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| Titel |
Phosphorus and other trace elements from secondary olivine in composite mantle xenoliths (CMX) from Cima Volcanic Field (CVF; California, USA): implications for crystal growth kinetics |
| VerfasserIn |
Ioannis Baziotis, Paul Asimow, Theodoros Ntaflos, Jeremy Boyce, Antonios Koroneos, Diego Perugini, Yongsheng Liu, Stephan Klemme, Jasper Berndt |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250113563
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| Publikation (Nr.) |
 EGU/EGU2015-13780.pdf |
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| Zusammenfassung |
| Phosphorus(P)-rich zones in olivine may reflect excess incorporation of P during rapid
growth; zoning patterns may then record growth rate variations (Milman-Barris et al.,
2008; Stolper et al., 2009). We report data on interior cuts of two CMX from alkali
basalt flows (Mukasa & Wilshire, 1997) in the CVF with second-generation P-rich
olivines.
In Ci-1-196, a dark layer (~200 μm wide) between lherzolite and websterite is interpreted
as a rapidly crystallized melt layer (ML), consisting of Ol+Gl+Pl+Spl+Cpx+Ap+Ilm. Glass
(~15 vol%) is variable in composition (P2O5 ≈¤1.2 wt%, Li 8.22-20.0 ppm). Olivines in the
layer have 0.03-0.62 wt% P2O5; P-rich Ol (P2O5 >0.1 wt%) are Fo85-89.3. The lowest P
concentrations are consistent with equilibrium with liquid parental to Gl, but the higher
concentrations are not. Li concentrations, zoned from 3.84 to 4.90 ppm (core-rim), indicate
equilibrium incorporation during crystal growth from a small, evolving melt pool and
preservation of this rapidly relaxing gradient. REEs are mostly consistent with equilibrium
growth from liquids evolving towards the observed LREE-enriched glass. Most of the
clinopyroxenes are diopsides with some augites. Apatite inclusions occur in the rim of P-rich
Fo85 and in An54.
In Ki-5-301, a dark-coloured area of irregular shape (~200 μm wide) is present along the
contact between lherzolite and orthopyroxenite, consisting of Ol+Pl+Gl+Cpx+Spl+Ilm+Ap.
It resembles a tabular dyke but is connected to melt-patches infiltrating the host rock.
Widespread Glass in the layer has variable composition with two populations not related by
fractional crystallization: 1) P2O5 1.02-1.09 wt% and 2) P2O51.62-2.35 wt% (a Gl
inclusion in Ol has P2O5 3.57 wt% may have captured melt from the P-rich boundary
layer at the interface with the rapidly growing olivine). REEs cluster in the same
two groups. Li is as low as 3.66 ppm group 1 and 3-4x higher (9.64-13.3 ppm) in
group 2. Olivine occurs as small idiomorphic crystals embedded in Gl and as large
(~100 μm) idiomorphic to hypidiomorphic crystals with Gl and Spl inclusions;
Mg# ranges from Fo74.5 (rim in contact with Gl) to Fo90.3; P2O5 reaches 3.5 wt%
(in a ~Fo84 rim); Li varies from 2.80 (core) to 6.35 ppm (rim). Clinopyroxene
(Wo41-43En50-54Fs5-8; P2O5 0.04-0.08 wt%; Li 3.33 ppm) is found both within the ML
and as a reaction product between melt and matrix Opx. Trace element geochemistry shows
possible equilibrium with ML glass for some elements, but clear disequilibrium for
others. Apatite occurs as large (~100 μm) crystals in contact with Ol or Gl, as
near-rim inclusions in P-rich Fo84 and as tiny prismatic crystals in Gl; REEs show
slight negative Eu anomalies (Eu/Eu*=0.79-0.86) due to prior crystallization of
plagioclase.
High-resolution X-ray mapping of P in Ol from Ci-1-196 reveals 3-7 μm wide P-rich
bands parallel to facets. P2O5 correlates negatively with Si and Mg+Fe+Ca, suggesting a
substitution Mg2SiO4 + 1 /2 P2O5 /Mg1.5[]0.5PO4 + 1 /2MgO+SiO2. P-Al-rich areas may
grow in minutes, whereas P-Al-poor over few weeks (Jambon et al., 1992). At such rates,
dendritic growth (Welsch et al., 2014) implies that core to rim zoning may not be simple
growth stratigraphy. A slight correlation between P and Al in our data implies either diffusive
relaxation of Al gradients or, judging by dynamic experiments (Grant & Kohn, 2013), cooling
rates >10Ë C/h that generate disequilibrium solute trapping of P but near-equilibrium
incorporation of Al.
The petrogenetic history following melt intrusion requires rapid cooling and reaction with
matrix minerals and crystallization sequence Ol/Cpx/Pl/Ap/Fe-Ox/quench of Gl. P
and Li concentrations set upper and lower limits on growth rates after intrusion of melt into
CVF xenoliths. Early-crystallized olivine grew rapidly enough that sluggish P became
over-enriched but not so fast as to over-enrich other elements. Cpx formed later either as
neoblasts or reaction rims in which P was homogeneous (Baziotis et al. 2014) and growth
was slower compared to Ol but fast enough to preserve the Li zoning. Li diffuses in Ol a
factor of 3 faster than Mg-Fe (Qian et al., 2010) and hence sets a lower limit on time from Ol
growth to eruption. |
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