Ordinary chondritic micrometeorites from the Indian Ocean
M. SHYAM PRASAD1*, N. G. RUDRASWAMI1, Agnelo DE ARAUJO1, E. V. S. S. K. BABU2,and T. VIJAYA KUMAR2
1CSIR—National Institute of Oceanography, Dona Paula, Goa 403004, India2LAM-ICP-MS National Facility, CSIR-National Geophysical Research Institute, Uppal Road,
Abstract–Extraterrestrial particulate materials on the Earth can originate in the form ofcollisional debris from the asteroid belt, cometary material, or as meteoroid ablationspherules. Signatures that link them to their parent bodies become obliterated if thefrictional heating is severe during atmospheric entry. We investigated 481 micrometeoritesisolated from ~300 kg of deep sea sediment, out of which 15 spherules appear to haveretained signatures of their provenance, based on their textures, bulk chemical compositions,and relict grain compositions. Seven of these 15 spherules contain chromite grains whosecompositions help in distinguishing subgroups within the ordinary chondrite sources. Thereare seven other spherules which comprise either entirely of dusty olivines or contain dustyolivines as relict grains. Two of these spherules appear to be chondrules from anunequilibrated ordinary chondrite. In addition, a porphyritic olivine pyroxene (POP)chondrule-like spherule is also recovered. The bulk chemical composition of all thespherules, in combination with trace elements, the chromite composition, and presence ofdusty olivines suggest an ordinary chondritic source. These micrometeorites have undergoneminimal frictional heating during their passage through the atmosphere and have retainedthese features. These micrometeorites therefore also imply there is a significant contributionfrom ordinary chondritic sources to the micrometeorite flux on the Earth.
INTRODUCTION
In view of the sheer quantity that rain on the Earth,micrometeorites overlap and extend the range of theknown meteorite types, therefore constitute anextremely valuable resource of extraterrestrial material.Estimates on the quantity of micrometeorites thatimpact the top of Earth’s atmosphere are varied (Loveand Brownlee 1993; Peucker-Ehrenbrink and Ravizza2000; Cremonese et al. 2012). More than 90% of thismaterial undergoes frictional heating and vaporizationduring atmospheric entry; therefore, the survivingparticulate matter provides an important window tounderstand the components of the Earth-crossingmeteoroid complex. Preservation of micrometeoritesthat survive atmospheric entry depends on theenvironments where they are collected. The polarregions, in view of the prevailing cryogenic conditions,are now realized as the treasure houses of vast numbers
and varieties of micrometeorites (e.g., Kurat et al. 1994;Taylor et al. 2000, 2011; Van Ginneken et al. 2012).The fluxes of micrometeorites that survived atmosphericentry recorded in the polar regions are also among thehighest in terrestrial environments (e.g., Yada et al.2004). Due to low sedimentation rates, deep sea regionsfacilitate the collection of micrometeorite material. Theyalso have the potential to provide information on themicrometeorite flux and components on the scale oftens to hundreds of thousands of years (Taylor andBrownlee 1991; Prasad et al. 2013).
Micrometeorites are found either as unmelted/partially melted or as fully melted cosmic spherules.Some melted cosmic spherules produce refractorynuggets during atmospheric entry (Brownlee et al. 1984;Rudraswami et al. 2011). The spherules that areablation products of meteorites, although rare, can bedistinguished generally by their major element ratios;however, if they undergo severe heating and ablation
Meteoritics & Planetary Science 50, Nr 6, 1013–1031 (2015)
during atmospheric entry, then they develop glassytextures and would be indistinguishable from cosmicspherules from other sources (Genge and Grady 1999).The unmelted particles provide direct evidence for typesof meteorites that they were a part of (Suavet et al.2011; Van Ginneken et al. 2012); such information isdifficult to ascertain from most cosmic spherules. Theexception to this rule is scoriaceous micrometeorites,which, in view of their matrix-dominant textures can beequated with CI carbonaceous chondrite type parentbody (Kurat et al. 1994; Engrand and Maurette 1998;Maurette et al. 2000) A majority of micrometeorites inthis genre have chemical compositions that enable theiridentification as being parts of CI or CM chondrites(Brownlee et al. 1997) with smaller percentages of othermeteorite types in the total flux (Taylor et al. 2007).However, the larger meteorites show a reverse trend,where, the ordinary chondrites dominate the flux with87% and the carbonaceous chondrites comprise aminiscule ~3% of the meteorite flux (Krot et al. 2003).More recently, micrometeorites that have an ordinarychondrite (Genge 2008; Suavet et al. 2010; VanGinneken et al. 2012) or achondritic (Cordier et al.2011) provenance have been identified. These findingsindicate more widespread sources for the cosmicmaterials that rain onto the Earth in the form of dust.
Relict grains in melted micrometeorites providestrong evidence for pinpointing the parent meteoritetypes. Blanchard et al. (1980) reported ~10% of thecosmic spherules they observed contained relict grainsthat were mostly zoned olivines having Mg-rich cores;the other relict minerals identified were ferrous spinel,chromite, and pentlandite. Beckering and Bischoff(1995) carried out detailed investigations on the relictgrains in micrometeorites from the polar regions;olivines and pyroxenes dominated the relict grains inaddition to perovskite, chromite, Fe-Ni metal, etc. Theysuggested carbonaceous chondritic precursors for amajority of the relict grain-bearing particles, a fewcould also be from ordinary chondrites and none fromachondrites or mesosiderites. However, subsequentinvestigations (e.g., Delaney et al. 2004; Gounelle et al.2009; Badjukov et al. 2010) identified achondriticmicrometeorites. Furthermore, Taylor et al. (2011)reported that relict olivines and pyroxenes dominatedamong the relict grain-bearing particles; however, theyalso identified feldspars and spinel some of whichhelped identify achondritic particles among theircollection. Imae et al. (2013) analyzed olivines andpyroxenes from micrometeorites and compared themwith those found in meteorites. Their results showed amajority of the micrometeorites are from carbonaceouschondritic source; however, they report 22% of themicrometeorites have an ordinary chondrite precursor,
more probably an unequilibrated ordinary chondrite.There have not been many investigations on chromitesin micrometeorites or on dusty olivines. Parashar et al.(2010) identified an unequilibrated ordinary chondriticparent body for a chromite-bearing micrometeoritein their collection. We analyzed 481 meltedmicrometeorites from the Indian Ocean isolated from 10close-spaced surficial sediment samples (Prasad et al.2013); in this collection, several relict grains were foundsuch as forsterite (dominant), pentlandite, troilite,chromites, and reversely zoned olivines. Whileforsterites and sulfur-bearing minerals have beencommonly identified among all such collections andhave been attributed a CI or CM chondritic parentsource (e.g., Taylor et al. 2011), the chromites andreversely zoned olivines that contain small blebs ofmetal, which are otherwise known as “dusty olivines,”can be directly linked to ordinary chondritic parentbodies of different metamorphic grades. In the presentstudy, we present our investigations on chromites and“dusty olivines” found in melted micrometeorites fromthe Indian Ocean.
SAMPLING AND ANALYSES
The spherules were isolated from 10 deep seafloorsediment samples covering a total surface area of2.5 m2; the details of sampling and spherule isolationtechniques employed are described in Prasad et al.(2013). Of the 481 spherules that were examined, relictgrain-bearing and scoriaceous spherules togetherconstitute 8.5%. Chemical composition of the spheruleswas determined with a CAMECA SX-5 ElectronProbe Microanalyser at the National Institute ofOceanography and CAMECA SX-100 at PhysicalResearch Laboratory, Ahmedabad. Silicate and oxidestandards were used for all the analyses at anacceleration voltage of 15 kV, a beam current of 12 nA,and beam diameter of 2 lm especially when analyzingthe relict grains, whereas for bulk analyses the beamdiameter was 10 lm. Several spots on each spherule orfeature were obtained to arrive at average compositions.The data reduction and correction was done using PAPmodel (Pouchou and Pichoir 1991).
The laser ablation inductively coupled plasma–massspectrometer (LA-ICP-MS) at National GeophysicalResearch Institute (CSIR-NGRI, Hyderabad) was usedfor high precision situ analyses of trace elements inmicrometeorites. Seven spherules were chosen for thisanalysis: three spherules with chromite grains, three withdusty olivines (out of which two spherules comprisingwholly of dusty olivine grains), and an additionalspherule which had both dusty olivine and chromite asrelict grains. The ablation was done using New Wave UP
1014 M. Shyam Prasad et al.
213 Nd:YAG laser with beam size of ~60 lm with arepetition rate of 10 HZ that provides an energy of ~3–6 mJ. The analyses generate a pit of size ~40 lm. Theabove parameter is the same for all the spherulesanalyzed in this work. The laser was coupled to ThermoX SeriesII Quadrupole. The analyses also included 45 sgas blank background before the start of the analyses.The ablation was done for 85 s with dwell time of ~20 msfor each analysis. The details on the analyses are alsoprovided in Rudraswami et al. (2012). SRM 612 was usedfor bracketing the error. For every 5–10 sample analyses,the SRM 612 was used for bracketing using two analysesbefore and after the unknown analyses. USGS BCR-2Gand BHVO-2G was also included during the set ofanalyses to verify the data in between the analyses.Internal calibration is done using Ca for the quantitativeanalyses. The sets of standards were repeated multipletimes to verify the precision of the data sets before andafter the analyses. The data are processed using Glittersoftware. The standard errors in a majority of the traceelements have ranges from ~10 to 20%, but in someelements can go as high as ~30%. The Glitter softwarecalculated the detection limits for individualmeasurements in each run using Poisson countingstatistics (refer to http://gemoc.mq.edu.au/AnMethods/AnlyticalMeth.html#3 for more details).
DESCRIPTION OF MICROMETEORITES
This study focuses on relict grain-bearing spherulesespecially those with dusty olivines or chromites. Themost prominent relict grains observed are forsteriteswhether they are in a fine-grained matrix or inporphyritic spherules, they are followed by Fe-Ni-Sminerals such as troilite and pentlandite, we have notcome across pyroxene grains in this collection althoughwe found glassy spherules having bulk compositionswhich are pyroxene normative (Rudraswami et al.2012). In all, seven spherules are observed to containchromites, seven spherules are observed to contain“dusty” reverse zoned olivine grains. Three spherulescontain both chromites and dusty olivines. One spherulehas a cluster of olivines and pyroxenes and in a glassymatrix and appears to be a relict of a porphyriticolivine–pyroxene (POP) chondrule. All the spherules arein the diameter range of 173–430 lm; however, onlytwo spherules are the extreme ends of the size range, allthe others are close to the average diameter in thiscollection (265 lm) (Prasad et al. 2013) (Table 1).
CHROMITE-BEARING SPHERULES
Seven spherules are found to enclose chromitegrains (Table 1). Five of them have one chromite grain
in the cross section observed; however, two spheruleshave more than one: AAS38-155#1-P6 contains 10chromite grains, the largest of which is ~80 lm(Figs. 1A and 1B), and AAS38-178#1-P3 has twochromite grains, respectively (Figs. 1E and 1F).
The spherules that contain chromites have adiameter range of 230–443 lm (majority of thembetween 230 and 300 lm). Of the seven spherules thatenclose chromites, five have a criss-cross barred texturewith the gray areas in between the bars having higher Siand Al contents than the bars. Of the other twospherules, one is a glassy spherule and the other is arelict-grained spherule with a dusty olivine inclusion. Thespherules are not barred in the conventional sense, i.e.,the normal barred spherules have olivine lathes arrangedin parallel orientations with the interstitial areas occupiedby glass and magnetite grains. Here, the bars are criss-cross and magnetites are absent. The light colored (BSEimage) area comprises of higher Ca, Fe, Al, and Sicontents. The glassy portions between olivine lathes inbarred spherules were also described as containing Ca,Fe, Al, Si glass by Blanchard et al. (1980).
The presence of chromite (melting temperature is~2000 °C: Greenwood and Earnshaw 1997) implies thespherules have not reached this temperature duringatmospheric entry. One chromite-bearing spherule has atroilite inclusion. The presence of FeS is an indicationthat this spherule was not severely heated during entry,sulfide survival sets an upper limit to the temperatureexperienced by this spherule. The same spherule has asmall chromite inclusion (Figs. 1C and 1D) which ispartly resorbed into the melt.
Jackson (1969) suggested the compositions of olivineand chromite in a given rock help identify their relativeformation temperatures. Therefore, the Fe/(Fe+Mg) ofolivine versus Cr/(Cr+Al) of chromites has beentraditionally used as a formation temperature indicatorfor meteorites for over the last few decades (Bunch et al.1967). In our case, the chromites are mostly found inspherules which have undergone melting duringatmospheric entry and the olivine grains equilibrated inthe spherule as barred olivines, therefore it is notpossible to compare chromite composition with that ofolivine in cosmic spherules. Furthermore, chromite has anarrow compositional range within chondrites, and it isuniform in each chondrite (Snetsinger et al. 1967).Extensive analyses of chromites in meteorites haveconfirmed that chromites in meteorites in view of theirnarrow and characteristic compositions with differentchondrites can be used to classify meteorites (Bunchand Olsen 1975). Over a period of time, Birger Schmitzand his group (Schmitz et al. 2001, 2003; Schmitz andH€aggstr€om 2006; Alwmark and Schmitz 2009) havesystematically analyzed several hundred chromites from
Fig. 1. A) Image of a polished section of spherule showing numerous chromites, three large vesicles are also seen whichcould have been made by the escaping volatile elements during atmospheric entry. Note the criss-cross texture of the spherule.B) Chromite grains shown in (A) under higher magnification, at least 10 chromite grains can be seen having different sizes. Thelight gray areas in between the criss-cross patterns are enhanced in Ca, Al, and Si contents. This is observed consistently in allchromite-bearing spherules. C) Image of a spherule containing a small, rounded chromite (in the circle), a large vesiclecomprising partly of troilite is also seen. Criss-cross texture is also observable in this spherule. D) Image of chromite shown in(C) magnified. This chromite appears to have been resorbed partly into the spherule, troilite grains can be seen at the top rightcorner of the image. E) Image of a spherule containing two chromite grains (at the bottom). Similar criss-cross texture is seen inthe spherule as in the earlier spherules. F) Arrows point toward the two chromite grains shown in (E) under highermagnification. G) Image of a spherule with large vesicles and a chromite which is partly resorbed. The criss-cross pattern ispersistent in this spherule as well. H) The chromite shown in G magnified, it has become subhedral due to melting and heating.All the images shown in Fig. 1 are SEM backscattered electron images.
Ordinary chondritic micrometeorites 1017
fossil meteorites which helped identify the type andsubtypes of the fossil ordinary chondrites discovered bythem. This aspect once again emphasizes the diagnosticimportance of chromites in identifying and classifyingordinary chondrites.
In the present investigation, 17 chromite grains from7 spherules have been analyzed, their major oxide plotsi.e., V2O5 versus TiO2; TiO2 versus Al2O3; V2O5 versusAl2O3; TiO2 versus Cr2O3; and, Al2O3 versus Cr2O3 arepresented in Fig. 2. It is observed that the chromite datafrom the literature presented in Fig. 2 define characteristicareas for different meteorite groups/subgroups ofordinary chondrites in these plots. Major oxide ratios(bulk compositions) of the chromite-bearing spherules arealso plotted to help decipher their provenance (Fig. 3).
There are seven spherules that enclose chromites,with AAS38-155#1-P6 containing 10 chromites(Figs. 1A and 1B), out of which 6 could be analyzedwith the microprobe.
All chromites (but one) in this spherule show similarcompositions (Table 2), confirming the observation(Bunch and Olsen 1975) that chromites from individualchondrites have similar compositions, thereforehighlighting their diagnostic value. The chromites fromthis spherule plot along with the chromites from the highmetamorphic grade chondrites, they plot close to L5, L6,and H5 chondrites in all the oxide plots (Figs. 2 and 3).The high Ti contents of the chromites strongly suggestan L5 or L6 parent body. The TiO2 contents of ordinarychondrites shows an increasing trend from H to L to LLand also from increasing metamorphic grades from 3 to6 (Alwmark and Schmitz 2009). A similar trend is seen inthe chromite enclosed in the spherule AAS38-165#III-P32 in view of its Ti content (Table 2) where the Ti/Crand Ti/Al ratios fall close to L6 chondrites (Figs. 2 and3). However, this spherule also contains a dusty olivineinclusion and the Al/Cr ratios fall close to H3chondrites.
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Fig. 2. Oxide ratios of chromites in the present study compared with those in ordinary chondrites. Chromite data for ordinarychondrites from Bunch et al. (1967) and Wlotzka (2005).
1018 M. Shyam Prasad et al.
It is also seen from the literature (Bunch et al. 1967;Wlotzka 2005) that the chromites from unequilibratedchondrites have low TiO2 and low vanadium contents
as well and they plot in the extreme lower and left ofthe plot of V2O5 versus TiO2 (Fig. 2). We find there areat least three chromites from three different spherules
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Fig. 3. CI-normalized (atom) major element ratios (bulk composition) of the spherules (n = 15) in this study. The areas wherebulk compositions of L and H chondrites occur are flagged. H and L chondrite averages from Wasson and Kallemeyn (1988).
Ordinary chondritic micrometeorites 1019
that plot close in this area. One of them (AAS63-54-P10) contains a large troilite inclusion (Figs. 1C and1D) and the chromite is diffuse and small and appearsto be mostly equilibrated with the melt. Two chromitesare observed in AAS38-178#1-P3 (Figs. 1E and 1F).Both of these chromites plot in two extremes of the Vversus Ti plot (Fig. 2). One chromite plots close to theL3 chromite area of the plot and the other plots in theextreme right side of the diagram. However, both thesechromites are at the rim of the spherule and seaflooretching, in addition to entry heating, could have alteredtheir compositions. However, the larger of the twochromites for which we were able to obtain relativelyreliable analysis plots close to the L3 area of the Vversus Ti plot. Therefore, it can assumed that thisparticular spherule could be a part of an unequilibratedchondrite, as does AAS63-54-P10, which containstroilite.
The spherule AAS38-165-#III-P32 has a dustyolivine relict grain and a diffuse chromite (Fig. 4C).In view of its high Ti contents (Table 2), the Ti/Cr andTi/Al ratios plot in the area of L6 chromites, however,the Al/Cr ratios plot close to H3 chondrites.
The chromite in the spherule AAS38-178#2-P16(Fig. 1F from Rudraswami et al. 2012) is the only onefound in a spherule with a barred texture with parallelolivine lathes. The chromite is large in size (~50 lm)and the Al contents of this chromite are the lowest inthis collection. The chromite grain margins appear to berounding and undergoing resorption into the melt,therefore one cannot be certain of the chemicalalteration undergone atmospheric entry. This chromite
also plots close to chromites in L4 chondrites Ti versusAl plot. In the V versus Al plots close to L4 chromitearea, and in the Ti/Cr plot it again falls close to L4chromites.
Beckering and Bischoff (1995) carried out detailedinvestigations on relict grains in over 200micrometeorites from polar regions (Greenland andAntarctica), 66 of them having relict grains. Theyidentified 140 forsterite grains either in fine-grained orporphyritic micrometeorites and pyroxenes next inabundance (84 relict pyroxenes). The other relict grainsidentified by them are perovskite, chromite, Fe-Nimetal, magnetite, and sulfides. The chromites identifiedby them are small in size and present in spherules havingFa-rich olivine relict grains (similar to the dusty olivinegrains as shown in this study) and the presence ofchromite in a pyroxene normative mesostasis wasreported by them. In addition, they also reported partlyresorbed chromite-bearing relict olivines in a fine-grained to glassy groundmass. However, the chromitesanalyzed by them had Mn contents up to 9 wt% similarto those found in CI chondrites (Endress and Bischoff1993). We did not observe any chromite withenhanced Mn contents. More importantly, the Al2O3
concentrations of all the chromites (Table 2) are in therange 5–6 wt%, which are typical for UOCs and EOCs(Wlotzka 2005; Nehru et al. 1997). The Al2O3 contentsof chromites in carbonaceous chondrites are muchhigher, up to 15–16 wt% (Brearley and Jones 1998).
In summary, two spherules show strong indicationsfor an L6 parent body (AAS38-155#1-P6 and AAS38-153II#1-P9; Table 2). The chromite composition of
Table 2. Chemical compositions of chromites in micrometeorites (wt%).
four spherules agrees with an L3 or H3 parentbody (Table 2). One spherule (AAS-38-165-#III-P32)enclosing a dusty olivine inclusion (Fig. 4C) showsmixed signals. It has high Ti content typical of highmetamorphic grade ordinary chondrites; however, theAl/Cr ratios of the chromite in this spherule indicate aH3 parent body, the dusty olivine inclusion alsosupports an unequilibrated chondrite. The majorelemental ratios (atomic) normalized to CI chondrites ofa majority of the spherules, especially their Mg/Si/CI,Al/Si/CI, and Ti/Si/CI ratios, are close to those ofordinary chondrites (Fig. 3).
Parent Bodies of Chromite-Bearing Spherules
Chromite is a common trace mineral in ordinarychondrites at 0.5–5 vol%, and it is rare or absent incarbonaceous and enstatite chondrites as well as ironmeteorites (Ramdohr 1967; Rubin 1997). Out of theseven spherules that contain chromites two of themshow a strong indication of an L6 parent body, fourspherules indicate unequilibrated chondrites (L3 orH3) as parent bodies and one spherule having adusty olivine inclusion indicates either an L6 (inview of high Ti contents) or H3 parent body in viewof Al/Cr ratios and presence of a dusty olivineinclusion. Overall, the spherules appear to be derivedfrom two different types of ordinary chondritic parentbodies.
In chondrites, chromite occurs in two main forms:Interstitial grains in the matrix and as xenomorphicgrains against silicates and show idiomorphic shapesonly (Wlotzka 2005). Most of the chromites in thepresent investigation appear to be idiomorphic inshapes, some of which appear to be in the process ofundergoing melting and resorption during entry. As perthe observation of Wlotzka (2005) the spherules thatcontain the chromites could be part of ordinarychondritic matrix.
Chondrite-normalized major element bulkcomposition of the spherules that contain chromitesshow Mg/Si, Al/Si, and Ti/Si ratios conforming to thoseof L and H chondrites (Fig. 3). Fe/Si shows depletion,probably because Fe being a relatively volatile elementit could have been depleted during atmospheric entry.However, the Ca/Si also shows depletion with respectto chondrite-normalized ratios. Brownlee et al. (1997)suggested a CM-like parent body for most cosmicspherules; however, they also suggested that thechemical compositions of those belonging to ordinarychondrites could be distinguished based on their majorelement ratios. Most of the major element ratios (e.g.,Mg/Si, Fe/Si, and Ca/Si) indicate their affinity toward Land H chondrites.
AAS38-147-REMSPH-P11
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Fig. 4. A, B) BSE image of a spherule with a large vesicle(making it appear crescent-shaped), a small chromite is seen atthe top right edge of the spherule, which is magnified in (B).C) BSE image of a spherule showing a large dusty olivinerelict grain and a partly resorbed chromite. A large vacuole isseen in this spherule as well. It is discernible that the otherolivines are zoned “normally” in a glassy matrix in the rest ofthe spherule. The vacuoles have been made by the volatileelements which expanded because of heating experiencedduring atmospheric entry.
Ordinary chondritic micrometeorites 1021
DUSTY OLIVINE SPHERULES
Seven spherules are observed to enclose dustyolivines. Two of these are porphyritic and compriseentirely of dusty olivine grains in a glassy matrix(Figs. 5A and 5B), four spherules contain dusty olivineinclusions and have textures which are criss-crossbarred (one of which is a RGB in a fine-grainedmatrix) (e.g., Figs. 5C and 5D), and one spherule hashalf of the olivines reversely zoned and contain smallblebs of almost pure Fe (Figs. 5E–G) and the otherhalf of the olivines are normally zoned (i.e., with aMg-rich core and a Fe-rich rim). This is reverse of thezoned olivines commonly observed in the porphyriticspherules and the olivine observed in the scoriaceousand RGB spherules—where the interior is forsteriticand the exterior is more fayalitic. All the spherulesconform to the description of dusty olivines asproposed by several investigators (Rambaldi andWasson 1982; Jones and Danielson 1997; Hewins1997), in the sense the olivines show a reverse zoningwith an Fe-rich core and an Mg-rich rim and containsmall blebs of almost pure metal or sometimeschrome-spinel inclusions.
Out of the two spherules that comprise entirely ofdusty olivine grains (Figs. 5A and 5B), there are severalcommon features: both have euhedral reversely zonedolivines that enclose chrome-spinel grains, in a glassymatrix. The olivines in both the spherules show vastdifferences in their core and rim compositions with Facontents in the cores of zoned olivine grains reaching≥30 mole% (Table 3). However, the zoning is morepronounced in AAS-38-155-P2, out of five olivine grainsthat were analyzed with the EPMA the Fo contents inthese grains range from 54 to 75 mole%. In addition,one euhedral Fe-rich olivine grain (Fa 69.91) is seen asan inclusion and the largest olivine grain in thisspherule is close to 100 lm in its longest dimension andencloses a vug-like feature comprising of enstatitepyroxene grains (Fig. 5A).
All investigations suggest dusty olivine formation ina reducing environment (e.g., Fox and Hewins 2005);therefore, the presence of an Fe-rich olivine (hortonolitecomposition) close to that of a fayalite could imply thatthe fayalitic grain was incorporated after the formationof the spherule, because fayalite is known to form underoxidizing conditions.
One spherule shows approximately half of theolivines which are dusty in appearance and the rest ofthe olivines show normal zoning (Figs. 5F and 5G). Thedusty olivines in this porphyritic spherule containseveral large blebs of almost pure Fe (Fig. 5G).Another relict grain-bearing spherule contains a verylarge reversely zoned olivine (140 lm) in size and
several other similarly zoned smaller olivines in a glassymatrix (Fig. 5H).
Four spherules have dusty olivines as inclusions in amatrix which has a criss-crossed texture similar to theone described for the chromite-bearing spherules in thisstudy (e.g., Figs. 5C and 5D). The matrix has Ca, Fe,Al, Si glassy composition and the darker “bar” portionshave an olivine normative composition. Magnetite,which is common for all conventional barred cosmicspherules, is once again conspicuous by its absence inthese spherules.
There have been observations of dusty olivines inmicrometeorites. Taylor et al. (2011) presented spheruleshaving similar relict reversely zoned olivines. Beckeringand Bischoff (1995) identified several such olivine-bearing spherules where the Fa mole% was observed tobe dominantly between 12 and 30 mole% with a rarefew greater than 30 mole%. However, Brownlee et al.(1983) report on a scoriaceous spherule which encloses arelict forsterite which contains bright (dusty) metals thatwere devoid of nickel, they suggested that the metalformed due to reduction in silicates and/or thedecomposition of sulfide in a carbonaceous chondriticparent body.
Origin of Dusty Olivine-Bearing Spherules
Dusty olivines were first reported to containNi-poor nanometer-sized metal inclusions (Nagahara1981; Rambaldi 1981; Rambaldi and Wasson 1982).They were suggested to be solid state alterationproducts of a previous generation of chondrules due tothe in situ reduction in Fe from the host olivines(Rambaldi and Wasson 1982; Hewins 1983; Jones andDanielson 1997; Rubin 2006). Therefore, dusty olivineshave a role in understanding chondrule recycling(Ruzicka et al. 2007). Dusty olivine grains withinchondritic meteorites have been proposed as suitablecandidates for retaining a faithful memory of pre-accretionary magnetic fields (Uehara and Nakamura2006; Lappe et al. 2012). Dynamic crystallizationexperiments confirm that dusty olivine can be producedby reduction in FeO-rich olivine in unequilibratedordinary chondrite (UOC) material (Lofgren and Le2002; Hewins 1983; Uehara and Nakamura 2006). Thereduction that formed the Fe-Ni inclusions is thought tobe caused by the presence of organic or graphiticcarbon in the chondrule precursor (Connolly et al.1994). It requires low oxygen fugacity (below theFe-w€ustite buffer) and temperatures between 950and 1500 °C (Boland and Dube 1981). Pack and Palme(2003) observed orthoenstatite as by-product of thisreaction. This perhaps explains the presence of pyroxeneenclosed in one of the large dusty olivine grains in
1022 M. Shyam Prasad et al.
AAS-62-54,#2,P33
AAS-38-155-P2
AAS-38/153,1,P10
AAS-38-178-#2-P2
Fa
px
A B
C D
E F
GG H
Fig. 5. A) Porphyritic spherule comprising entirely of dusty olivine grains in a glassy matrix. Arrow points toward a subhedralfayalitic grain seen in the spherule (Fa). Another arrow points toward a vug-like feature containing pyroxenes (px) enclosed bythe largest dusty olivine grain. B) Another porphyritic spherule comprising entirely of dusty olivines. The individual bright spotsin many grains are chrome-spinel. In addition, small bright areas are metal in the grains. Reverse zoning (Fe-rich interior andMg-rich rim) is seen in all the grains. C) Image of a relict grain-bearing spherule with two dusty olivine grains, in a criss-crosspattern background. D) Dusty olivine relict grain shown in (C) under higher magnification. Reverse zoning and numerous smallblebs of metal are discernible. E) Porphyritic spherule comprising partly of dusty olivines and the others are normally zonedolivines. F) Magnified image of dusty olivine grains with metal blebs and the glassy matrix is also visible. G) Pure Fe metalcontained in one of the dusty olivine grains shown in (F) under high magnification. H) Relict grain-bearing spherule containing alarge dusty olivine inclusion and few other small reversely zoned grains. The matrix is partly glassy and is also sulfur-rich in thetop left of the image. All the images shown in Fig. 5 are SEM backscattered electron images.
AAS-38-155-P2 (Fig. 1A). Semarkona, an L3 chondrite,has chondrules that range in size from a few mm tomicrochondrules measuring <10 lm in diameter(Hutchison et al. 1987) therefore the present range indiameter of the dusty olivine-bearing spherules comeswell within this range in terms of size.
Pyroxene crystals similar to those observed in thevug-like feature in AAS38-155-P2 (Fig. 5A) areobserved in dusty olivine-bearing chondrules of UOCs.Olivine occurring as chadacrysts inside pyroxene grainsin chondrules of ordinary chondrites (L3, Semarkona)have been reported (Jones 1996). In addition to dustymetal in chondrules, Rambaldi and Wasson (1982) alsoobserved minor orthopyroxene and Ca, Al-rich glasswithin the chondrules. Boland and Dube (1981)produced Ni-poor, pure Fe crystals in the interiors ofolivines by experimentally heating olivine underreducing conditions, in addition they also observedpyroxene or SiO2. Rubin (2006) observed a porphyriticolivine chondrule in Semarkona (LL3.0) that containednumerous dusty olivines, some of them enclosing low-Ca-pyroxene similar to the one enclosed in AAS39-155-P2 (Fig. 5A).
Cosmic Spherules or Chondrules (?)
Dusty olivines can be found in ordinary chondrites,CM2, CV3, and CO3 chondrites, but always in type 3chondritic meteorites (Kracher et al. 1984). Dustyolivine grains have been observed in chondrules,more commonly in those of unequilibrated ordinarychondrites such as Semarkona, Chainpur, Inman,Ragland, etc. (Jones and Danielson 1997), and alsorarely as isolated grains in the matrices (Kracher et al.1983). Kracher et al. (1984) describe porphyritic olivinechondrules that consist almost entirely of dusty olivines(e.g., St. Mary’s County LL 3.3 meteorite), the authorsdescribe no other meteorite containing such chondrulesthat comprise entirely of dusty olivines. In the presentinvestigation, two such spherules are found: AAS38-155-P2 (this spherule also contains a fayalite inclusion,and a large hopper dusty olivine that encloses pyroxenecrystals); the low-Ca pyroxene poikilitically enclosed ina dusty olivine AAS-38-155-P2 suggests that thepyroxene also crystallized in situ. AAS38-178-#2-P2(most of the dusty olivines in this spherule containchrome-spinel inclusions). All the olivines in these twospherules show a decrease in Fe/(Fe+Mg) ratio towardthe borders of the grains, which according to Kracheret al. (1984) takes place when a chondrule reacts witha reducing gas. These two spherules appear to bechondrules and were apparently heated to temperaturesnot much above the solidus, because the presence of afayalitic grain in one of them indicates a heating which
is not greater than its crystallization temperature of1205 °C (Deer et al. 1992). Or the fayalitic grain couldhave been captured subsequent to the formation of thespherule.
However, simulation experiments have indicated aminimum temperature of 1250 °C would be needed inorder to form the scoriaceous textured cosmic spherules.Spherules that display porphyritic, barred, or glassytextures would have undergone greater temperaturesduring entry (Fox and Hewins 2005). The presence of afayalitic grain in AAS-38-155-P2 (Fig. 5A) suggests thatthis spherule has survived atmospheric entry withoutattaining such temperature. The spherules are devoid ofa rim of FeO-rich material which is commonly seen onspherules. Such a rim is indicative of heating andoxidation during atmospheric entry, the absence of themagnetite rim also indicates that these two spheruleshave not undergone significant melting during entry.
These chondrules could have been reduced duringcrystallization, so that the FeO does not show normalzoning. This observation of Kracher et al. (1984)appears to be relevant especially for the aboveporphyritic spherules which comprise entirely of dustyolivine grains. In addition, most dusty olivine grains inboth these spherules contain chrome-spinel inclusions.
It appears that the two porphyritic spherules thatcomprise wholly of dusty olivines (AAS38-155-P2 andAAS38-178-#2-P2) can qualify as chondrules and as forthe others, similarity in composition and in appearance(e.g., Figs. 5C–G) between the dusty olivines and thematrix in at least four spherules suggests that theyare all of the same origin—these could have beenchondrules altered during atmospheric entry with a fewdusty olivine grains that still remained unoxidized,probably because the temperatures attained during entrywere not sufficiently high. In only one spherule, a largedusty olivine inclusion in a cryptocrystalline/glassymatrix (Fig. 5H) suggests that this grain is a relict andcould have been part of a matrix or a chondrule.
POP CHONDRULE-LIKE SPHERULE
Van Ginneken et al. (2012) identified unmeltedparticles from Transantarctic Mountains which have agranoblastic texture of olivine and enstatite, which theyidentified as POP chondrule-like materials. Onespherule in this collection could belong to this category:AAS38-186-P8–REMSPH (Figs. 6A and 6B). Thisspherule has grains that comprise an equal mixture ofolivine and pyroxene therefore, it is similar toporphyritic olivine pyroxene chondrule-like materials ina glassy matrix (Figs. 6A and 6B). The olivine andpyroxene crystals are subhedral and appear to be partlyresorbed. The pyroxene is Ca-poor and is enstatitic in
Ordinary chondritic micrometeorites 1025
composition. This spherule could be a POP chondruleof type II.
Porphyritic chondrules contain abundant, relativelylarge (up to one half of the chondrule diameter) fairlyuniformly sized crystals in a fine-grained glassymesostasis. Porphyritic chondrules that are dominatedby olivine (>10/1 volume ratio) are referred to asporphyritic olivine chondrules (Gooding and Keil 1981).Porphyritic chondrules happen to be olivine-rich (PO)or pyroxene-rich (PP) or pyroxene-olivine-rich (POP)with 5–10% of normative feldspar. POP chondrules aremuch rarer (Hutchison 2004). Porphyritic, pyroxene-and olivine-rich (POP) chondrules and porphyritic,pyroxene-rich (PP) chondrules constitute approximately50 vol% and 10 vol% of all chondrules in ordinarychondrites, respectively (Gooding and Keil 1981; Jones1994).
Trace Element Compositions
Seven spherules were analyzed for trace elements(Table 5; Fig. 7), three of which contain chromites,three others contain dusty olivines, and one spherulecontains chromite and dusty olivine inclusions. Twospherules show UOC characteristics; i.e., AAS38-155-1-P2, AAS38-178-2-P2. Both of these spherules comprisewholly of dusty olivines and are suggested here tobelong to UOC parent source (by way of chromitecomposition or presence of dusty olivines): the Rb andZn contents do not show as much depletions as dothose that show characteristics of L5 or L6 chondrites(e.g., AAS38-153-II-P9 and AAS38-155-1-P6; Fig. 7).
Wang and Lipschutz (2007) suggested a consistencyin the trace elemental compositions of all UOCs (H,Lor LL), and except for the highly volatile elements suchas Cd, Bi, and Tl. Rb in their analysis shows a narrowrange (CI-normalized) with not much depletion withrespect to CI for UOCs. Anders and Zadnik (1985)suggested the contents of the volatile traces in ordinarychondrites decreases with increase in metamorphic type.In contrast, the bulk contents of refractory elementsremain remarkably uniform for all chondritemetamorphic types (Ebihara et al. 1996).
Micrometeorite internal texture has been identifiedas a qualitative indicator of the amount of heatingundergone during atmospheric entry (Taylor et al.2000). Simulation experiments have substantiated theseobservations (Toppani and Libourel 2003). If weconsider the above, the textures of the chromite-bearingspherules (at least 4–5 of them) are similar, whereastheir chromite chemistry indicates different subgroupsof ordinary chondrites. Their volatile trace elements(Rb, ZN, Pb) also show complementary differences.Strong depletion of Ba, Rb, Zn, and Pb is seen for thetwo spherules which indicate an L5/L6 parent body(AAS38-153-II-P9 and AAS38-155-I-P6), whereas forthe other spherules for which r parameters such aschromite composition, presence of dusty olivines, etc.show an UOC parent linkage, the depletion of thevolatiles is not that strong (Fig. 7).
Th, U, and Ba show enhancements which reach109 of CI values in some spherules (Fig. 7). Thesespherules had long residence times on the deep seafloorwhich could be up to 50,000 yr (Prasad et al. 2013).Pattan et al. (2005) and Pattan and Jauhari (2001)measured trace elements from the sediments in this partof the Indian Ocean. The sediments here containTh = 11–12 ppm, U = 1–1.5 ppm, and Ba = 3000–4000 ppm. In contrast chondrites have Th and U at ppblevels, and Ba is a few ppm (Mason 1971), which areseveral orders of magnitude less. Therefore, theenhancements in Th, U, and Ba contents of the
OL
PX
PX
PX
PX
PX
PXOL
OL
OL
OL
PX
PX
AAS38-186-P8 -REMSPH
A
B
Fig. 6. A) BSE image of a porphyritic olivine pyroxenespherule. A cluster of olivines and pyroxenes are seen in thecenter of the spherule in a glassy matrix. The mineral grainsappear to be partly resorbed in the matrix. B) The olivine andpyroxene cluster magnified (PX = pyroxene; OL = olivine) theolivine is forsteritic and the pyroxene is mostly enstatite (datapresented in Table 4).
spherules appear to be due to incorporation of theseelements from terrestrial sources.
DISCUSSION AND CONCLUSIONS
Most investigations on micrometeorites involvinglarge numbers of particles have proposed a dominance ofcarbonaceous chondritic material in the particulate fluxon the Earth which is CM or CI chondrite (Brownleeet al. 1997; Prasad et al. 2013). Genge and Grady (2002)proposed 70% contribution from carbonaceouschondrites and 30% from an ordinary chondritic source;whereas Taylor et al. (2000) supported this division, butsuggested a small (1–2%) contribution from ironmeteorites and an even smaller contribution (0.5%) fromachondrites. This contrasts with the meteorite fluxwhere ordinary chondrites (87%) dominate the fluxon the Earth (Krot et al. 2003). This difference isattributed to physical properties of the carbonaceouschondrites which tend to fragment much more easilyduring asteroidal collisions and produce largerquantities of dust-sized materials (Flynn 1989). Besidesfragmentation in space, atmospheric breakup plays amajor role in determining what types of materials cansurvive atmospheric entry to make micrometeorites.Revelstoke and Tagish Lake are excellent examples ofmaterials that were almost entirely disrupted into dustand vapor during entry. However, in more recentinvestigations, ordinary chondritic materials have beenobserved to comprise at least 33% of the large-sized(>500 lm) particles (Suavet et al. 2010, 2011; VanGinneken et al. 2012). Genge (2008) suggests a substantialcontribution from ordinary chondrites (~20%) to themicrometeorite flux.
In addition, meteorites comprise of chondrules,matrix, AOAs, and CAIs. In contrast, matrix-dominantmaterials dominate the micrometeorite populationswhere it is observed that at least 25% of most majorunbiased collections comprise of scoriaceous materialswhich have a provenance of CI chondritic parent bodies(Taylor et al. 2000; Prasad et al. 2013). However, amongthe unmelted populations of micrometeorites, the parentbodies of many materials could be ascribed with greatercertainty and among these populations ordinarychondrites (e.g., Genge 2008), achondritic materials, andeven fragments containing chondrules have beenobserved (Gounelle et al. 2009; Van Ginneken et al.2012). It has been possible to identify chondrules amongthe melted micrometeorite populations, recently Imaeet al. (2013) identified four CO3 chondrules. We haverecently attempted to do so using oxygen isotopes(Rudraswami, personal communication). In the presentinvestigation, perhaps for the first time, we find a strongevidence for ordinary chondritic parent sources amongthe spherules and also possible chondrules fromunequilibrated ordinary chondrites. An individualchondrule enclosed in a cosmic spherule has beenidentified earlier (Reshma et al. 2013), but that has beenan isolated occurrence.
In this study, seven spherules with chromites, sevenspherules with dusty olivines, and one POP spherule arepresented. Their morphologies, major elementcompositions, trace element compositions, and moreimportantly, chromite composition enables us toidentify them to be of ordinary chondritic provenance.These constitute 15 of 481 spherules investigated andcomprise only ~3% of the total number of spherules.Prasad et al. (2013) investigated all the 481 spherules in
Fig. 7. CI chondrite-normalized trace element compositions of micrometeorites (CI chondrite values from Anders and Grevesse1989). The data are presented in the order of increasing volatility from left to right.
1028 M. Shyam Prasad et al.
this collection and observed a majority of the spheruleshad undergone alteration during atmospheric entrywhich was revealed in their textures such as barred,cryptocrystalline, and glassy; however, based on theirmajor element ratios these spherules could be identifiedas chondritic and possibly either CI or CM. In addition,they also found 8.5% of these spherules to bescoriaceous in texture and could have had CI chondriticparent sources. The I-type and G-type spherulestogether constituted an additional 8.5% which duringtheir recent investigations using the PGE contents ofFe-Ni beads Rudraswami et al. (2014) could identify achondritic parent source. Prasad et al. (2013) furtheridentified one spherule in this collection to beachondritic. It is possible there could be a largernumber of particles with ordinary chondriticprovenance, however, due to the alteration undergoneduring atmospheric entry they are not recognizable.While all the 15 particles in this investigation appear tobe from ordinary chondrites, their chromite compositionallows us to make the distinction that at least one ofwhich (AAS38-155-#1-P6; Figs. 1A and 1B) could befrom an L5 or L6 chondrite, and at least two of thedusty olivine-bearing particles (Figs. 5A and 5B) appearto be chondrules from an unequilibrated ordinarychondrite. In view of the large volumes occupied bychondrules in ordinary chondrites, there should bemany more chondrules from ordinary chondritic sourceamong the micrometeorite flux.
Acknowledgments—The authors are grateful to theDirector, NIO for the encouragement and the facilities.MSP, NGR, and AAR are grateful to PRL,Ahmedabad for the PLANEX project onmicrometeorites and to the CSIR XIIth Plan projectGEOSINKS. EVSSKB and TVK are grateful to theDirector NGRI, Hyderabad for the facilities. Theyfurther acknowledge financial support through MLP-6513-28-EVB and PSC-0204 (INDEX)-WP-2.1 projectgrants. We acknowledge with gratitude the helprendered by Vijay Khedekar and Samena Balgar duringthe analyses and sample preparation, respectively. Weare grateful to an anonymous reviewer for thecomments and suggestions; and Christian Koeberl andDon Brownlee for the detailed, painstaking reviews andsuggestions which helped improve the manuscriptconsiderably. The samples were collected during thecruises for the project: Surveys for Polymetallic Nodules(PMN) by one of us (MSP), thus we thank the Ministryof Earth Sciences for the funding of the PMN Project.This is NIO’s contribution number 5716.
Editorial Handling—Dr. Donald Brownlee
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