Troktolite
Category: Plutonic
Type Gabbroic rock.
Commons Troctolite is a mafic intrusive rock. It is a variety of gabbro composed predominantly of plagioclase and olivine. It may also contain small amounts of pyroxene. It is distinguished from typical gabbro by the absence of pyroxenes.
Name origin Troctolite is named after the Greek word for trout. It was first mentioned in 1875 by Arnold von Lasaulx, a German mineralogist and petrologist.
Locality Hualalai Volcano, Island of Hawaii (Hawaiian Islands), Kaupulehu lava flow from 1800 (sample donated by J. Lexa)
GPS: 19° 45' 50,22" N, 155° 58' 44,18" W
Major minerals Plagioclase rich in Ca and olivine.
Accessory minerals Pyroxene.
Classification Classification of the rock is based on the modal composition of mafic minerals—pyroxenes (Px), plagioclase (Plag), and olivine (Ol)—using the triangular classification diagram for gabbroic rocks (Streckeisen, 1976).
Colour White to light gray with green olivine crystals.
Structure Massive (compact).
Granularity Medium-grained (1–3 mm) with large, isolated olivine crystals up to 1 cm in size.
Texture Phaneritic, equigranular.
Alterations Rock is unaltered.
Petrographic characteristics A 7 cm troctolite xenolith is found within the alkalic basalt of the 1800 Kaupulehu lava flow, which erupted from the northwest rift (at an elevation of 1700 m above sea level) of the Hualalai volcano. Troctolite is distinctive in that it is composed solely of plagioclase and olivine. Although it belongs to the gabbro group, rocks that are typically dark, troctolite is very light in color. White plagioclase and green olivine grains can be distinguished macroscopically. The Kaupulehu flow is one of the youngest lava flows of the Hualalai volcano. At the time of the eruption, the basaltic lava flow exhibited very low viscosity and high velocity, exceeding 10 m/s (40 km/h) and causing extensive damage. The total length of the lava flow from the rift vent to the sea is 15 km (Guest et al., 1995). Xenoliths of troctolite and other mafic and ultramafic rocks, such as dunite, wehrlite, websterite, gabbro, and anorthosite (Bohrson & Clague, 1988; Chen et al., 1992; Schamberger & Hammer, 1996)—formed accumulations within the lava flow (Richter & Murata, 1961; Jackson et al., 1981; Guest et al., 1995).
Usage Troctolite has no practical application. However, from a petrological point of view, it is a highly interesting rock because it forms through the accumulation of olivine and plagioclase from magma of ultrabasic to basic composition and this type of rock is relatively rare. Troctolites are also among the most beautiful rocks brought back from the Moon. Nevertheless, some cumulate rocks can be of economic interest. Very pure anorthite is occasionally mined from anorthosite complexes, while chromite, magnetite, and ilmenite can accumulate in layered intrusions. Economically, however, accumulations of sulfide minerals, which serve as sources of nickel, cobalt, copper, and platinum-group elements, are more significant within layered intrusions. In these settings, sulfide minerals form an interstitial matrix between silicate minerals. Notable examples of ultramafic layered intrusions include the Bushveld Complex in South Africa, the Skaergaard intrusion in Greenland, and the Rum intrusion in Scotland.
Literature Bohrson, W.A. & Clague, D.A., 1988: Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai volcano. Hawaii. Contribution to Mineralogy and Petrology, 100, 2, 139-155. Chen, Ch.-H., Presnall, D.C. & Stern, R.J., 1992: Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow, Hualalai Volcano, Hawaii. Journal of Petrology, 33, 1, 163-202. Cox K.G., Bell J.D., Pankhurst R.J., 1979: The Interpretation of the Igneous Rocks. George Allen & Unvin, London, 450 p. Fodor, R.V. & Galar, P., 1997: A View into the Susurface of Mauna Kea Volcano, Hawaii: Crystallization Processes Interpreted through Petrology and Petrographz of Gabbroic and Ultramafic Xenoliths. Journal of Petrology, 38, 5, 581-624. Gao, R., Lassiter, J.C., Barnes, J.D., Clague, D.A. & Bohrson, W.A., 2016: Geochemical investigation of Gabbroic Xenoliths from Hualalai Volcano: Implications for lower oceanic crust accretion and Hualalai Volcano magma storage system. Earth and Planetary Science Letters, 442, 162-172. Guest, J.E.., Spudis, P.D., Greeley, R., Taylor, G.J. & Baloga, S.M., 1995: Emplacement of xenolith nodules in the Kaupulehu lava flow, Hualalai Volcano, Hawaii. Bulletin Volcanology, 57, 179-184. Hall, A., 1987: Igneous Petrology, Longman, p. 228-231. Jackson, E.D., Clague, D.A., Engleman, E., Friesen, W.B. & Norton, D., 1981: Xenoliths in the alkalic basalt flows from Hualalai Volcano, Hawaii. US Geol Survey Open-File Report 81-1031, 1-32 p. Lasaulx, A.von., 1875: Elemente der Petrographie. Strauss, Bonn. 486 pp. Rhodes, J.M., Rodgers, K.V., Shih, C., Bansal, B.M., Nyquist, L.E., Wiesmann, H. & Hubbard, N.J. 1974: The relationships between geology and soil chemistry at the Apollo 17 landing site. Proc. 5th Lunar Sci. Conf., 1097-1117. Schamberger, P.J. & Hammer, J.E., 1996: Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawaii. Journal of Petrology, 47, 9, 1785-1808. Streckeisen, A. , 1976: To each plutonic rock its proper name. Earth Science Reviews. 12, 1–33.
Photomicrographs
Troctolite is composed of olivine (Ol) and plagioclase (Pl). Under plane-polarized light, the olivine appears transparent and lacks cleavage. In the plagioclase, polysynthetic twinning according to the albite law is clearly visible under crossed polars. The relatively wide and sparse lamellae indicate a basic composition for the plagioclase, reflecting a higher anorthite content. Based on the terminology for cumulate rocks regarding the ratio of cumulus minerals to groundmass (Hall, 1996), this troctolite is classified as an adcumulate, as it contains 93–100% accumulated magmatic minerals. The width of the photomicrographs is 2.2 mm.
Normative composition
Troctolite is a cumulate rock, and its composition does not represent the composition of the magma from which it crystallized. Troctolite is a rock with normative olivine - ol and anorthite an. Its low SiO2 content is reflected in the presence of normative nepheline - an, unlike average gabbro, which is quartz-normative q. The CaO content is obvious from the presence of the normative anorthite an and diopside di. The entire TiO2 content is projected in the normative ilmenite di).
Normative minerals
SiO2
TiO2
ZrO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
F
S
CO2
Total
Molar proportion of normative mineral
Molecular mass of normative mineral
Weight % of normative mineral
Oxide
(wt. %)
46.01
0.93
19.81
9.56
0.11
10.97
9.00
2.73
0.09
0.02
99.23
Molecular
weight
60.08
79.88
101.96
71.85
70.94
40.31
56.08
61.98
94.20
141.95
Molecular
proportion
0.7658
0.0116
0.1943
0.1346
0.0016
0.2721
0.1605
0.0440
0.0010
0.0001
ap
0.0005
0.0001
0.0001
328.68
0.05
il
0.0116
0.0116
0.0116
151.75
1.77
or
0.0057
0.0010
0.0010
0.0010
556.67
0.53
ab´
0.2643
0.0440
0.0440
0.0440
524.46
23.10
an
0.2986
0.1493
0.1493
0.1493
278.21
41.53
zvyšky
0.1230
0.2721
di
0.0215
0.0033
0.0074
0.0107
0.0107
226.38
2.43
hy´
0.3844
0.1196
0.2648
0.3844
110.21
42.36
ol
0.1922
0.1196
0.2648
0.1922
160.32
30.81
ne
0.0083
0.0021
0.0041
0.0041
284.11
1.17
ab
0.2397
0.0420
0.0399
0.0399
524.46
20.95
D: 0.2086
D1: 0.0164
D2: 0.0164
Mg/(Mg+Fe2+): 0.689
Total of normative wt. % 99.23
Comment The SiO2 undersaturation of the rock caused that the entire provisional hypersthene hy´ was converted into olivine ol, while a portion of the provisional albite ab´ was transformed into the normative nepheline ne and actual albite ab was formed.
Chemical composition
Troctolite and other gabbroic xenoliths were formed by fractional crystallization of magma generated during the shield and post-shield stages of Hualalai volcano's evolution. Geochemical and petrological characterization of the gabbroic xenoliths confirmed the existence of a magma reservoir located in the lower to deep oceanic crust during the shield stage of its development (Gao et al., 2016). Petrological modeling confirmed the presence of a magma reservoir at a depth of 8–17 km (2.5–5.0 kbar). The chemical composition of the troctolite varies depending on the modal abundance of olivine and plagioclase. Based on its SiO2 content, the troctolite is classified as a basic to ultrabasic rock. Compared to the average analysis of gabbro (Cox et al., 1979), it exhibits higher contents of Al2O3, MgO, and FeO.
Mauna Kea, xenolith, Hawaii - C6
SiO2
46.01TiO2
0.93Al2O3
19.81FeO
9.56MnO
0.11MgO
10.97CaO
9.00Na2O
2.73K2O
0.09P2O5
0.02Total
99.23Mg(Mg/Fe2+)
0.67
Fodor & Galar (1997).
Mauna Kea, xenolith, Hawaii - C49
SiO2
40.45TiO2
5.95Al2O3
16.44FeO
13.61MnO
0.12MgO
11.93CaO
7.66Na2O
2.35K2O
0.11P2O5
0.02LOI
0.33Total
99.55Mg(Mg/Fe2+)
0.61
Fodor & Galar (1997).
Lunar troctolite - sample 76535
SiO2
42.88TiO2
0.05Al2O3
20.73FeO
4.99MnO
0.07MgO
19.09CaO
11.41Na2O
0.23K2O
0.03P2O5
0.03Total
99.51Mg(Mg/Fe2+)
0.87
Rhodes et al., 1974