93:
36:
554:
146:
315:
369:
The rock that is made of the accumulated minerals will not have the same composition as the magma. In the above example, the cumulate of anorthite + enstatite is rich in calcium and magnesium, and the melt is depleted in calcium and magnesium. The cumulate rock is a plagioclase-pyroxene cumulate (a
684:
rocks become sulfur saturated and form sulfide segregations. In this case, the typical result is a disseminated form of sulfide mineral, usually a mixture of pyrrhotite, pyrite and chalcopyrite, forming copper mineralisation. It is very rare but not unknown to see cumulate sulfide rocks in granitic
381:
It can be seen that the effect on the composition of the residual melt left behind by the formation of the cumulate is dependent on the composition of the minerals which precipitate, the number of minerals which co-precipitate at the same time, and the ratio of the minerals which co-precipitate. In
458:
One way to infer the composition of the magma that created the cumulate rocks is to measure groundmass chemistry, but that chemistry is problematic or impossible to sample. Otherwise, complex calculations of averaging cumulate layers must be utilised, which is a complex process. Alternatively, the
342:
The chemistry of a cumulate can inform upon the temperature, pressure and chemistry of the melt from which it was formed, but the number of minerals which co-precipitate need to be known, as does the chemistry or mineral species of the precipitated minerals. This is best illustrated by an example;
470:
These methods have their drawbacks, primarily that they must all make certain assumptions which rarely hold true in nature. The foremost problem is that in large ultramafic intrusions, assimilation of wall rocks tends to alter the chemistry of the melt as time progresses, so measuring groundmass
377:
In the above example, the plagioclase and pyroxene need not be pure end-member compositions (anorthite-enstatite), and thus the effect of depletion of elements can be complex. The minerals can be precipitated in any ratio within the cumulate; such cumulates can be 90% plagioclase:10% enstatite,
366:) removes calcium from the melt, which becomes more depleted in calcium. Enstatite being precipitated from the melt will remove magnesium, so the melt becomes depleted in these elements. This tends to enrich the concentration of other elements - typically sodium, potassium, titanium and iron.
305:
Cumulate terminology is appropriate for use when describing cumulate rocks. In intrusions which have a uniform composition and minimal textural and mineralogical layering or visible crystal accumulations it is inappropriate to describe them according to this convention.
382:
nature, cumulates usually form from 2 mineral species, although ranges from 1 to 4 mineral species are known. Cumulate rocks which are formed from one mineral alone are often named after the mineral, for example a 99% magnetite cumulate is known as a magnetitite.
137:; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded appearance.
600:
These oxide layers form laterally continuous deposits of rocks containing in excess of 50% oxide minerals. When oxide minerals exceed 90% of the bulk of the interval the rock may be classified according to the oxide mineral, for example
333:
of a parental magma, should not be used to infer the composition of a magma from which they are formed. The chemistry of the cumulate itself can inform on the residual melt composition, but several factors need to be considered.
584:
These conditions are created by the high-temperature fractionation of highly magnesian olivine or pyroxene, which causes a relative iron-enrichment in the residual melt. When the iron content of the melt is sufficiently high,
471:
compositions may fall short. Mass balance calculations will show deviations from expected ranges, which may infer assimilation has occurred, but then further chemistry must be embarked upon to quantify these findings.
474:
Secondly, large ultramafic intrusions are rarely sealed systems and may be subject to regular injections of fresh, primitive magma, or to loss of volume due to further upward migration of the magma (possibly to feed
675:
Sulfide mineral segregations can only be formed when a magma attains sulfur saturation. In mafic and ultramafic rocks they form economic nickel, copper and platinum group (PGE) deposits because these elements are
459:
magma composition can be estimated by assuming certain conditions of magma chemistry and testing them on phase diagrams using measured mineral chemistry. These methods work fairly well for cumulates formed in
261:
Cumulate rocks are typically named according to the cumulate minerals in order of abundance, and then cumulate type (adcumulate, mesocumulate, orthocumulate), and then accessory or minor phases. For example:
565:
Oxide mineral cumulates form in layered intrusions when fractional crystallisation has progressed enough to allow the crystallisation of oxide minerals which are invariably a form of
358:
pyroxene is changing composition by the removal of the elements which make up the precipitated minerals. In this example, the precipitation of anorthite (a
483:
swarms). In such cases, calculating magma chemistries may resolve nothing more than the presence of these two processes having affected the intrusion.
378:
through to 10% plagiclase:90% enstatite and remain a gabbro. This also alters the chemistry of the cumulate, and the depletions of the residual melt.
498:
The economic importance of cumulate rocks is best represented by three classes of mineral deposits found in ultramafic to mafic layered intrusions.
657:
and platinum-tellurium sulfides. These deposits are formed by melt immiscibility between sulfide and silicate melts in a sulfur-saturated magma.
203:
330:
186:
92:
897:
857:
79:
57:
792:"Petrology and geochemistry of igneous inclusions in recent Merapi deposits: a window into the sub-volcanic plumbing system"
597:
is generally formed during pyroxene fractionation at low pressures, where chromium is rejected from the pyroxene crystals.
192:
chamber. These accumulations typically occur on the floor of the magma chamber, although they are possible on the roofs if
613:. Strictly speaking, these would be magnetite orthocumulate, ilmenite orthocumulate and chromite orthocumulates.
904:
Petrology of layered mafic-ultramafic intrusions of the Giles
Complex, western Musgrave Block, central Australia
558:
50:
44:
919:
694:
660:
They are not strictly a cumulate rock, as the sulfide is not precipitated as a solid mineral, but rather as
630:
924:
61:
868:
J. Leuthold, J. C. Lissenberg, B. O’Driscoll, O. Karakas; T. Falloon, D.N. Klimentyeva, P. Ulmer (2018);
557:
Dark layers of chromite-rich cumulate rock alternating with light layers of plagioclase-rich rock in the
714:
467:). Investigating magma conditions of large layered ultramafic intrusions is more fraught with problems.
153:. While cooling, the magma evolves in composition because different minerals crystallize from the melt.
804:
753:
386:
789:
Chadwick, J. P.; Troll, V. R.; Waight, T. E.; van der Zwan, F. M.; Schwarzkopf, L. M. (2013-02-01).
553:
393:. At Skaergaard a 2500 m thick layered intrusion shows distinct chemical and mineralogic layering:
234:
Cumulates are named according to their dominant mineralogy and the percentage of crystals to their
134:
828:
27:
Igneous rocks formed by the accumulation of crystals from a magma either by settling or floating.
664:
sulfide liquid. However, they are formed by the same processes and accumulate due to their high
517:
Silicate minerals are rarely sufficiently valuable to warrant extraction as ore. However, some
893:
853:
820:
771:
709:
812:
761:
665:
235:
145:
668:, and can form laterally extensive sulfide 'reefs'. The sulfide minerals generally form an
450:
The
Skaergaard is interpreted to have crystallised from a single confined magma chamber.
245:
are rocks containing ~100–93% accumulated magmatic crystals in a fine-grained groundmass.
808:
757:
181:: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rock forms.
487:
480:
370:
gabbro) and the melt is now more felsic and aluminous in composition (trending towards
314:
486:
Though crystallized at high temperature, cumulate can remelt when later intruded by a
913:
832:
699:
661:
534:
669:
642:
122:
766:
741:
185:
Cumulate rocks are the typical product of precipitation of solid crystals from a
149:
Schematic diagrams showing the principles behind fractional crystallisation in a
677:
638:
518:
267:
196:
174:
873:
816:
719:
646:
538:
530:
438:
298:
824:
775:
791:
704:
654:
586:
542:
522:
464:
390:
355:
351:
290:
215:
211:
193:
133:
either by settling or floating. Cumulate rocks are named according to their
621:
Sulfide mineral cumulates in layered intrusions are an important source of
257:
are rocks containing between 85 and 75% accumulated minerals in groundmass.
594:
590:
578:
574:
526:
476:
371:
363:
271:
166:
17:
460:
359:
275:
251:
are rocks with between 93 and 85% accumulated minerals in a groundmass.
223:
158:
126:
97:
681:
650:
634:
626:
622:
566:
347:
319:
279:
219:
637:. Deposits of a mixed massive or mixed sulfide-silicate 'matrix' of
680:
and are strongly partitioned into the sulfide melt. In rare cases,
552:
313:
189:
150:
144:
130:
91:
593:
crystallise and, due to their high density, form cumulate rocks.
570:
323:
207:
870:
Partial melting of the lower oceanic crust at spreading ridges.
569:. This can happen due to fractional enrichment of the melt in
29:
444:
MgO 11.6% base to 1.7% top; FeO 9.3% base to 22.7% top
872:
Frontiers in Earth
Sciences: Petrology: 6(15): 20p;
892:2nd ed., pp. 123–132 & 194–197, Freeman,
790:
890:Petrology: Igneous, Sedimentary and Metamorphic,
350:composition that is precipitating cumulates of
199:is able to float free of a denser mafic melt.
8:
740:Emeleus, C. H.; Troll, V. R. (August 2014).
874:https://dx.doi.org/10.3389/feart.2018.00015
902:Ballhaus, C.G. & Glikson, A.Y., 1995,
888:Blatt, Harvey and Robert J. Tracy, 1996,
797:Contributions to Mineralogy and Petrology
765:
80:Learn how and when to remove this message
43:This article includes a list of general
732:
525:concentrations that they are mined for
844:
842:
96:Close-up view of a cumulate rock from
289:A rock consisting of 80% olivine, 5%
7:
906:. AGSO Journal, 16/1&2: 69–90.
742:"The Rum Igneous Centre, Scotland"
206:, in the base of large ultramafic
49:it lacks sufficient corresponding
25:
329:Cumulate rocks, because they are
202:Cumulates are typically found in
278:and 5% groundmass (in essence a
34:
672:matrix to a silicate cumulate.
284:plagioclase-pyroxene adcumulate
653:are formed, occasionally with
412:CaO 10.5% base to 5.1% top; Na
125:formed by the accumulation of
100:(scale: about 45 millimetres (
1:
521:intrusions contain such pure
767:10.1180/minmag.2014.078.4.04
617:Sulfide mineral segregations
852:1987, Longman, p. 228-231,
441:percentage of the olivine)
941:
513:Silicate mineral cumulates
502:Silicate mineral cumulates
397:Plagioclase varies from An
385:A specific example is the
346:As an example, a magma of
817:10.1007/s00410-012-0808-7
318:Layers of cumulate rock (
293:and 15% groundmass is an
559:Bushveld Igneous Complex
409:= anorthite percentage)
695:Igneous differentiation
631:platinum group elements
549:Oxide mineral cumulates
537:and other sundry uses (
505:Oxide mineral cumulates
454:Residual melt chemistry
420:O 2.3% base to 5.9% top
286:with accessory olivine.
222:flows and also in some
64:more precise citations.
746:Mineralogical Magazine
562:
508:Sulfide melt cumulates
425:Olivine varies from Fo
326:
182:
115:
715:List of rock textures
556:
317:
204:ultramafic intrusions
148:
95:
387:Skaergaard intrusion
295:olivine mesocumulate
282:) would be termed a
809:2013CoMP..165..259C
758:2014MinM...78..805E
494:Economic importance
429:near the base to Fo
401:near the base to An
850:Igneous Petrology,
563:
490:or dyke of magma.
463:conditions (i.e.;
327:
183:
116:
710:Layered intrusion
354:plagioclase plus
266:A layer with 50%
114: in) across)
90:
89:
82:
16:(Redirected from
932:
876:
866:
860:
846:
837:
836:
794:
786:
780:
779:
769:
737:
666:specific gravity
405:near the top (An
297:, (in essence a
113:
112:
108:
105:
85:
78:
74:
71:
65:
60:this article by
51:inline citations
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374:compositions).
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173:: pyroxene and
143:
110:
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86:
75:
69:
66:
56:Please help to
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39:
35:
28:
23:
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12:
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5:
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922:
920:Plutonic rocks
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911:
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861:
838:
803:(2): 259–282.
781:
752:(4): 805–839.
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561:, South Africa
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535:semiconductors
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433:at the top (Fo
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339:
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255:Orthocumulates
252:
246:
238:(Hall, 1996).
231:
228:
165:: olivine and
161:crystallizes;
142:
139:
119:Cumulate rocks
88:
87:
42:
40:
33:
26:
24:
14:
13:
10:
9:
6:
4:
3:
2:
937:
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925:Igneous rocks
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898:0-7167-2438-3
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858:0-582-30174-2
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700:Igneous rocks
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253:
250:
249:Mesocumulates
247:
244:
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237:
229:
227:
225:
221:
217:
213:
209:
205:
200:
198:
195:
191:
188:
187:fractionating
180:
177:crystallize;
176:
172:
169:crystallize;
168:
164:
160:
156:
152:
147:
140:
138:
136:
132:
128:
124:
123:igneous rocks
120:
99:
94:
84:
81:
73:
70:February 2008
63:
59:
53:
52:
46:
41:
32:
31:
19:
903:
889:
869:
864:
849:
800:
796:
784:
749:
745:
735:
685:intrusions.
674:
670:interstitial
659:
643:chalcopyrite
620:
610:
606:
602:
599:
583:
564:
531:refractories
516:
497:
485:
473:
469:
457:
449:
384:
380:
376:
368:
345:
341:
331:fractionates
328:
310:Geochemistry
304:
294:
283:
260:
254:
248:
242:
233:
226:intrusions.
201:
184:
178:
170:
162:
154:
118:
117:
76:
67:
48:
678:chalcophile
639:pentlandite
603:magnetitite
519:anorthosite
268:plagioclase
243:Adcumulates
230:Terminology
197:plagioclase
175:plagioclase
62:introducing
914:Categories
727:References
720:Ultramafic
662:immiscible
647:pyrrhotite
611:chromitite
607:ilmenitite
539:toothpaste
465:komatiites
439:forsterite
362:aluminium
299:peridotite
236:groundmass
45:references
833:128817557
825:1432-0967
776:0026-461X
705:Komatiite
655:cobaltite
587:magnetite
545:, etc.).
543:cosmetics
523:anorthite
479:vents or
391:Greenland
356:enstatite
352:anorthite
338:Chemistry
291:magnetite
216:magnesium
212:komatiite
210:tubes in
194:anorthite
141:Formation
689:See also
595:Chromite
591:ilmenite
579:chromium
575:titanium
527:feldspar
477:volcanic
461:volcanic
372:andesite
364:feldspar
272:pyroxene
224:granitic
167:pyroxene
127:crystals
18:Cumulate
883:Sources
805:Bibcode
754:Bibcode
649:and/or
360:calcium
276:olivine
159:olivine
135:texture
129:from a
109:⁄
98:Montana
58:improve
896:
856:
831:
823:
774:
682:felsic
651:pyrite
635:cobalt
627:copper
623:nickel
567:spinel
348:basalt
320:gabbro
280:gabbro
270:, 40%
220:basalt
47:, but
829:S2CID
722:rocks
416:O + K
322:) in
274:, 5%
218:rich
190:magma
151:magma
131:magma
894:ISBN
854:ISBN
821:ISSN
772:ISSN
633:and
571:iron
488:sill
481:dyke
324:Oman
214:and
208:lava
121:are
813:doi
801:165
762:doi
609:or
589:or
577:or
389:in
916::
841:^
827:.
819:.
811:.
799:.
795:.
770:.
760:.
750:78
748:.
744:.
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641:,
629:,
625:,
605:,
581:.
573:,
541:,
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435:xx
427:57
407:xx
403:30
399:66
301:).
157::
835:.
815::
807::
778:.
764::
756::
431:0
418:2
414:2
179:4
171:3
163:2
155:1
111:4
107:3
104:+
102:1
83:)
77:(
72:)
68:(
54:.
20:)
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