Welcome!
Hi, I’m Crystal T Luna. I’m a PhD student at Louisiana State University and am currently advised by Dr. Matthew Loocke.
I previously completed a MSc in Geology at the University of Wyoming under the advisement of Dr. Michael Cheadle and Dr. Barbara John. This page describes my master's project.
About me:
I'm a native Texan and served in the Army National Guard for 8 years as an Intelligence Analyst. I worked as a mathematics supplemental instructor/instructional assistant for 5 years at Tarrant County College. I also worked for Shimadzu Institute in their Environmental, Forensics and Material Science Lab for 2 years. Additionally, I've done GIS work as a Telecom Engineer and during my time as an Intelligence Analyst.
Pito Deep
This research project focused on fresh, plutonic rock samples from Pito Deep, a site at the bottom of Pacific Ocean, near Easter Island with the goal of understanding how this crustal section formed.
A thin section, a paper-thin slice, scan of a rock sample under cross-polarized light (XPL). Olivine gabbro containing olivine, plagioclase, and clinopyroxene.
The gabbro glacier and sheeted sill models are the current end member models describing the processes leading to the formation of fast-spread ocean crust.
The sheeted sill model proposes melt emplaced at various depths into multiple sills.
The gabbro glacier flow model proposes constant melt migration into an axial melt lens (AML) with accompanying flow of crystals in a downward and outward motion.
Schematic diagram highlighting differences between end-member models for formation of fast-spread ocean crust (by Gess, after Coogan, 2007).
This research project focused on the element analysis of plagioclase, olivine, and pyroxene. Chemical compositions are indicators of the type of melt the minerals crystallized from (e.g. primitive vs evolved) which suggest processes leading to their formation (e.g. gabbro glacier flow vs sheeted sill). Specifically, I will be looking at major element analyses of anorthite content (An content) in plagioclase, forsterite content (Fo content) in olivine as well as Mg# (Mg/(Fe+Mg)) in clinopyroxene. Also, some minor element analyses of Fe2O3 and NiO in plagioclase, TiO2 in clinopyroxene, and Cr2O3 in olivine will be evaluated. More primitive melts (e.g. high Mg#, An content, and Fo content) may suggest a gabbro glacier flow type of model while "sawtooth" trends in primitive to evolved compositions up section may reflect sill emplacement as in the sheeted sill model. Although, there's also the possibility that a hybrid or other model may be necessary to describe results from analyses.
Significance
Ocean crust forms 60% of the Earth’s surface, 35% of which formed at fast spreading ridges, although we still do not fully understand how fast-spread crust forms. This work can provide answers to the long-unanswered questions about how this crust forms.
Findings
Major conclusions regarding the origin and nature of lower oceanic crust at Pito Deep from this work and previous studies (Perk et al., 2007; Brown et al., 2019; Gess, 2020; Doorn 2022) on samples collected during the AT11-23 and AT37-08 cruises include:
i) The sampled exposure of lower oceanic crust at Pito Deep reveals two conformable, but compositionally different magmatic suites: the Northern Magmatic Suite and the Southern Magmatic Suite. Consequently, there is lateral compositional variation, and therefore the top ~1.5 km of fast spread lower crust at Pito Deep is not 1-D as suggested by the simple Penrose model (Penrose Conference Participants, 1972).
ii) Both suites have ~100m of more evolved gabbroic rocks at their top which may correspond to the roof zone of an axial melt lens, beneath the sheeted dikes (i.e., the RZSDC, a contact zone between the gabbro unit and the sheeted dikes).
iii) Downward increasing deformation and the inclined dip of magmatic fabrics within both suites supports a motion of downward and outward flow of crystal mush formed at the bottom of the axial melt lens. If correct, this requires rotation and simple shear within the mush; present-day vertical profiles through the lower crust may therefore correspond to near horizontal profiles through the crystal mush originally beneath the ridge axis.
iv) Both suites exhibit complex lithological variation, however, mineral compositions overall, evolve upwards. Less upward variation in the Southern Magmatic Suite may imply near continuous supply of relatively primitive melts to the axial melt lens. In contrast, the systematic, cyclic variation in mineral chemistry with depth throughout the Northern Magmatic Suite requires a pulsed supply of more evolved melt. The transition from one magmatic suite to the other is best explained by temporal variation in melt supply from the mantle.
v) Clear evidence for melt-mush reaction in the rocks of the Southern Magmatic Suite; however, fractional crystallization is recorded throughout lower crustal Pito Deep rocks. It is not clear from the available data whether melt-mush reaction is solely due to the expulsion and upward migration of evolved interstitial melt through the subsiding crystal mush or whether additional external evolved melt is required.
References
Brown, T.C., Cheadle, M.J., John, B.E., Coogan, L.A., Gee, J.S., Karson, J.A. and Swapp, S.M., 2019. Textural character of gabbroic rocks from pito deep: A record of magmatic processes and the genesis of the upper plutonic crust at fast-spreading mid-ocean ridges. Journal of Petrology, 60(5), pp.997-1026; doi: https://doi.org/10.1093/petrology/egz022
Doorn, C.J., 2022. New Insights into Accretion and Cooling of Lower Ocean Crust at Ultraslow-and Fast-Spreading Mid-Ocean Ridges. University of Wyoming, pp. 118-5
Gess, M.R., 2020. The Dynamic Accretion of Fast-Spread Oceanic Crust: Insights from Pito Deep (Southeast Pacific). University of Wyoming, pp. 1-744.
Luna, C.T., 2024. Petrologic and Geochemical Complexities Revealed in Gabbros From Fast-Spread Lower Oceanic Crust at Pito Deep - ProQuest. University of Wyoming, pp. 1-744; https://www.proquest.com/docview/3057596552
Perk, N.W., Coogan, L.A., Karson, J.A., Klein, E.M. and Hanna, H.D., 2007. Petrology and geochemistry of primitive lower oceanic crust from Pito Deep: implications for the accretion of the lower crust at the Southern East Pacific Rise. Contributions to Mineralogy and Petrology, 154, pp.575-590; doi: https://doi.org/10.1007/s00410-007-0210-z