Nickel-iron in lunar anorthosites

EARTH AND PLANETARY SCIENCE LETTERS 8 (1970) 387-392 . NORTH-HOLLAND PUBLISHING COMPANY

NICKEL-IRON IN LUNAR ANORTHOSITES John S. DICKEY, Jr.

Smithsonian Astrophysical Observatory, Cambridge, Massachusetts 02138, USA Received 7 May 1970 Nickel-iron grains in anorthositic particles found in lunar soil from Mare Tranquilitatis contain 6 to 29% Ni . Similar metal grains in the more common Ti-rich basalts are nearly pure Fe. Either the anorthosites have been selectively contaminated by meteoritic metal, or Ni has been magmatically fractionated . If the anorthosites and basalts of Mare Tranquilitatis are related by magmatic fractionation, the process probably involved fractional crystallization followed by partial fusion of pyrorene-rich cumulates .

1 . Introduction The sample collected at Tranquility Base by the Apollo 1 1 astronauts comprised basaltic igneous rocks, microbreccias, and lunar soil (11 . Among the igneous rocks, dark Ti-rich basalt, composed essentially of ilmenitc, calcic plagioclase, and Ca-poor clinopyroxene, predominated . Fragments of Ti-rich basalt also dominated the 1 to 5 mm size fraction of the soil ; however, 4% of this particulate material is of a distinctive, anorthositic suite 121 . These particles, consisting of anorthite plus subordinate olivine and pigeonite, range from true anorthr"sites (plagioclase > 90%) to anorthositic gabbros (plagioclase < 7717) . They are less dense (2 .9 vs. 3 .3 g/cm3 ) and lighter in color than the basalts . Their chemical compositions, t ich in Ca and Al and poor in Ti, resemble the composition of Tycho ejecta analyzed by the Alpha Scattering Analysis Instrument on Surveyor V11 131 . This chemical similarity, the light colors and low densities led Wood et al. 121 to suggest that the anorthositic particles are impact ejecta, tossed into Mare Tranquilitatis from the lunar highlands . An alternate possibility, that the anorthosites crystallized in Marc Tranquiftatis, perhaps as cumulates %%thin the basalts, seemed unlikely to them because of the marked difference in Ti content between the basaltic and anorthositic suites . Even the most mafic anorthositic gabbro contains less than l 'n 7 i0? (versus 8 to 1 12%, in the basalts 14---71 ), and none contains

ilmenite. A more compelling distinction between the basaltic and anorthositic suites has now appeared : both the Ti-rich basalts and the Ti-poor anorthosites contain grains of Fe-rich metal, but whereas in the basalts the metal is nearly pure Fe, the metal in the anorthositic rocks contains 6 to 29% Ni. 2. Metal compositions Fig . 1 shows the range and distribution (but not the frequency) of Ni cor:centrations in metal grains in small (I to 5 mm) particles from the Apollo I 1 bulk sample (LRL Sample No. 10085,24) . The ranges shown are based on metal grains in only a few (29) particles, for although most particles contain metal the grains are seldom large enough for quantitative analysis . In both the basalts and the anorthosites metal is a minor accessory which occurs interstitially with troilite . In the basalts the metal forms globular inclusions in troilite; in some anorthosites there are independent metal grains as well as inclusions in troilite . In glass particles the metal typically occurs as tiny spherules . The breccias contain metal iii lithic fragements, in glass, and as isolated grains. The distribution of Ni in the anorthositic metals is discontinuous : its in many meteorites, a low Ni phase, kamacite (Ni G 7.517n), coexists with a high Ni phase, tacnite . This partitioning of Ni, found between homogeneous grains and within individual,

JS .DICKEY, Jr .

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BASALTS

GLASSES

ISRECC 1 AS 000000 ANORTHOSITES

t o

00

000000000

0000

10

20

ao

WEIGHT % Ni

Fig. 1. Ni concentrations in metal grains from various types o.° particles in soil from Tranquility Base . Note that this is not a frequency diagram. The ranges shown are based upon the following number of particles/analyses : basalts (8/11), glasses (5/ IS). breccias (10/149), anorthosites (6/159). In addition to Ni and t"e these alloys contain about 0.5% Co . Analyses by MAC Model 400 electron microprobe, using 20 kV accelerating voltage, 0.01 pa sample current, 30 sec courting times, and Ni ., 5Fe"6S and pure Co metal standards. Total estimated uncertainty = ± 0.3`1 Ni . Filled circles, basaltic compositions ; open circles : anorthositic composi tions.

multiphase grains, is a consequence of cooling in the Ni-Fe system [8, 9j . During cooling Ni-Fc 2 ~oys pass from ., one-phase, taenite, field into a two-phase, taenite + kamacite, field . With continued cooling the Ni content of loth phases and the relative amount of kamacite, the low Ni phase, increase . This process depends upon diffusion of Ni away from the kamacitctaenite interface and is, therefore, facilitated by slow cooling.

3. Cooling rates Using the Ni-Fe phase diagram, coefficients of diffusion, and Fick's diffusion equation, Wood [i3, 101 showed that Ni-Fe partitioning between coexisting metal phases may be a quantitative indicator of the cooling histories of octalledrites and other nickeliroun-bearing meteorites . Unfortunately, no lunar particle yet examined contains enough analyzable metal to apply Wood's method rigorously . One fragment of anorthositic gabbro, however, contains

metal grains which are susceptible to qualitative interpretation . This anorthositic gabbre~ (fig . ?) is an undeformed, hc>locrysialline rock containing 77'7 anorthite, 11"X pigeonite, and 470 olivine. The rock has a poikilitic texture created under stagnant conditions by coprecipitation and accumulation of olivine (F 0 67 ,70) and anorthite (An96-98) followed by crystallization of the intercumulus liquid as optically continuous pigeonite . (Chemical analyses of all visible phases will he found in [ I I J ) . Troilitc and nickel-iron constitute less than 0 .5% of the rock . They occur interstitially, typically together (fig . 3) . Most of the metal grains are homogeneous, but some are not . Fig. 4 illustrate .Ni distribution in a zoned metal grain . Profiles of Ni concentration me :usured neriticc vrrv slowly cooled nickel-iron assemblages are characterized by high, sharp taenite peaks standing above broad stretches of kamacitc . The profile in fig. 4c is not of that form . This low, broad profile indicates limited Ni diffusion and, therefore, fairly rapid cooling . By comparison with diffusion models computed

NICKEL-IRON IN LUNAR ANORTNOSITES

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Fig . 2 . Anorthositic gabbro particle (38 " 1) from Tranquility Base . Subidiomorphic anorthite (white to light gray) and olivine (light gray) crystals lie in a field of optically continuous pigeonite (dark gray) . The large anorthite and olivine crystals contain, respectivcly, inclusions of olivine and inorthitc . Viewed by transmitted light with partly crosses nicols . lVidih of field : 2 .5 mm .

by Wood [S, 101 it appears that an appropriate cooling rate estimate for this lunar gabbro is of the order of I °C per year . Such a cooling rate corroboraies the textural evidence that the parent body . v.s ., r e. ., t1. k.W :... . ., :. .._ HtI h OI ICiti\J~7 Intrusion . 4. Discussion The presence of nickel-iron in the anorthosites bitt not in the bas,,tlt,-. from Marc Tranquilitatis indict tes

that either the anorthosites contain a meteoritic component which the basalts lack or else indigen )us, lunar Ni has been fractionated . The possibility meteoritic Ni in the anorthosites can not be discounted, especially it they (to come from the highlands. The density of impact craters in the lunar highlands is about 30 times that observed in the maria 1121, so the highlands are assumed to be older than the maria and to contain more meteoritic debris . Meteoritic metal grains in ancient highlands soil might have been assimilated by lunar magmas and rt. precipitated . In a

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J.S . DICKEY, Jr

Fia, . 3 . Nickel-iron (M) and troilite (T) amid intercumulus pigeonite (gray) and cumulus anorthite and olivine crystals (dark gray) of anorthositic gabbro (38-1) . Viewed by transmitted and reflected light . Width of field : 650 microns.

such rocks a meteoritic contribul ion would be difficult, perhaps impossible, to distinguish from iwligenous Ni . The plausibility of selective contamination of the anorthosites has been reduced, however, by the discovery of nickel-iron in olivine basalts of the Apollo 12 sample fronâ nccanuis Procellarum . Reid et al. 1131 report that metal grains in rocks 12004 and 12022 contain up to 30% Ni. Furthermore, they have found evidence that the metal phase was constantly precipitating and extracting Ni from the magma : metal inclusions in early formed olivine and

chromite crystals contain 15 to 30"h Ni ; subsequent metal grains, contain progressively less Ni ; and the last metal to precipitate was nearly pure Fe. Wasserburg 1141 has found, by the Rb-Sr method, that basalts from Mare Tranquilitatis and il __ .._ ..~ n_,. . ._tt_. ..... ...tt"..- .t . . . ._t . . .t ._ VcealILIS l Iocellalul(t 4.rylla1llC.CU al appruÄllllatuly lil<' X same time (- 3.4 to 3.6 101 years ago) . Thus their different metal compositions are not likely t« have been caused by different amounts of meteoritic contamination . The evidence points, rather, toward fractionation of indigenous, lunar Ni . Ni can be strongly fractionated in basic igneous

NICKEL-IRON IN LUNAR ANORTHOSITES

39 1

l ~r ä

z R W

I ls

MICRONS

zo

zs

Fig . 4 . A nickel-iron grain in anorthositic gabbed (38-1) which contains coexisting kamacite (7% NO and tacnite (8 .6 to 17 .6% NO . (a) Viewed by reflected light . Line A-Ii indicates the course of the tranverse shown in (c), but the outline of the grain has been altered by subsequent polishing . Before the latest polishing minute amounts of troilite were visible along the margins of the grain . (b) Areal display of Ni 1`Ck X-ray emission . Ni concentration is proportional to the brightness . (c) Profile of Ni concentrations along traverse A-B . Analyses by electron microprobe .

rocks. In the crust of the earth the Ni-' ion substitutes for Mg`+ and pet+ in early formed olivines and pyroxenes; and in differentiated igneous bodies, such as the Skaergaard intnision 1151, virtually all of the Ni in the ntagtna is concentrated in the early precipitates . Under the low oxvrzen fugacities of lunar nlagnlas (less by a factor of - 105 than f( in comparable terrestrial magmas I 1 ~) even more efficient Ni fractionation can be u~complished by nickel-iron precipitation . h'ractionation of Ni in the basalts of Oceanus 1'rocellarutn sul.;gests tliat a similar process could

produce the different Ni concentrations found in the a iortliosites and Ti-rich basalts of Mare Tranquilitatis . Indeed, were it not for the absence of plagioclase on the liquidus of the Ti-rich basalts 116, 171 they and the anorthosite, might be ini;rpretted as com trnlement "lrv ... ..or .,ll'~ ..r' " . .. ""lrvi ~ --d-c- of "mw" w.efioWaaa1m.y oactWit . : .tViÎ . Since the basal ; liquids are not saturated with 1)iagioclase however, their relationship to dhc anorthocites must involve at least two magmatic episodes . The simplest two-stage process would be : ( I) differentiation of gabbroic magma into anorthiteand pyroxene-rich layers followed by (?) partial

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J.S .DICKEY, Jr .

fusion of the pyroxene-rich layer to form Ti-rich basalts . Such a scheme, although simplistic, can account for two striking characteristics of the basalts [I I, high Ti concentrations and negative Eu anomalies; for, in ad,,-'ition to providing magmas which need not be saturated with plagioclase, the pyroxene-rich soyirce layer would be already enriched in Ti and deple- .ed in Eu (relative to the other rare earths). Acknowledgements 1 thank my colleagues, J.A.Wood, U .B .Marvin, and B.N .Powell, for their assistance and advice . This work has beer supported by NASA grant NAS9-8106 (J.A. Wood, Principal Investigator) . References [11 Lunar Sample Analysis Planning Team, Summary of Apollo 11 Lunar Science Conference, Science 167 (1970) 449-451 . [21 J.A .Wood, J .S .Dickcy, Jr ., U.B .Marvin, and B .N .Powell, Lunar anorthosites, Science 167 (1970) 602--604 . [31 A .L .Turkevich, E.J .Franagrote, and J.H .Patterson, Chemical analysis of the Moon at the Surveyor VII landing site : preliminary results, Science 162 (1968) 117--11-1 . ( 4 1 A .F .J .Er gel and C.G .Engel, Lunar rock cot ipositiuns ;jnd som : int---rpretations, Science 167 (19; 0) 527 - 528. [~ ; I.A .Maxwell, S.Abbey, and W .H .Champ, Cl-emical composition of lunar materia3, Science 167 (19 0) 530-531 .

[61 H .B .Wiik and P.Ojanpera, Chemical analyses of lunar samples 10017, 10072 and 10084, Science 167 (1970) 531-532. [7] L.C.Peck and V.C .Smith, Quantitative chemical analysis of lunar samples, Science 167 (1970) 532. [8] J.A .Wood, The cooling rates and parent planets of several iron meteorites, Icarus 3 (1964) 429-459. [9] J .I .Goldstein and R.E .Ogilvic, The growth of tt.e Widmanstätten pattern in metallic meteorites, Geochim. Cosmochim. Acta 29 (1965) 893 --920. 1101 J.A .Wood, Chondi . , : their metallic minerals, thermal histories, and parent planets, Icarus 6 (1967) 1-49. III] J .A .Wood, J.S .Dickey, Jr ., U.B .Marvin, and B.N .Powell, Lunar anorthosites and a geophysical model of the moon, Geochim. Cosmochim. Acta (in press) . [121 M.Eimer, Lunar environment, In : Lunar Missions and

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Exploration, eds. C .T .Leonides and R.W .Vance (New York, John Wiley, 1964) 60-85. A.M .Reid, C.Mcycr, Jr., R.S .Harmon, P.Butler, Jr ., and R.Brett, Metal in two Apollo 12 igneous rocks (abstract), Trans. Amer. Geophys. Union (in press) .

[141 G .J .Wasserburg, The moon : observations and conclusions from Apollo 11 and 12, paper presented at the 51 st annual meeting of the American Geophysical Union, Washington, D.C ., Aptil 1970 . 1151 L.R .Wager and R .L .Mitchell, The distribution of trace elements during strong fractionation of basic magma a further study of Skaergaard intrusion, East Greenland, Geochim . Cosmochim. Acta 1 (1951) 129-- 208. M.J .O'Flara, G.M .Biggar and S .W .Richardson, h xperimental petrology of lunar material : the nature of mascons, seas, and the lunar interior, Science 167 (1970) 605 - 607 . 1171 A .h. .Ringwvod and h:+:,sene, Petrogenesis of lunar basalts and !ie internal constitution and origin of the moon, Science 167 (1970) 607-610.

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