Jökull - 01.01.2019, Page 91

Nikkola et al.
Melt inclusions in olivine
Representative compositions of melt inclusions in
the Brattaskjól and Hvammsmúli olivine macrocrysts
are shown in Table 1. At Brattaskjól, the melt in-
clusions in Fo<80.9−87.0 olivine macrocrysts corre-
spond to mildly alkaline basalt (1–3 wt% norma-
tive nepheline) with Mg#melt (Mg#melt = molar
100Mg/(Mg+0.9Fetot)) of 56.1–68.5, whereas the
four melt inclusions in more primitive Fo87.9−88.6
olivines are silica saturated with low FeOtot and
Mg#melt of 68.8–73.0. Most of the homogenized
Brattaskjól melt inclusions are in equilibrium with
their host olivine (KdMg−Fe
Ol−Liq = 0.30±0.3, Toplis
2005) and do not require post-entrapment crystal-
lization (PEC) correction. For those melt inclusions
that require it, a PEC correction of 1–6 wt% was
made using the Petrolog software of Danyushevsky
and Pletchov (2011). For these calculations, FeO*
(FeO content before post-entrapment crystallisation
and diffusion) of the melt inclusions was derived from
the relationship of Fo in olivine and FeO in melt
in suitable Eyjafjallajökull magmas (this study and
Moune et al., 2012). The melt inclusions in olivines
from Hvammsmúli often exhibit crystals, even after
heating to 1240◦C, and it was thus not possible to per-
form proper PEC and diffusion correction for them.
In addition, all Hvammsmúli melt inclusions have so
low FeOtot in relation to their high Mg#melt that they
are unlikely to represent any reasonable near-primary
parental melt. Most likely, these melt inclusions have
been modified by post-entrapment olivine crystalliza-
tion and solid-state diffusion of elements from their
host olivine (see Danyushevsky et al., 2000). For ad-
ditional Hvammsmúli melt inclusion data, see Björns-
son (2019).
Clinopyroxene
Clinopyroxene macrocrysts in the Brattaskjól (n=31)
and Hvammsmúli (n=20) ankaramites have simi-
lar core compositions of Ca-rich augite, although
only in the case of Brattaskjól, a subpopulation
of magnesium-rich (Mg#cpx 89.7–89.8; Mg#cpx =
cation fraction Mg/(Mg+Fetot) clinopyroxene was
identified (Figure 4). Interestingly, as the clinopyrox-
ene macrocrysts become more ferrous (Mg#cpx de-
creases), CaO and Al2O3 contents gradually increase
until Mg#cpx 84.5, followed by a decrease in these
oxides in more evolved clinopyroxene crystals (Fig-
ure 4c and d). The enrichment in CaO and Al2O3,
together with simultaneous decrease in SiO2 and no
variation in Na2O, points to an increase in the Ca-
Tschermak (CaTs) component (Figure 4b). The vari-
ation in Na2O is low, hence jadeite (Jd) component
is unchanged against Mg#cpx in clinopyroxene cores
(Figure 4e). In contrast, TiO2 shows negative (Fig-
ure 4f) and Cr2O3 positive correlation with Mg#cpx.
Compared to clinopyroxene cores, the rims are typi-
cally richer in the ferrosilite (Fs) component and TiO2
and have lower Mg#, Jd, CaTs (Figure 4) and Cr2O3.
4. mynd. – Efnasamsetningar klínópýroxendíla í ankaramíti frá Brattaskjóli og Hvammsmúla. Samsetningar kjarna eru
meðaltal þriggja efnagreininga á hverjum kjarna, en samsetningar kristalrima meðaltal tveggja til þriggja greininga gerðar
nærri rimanum. Klínópýroxendílarnir hafa óðul með breytilega samsetningu og þessi breytileiki í samsetningu einstakra
klínópýroxenkristalla er gefinn til kynna með fylltu gráu ferningunum (óbirt gögn). Svörtu örvarnar gefa til kynna áætluð
áhrif af kristaldiffrun ólivíns (ol), Ca-ágíts (cpx) og plagíóklass (plg), þar sem gert er ráð fyrir að KdMg−Fe
Ol−Liq = 0.30,
KdMg−Fe
Cpx−Liq = 0.27, DCa
Ol−Liq = 0, DCa
Cpx−Liq = 1.7, DAl
Ol−Liq = 0 og DAl
Cpx−Liq = 0.15–0.25 (0,005 aukning þegar hvert
mól% er fjarlægt úr bráðinni til að líkja eftir hitaháðum áhrifum á dreifingu Al milli bráðar og klínópyroxens). Kristald-
iffrun plagíóklass minnkar styrk CaO og Al2O3 í bráðinni en hefur ekki áhrif á Mg-tölu ((Mg#cpx = katjónahlutfallið
100Fe/(Fe+Mg)). Brotna örin er vektor sem sýnir áhrif diffrunar ólivíns og klínópýroxens í jöfnu hlutfalli. a) Klínópýroxen-
fjórhliðungur; Di = díopsíð, Hd = hedenbergít, En = enstatít, Wo = wollastonít, Fs = ferrósilít; b) Ca-tschermak’s (CaTs)
kristalþáttur á móti Mg#cpx; c) CaO á móti Mg#cpx; d) Al2O3 á móti Mg#cpx; e) Jaðeít (Jd) kristalþáttur á móti Mg#cpx;
d) TiO2 á móti Mg#cpx.
90 JÖKULL No. 69, 2019