A ternary-diagram approach is used to show on a single map as much detailed geochemical information concerning petrologic variations within the lunar crust as is possible. The classification map shows the global spatial distributions of end-member compositions, the transitional spatial relations between end-member compositions, and quantitative estimates of relative proportions of each end member at each pixel location within the orbital groundtracks. The use of elemental ratios in this analysis, instead of the commonly used elemental bivariate diagrams, shows geologic information that is otherwise hidden in individual elemental databases

Davis, Philip A.

Spudis, Paul D.

English

NASA Technical Reports Server

Global Pet ro logic  Var ia t ions  on the  Moon: A Ternary-Diagram Approach 
P h i l i p  A, Davis and Paul D. Spudis, Branch of Astrogeology, U,S, Geological 
Survey, F l a g s t a f f ,  Arizona 86001 
Apgmach, We have used the ternary-diagram approach outl ined here in  i n  an 
attempt t o  show on a s i n g l e  map a s  much de ta i l ed  geochemical information 
ind ica t ing  pet ro logic  v a r i a t i o n s  wi th in  the  lunar  c rus t  a s  poss ib le ,  We 
confine the  presenta t ion  of our ternary-diagram ana lys i s  and subsequent 
discussion of the  r e s u l t s  t o  use of the  Fe (wt%) versus (Th/Ti)c v a r i a t i o n  
diagram, because these  da ta  include only the  Apollo gamma-ray o r b i t a l  da ta ,  
which have more g lobal  coverage (about 19%) than t h a t  of the  X-ray o r b i t a l  
da ta  (about 9%). The (Th/Ti) r a t i o  represents  the  observed Th/Ti r a t i o  
normalized t o  C1 chondri te  varues. We produced an e r r o r  database f o r  Fe and 
(Th/Ti), by the  standard method of determining e r r o r  from counting s t a t i s t i c s  
t o  take i n t o  account the  a n a l y t i c a l  e r r o r s  associa ted  with the  Fe and (Th/Ti)c 
da ta  i n  the  present  analys is ,  
The method s t a r t s  by es t ab l i sh ing  a te rnary  reference diagram whose th ree  
s ides  a r e  each divided i n t o  e igh t  segments. Each ternary  subdivision is  
assigned a d i s t i n c t  color ;  t h e  colors  represent  a s p e c t r a l  continuum from red 
t o  green t o  blue t o  red, The center  of the diagram is a t r i a n g l a r  a r e a  
representing approximately equal proport ions of each apex, and it is  thus 
assigned a gray color .  Assignment of rock end-member compositions t o  the  
three  apexes allows rock o r  s o i l  compositions t h a t  a r e  binary o r  te rnary  
mixtures of these three  end members t o  be represented a s  continuous colors  i n  
the  v i s i b l e  spectrum. 
Each of the  end-member Fe and (Th/Ti), compositions is  an average 
ca lcula ted  from values reported in' the  l i t e r a t u r e .  The &-suite ( t r o c t o l i t e  
and n o r i t e )  and KREEP rocks a r e  represented by the  red apex, the  mare basa l t  
by green, and the  fer roan anor thos i t e  by blue. Thus, we can now ass ign a 
color  from the  ternary  reference diagram t o  each p ixel  i n  the  o r b i t a l  
geochemical databases,  using the  Fe-concentration and (Th/Ti),-ratio values of 
the  p ixe l  and the  te rnary  apexes. The r e l a t i v e  proport ion of each of the  
three  compositional end members (apexes) needed t o  produce the  observed 
composition of a p a r t i c u l a r  p ixe l  i s  determined tr igonometrical ly,  Once the  
p ixe l ' s  loca t ion  wi th in  the diagram has been determined, the  color  a t  t h a t  
loca t ion  is  assigned t o  t h a t  pixel"  pos i t ion  i n  a new image o r  map f i l e ,  
This process is  then repeated f o r  every p ixe l  within the  o r b i t a l  databases,  
Also, the  frequencies of occurrence of p ixe l s  a t  a p a r t i c u l a r  t e rna ry  
composition a r e  accumulated within a t e rna ry  scattergram. 
Certain p ixels  within the  Fe and (Th/Ti) databases have high 
uncertaintie?,mostly because of t h e i r  low o r b z t a l  accumulation times. To 
determine the  e f f e c t s  of these e r r o r s  on the  a r e a l  abundance of u n i t s  wi th in  
the  c l a s s i f i c a t i o n  map, we decreased by increments the  amount of e r r o r  t h a t  
p ixe l s  could have i n  terms of Fe and (Th/Ti)c before being excluded from the  
c l a s s i f i c a t i o n  map. The following discuss ion pe r t a ins  t o  the  c l a s s i f i c a t i o n  
map (not shown) i n  which p ixels  t h a t  have e r r o r s  g r e a t e r  than 75% a r e  
excluded. This l e v e l  of e r r o r  exclusion provides a reasonable amount of 
c e r t a i n t y  f o r  the  remaining u n i t  p ixe l s  and t h e i r  a r e a l  abundances, 
MscussPon, Examination of the  c l a s s i f i c a t i o n  map allows easy determination 
of (1) the  g lobal  s p a t i a l  d i s t r i b u t i o n  of end-member compositions, ( 2 )  the  
t r a n s i t i o n a l  s p a t i a l  r e l a t i o n s  between end-member compositions, and ( 3 )  
q u a n t i t a t i v e  es t imates  of the  r e l a t i v e  proport ions of each end member a t  each 
p ixel  loca t ion  within the  o r b i t a l  groundtracks, The use of elemental r a t i o s  
i n  our analyses ,  ins t ead  of the  commonly used elemental b i v a r i a t e  diagrams 
https://ntrs.nasa.gov/search.jsp?R=19870013974 2020-03-20T10:25:20+00:00Z
[1 ,2] ,  shows geologic information t h a t  i s  otherwise hidden i n  individual  
elemental databases,  
The Apennine Bench region i s  shown t o  have a composition corresponding t o  
a mixture of KREEP and mare b a s a l t ,  which i s  consis tent  with the r e s u l t s  of 
previous s t u d i e s  [3,4],  Other a reas  of Mg-suite/KREEP mate r i a l  a r e  i n  the  
f a r s i d e  highlands near  Van de Graaff (18O-29OS9 175°-1710W), within the  
Hertzsprung bas in  (3'-5OS, 125°-1300W), and south of Mare Smythii and west of 
Pasteur Crater (7O-15OSS 76O-98"E). The f i r s t  two of these  KREEP-rich 
highland areas  coincide with areas  of highland c r u s t a l  thinning [5,6] t h a t  a r e  
covered by the  o r b i t a l  gamma-ray da ta ,  An inverse  r e l a t i o n  between Th 
concentrat ion and highland c r u s t a l  thickness has been reported by [7] ,  The 
preliminary elemental concentrat ions obtained by [8] suggest t h a t  the  Van de 
Graaff region may have a "g ran i t i c"  rock composition, s imi la r  t o  t h a t  of 
sample 12013, Generally, the  average composition of lunar g r a n i t e s  is  lower 
i n  Fe, Ti ,  and @ and s i g n i f i c a n t l y  higher i n  K and Th than t h a t  of KREEP 
basa l t s .  The Fe and Ti concentrat ions of these  three  highland areas  a r e  
indeed lower than t h a t  a t  any of the  th ree  nearside high-KREEP a reas ,  possibly 
because of the  proximity of maria t o  the  KREEP-rich a reas ;  however, the th ree  
highland KREEPy areas  do not appear t o  be associa ted  with extensive mare 
deposi t s .  For K, [9] have presented preliminary gamma-ray data  showing the  
Van de Graaff region t o  have only 880 ppm K, whereas they repor t  the  Fra Mauro 
region a s  having 2680 ppm K, This s i g n i f i c a n t l y  lower K value f o r  Van de 
Graaff s t rongly  suggests  t h a t  these KREEP/Mg-suite highland areas  ( a t  l e a s t  
those near Van de Graaff)  a r e  not composed of "g ran i t i c"  rock. They a r e  most 
l i k e l y  e i t h e r  "KREEPy basa l t s "  [ l o ]  r e s u l t i n g  from volcanism propagated by 
c r u s t a l  thinning i n  these  areas ,  o r  the  remnants of an Mg-suite pluton exposed 
by an e a r l y  impact event (such a s  the  South Pole-Aitken basin;  [ I l l ) .  
Our c l a s s i f i c a t i o n  map a l s o  shows t h a t ,  a t  the  s p a t i a l  resolu t ion  (about 
100 km) of the  gamma-ray instrument, the  c e n t r a l  regions of most major maria 
have r e l a t i v e l y  pure mare-basalt compositions. Only Mare T r a n q u i l l i t a t i s  
appears t o  have compositions t r a n s i t i o n a l  between mare basa l t  and fer roan 
anor thos i te ,  which i s  probably the  r e s u l t  of the addi t ion  of underlying 
a n o r t h o s i t i c  highlands debr i s  t o  mare-basalt r ego l i ths  by v e r t i c a l  mixing 
through r e l a t i v e l y  t h i n ,  young, blue mare-basalt flows [12,13]. A t  
Aristarchus,  the  u n i t  map ind ica tes  a mixture of KREEP, mare b a s a l t ,  and 
fer roan anor thos i t e ,  which grades i n t o  a more KREEP- and mare-basalt-rich u n i t  
a t  the  north border of the  groundtrack. The presence of these  two u n i t s  can 
be a t t r i b u t e d  t o  the  r e l a t i v e l y  t h i n ,  young, blue mare-basalt flows i n  these  
two areas  [14] t h a t  mixed with underlying KREEP- o r  Q-sui te- r ich  highland 
t e r r a i n .  This underlying mater ia l  i s  present  a t  r e l a t i v e l y  shallow depth, a s  
indica ted  by i t s  exposure wi th in  Aristarchus Crater [15,16], 
A s e r i e s  of r e l a t i v e l y  young lava flows with well-developed flow f r o n t s  
occur i n  southwestern Mare Imbrium [17]. I n  addi t ion  t o  t h e i r  s t r i k i n g  
morphological development, remote-sensing data  i n d i c a t e  t h a t  these lava  flows 
a r e  r i c h  i n  Ti  [14], r e l a t i v e l y  r i c h  i n  Th (8.0 ppm; [18]) ,  and young ( l e s s  
than 2.0 bey. o ld ;  [19]).  These high-Ti, KREEP-rich lavas  a r e  represented on 
our pe t ro logic  map by two u n i t s  of mostly mare basa l t  with some KREEP 
component. 
Another i n t e r e s t i n g  area  is  wi th in  and near  the  Balmer basin on the  lunar  
eas te rn  limb (10°-150S, 75OE). The pos i t ion  of t h i s  zone c o r r e l a t e s  with the  
l i g h t  p la ins  f i l l  of the  Balmer basin,  which has been described previously a s  
KREEP-rich, mare-like deposi t s  [10,20], Our c l a s s i f i c a t i o n  map shows these  
p la ins  t o  represent  roughly an equal mixture of anor thos i t e ,  mare b a s a l t ,  and 
KREEP/Mg-suite ma te r i a l e  The i d e n t i f i c a t i o n  of dark-halo c r a t e r s  i n  t h i s  
r e g i o n  [20,21]  s u p p o r t s  t h e  s u g g e s t i o n  t h a t  l i g h t  p l a i n s  i n  t h e  r e g i o n  t h i n l y  
mant le  b u r i e d ,  KREEP-rich mare-basal t  f lows .  These b a s a l t  f lows  a r e  p robab ly  
o l d e r  t h a n  3.9 b e y .  because  t h e y  a r e  b u r i e d  by h igh land  p l a i n s  of Imbrian t o  
N e c t a r i a n  a g e  [22].  It t h u s  a p p e a r s  t h a t  t h e  a n c i e n t  l u n a r  maria ( o l d e r  t h a n  
3.9 b .y . )  had a  d i v e r s i t y  of chemical  compos i t ions ,  r a n g i n g  from "normal" 
c h o n d r i t i c  Th/Ti v a l u e s  t o  more KREEP-rich v a r i e t i e s .  
The l u n a r  s u r f a c e  r e p r e s e n t e d  by t h e  Apol lo  o r b i t a l  g r o u n d t r a c k s  i s  shown 
t o  c o n s i s t  o f  8.4% r e l a t i v e l y  p u r e  (85%) f e r r o a n  a n o r t h o s i t e ,  even though 
p i x e l s  t h a t  have h i g h  compos i t iona l  u n c e r t a i n t i e s  were excluded. D e l e t i n g  t h e  
maria  from t h e s e  d a t a  r a i s e s  t h i s  v a l u e  t o  12.9%. Most of t h e  l u n a r  h i g h l a n d s  
i s  composed of f o u r  u n i t s .  Cons ider ing  t h e  a r e a l  p e r c e n t a g e s  of t h e  
g roundt racks  and t h e  modal amounts of t h e  end-member components f o r  t h e s e  f o u r  
u n i t s  r e s u l t s  i n  a n  a v e r a g e  h igh land  composi t ion of 68% f e r r o a n  a n o r t h o s i t e ,  
29% mare b a s a l t ,  and  3% KREEP/Mg-suite rocks .  Th is  r e s u l t a n t  rock  compos i t ion  
approx imates  t h a t  of " a n o r t h o s i t i c  gabbro" and  i s  c o n s i s t e n t  w i t h  o u r  p r e v i o u s  
a n a l y s e s  [23] .  Th i s  compos i t ion  may r e p r e s e n t  t h e  a v e r a g e  composi t ion of t h e  
upper h a l f  of t h e  h i g h l a n d s  c r u s t  [ 2 4 ] .  
S i g n i f i c a n t  amounts of mare b a s a l t  (21.1% of t h e  Apollo g r o u n d t r a c k )  
occur  w i t h i n  t h e  h i g h l a n d s  (most ly  on t h e  e a s t e r n  l imb and  f a r s i d e  h i g h l a n d s ) ,  
a s  i n d i c a t e d  by t h e  a r e a l  d i s t r i b u t i o n  of a  u n i t  composed o f  65% f e r r o a n  
a n o r t h o s i t e ,  3% KREEP/Mg-suite, and 32% mare b a s a l t .  I ts a r e a l  d i s t r i b u t i o n  
c o i n c i d e s  w i t h  mapped o c c u r r e n c e s  of h igh land  p l a i n s  t h a t  d i s p l a y  dark-halo 
c r a t e r s  [19 ,21] ,  f o r  which s p e c t r a l  d a t a  i n d i c a t e  t h e  p resence  of excava ted  
mare b a s a l t  [25] .  This  c o i n c i d e n c e  s u g g e s t s  that mare volcanism o c c u r r e d  
w i t h i n  t h e s e  h igh land  a r e a s  b e f o r e  t h e  end of t h e  f i n a l  heavy bombardment. W e  
do n o t ,  however, d i s m i s s  t h e  p o s s i b $ l i t y  t h a t  t h i s  u n i t  may r e p r e s e n t  some 
t y p e  of h igh land  gabbro o r  a  m i x t u r e  of Mg-suite r o c k s  w i t h  a n  as-yet-  
unsampled maf ic  r o c k  t y p e  t h a t  h a s  a s u b c h o n d r i t i c  Th/Ti r a t i o .  
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Global petrologic variations of the Moon: A ternary-diagram approach

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