Earth and Planetary Sciences ETDs

Publication Date

Summer 6-29-2018

Abstract

Chapter I:

The 2007-8 eruption at Lengai was highly explosive, reaching plinian proportions, and the anhydrous nature of the nephelinite magma at Lengai, does not explain this highly volatile behavior. The increase in volatiles in a low H2O nephelinite magma could occur from decompression melting of magma injection from a deeper source. Two distinct nephelinite compositions were identified in a mineralogical analyses of the ash erupta: a highly evolved nephelinite (OL2), with less than 3% glass from the magma chamber, as indicated by the highly peralkalinic feldspathoid: combeite (Na2Ca2Si3O9), commonly found in Lengai eruptive products (Dawson 1966, 1998), and a less evolved nephelinite magma, with up to 17% glass (ASHES) that did not contain combeite, with significantly higher Si, Al, Mg, and Mn content. Phase abundances, mineral formulas and endmember components are calculated for both assemblages. Phenocrysts encountered in both nephelinite assemblages are nepheline, augite (CPX), titanium andradite, wollastonite, apatite, and iron oxides. Magma mixing of the two nephelinites are evidenced by sudden changes in the melt chemistry in both ash sample sets. In the combeite-wollastonite-nephelinite (OL2), combeite microlites exhibit resorbtion rims indicative of mineral instability, and nepheline from this assemblage has a distinct chemical boundary withinrim, evidenced by Mg overgrowth. The wollastonite-nephelinite contains almost fully resorbed CPX, and resorbtion rims on Ti-andradite. Chemical changes resulting from a decrease of in Ca in the melt were detected in the rims of the wollastonite via electron microprobe WDS mapping. Two large CPX mineral grains with very differing composition and crystallization histories were found alongside each other in the wollastonite-nephelinite. Primary compositional differences between the two CPX grains are Ti and Mg content, the CPX mineral grain exhibiting disequilibrium features (ASH15-DISEQ) had higher total Mg (Mg content as high as 0.87 c.p.f.u., with an average of 0.72 c.p.f.u. as opposed to an average of 0.52 c.p.f.u.) and lower Ti (on average 0.00 c.p.f.u., as opposed to 0.02 c.p.f.u. in the second grain), than the zoned CPX (ASH15-EQUIL). The Ti-enriched CPX (ASH15-EQUIL) exhibits oscillatory compositional zoning, with few inclusions. The second (ASH15-DISEQ) is richer in Mg, and contains abundant inclusions, suggesting a high degree of disequilibrium. Both CPX and nepheline microlites and rims are enriched in Al, Mg and Mn, elements typically depleted in the highly peralkaline magma chamber. For both ash types the crystal size distribution is bimodal indicative of two stage cooling: an initial stage of slow cooling, with low nucleation and high growth rates producing large crystals (longest axes up to 1.5mm), followed by a stage of rapid cooling with high nucleation and low growth rates as the magma migrated to the surface. The large volume of visible interstitial glass vesicles in OL2 scoria is indicative of rapid degassing and subsequent crystallization in the magma chamber.

Chapter II:

Oldoinyo Lengai is the world’s only active natrocarbonatite-nephelinite mixed-magma system on earth. Recent volcanic activity and geochemical studies suggest there may be two nephelinite magmas mixing prior to the 2007-8 eruption.

In this study, we present scanning electron microscope (SEM) analyses from 2006 natrocarbonatite deposits, electron microprobe (EMPA) melt analyses for the 2007-8 eruptive nephelinite deposits: combeite-wollastonite nephelinite (CWN) and wollastonite nephelinite (WN). We also present laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS) trace and rare earth element data (ppm) for a xenolith sample (consisting of CPX and apatite), melt phenocrysts (andradite, and CPX), and matrix (a non-vitrified, non-crystalline, ultrafine ash representative of the pre-eruptive melt composition). Rare earth and trace element data presented for: V, Cr, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Pb, Th, and U. In addition, mineral/matrix partition coefficients (Kds) are presented for andradite and CPX. From the total alkali vs. silicate (TAS) diagram and Harker’s diagrams two distinct melt compositions were identified. These two melt compositions are characterized by different REE and trace element abundance patterns for the melt and phenocrysts, both of which demonstrate differences of up to 3 orders of magnitude in concentration (ppm), especially in the LREE.

Similarity in trace- and rare-earth-element-normalized abundance patterns for both matrix and andradite phenocryst analyses suggest they share a common source and may originate from the same parental magma. However the broad range in values suggests that the WN may be more recently evolved from the parental magma than the CWN, which demonstrates evidence of contact with natrocarbonatite in the form of resulting enrichments of HREE, Th and U.

However, interaction with the natrocarbonatite was not indicated by the CPX patterns, which show significant differences in concentration (ppm; normalized to CI chondrite), in addition to a pronounced negative K anomaly and a positive Y anomaly displayed by some samples.

Overlap in melt compositions is interpreted as the chemical signature of magma mixing, especially in combination with evidence of other disequilibrium features, as documented by Thomas et al (2018), such as CPX and garnet resorbtion, zoning, and the two differentmineral assemblages (CWN and WN).

The data from this study support the presence of a deeper nephelinite source (WN) injecting Lengai’s primary nephelinite chamber (CWN) causing the 2007-8 eruption. A time series of seismic and eruptive events at Lengai supports the hypothesis that all explosive eruptions are triggered by injection of deeper magma (WN) which is preempted by a series of significant seismic events (ISC., 2001, Baer et al., 2008, and GVP., 2014), as supported in the most recent eruption by InSAR studies (Biggs et al 2009., 2013).

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Tobias P. Fischer

Second Committee Member

Adrian J. Brearley

Third Committee Member

Brandon Schmandt

Language

English

Keywords

Magma mixing, OlDoinyo Lengai, Nephelinite, Trace Element Geochemistry, Peralkalinic Mineralogy, Mixed Magma Systems

Document Type

Thesis

Comments

Please note: Thesis is presented in two chapter format. Chapter I is the mineralogical analysis, Chapter II is the trace element analysis.

Available for download on Tuesday, July 28, 2020

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