Allende
0.03 and 0.39 and Hcr/Hc ratios between chondrite may permit to better understand the
0.1 and 9. Mokoia chondrules plot in the PSD coercivity distribution (Flores-Gutierrez and
field showing a restricted trend, while Allende Urrutia-Fucugauchi, 2002). The connections to
chondrules show more variability plotting in internal chondrule structure and morphologies,
the PSD and MD fields, similar to other CV3 with compound chondrules, alteration rims,
and C chondrites. Mokoia chondrules show fracturing can be used for sample selection for
variation trends suggesting mixtures of SD and paleointensity experiments.
Fig. 3. Example of magnetic hysteresis data for chondrules and matrix samples from Allende and Mokoia meteorites,
plotted in a hysteresis parameter ratio plot (from Emmerton et al., 2011). Domain state fields are indicated
(Day et al., 1977); observe data points plot in pseudo-single domain (PSD) and multidomain (MD) fields. Mokoia
chondrules show a mixture trend of single domain (SD) and MD behavior; in contrast to the scattered pattern for
Allende chondrules (see text for discussion).
July-September 2014 349J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
Allende Chondrite in chondrule rims. Fe forms FeNi alloys and trolite.
Si is a major element in chondrule composition;
The Allende meteorite is an oxidized CV3 in low concentrations it tends to be uniformly
carbonaceous chondrite (Figure 1). These distributed and in higher concentrations it is
chondrites are relatively rare as compared to distributed in wide regions and in the rims.
other types such as the ordinary chondrites. In one of the chondrules analyzed, Si shows
Its fall in the area of Allende town, Chihuahua, higher relative contents in the interstitial zones.
northern Mexico on February 1969 was well Nickel distribution in Ni-rich minerals and alloys
documented, with a relatively large amount of seems an important indicator of alteration, with
fragments up to 2 tons recovered (Clarke et al., metamorphism allowing Ni to migrate, recording
1970). At the time, samples were distributed metamorphic temperatures in pentlandite-
to numerous laboratories, making Allende one bearing chondrites. The mineralogy suggests
of the most intensively investigated meteorites. secondary events following chondrule formation
Several major findings have been first reported in the solar nebula. Kring (1991) considered the
in Allende samples, including evidence for high-temperature chondrule rims as indicators
short lived Mg-Al isotope system supporting a of fluctuating conditions, with accretion of dust
supernova explosion at the early stage of nebula rims occurring under turbulence in particle rich
condensation (Lee et al., 1976). regions up to several hundred of kilometers in
scale (Cuzzi and Alexander, 2006).
Allende is characterized by high magnesium
and iron contents, with olivine (MgO 47-49 Analysis of hysteresis ratio plots and
%, FeO 7-9%, SiO2 42-44%), awaruite (FeO compilation of hysteresis data show a relatively
28-32%, NiO 68%), and enstatite (MgO 35- higher scatter for the Allende and other
36%, FeO 0.8%, Al 2 O 3 1.2%, SiO 2 60%). chondrites, in contrast to data for chondrites
Other minerals present are kamacite, taenite, characterized by less variability and trends of
troilite, mackinawite, antigorite, majorite and SD and MD domain particles (Figure 3). Flores-
pentlandite, with magnetite, chromite, Ni-Fe Gutierrez and Urrutia-Fucugauchi (2002) and
alloys, sulfides and sulfates and anorthite as Flores-Gutierrez et al. (2010a) investigated the
accessory minerals. Chondrules show different domain state pattern for Allende chondrules and
textures and mineral compositions (e.g., Figure found that Allende is characterized by a variation
1B), characterized by anhedral, euhedral and range in hysteresis parameter ratio plots related
recrystallized olivine chondrules, barred olivine to chondrule size (Figure 4).
chondrules, pyroxene olivine chondrules and
pyroxene chondrules. Major magnetic minerals Hysteresis magnetization ratios (Mr/Ms) vary
are magnetite, taenite and iron sulfides. from 0 to 0.22, showing a linear relation as a
function of coercivity (Hc), which varies from
Two different chondrules types are observed: 3 to 24 mT. The variation trend correlates with
FeO-poor reduced type I chondrules with iron internal microstructure and composition, with
metal blebs and olivines and FeO-rich oxidized compound chondrules showing higher hysteresis
type II chondrules with no metal blebs and olivines ratio and parameter values. Chondrules with
(McSween, 1979, 1987). The Allende chondrite hysteresis parameters falling outside the major
is characterized by FeO-poor chondrules and trend show internal structures, composition
low-volatile rich. Divisions of CV chondrite and textures of compound, fragmented or
sub-groups depend on relative contents of metal altered chondrules. The relationships between
and magnetite, which is reflected on the modal chondrule size and hysteresis parameters were
ratios of metal to magnetite. not expected from previous studies, suggesting
possible deeper associations with chondrule
Magnetite formation in oxidized and reduced composition. Flores-Gutierrez et al. (2010a)
chondrites have been intensively studied, with analyzed the internal chondrule structures
different mechanisms proposed, including and compositions using scanning electron
crystallization from oxidized melts, oxidation microscope on individual chondrules. Separation
of metallic nodules in the solar nebula, and of chondrules and CAIs from the fine-grained
secondary oxidation of metallic chondrule silicate matrix is time consuming, and studies
nodules in planetesimals (Krot et al., 1995). use generally a few chondrule samples. Results
In the Allende, magnetite appears mainly in from 100 individual chondrules permit to evaluate
porphyritic olivine chondrules as spherical the relations of chondrule size, density and
nodules, in association with Ni rich alloys and hysteresis parameters and parameter ratios
sulfides. Flores-Gutierrez et al. (2010a) study (Figure 4). Chondrule densities vary from
shows that Fe is associated with Si and Mg about 2 to 3.5 kg/m3, showing a rough trend
(fosterite to fayalite), with higher concentrations to decrease with increasing chondrule size.
350 Volume 53 Number 3Geofísica Internacional
Figure 4. (A) Plot of mass as a function of chondrule radii (B) density as a function of chondrule radii (C)
magnetization ratio as a function of coercivity (D) Remanent magnetization as a function of remanence coercivity
(after Flores-Gutierrez et al., 2010a,b). Selected chondrules marked are also shown in the microprobe analyses
in Figure 5 B (see text for discussion).
Plot as a function of chondrule radii show a for retrieval of paleointensity estimates for
wide scatter, with a decreasing trend. Some the early planetary magnetic fields. Reliable
chondrules show lower densities, particularly paleointensity estimates might be better
the larger ones with radius larger than 0.07 obtained from high coercivity spectra.
mm and up to 0.16 mm. Magnetic hysteresis
parameters were associated with chondrule Analysis of chondrules within the linear
structure, mineralogical composition and relation field and outliers show that magnetic
morphology. Analyses of chondrules within the properties correlate with chondrule internal
scatter plot relations and outsiders documented structure and mineral composition (Figure
that chondrules showed characteristic behaviors 5). Chondrules falling outside the relation are
related to their internal structure and properties. compound, with partial aggregation rims and/
or fractured (chondrules c40, c27 and c23), in
Magnetic hysteresis parameter ratios contrast to those chondrules with low coercivity
show simple linear relationships between the Hc and magnetization Mr/Ms ratios, which appear
magnetization ratio and coercivity (Figure homogeneous (chondrules c53 and c55). Silica
4). Magnetization ratios are low, lower than maps characterize chondrule structures, which
0.225, with the linear relationship present up can be correlated with chondrule size, external
to 0.17. Coercivity is low, lower than 24 mT. morphology and shapes.
The linear relationship covers the range from
about 2 mT to 17 mT. These low values suggest Results show that closely-spaced chondrules
that chondrules are susceptible to alteration in given fragments of Allende are characterized
and remagnetization, which has implications by highly variable Fe, Ni and S composition.
July-September 2014 351J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
Differences in magnetic properties relate to The relationship uncovered for the
characteristic sizes, textures and mineralogy. magnetization ratio and coercivity (Figure
Spatial distributions suggest chondrules record 5A) permit to better constrain the coercivity
distinct shock and alteration processes during distribution and relations to internal structure,
formation stages. Scanning electron microscope with compound chondrules, alteration rims,
(SEM) analyses constrain the composite nature, fracturing appear useful criteria for sample
with fragmented chondrules, different accretionary selection in paleointensity experiments. The
rims, mineralogic and elemental compositions combined hysteresis data and microscope
and coalesced compound chondrules. observations permit to characterize populations
of chondrules, with distinct magnetic mineralogy,
morphology and internal structure.
Figure 5. (A) Remanent magnetization-coercivity diagram, showing interpreted chondrule distributions. Compare
with diagram in Figure 4 D. (B) scanning electron microscope and elemental analyses (Flores-Gutiérrez et al.,
2010b).
352 Volume 53 Number 3Geofísica Internacional
Observations provide further evidence on Chondrules and CAIs formed from dust
multiple heating events and long time scales agglomerates by short-lived heating events that
involved in chondrule formation (Scott, 2007). affected the gas nebula. Transient temperatures
The wide range of internal structures and may have reached up to 1400o to 1800o C with
mineralogical compositions in a single fragment rapid heating/cooling. Mechanisms proposed for
of the Allende indicate that Allende chondrite melting of fine-grained solid matter include a
formed from a heterogeneous assemblage of wide range of processes. Chondrules appear to
chondrules, which underwent complex pre- have been formed at a later time than refractory
accretionary histories (Figure 6). This is further inclusions, as suggested from isotopic analyses
examined below in relation to the chronological and petrographic and microstructural observations
constrains on the early stages of evolution of (Figure 7). Isotopic analyses of the Al and Mg
the protoplanetary disk. systematics completed in Allende chondrite
provided constraints for the event chronology of
Chronology of solar system early evolution CAIs and chondrules and solar nebula conditions
(Lee et al., 1976; Scott, 2007). Analyses of 26Al
The age of ~4,568 millions of years derived from isotopic differences in CAIs and chondrules have
radiometric studies of meteorites is obtained been discussed in terms of orderly or overlapping
from dating CAIs and chondrules from chondrites sequence of events (Itoh and Yurimoto, 2003;
and other primitive meteorites (Connelly et al., Bizarro et al., 2004; Krot et al., 2005). Refractory
2012). Chondrules and refractory inclusions inclusions appear having started to form 1 to
in carbonaceous chondrites give the oldest 4 million years earlier (Scott, 2007). Additional
radiometric dates, providing a chronology of detailed studies of CAIs and chondrules are
events in the accreting protoplanetary disk. required to constrain the chronology of early
Radiometric dating on primitive angrites, events, processes and their spatial relationships
eucrites, and chondrules, refractory inclusions (Itoh and Yurimoto, 2003; Bizarro et al., 2004;
and olivine aggregates from chondrites provides Krot et al., 2005). Experimental evidence on
a chronology for solar system early stages an early origin of CAIs includes the findings of
(Amelin et al., 2002; Bouvier and Wadhwa, relict CAIs inside chondrules. Studies have also
2010; Connelly et al., 2012). reported findings of a chondrule inside a CAI,
Figure 6. (A) Model for the accretion disk, showing the chondrule and refractory calcium-aluminum inclusions
formation sectors. (B) protoplanetary disk zone of crystalline and amorphous condensates, showing the distinct
chondrule morphologies (modified after Scott, 2007).
July-September 2014 353J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
Figure 7. Age plot for the first millions of years, for formation of refractory inclusions, chondrules and iron,
chondrite, angrite and eucrite parent planetesimals (after Elkins-Tanton et al., 2011).
which support contemporaneous formation of was marked by collisions, which fragmented the
CAIs and chondrules, and an extended period of planetesimals, leading to the partial or complete
chondrule formation (Itoh and Yurimoto, 2003; disruption, producing the differentiated and
Krot et al., 2005). undifferentiated meteorite types (Figure 8B).
Depending on the crustal level, distinct chondrite
A chronology of events that incorporates the types originate, marked by varying hydrous
rapid accretion of differentiated palanetesimals alteration and metamorphic degrees (Figure
within the first millon years, constrained by ages 8A). CK chondrites come from deep crustal
from different isotopic systems, is illustrated levels, whereas Allende CV3 chondrite comes
in Figure7 (Kleine et al., 2002; Weiss and from an intermediate crustal level.
Elkins-Tanton, 2013). In this model, accretion
of chondrite parent planetesimals occurred Use of short lived radioisotopes like the
shortly after chondrule formation with an Al-Mg system provides temporal resolution
extended period marked by hydrous alteration for asteroid accretion, differentiation and
and thermal metamorphism. Carbonaceous secondary alterations (Nyquist et al., 2009).
chondrites developed in the crust of partly Isotope geochemical studies on the Allende
differentiated planetesimals, large enough to carbonaceous chondrite and other chondrites
have metallic cores and internal magnetic fields documented presence of the radioactive isotope
(Figure 8; Elkins-Tanton et al., 2011; Carporzen aluminium-26, which indicates that a supernova
et al., 2011; Sahijpal and Gupta, 2011). Dynamo exploded in the vicinity of the solar nebula,
action was sustained for several millions of initiating compression of the gas and dust
years, consistent with thermal evolution and molecular cloud.
rapid differentiation processes. The early stage
354 Volume 53 Number 3Geofísica Internacional
Figure 8. (A) Schematic model for a chondrite
parent planetesimal, with a differentiated
interior, convecting magma mantle and a
metallic core capable of sustained dynamo
action (Elkins-Tanton et al., 2011). Distinct
chondrititic meteorites showing variable aqueous
alteration and metamorphic degrees originated
from different crustal levels. The Allende CV3
chondrites come from an intermediate crustal
level. (B) Schematic model for formation of
different meteorite types from collision of
the parent planetesimals with variable size
impactors (Weiss and Elkins-Tanton, 2013). Note
formation of undifferentiated and differentiated
meteorites resulting from small to large impacts.
CAI fragments inside chondrules are consistent chondrule fragments inside CAIs add support for
with the older ages for CAIs, also indicating that overlapping chondrule and refractory inclusion
chondrule formation occurred in a nebula sector formation, within given sectors of the nebula
with CAIs present. There are different models for (Itoh and Yurimoto, 2003; Bizarro et al., 2004).
formation of chondrules and refractory inclusions,
some involving distinct separate sectors in the Discussion
nebula (Figure 6A). Radiometric dates constrain
the period for CAI and chondrule formation. In recent years, improvements on instrumentation
Chondrule size varies among different chondrite and analytical techniques provide increased
classes, which has been interpreted in terms of resolution in studying and characterizing
distinct regions for chondrite formation and/ the various classes of meteorites. Use of
or chondrule sorting mechanisms. Chondrules microanalytical methods permits to investigate
showing fragmentation, coalesced composite meteorite constituent minerals at increasing
chondrules, chondrule fragments inside detail. In the case of chondrites studies focus
chondrules and chondrules with rims (Figure 6B) on the CAIs, chondrules, metal grains and matrix,
support relatively large spans for the formation to investigate the early evolution stages of the
with multiple fast heating events. solar system. Radiometric dating on CAIs and
chondrites give the oldest dates, constraining
The observations on individual chondrules the chronology of accretion. Chemical and
from a fragment of the Allende meteorite isotopic analyses on CI and carbonaceous
show that highly diverse types coexist with chondrites show they preserve the composition
fragmented, rimmed and composite chondrules of the solar nebula.
characterized by distinct mineralogy (Figure
5B). Findings support a dynamic evolution of Chondrites are characterized by a trend from
chondrule formation, fragmentation and partial ‘reduced’ with Fe in metallic form to ‘oxidized’
metasomatism before and during chondrule with Fe in silicates and oxides. Magnetic minerals
aggregation (Scott, 2007). The recent reports of in CV3 chondrites are FeNi, iron-titanium
July-September 2014 355J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
oxides and magnetite in olivines, with magnetite fields rests on the magnetization record and
formed by aqueous alteration. The results from remanence acquisition mechanism. Carporzen et al.
the hysteresis analyses of individual chondrules (2011) summarized the records for unidirectional
show magnetite as dominant phase with PSD magnetizations in the Allende, residing in
and MD domain states, which along with taenite middle unblocking temperature components,
and iron sulfides make the magnetic signal of interpreted as up to ~290 C partial TRM (pTRM)
Allende. or thermochemical (CTRM) magnetizations (e.g.,
Figure 2B). This secondary component is carried
Chondrules examined in a fragment of the in magnetite, pyrrothite and awaruite, which
Allende chondrite display relations between formed as a result of hydrous alteration (which is
mass and size (Figure 4A). The relation is further discussed below). The ambient magnetic
partly dependent on density (Figure 4B), with field under which the magnetization was blocked
density decreasing with size. Examination of is constrained from paleointensity estimates
the apparent scatter in the size-density plot from both Thellier and laboratory imparted AF
shows a dependency on internal structure and demagnetization methods, which Carporzen et
mineralogical assemblage. The composite al. (2011) set at minimum fields of 20 mT and
and fragmented nature of some chondrules is up to about 60 mT. The high magnetic field
related to outliers in the plots. The study also magnitude is compatible with internal fields.
uncovered simple relationships among the The other requirement in this interpretation of
magnetic hysteresis properties, which need the Allende paleomagnetic record is the timing
further consideration using detailed petrographic and duration for remanence acquisition, which
and geochemical analyses on a larger group are constrained from the isotope chronometers.
of chondrules. The magnetization ratio shows In the study, chondrules are considered to have
a dependency on the coercivity (Figure 4C). formed over an interval of 1.2 to 3 Ma, starting
The plot of remanent magnetization as a at about 200 ka after formation of refractory
function of remenent coercivity shows a trend inclusions. Carporzen et al. (2011) discussed the
of increasing intensity with increasing coercivity temporal constrains in further detail, concluding
(Figure 4D). There is however significant scatter, that an extended period of elevated temperatures
in contrast to the pattern shown using the and hydrous alteration was involved in the
magnetization ratio. The examination of the acquisition of the paleomagnetic record.
internal structure and mineralogical assemblage
uncovers correlations between the magnetic The scenario for magnetization acquisition in
properties, variation patterns and chondrule the crust of a differentiated parent planetesimal
characteristics (Figure 5A, B). appears consistent with magnetite formation as
an alteration product. CV chondrites have been
The wide range of variation in chondrule divided into magnetite-rich and magnetite-
structures, composition and hysteresis properties poor subgroups, marked by the modal ratio of
has implications for the paleomagnetic and magnetite to metal (McSween, 1977; McMahon
paleointensity studies (Flores-Gutierrez et and Haggerty, 1980). Different mechanisms for
al., 2010a, b), indicating that the differences magnetite formation have been proposed, with
in magnetization vector components and some occurring in the solar nebula and some
paleointensity record relate to chondrule in planetesimals (Krot et al., 1995). Choi et al.
structure and mineralogy. Magnetic hysteresis (1997) analyzed the oxygen isotope compositions
parameters and domain structure are related to of magnetites and olivines in the Allende, where
chondrule morphology, size and density, which they are closely related. Magnetite is present
in turn reflect internal structure and mineralogic as spherical nodules in porphyritic-olivine
differences (Figure 5A, B). A next step is to chondrules, associated with Ni-rich metal and
obtain paleointensity estimates and expand the sulfides. The oxygen isotopes differ, implying
analyses and correlations between magnetic that magnetite and olivine are in isotopic
hysteresis properties and chndrule structure disequilibrium, with more negative magnetite
and mineralogical composition. δ-O than those expected for the solar nebula.
Choi et al. (1997) interpreted that magnetite
The studies, reviewed in the previous formed by aqueous oxidation of metal, which
section on paleomagnetic records in chondrites, had already undergone oxygen isotope
show that they are secondary with remanent exchange with fine-grained silicate material in
magnetizations acquired in the planetesimals the parent planetesimal.
under ambient fields generated by active
internal dynamo action (Carporzen et al., 2011; Modeling of the size and internal structure
Elkins-Tanton et al., 2011). The paleomagnetic of differentiated bodies with iron cores and
evidence for differentiated planetesimals with undifferentiated crusts has been used to further
iron cores and sustained internal magnetic constrain the early thermal state evolution
356 Volume 53 Number 3Geofísica Internacional
(e.g., Elkins-Tanton et al., 2011; Sahijpal and bodies were later destroyed by collisions or
Gupta, 2011; Storenberg and Crowley, 2013). other disturbances such as tide forces. Part of
Models show that dynamo action might have the differentiated bodies have been preserved
been sustained for several million years. Further forming the planets, satellites, dwarf planets and
modeling incorporating stringent boundary specially the large asteroids Ceres and Pallas in
conditions on thermal state and heat transport the asteroid belt (Elkins-Tanton et al., 2011).
are needed to constrain the evolution of the
planetesimal dynamos. The crustal structure and The studies on eucrite meteorites support
thickness and temperature-pressure profiles, that their acquired remanent magnetizations
fluids and layering in parent planetesimals in a magnetic field, consistent with an internal
remain to be incorporated in the modeling, field with surface magnitude of around 2
including the magnetization acquisition microtesla (Fu et al.,2012). The asteroid Vesta
mechanisms at different crustal levels. in the asteroid belt is the smallest differentiated
body known. The paleomagnetic record suggests
Paleomagnetic data for meteorites inferred that Vesta might had had a convecting metallic
to originate at various levels marked by distinct core in the early stages.
hydrous alteration and metamorphic grades
(Figure 8A) might allow testing and refining the Stony-iron meteorites have been assumed
differentiated planetesimal model. Paleomagnetic to form in the interior of differentiated
data for terrestrial lower crust, metamorphic planetesimals, near the core-mantle boundary.
lithologies and crustal and mantle xenoliths Therefore, they come from differentiated bodies
might provide some potentially useful analogies. with developed cores that could sustain dynamo
Modeling of expected paleomagnetic records action. Paleomagnetic analysis of stony-iron
and rock magnetic property variations with meteorites such as the main group pallasites
depth within the crust might allow testing the could provide information on planetesimals´
remanent acquisition mechanisms. The effects magnetic fields Paleomagnetic analyses of
of the large impacts involved in fragmenting pallasites characterized soft coercivity highly
the planetesimals (Figure 8B), including shock anisotropic magnetizations, likely associated
remagnetization, need also to be investigated with the Fe-Ni metal (Nagata, 1979). Pallasites,
in the paleomagnetic records of the different characterized by metal-silicate assemblages
meteorites. composed of olivines and Fe-Ni are particularly
interesting, since olivine and metal phases tend
Forward modeling and inversion of crustal to separate, so a mechanism seems required to
magnetic anomalies on the Moon and Mars have keep them without segregation.
been used to estimate paleomagnetic poles
and derive constraints on the internal dynamo The different types of iron meteorites
generated magnetic fields (e.g., Cisowski et and achondrites indicate different parent
al., 1983; Scott and Fuller, 2004; Lillis et al., planetesimals, which supports widespread
2013). Occurrence of polarity reversals of the formation of differentiated bodies in the early
core dynamos has also been analyzed from stages. Burbine et al. (2002) estimated that
modeling of crustal magnetic field anomalies more than 100 parent planetesimals are
(Arkani-Hamed, 2001). This cannot be required to account for the diversity in
accomplished for the fragmented chondrite achondrites and iron meteorites, including
parent planetesimals, but might be possible the early estimates by Wasson (1990) on the
to be eventually investigated for the large probable large numbers of iron meteorite parent
differentiated asteroids Ceres and Pallas. planetesimals. Ungrouped iron meteorites have
been related to some 50 parent planetesimals
Paleomagnetic analyses of different meteorite (Goldstein et al., 2009), as suggested by the
types, including carbonaceous chondrites, differences in sulfur compositions. Goldstein et
eucrites and pallasites, suggest that their al. (2009) propose that iron meteorites were
acquired remanent magnetizations under the derived from relatively large parent bodies, with
influence of internal magnetic fields (Weiss sizes up to 1000 km or more, which formed
et al., 2008; Carporzen et al., 2011; Tarduno before chondrite accretion outside the asteroid
et al., 2012; Weiss and Elkins-Tanton, 2013). belt zone.
This implies differentiated planetesimals with
convecting cores capable of sustaining dynamo The widespread early formation of differentiated
action for extended periods. To sustain dynamo planetesimals and apparent lack of such
action an energy source is required, plus a thermal bodies in the asteroid belt has been discussed
state and evolution that allows the crust and in several studies. Surface weathering and
upper mantle to cool enough for magnetization impact debris cover may have masked the
acquisition. Most of these differentiated spectroscopic observations (e.g., Nakamura
July-September 2014 357J. Urrutia-Fucugauchi, L. Pérez-Cruz and D. Flores-Gutiérrez
et al., 2011). Other effects may include the Evidence on the nature of magnetic fields
preservation of primitive undifferentiated crusts in the early solar system has been derived
(Weiss end Elkins-Tanton, 2013). The layered from chondritic meteorites. Chondrites and
internal structures in planetesimals offers an in particular chondrules, which are millimeter
explanation of the genetic relations uncovered sized silicate spherules formed by rapid melting
for distinct meteorite classes, where they and cooling, have remanent magnetization
originate upon collision fragmentation from records which in some cases date from the
different levels from the iron cores, mantles time of chondrule formation and accretion of
and crusts. The paleomagnetic record for the planetesimals. Studies on different meteorite
Allende chondrite supports an origin from an classes, including ordinary and carbonaceous
intermediate crustal level in a partly differentiated chondrites, have provided contrasting results,
parent planetesimal. Carporzen et al. (2011) with a range of estimates for proto-planetary
showed that the magnetization in the Allende magnetic fields.
was likely acquired over an extended period
under the influence of an internal magnetic Developments on instrumentation and
field. The characteristic magnetization is techniques for rock magnetic and paleointensity
univectorial acquired in a magnetic field of analyses are allowing increasing precision.
about 20 mT, likely corresponding to a partial Micromagnetic measurements and a range of
TRM or a thermochemical C-TRM. The magnetic geochemical and electronic microscopy analyses
carriers, magnetite, pyrrothite and awaruite, are are providing unprecedented resolution,
consistent with this interpretation, corresponding characterizing rock magnetic properties at
to secondary minerals formed from hydrous magnetic domain scales. Magnetic hysteresis
alteration and thermal metamorphism. analyses on individual chondrules from the
Allende CV3 chondrite reveal relationships
Fu and Elkins-Tanton (2013) examined the among hysteresis parameters and physical
thermal evolution of planetesimals in which properties (Figure 4). Coercivity, remanent and
short lived radiogenic isotopes provided saturation remanence parameters correlate
sufficient heat for differentiation with formation with chondrule size and density; related to
of layered structures. In those bodies, heat internal chondrule structure, mineralogy and
dissipation involved magmatic processes morphology (Flores-Gutierrez et al., 2010a,b).
resulting in magmas ascending through the Compound, fragmented and rimmed chondrules
crust. Modeling by Fu and Elkins-Tanton (2013) have distinct hysteresis properties, related to
supports that magmas were volatile-depleted, mineral composition (Figure 5).
and likely emplaced through enstatite crusts but
likely not through CV and CM crusts, as indicated The remanent magnetization record and
by the paleomagnetic constraints. paleointensity estimates derived from the
Allende and other chondrites support remanent
Conclusions acquisition under influence of internal magnetic
fields within parent planetesimals. Rapid
In recent years, planetary missions, Earth- differentiation in the first 10 Ma following
based astrophysical observations, use of a formation of refractory inclusions and chondrules
range of analytical techniques of microscopy, gave rise to planetesimals with metallic cores
isotope geochemistry and magnetic properties, (Fig. 7), capable of generating and sustaining
discovery of exoplanets, and development of dynamo action for million year periods (Weiss
evolutionary models for planetary nebula have and Elkins-Tanton, 2013). Observations on
opened new research multidisciplinary fields, turbulent circumstellar disks also indicate rapid
with the potential of integrative approaches. planetesimal formation (Johansen et al., 2007),
providing a broad perspective for evolution
Meteorites represent the earliest rocks of planetary systems. Paleomagnetic and
preserved from the initial stages of formation thermal modeling constrain the evolution of
of the solar system, providing the record on early planetesimals, which might presently be
the conditions, processes and chronology for represented by large sized asteroids such as
formation of first solids, planetesimals and Vesta (Fu et al., 2012). The Allende chondrite
differentiated bodies. Refractory calcium- and other undifferentiated meteorites may
aluminum inclusions and chondrules give the have formed by fragmentation from collisions,
oldest radiometric dates. New dating studies representing different crustal levels in their
provide higher resolution on the chronology parent planetesimals (Figure 8; Carporzen et
of events, constraining the formation of CAIs al., 2011; Elkins-Tanton and Weiss, 2011).
and chondrules and accretion of differentiated Constraining the crustal levels, temperature/
planetesimals. pressure profiles, role of fluids and conditions
for aqueous alteration and metamorphism are
358 Volume 53 Number 3Geofísica Internacional
part of the major open questions remaining. Bouvier A., Wadhwa M., 2010. The age of the
Models of heat sources and transfer mechanisms solar system redefined by the oldest Pb-
and thermal state evolution are being used to Pb age of a meteorite inclusion. Nature
better define accretion processes and possible Geoscience, 3, p. 637-641.
boundary conditions (Elkins-Tanton et al., 2011;
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and layering may have been different, depending B., Keil K., 2002, Meteoritic parent bodies:
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in protoplanetary accretion disks are opening Butler R.F., 1972, Natural remanent magne-
new avenues. Integrative interdisciplinary tization and thermomagnetic properties
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Acknowledgments Astrophysics, 26, p. 441-472.
This study forms part of UNAM PaleoMeteor Carporzen L., Weiss B.P., Elkins-Tanton
research program. We thank Carlos Linares, L.T., Shuster D.L., Ebel D., Gattacceca
Raquel Díaz, Paloma Díaz and Avedis Urrutia for J., 2011, Magnetic evidence for a
collaboration in the studies and discussions. partially differentiated carbonaceous
Partial economic support from PAPIIT IN- chondrite parent body, Proc. National.
111112 grant is acknowledged. The Allende Academy Sciences, 108, 16, 6386–6389,
chondrite samples that are being studied are doi:10.1073/pnas.1017165108.
part of the meteorite collection of the Institute
of Astronomy, UNAM. Choi B.G., McKeegan K.D., Leshin L.A., Wasson
J.T., 1997, Origin of magnetite in oxidized CV
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